U.S. patent application number 12/183911 was filed with the patent office on 2008-11-20 for electric submersible pumps.
Invention is credited to Alan Thomas Fraser, Michael Andrew Yuratich.
Application Number | 20080284264 12/183911 |
Document ID | / |
Family ID | 27637121 |
Filed Date | 2008-11-20 |
United States Patent
Application |
20080284264 |
Kind Code |
A1 |
Yuratich; Michael Andrew ;
et al. |
November 20, 2008 |
ELECTRIC SUBMERSIBLE PUMPS
Abstract
A method of pumping wellbore fluid, comprising the steps of:
installing an electric submersible pump in a wellbore; and
operating the pump at more than 4,500 rpm to pump the wellbore
fluid. Pumping in this manner provides a number of advantages in
use in that the required high-speed motor and pump is shorter for a
given power than existing arrangements, and provides increased
reliability due to reduced complexity. A much shorter motor/pump
combination also allows such equipment to be used in deviated
boreholes with a reduction in damage due to mishandling and
bending, as well as facilitating assembly and testing in the
manufacturer's plant.
Inventors: |
Yuratich; Michael Andrew;
(Hamble, GB) ; Fraser; Alan Thomas; (Crowthorne,
GB) |
Correspondence
Address: |
William B. Patterson;MOSER, PATTERSON & SHERIDAN, L.L.P.
Suite 1500, 3040 Post Oak Blvd.
Houston
TX
77056
US
|
Family ID: |
27637121 |
Appl. No.: |
12/183911 |
Filed: |
July 31, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10562255 |
Dec 21, 2005 |
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PCT/GB04/02667 |
Jun 21, 2004 |
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12183911 |
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Current U.S.
Class: |
310/87 ; 29/596;
310/156.09 |
Current CPC
Class: |
H02K 5/132 20130101;
Y10T 29/49009 20150115; H02K 5/1285 20130101; E21B 43/128 20130101;
H02K 1/278 20130101; H02K 1/185 20130101; H02K 1/30 20130101 |
Class at
Publication: |
310/87 ;
310/156.09; 29/596 |
International
Class: |
H02K 5/132 20060101
H02K005/132; H02K 21/16 20060101 H02K021/16; H02K 15/02 20060101
H02K015/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 21, 2003 |
GB |
0314553.9 |
Claims
1. A motor, comprising: a rotor comprising: a central shaft; a
carrier sleeve (203) loosely fitted on the central shaft (201); and
support rings closely engaging the shaft and supporting the carrier
sleeve; and a stator coaxial with the rotor, comprising: a stack of
laminations; and radially spaced coils wound around the stack.
2. The motor according to claim 1, wherein the carrier sleeve is
keyed to the shaft to prevent relative rotation between the carrier
sleeve and the shaft.
3. The motor according to claim 2, wherein the rotor further
comprises a key extending outwardly from the shaft and engaging
complementary locating portions of the carrier sleeve and
associated support ring to prevent relative rotation between the
carrier sleeve, the support ring, and the shaft.
4. The motor according to claim 3, wherein the key is of relatively
short length by comparison with the length of the carrier
sleeve.
5. The motor according to claim 1, wherein the rotor further
comprises a plurality of carrier sleeves provided at axially spaced
locations along the shaft, the carrier sleeves rotationally locked
to the shaft.
6. The motor according to claim 5, wherein: the carrier sleeves are
supported on the shaft by the support rings, the carrier sleeves
and the support rings alternate on the shaft, and the rotor further
comprises a retainer constraining the carrier sleeves and support
rings on the shaft.
7. The motor according to claim 1, further comprising: a tubular
housing; and bearings disposed in the housing, wherein the shaft is
supported by the bearings.
8. The motor according to claim 7, wherein the bearings are
disposed between the support rings and an inside bore wall of the
stator.
9. The motor according to claim 1, wherein the rotor further
comprises a plurality of permanent magnets mounted on the carrier
sleeve and equiangularly spaced about the shaft.
10. The motor according to claim 1, further comprising a bearing
mounting the rotor to the stator; a resiliently biased projection
disposed on one of the stator and the bearing; and a receiver
disposed on the other of the stator and the bearing, wherein the
projection is operable, by rotation of the rotor, to engage the
receiver, thereby rotationally coupling the bearing and the
stator.
11. The motor according to claim 10, wherein the projection is
provided on the outer of the stator and the bearing and the
receiver is provided in the inner of the stator and the
bearing.
12. The motor according to claim 1, further comprising: a housing,
wherein the stator is mounted in the housing; an axial groove
formed in one of the stator and the housing; and an axial key
engaging the axial groove, thereby rotationally coupling the
housing and the stator.
13. The motor according to claim 1, wherein the motor is an AC
synchronous permanent magnet motor.
14. The motor according to claim 1, wherein the motor is capable of
reliably operating at speeds greater than 4,500 rpm.
15. An electric submersible pump incorporating a motor according to
claim 1.
16. A motor comprising: a rotor; a stator coaxial with the rotor; a
bearing mounting the rotor to the stator; a resiliently biased
projection disposed on one of the stator and the bearing; and a
receiver disposed on the other of the stator and the bearing,
wherein the projection is operable, by rotation of the rotor, to
engage the receiver, thereby rotationally coupling the bearing and
the stator.
17. The motor according to claim 16, wherein the projection is
provided on the outer of the stator and the bearing, and the
receiver is provided in the inner of the stator and the
bearing.
18. A motor comprising: a rotor; a housing; a stator coaxial with
the rotor and mounted in the housing; an axial groove formed in one
of the stator and the housing; and an axial key engaging the axial
groove, thereby rotationally coupling the housing and the
stator.
19. A motor comprising: an annular stator; a bearing disposed
within the stator and having a peripheral outer surface and an
axial groove formed in the peripheral outer surface; a rotor
mounted in the bearing and disposed within the stator; a
resiliently biased projection extending from an inner peripheral
surface of the stator; wherein the projection is operable, by
rotation of the rotor, to engage the groove, thereby rotationally
coupling the bearing and the stator.
20. The motor according to claim 19, wherein the projection is
provided within a recess in the inner peripheral surface of the
stator.
21. The motor according to claim 19, further comprising a retainer
rotationally coupling the projection and the stator.
22. The motor according to claim 19, wherein the projection is
resiliently biased so as to be capable of being retracted against a
spring force to allow insertion of the bearing.
23. The motor according to claim 19, wherein the projection is a
spring clip.
24. The motor according to claim 19, wherein the projection is an
open circular spring.
25. The motor according to claim 19, wherein the projection is a
spring loaded pin.
26. The motor according to claim 19, wherein the projection is a
cantilever.
27. The motor according to claim 19, wherein the projection is
disposed within a solid block disposed between laminations of the
stator.
28. The motor according to claim 19, wherein the rotor and the
bearing are mounted on a central shaft coaxial with the rotor and
the stator.
29. The motor according to claim 28, wherein the rotor comprises a
plurality of rotor parts spaced along the shaft with bearings
therebetween.
30. The motor according to claim 19, wherein the rotor comprises a
plurality of permanent magnets.
31. The motor according to claim 19, further comprising: a housing,
wherein the stator is mounted in the housing; an axial groove
formed in one of the stator and the housing; and an axial key
engaging the axial groove, thereby rotationally coupling the
housing and the stator.
32. The motor according to claim 19, wherein the motor is an AC
synchronous permanent magnet motor.
33. The motor according to claim 19, wherein the motor is capable
of reliably operating at speeds greater than 4,500 rpm.
34. An electric submersible pump incorporating the motor according
to claim 19.
35. A downhole elongate motor adapted to fit within an elongate
bore, the motor comprising: a rotor; a stator coaxial with the
rotor, and comprising: a tubular housing; a stator core disposed in
the housing and comprising: a stack of laminations having slots;
and windings formed on the laminations; a groove formed in the
housing and extending along the length of the housing; and an axial
key engaging the groove, thereby rotationally coupling the housing
and the stator core.
36. The downhole elongate motor according to claim 35, wherein
axial grooves are provided in both the stator core and the housing,
and the axial key engages within both axial grooves.
37. The downhole elongate motor according to claim 35, wherein the
axial key is in the form of an integral raised feature on the
stator core that engages within the axial groove in the
housing.
38. The downhole elongate motor according to claim 35, wherein the
stator core is locked within the housing by a series of axial keys
engaging within the groove in the housing.
39. The downhole elongate motor according to claim 38, wherein the
stator core comprises a series of stator sections axially spaced
within the housing, each stator section being locked within the
housing by a respective one of the axial keys engaging within the
axial groove.
40. The downhole elongate motor according to claim 35, wherein the
stator core comprises a series of stator sections axially spaced
within the housing (202) and separated by bearing surfaces, the
bearing surfaces being provided to support the rotor within the
stator.
41. The downhole elongate motor according to claim 35, wherein the
motor is an AC synchronous permanent magnet motor.
42. The downhole elongate motor according to claim 35, wherein the
motor is capable of operating at speeds greater than 4,500 rpm.
43. The downhole submersible pump incorporating an elongate motor
according to claim 35.
44. A method of fabricating a stator of an elongate downhole motor
adapted to fit within an elongate bore, the motor having a rotor
and a stator coaxial with the rotor, the method comprising: forming
a stator core by installing windings within slots in a stack of
laminations; and coaxially inserting the stator core with the
windings thereon into a tubular housing, wherein the stator core is
locked within the housing by an axial key engaging a groove formed
in the housing and extending along the length of the housing,
thereby preventing the laminations from turning relative to the
housing during operation of the motor.
45. The method according to claim 44, wherein an outside surface of
the stator core is ground prior to being inserted into the
housing.
46. The method according to claim 44, wherein the windings are
formed within the slots in the stack of laminations by threading
wire through the slots.
47. The method according to claims 46, wherein a mandrel
incorporating rebates is inserted within a bore in the stator core
to centre the laminations prior to the windings being formed within
the slots in the stack of laminations.
48. The method according to any one of claims 44, wherein the
elongate motor is incorporated into a downhole submersible pump.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/562,255 (Atty. Dock. No. MRKS/0141), which is the
National Stage of International App. No. PCT/GB04/02667, filed Jun.
21, 2004, which claims priority to GB 0314553.9, filed Jun. 21,
2003, which are herein incorporated by reference in their
entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to motors and electronic drives for
electric submersible pumps and compressors, and is concerned more,
but not exclusively, with centrifugal pumps.
[0004] 2. Description of the Related Art
[0005] Submersible pumping is a well-established technique for
extracting hydrocarbons from deep boreholes, where the natural
pressure in the reservoir is insufficient to lift the fluid or gas
to surface. The technique is also used in water production.
[0006] Typically the production requirement is to lift large
volumes of liquid against a pressure difference related to the
depth of the well in which the pump is installed. For very heavy
crude oils, slow-speed positive displacement pumps are suitable.
These are usually rotated by a motor at the surface connected to
the pump by a long flexible rod system. Centrifugal pumps have been
found most suitable for normal crude oils, gas and water. These
pumps are rotated by a submerged motor connected directly to the
pump, with electric power being delivered from the surface by a
long cable. Also, the use of electric cables makes installation
possible in deep or long horizontal wells which would otherwise not
be possible with the use of rotating rods.
[0007] The electric motors used for driving the centrifugal pumps
are very elongated, sometimes of a length of more than one hundred
times their diameter. The resulting complexity of such a device,
the difficulty of its manufacture and the quantity of the
degradable insulation materials it employs all reduce the system
reliability.
[0008] Electric motor shaft power output is defined as the product
of rotation speed and torque. For a given physical size and type of
motor there is a limit to the level of torque that can be produced,
typically due to self-heating. A high-speed motor therefore
provides a means for obtaining more power from the same length of
motor, or the same power from a shorter length.
[0009] The output of a pump is normally given in terms of its
hydraulic power, which is the product of flow rate and lifting
pressure (in rationalised units). Centrifugal pump technology is
characterised by the power output being proportional to the cube of
the rotational speed. This known relationship, sometimes termed the
"affinity law", means that a relatively small increase in the
rotational speed can give rise to a substantial power increase.
[0010] Centrifugal pumps are frequently made with hundreds of
impellers threaded on a common shaft, each impeller adding a little
to the lifting pressure. Reducing the number of impellers by
increasing the speed would therefore afford an improvement in
reliability.
[0011] The above demonstrates that a high-speed motor and pump
would, by being shorter for a given power, present direct
advantages in reliability due to reduced complexity, or
alternatively yield a higher output for a similar size. A large
proportion of boreholes are deviated from the vertical and commonly
even to the horizontal. A much shorter motor/pump combination would
also lead to a reduction in damage caused by mishandling and
bending during deployment through the curved sections of the
borehole. Furthermore, the much-shortened length would allow
motor/pump combinations to be assembled and tested in ideal
conditions at the manufacturer's plant prior to being transported
to the borehole location.
[0012] As will be described more fully below, innate limitations in
the established motor and motor controller technology used in the
electric submersible pumping industry have prevented the objective
of higher speed being recognised or addressed.
[0013] Historically, electric submersible motors used for
centrifugal pumping have been of the asynchronous, or induction,
type. The stator is made of steel laminations and copper windings,
and the rotor of steel laminations with copper bars forming the
winding known as a squirrel cage. The rotor laminations are keyed
to a shaft, this shaft providing the means of transmitting output
torque. The rotor poles are produced by induction or transformer
action between the stator and the rotor, using a portion of the
stator current. The stator, in addition, produces a rotating stator
field due to the alternating current in its windings. Since the
transformer coupling to the rotor requires an alternating field in
the rotor, the rotor must turn at a different (lower) speed than
synchronous speed, producing a so-called slip frequency for
induction. Electric submersible motors are made with two poles in
order to achieve the maximum rotating speed from a standard 60 Hz
utility supply. This speed is typically 3500 rpm, slightly less
than the unattainable synchronous speed of 60 Hz.times.1 pole
pair.times.60 s/min=3600 rpm.
[0014] It has become common to use variable speed drives to power
these motors, rather than direct connection to the utility supply.
Variable speed drives first convert utility AC power, typically at
60 Hz, to DC, and then by electronic switching convert the DC to a
variable frequency alternating voltage. The use of a variable speed
drive confers advantages during starting when it can limit the
motor current to a safe level, and during production when it can be
used to manage flow rates. The latter is important when the
changing characteristics of a reservoir are considered over its
producing life. Although variable speed drives, by creating an
artificial supply of 70 Hz or more, can operate the motor at higher
speed than when directly connected to the utility supply, this is a
limited capability. Firstly the elongated induction motor is not
suited to high-speed operations due to internal losses and small
mechanical clearances, and secondly at the medium voltages used
(often several thousand volts rms) drive losses become very high.
Performance is generally limited up to 80 Hz or about 4500 rpm.
[0015] In order to maximise the induced rotor pole strength it is
necessary to minimise the gap between the rotor and the stator.
Unless very hot, the oil in the gap is sheared by the rotor turning
yet remains in laminar flow. As a result the friction absorbs
several percent of motor power. Motor efficiencies above 90% are
sought, and this is an important source of loss in existing motors.
The internal heating caused by these losses, and the copper losses
in the squirrel cage, reduce motor life by aging the insulation
materials.
[0016] The small gap is also a cause of premature failure due to
mechanical causes. The limited diameter of boreholes is a natural
disadvantage to both motors and pumps, and as a result their design
is very elongated. A pump and induction motor assembly for
producing 250 HP may be 20 metres long. This slender assembly is
difficult to handle and particularly subject to damage when being
deployed into deviated or horizontal wells, since small deflections
of the motor housing can cause the rotor to impact on the stator.
Rotor vibration due to bearing wear or imbalance also increases the
chance of rotor impact.
[0017] The requirement for the rotor to be made of laminations and
the limited overall motor diameter act together to constrain the
diameter of the inner torque-carrying shaft. It is common practice,
for example, to couple two 250 HP motors of 5.62 inch diameter
together so as to make a longer 500 HP motor. Shaft strength
limitation prevents this being increased to 750 HP or 1000 HP.
[0018] To provide a high-speed electric pumping system, it is
desirable to increase the rotor clearance, and to reduce the
internal sources of power loss that increase with speed. It is also
necessary to use a drive technology which remains efficient at high
speed and at the different operating voltage levels needed for
different motor speeds required during the life of the well.
[0019] A further requirement of any high or low speed electric
submersible system using variable speed drives is to minimise the
deleterious effects of the electrical switching used to produce the
alternating output voltages. Switching events on the long cables
used in submersible cable propagate as wave fronts that reflect at
connections and most particularly at the motor terminals. These
reflections cause voltage transients that can approach twice the
original voltage, and hence destroy insulation to earth. Commonly
the motor voltage is presumed to be proportionally distributed
through the turns of the stator winding, and the inter-turn
insulation is less than that of the winding to earth. However a
wave front impinging on a motor terminal must travel through the
winding turn by turn before settling to its final value. Therefore
there are short periods in which one turn of a winding carries the
wave front at full voltage and an adjacent turn is unexcited. This
internal voltage difference can exceed the inter-turn insulation
rating, again causing premature failure. Increasing the insulation
level to overcome these problems reduces the space available for
the copper in the winding and also reduces the heat transfer from
the copper, so that the motor specification is reduced.
[0020] An associated consideration for transients is the
interference caused to data transmission systems used to convey
data from instrumentation located in the well bore.
[0021] The foregoing has emphasised high-speed centrifugal pumping
systems. However the same principles of reliable motor performance,
matching efficient drives and circumventing the effects of
transients on long cables are all applicable to positive
displacement pumping systems.
[0022] Positive displacement pumps have a flow rate essentially
determined by a characteristic volume per revolution multiplied by
rotation speed. The torque demand at the pump shaft is determined
by the back-pressure of the fluid column being lifted. These pumps
usually operate at low speeds of a few hundred revolutions per
minute. Since shaft power is the product of rotation speed and
torque, it follows that these pumps are also characterised by
extremely high torque demand. Where there is sand production with
the pumped fluid and where the wells are deviated or horizontal,
the rod connection to the surface has a very short working life. In
these cases it is desirable to use a downhole motor with the
positive displacement pump.
[0023] However, induction motors are inherently unsuited to low
speed and high torque (although variable speed drives have improved
their capabilities in this regard). Thus current installations rely
on a gearbox to match the normal motor running speed and torque to
the pump characteristics. This is also problematic as it is
extremely difficult to make a reliable high torque gearbox in the
small borehole diameter, and it is also expensive.
[0024] A motor having high torque at any speed including low speed
is therefore preferable.
SUMMARY OF THE INVENTION
[0025] It is an object of the invention to provide an efficient
electric submersible pump, comprising a reliable electric
submersible motor capable of operating at low, medium and high
speeds, and overcoming many of the above-described disadvantages of
existing motors.
[0026] It is a yet further objective of the present invention to
provide a high power electrical submersible pumping system of the
order of half the length of conventional equipment.
[0027] According to one aspect of the present invention, there is
provided a method of pumping wellbore liquid, comprising the steps
of: a) installing an electric submersible pump in a wellbore; and
b) operating the pump at more than 4,500 rpm to pump the wellbore
liquid.
[0028] It should be understood that references herein to "pumping
of wellbore liquid" are intended to encompass within their scope
the pumping of multiphase fluids, that is mixtures of water and/or
oil and gas, as well as the pumping of wellbore liquid in a
multi-lateral drilling environment in which the pump is operated to
draw the wellbore liquid from a plurality of lateral well bores
into a central sump.
[0029] According to further aspect of the present invention, there
is provided an electric submersible pump comprising a permanent
magnet motor having a rotor comprising a plurality of permanent
magnets equiangularly spaced about a central shaft, a plurality of
tubular elements supporting the permanent magnets spaced at
different axial locations along the shaft, a retaining sleeve
tightly fitted over the permanent magnets so as to retain the
permanent magnets on the shaft, and a stator coaxial with the rotor
comprising a stack of laminations and radially spaced coils wound
around the stack.
[0030] The invention also provides a motor having a rotor
comprising a carrier sleeve mounted on a central shaft, and a
stator coaxial with the rotor comprising a stack of laminations and
radially spaced coils wound around the stack, wherein the carrier
sleeve is a loose fit on the shaft and is supported on the shaft by
support rings tightly engaging the shaft.
[0031] The invention also provides a permanent magnet motor having
a rotor comprising a carrier sleeve mounted on a central shaft and
bearing a plurality of permanent magnets having axial ends, and a
retention sleeve extending over the magnets and having at least one
end turned in over at least one stress-relieving radially outwardly
extending abutment part on the carrier sleeve abutting an adjacent
axial end of the magnets to retain the magnets in position on the
carrier sleeve without damaging the axial end of the magnet.
[0032] The invention also provides a permanent magnet motor having
an elongate rotor provided with elongate permanent magnet means
extending therealong, and a stator coaxial with the rotor, wherein
the permanent magnet means incorporates axially laminated parts to
reduce eddy current losses.
[0033] The invention also provides a motor having a rotor and a
stator coaxial with the rotor, wherein the rotor is mounted in a
bearing, and one of the stator and the bearing is provided with
resiliently biased projection means for engaging within receiving
means provided on the other of the stator and the bearing to
prevent relative rotation therebetween when the rotor begins to
rotate with respect to the stator on starting of the motor.
[0034] The invention also provides a motor having a rotor and a
stator coaxial with the rotor, wherein the stator is mounted in a
housing, the stator being locked within the housing by an axial key
engaging within an axial groove in at least one of the stator and
the housing to prevent the stator from turning relative to the
housing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] For a better understanding of the present. invention and in
order to show how the same may be carried into effect, reference
will now be made, by way of example, to the accompanying drawings,
in which:
[0036] FIG. 1 schematically illustrates an electric submersible
pumping system;
[0037] FIG. 1A is a block diagram of a variable speed drive;
[0038] FIG. 2 illustrates an embodiment of a downhole motor in
accordance with the invention in cross-section;
[0039] FIG. 3 illustrates the embodiment of FIG. 2 in axial
section;
[0040] FIG. 4 illustrates the means of assembly of an elongated
motor;
[0041] FIG. 5 illustrates a possible construction of a motor stator
in a housing;
[0042] FIG. 6 illustrates an alternative construction of a motor
stator in a housing;
[0043] FIG. 7 illustrates a possible rotor journal bearing;
[0044] FIG. 8 illustrates a possible rotor assembly;
[0045] FIG. 9 illustrates a bearing that creates internal support
pressure;
[0046] FIG. 10 illustrates a known electrical representation of a
permanent magnet synchronous motor;
[0047] FIG. 11 shows the electrical waveforms of an idealised
permanent magnet synchronous motor;
[0048] FIG. 12 shows a known electrical circuit diagram for the
output stage of a variable speed drive;
[0049] FIG. 13 shows the typical electromotive force of a
trapezoidal-wound permanent magnet synchronous motor;
[0050] FIG. 14 shows the electrical waveforms of an idealised
permanent magnet synchronous motor operated as a brushless DC
motor;
[0051] FIG. 15 shows representative waveforms of the variable speed
drive of FIG. 12 for a motor when a variable output voltage or
current is required;
[0052] FIG. 16 shows representative waveforms of the variable speed
drive of FIG. 12 for a motor when a variable output voltage or
current is required, but with a low switching frequency;
[0053] FIG. 17 shows representative waveforms of the variable speed
drive of FIG. 12 incorporating practical switches when a high-speed
motor is driven and a variable output voltage or current is
required;
[0054] FIG. 18 shows the known idealised characteristics of
positive displacement pumps and centrifugal pumps, turbines and
fans;
[0055] FIG. 19 shows an electrical circuit diagram providing
efficient means for varying the speed of positive displacement
pumps and centrifugal pumps, turbines and fans by varying the
internal voltage of a variable speed drive;
[0056] FIG. 20 illustrates a means of providing the supply voltages
to a variable speed drive in accordance with the invention;
[0057] FIG. 21 illustrates the improvement in efficiency provided
by a variable speed drive in accordance with the invention;
[0058] FIG. 22 illustrates a known phasor diagram for the
interpretation of the operation of an idealised permanent magnet
synchronous motor according to FIG. 10;
[0059] FIG. 23 illustrates a means of optimisation of the control
of a permanent magnet synchronous motor by varying the variable
speed drive output voltage in accordance with the invention;
[0060] FIG. 24 illustrates a means of rotor assembly of an
elongated permanent magnet motor in a final position;
[0061] FIG. 25 shows a means of rotor assembly of an elongated
permanent magnet motor in an intermediate position;
[0062] FIG. 26 shows a means of stator assembly of an elongated
permanent magnet motor;
[0063] FIG. 27 shows a stator bore cross-section;
[0064] FIG. 28 shows a mandrel cross-section suited to the
manufacture of a stator assembly of an elongated permanent magnet
motor of the present invention;
[0065] FIG. 29 shows a bearing outer housing suitable for insertion
in an elongated permanent magnet motor of the present
invention;
[0066] FIGS. 30 and 31 show schematic end views of the stator
assembly of a motor in accordance with the invention,
[0067] FIG. 31A showing a detail within a slot of the assembly;
[0068] FIG. 32 schematically illustrates the assembly of such a
stator assembly;
[0069] FIG. 33 schematically illustrates an improved pumping system
according to the present invention;
[0070] FIG. 34 illustrates a motor for the pumping system of FIG.
33;
[0071] FIG. 35 shows a further electrical circuit diagram for the
output stage of a variable speed drive;
[0072] FIG. 36 shows an axial key between the stator and the
housing for use in a motor of the present invention;
[0073] FIG. 37 shows a bearing outer ring suitable for use in a
motor of the present invention;
[0074] FIG. 38 shows a cross-section through a coil suitable for
use in a motor of the present invention; and
[0075] FIGS. 39a and 39b illustrate a possible means of forming a
coil in such a motor.
DETAILED DESCRIPTION
[0076] With reference to FIG. 1, a representative installation of
an electric submersible pump (ESP) is shown. A borehole 101 drilled
in the earth is sealed with respect to the earth from the surface
to below a reservoir 102 with casing 103. The casing 103 is
perforated at 104 to allow reservoir fluid to enter the well. A
pump 107 is provided to lift fluid from the well up tubing 105 to
the surface. The tubing 105 is sealed to the casing 103 by packing
106 so that the reservoir fluid must go through the pump to reach
surface. A permanent magnet submersible motor 108 (PMSM) is mounted
beneath the pump 107. The connecting shaft of the motor 108 passes
through a seal and pump thrust bearing assembly 109, often termed a
`protector`. The pumped fluid passes over the motor 108 before
entering the pump 107 and thus provides a certain amount of cooling
of the motor 108.
[0077] A power cable 110 for the motor 108 is run up past the pump
107 and alongside the tubing 105 until it emerges at the surface
wellhead and passes to a variable speed drive 111. This drive 111
is powered by the utility supply 112 or a generator.
[0078] It will be appreciated that other configurations of the
installation are possible, such as mounting the pump below the
motor, and taking the cable up the tubing or making it an integral
part of the tubing. Arrangements, such as that disclosed in U.S.
Pat. No. 6,000,915, which accommodate the pump concentrically
within the motor bore will generally be found to make poor use of
the limited borehole cross section and are not preferred.
[0079] FIG. 2 shows a cross-section of an embodiment of PMSM in
accordance with the invention comprising a central rotor and
surrounding annular stator within a housing 202. The rotor has a
central shaft 201 for transmitting the output torque, and a
plurality of magnetically permeable sleeves 203 carrying permanent
magnets 204. The sleeves 203 are torsionally locked to the shaft
201 by keys, shrinkage or other means in the art. It is preferable
to make the sleeves 203 separate from the shaft 201 as shown, for
reasons of mechanical stability, to facilitate assembly and to
permit the optimum strength material for the shaft 201 to be chosen
independently of the sleeve material. The magnets 204 are
preferably of a samarium cobalt composition as this gives the best
economic performance at the temperatures commonly found in deep
boreholes used for hydrocarbon production. Other materials such as
neodymium iron boron may be used in appropriate circumstances, or
improved materials as they become available.
[0080] During high-speed rotation the magnets 204 experience
considerable centrifugal force, and the adhesive that bonds them to
the sleeves 203 may weaken with age. A retaining sleeve 205,
preferably of metal, provides a durable means of retention. To
avoid the use of materials which degrade during prolonged operation
at high temperature, it is preferable to make the sleeve 205 a
tight fit by shrinking it on, rather than depending upon tape,
adhesives and fillers. The sleeve 205 is preferably of one piece
although, for ease of assembly, several shorter rings may be fitted
adjacent to each other if required. U.S. Pat. No. 4,742,259
discloses a technique for fitting a sleeve with axial constraint.
This technique requires the fitting of end washers that are pressed
to the shaft to locate them without using positive abutments to do
so. In a preferred arrangement shown I\N FIG. 8a, rings 422,
preferably made of non-magnetic, non-conducting material, may be
slid onto the rotor sleeve 203, coming up against abutments 424,
and the retaining sleeve 205 over the magnets 204 may be rolled
over the outer faces of the rings 422, as at 425, thereby locking
the whole assembly in place axially without the need for adhesives.
Variations on this locking method are possible within the scope of
the invention, such as deforming the sleeve 205 with a punch into a
detent on the outer surface of the ring 422, or using a snap ring
and groove as a shoulder in place of machined feature 424.
[0081] The assembly so far described is termed the rotor, and the
length of motor delineated by a sleeve is termed a rotor stage.
FIG. 3 shows in axial section a single rotor stage.
[0082] The magnets 204 are circumferentially disposed about the
sleeve 203, and alternately poled in an essentially radial
direction to cause aspatially alternating magnetic flux to cross
the clearance gap 209. Other magnet arrangements will be known to
persons skilled in the art. The entire motor and hence the gap 209
are filled with a benign fluid, such as a highly refined mineral
oil, to balance the inside of the motor against the external
wellbore pressure.
[0083] Preferably the magnets 204 are plated, for example in a
vapour deposition process, with corrosion-resistant material such
as aluminium, so that they may resist corrosion from any ingress of
moisture into the motor or from other sources, and so that any
small loose particles of magnet material will be sealed into the
magnets and not come free to circulate within the motor bearing
system.
[0084] Contained within the tubular housing 202 is a stack of thin
magnetically permeable laminations 206 as may be seen more clearly
in FIGS. 3 and 4. Insulated wire, preferably made of copper coated
with high-integrity insulation such as polyetheretherketone or
polyimide materials, is wound through the slots 207, and looped
back 214 at the ends of the lamination stack as part of the coil
winding process. The wound lamination assembly constitutes the
stator of the motor.
[0085] The PMSM motor constructed as described will have many
desirable characteristics for submersible pumping, associated with
the general nature of permanent magnet motors. For example, by
providing a rotor flux from permanent magnets, there is no need to
energise the rotor, unlike the field winding that requires separate
power in an induction motor. This reduces the motor current by the
amount needed for rotor magnetisation, which therefore reduces the
ohmic loss in the stator windings and the power cable. It also
eliminates the rotor cage winding and thus an internal source of
heating. The copper within the stator is used only for the
production of the rotating stator flux. The inherent torque output
for the motor, which is derived from a product of space
utilisation, rotor flux and stator flux, is very high compared to
an induction motor. This torque is available at any speed.
[0086] Further aspects of the motor construction may be addressed
to give reliable high-speed performance. Firstly, as mentioned
above, a major source of inefficiency in induction motors is the
frictional drag in the necessarily small rotor-stator gap. In a
normal mass-produced PMSM the gap is also kept small in order to
economise on the amount of magnet material required. However, if it
is considered that permanently magnetic material is not itself
significantly magnetically permeable, then for magnetic purposes
the gap between the stator and the rotor is that between the
lamination tip 210 and the outer surface of the sleeve 203. The
mechanical clearance gap 209 is only a part of this. Thus, if, for
example, the magnet thickness was 3 mm and the clearance gap was
increased from 0.25 mm to 1.25 mm, or 500%, the magnetic gap would
only have increased 30%. With only a modest increase in the amount
of magnetic material it is possible to purposely design the motor
to ensure a sufficiently large mechanical clearance such that at
high speed the fluid in the clearance is turbulent. Above 5400 rpm
for a rotor of diameter more than 50 mm a gap greater than 1.25 mm
is preferred for this purpose. A designer may use the known
Reynolds number theory to estimate the needed gap size for other
operating conditions, fluids and motor sizes. Although the friction
loss is higher in turbulent flow than in laminar flow, turbulent
flow ensures much more effective heat transfer between the rotor
and the stator, so reducing the maximum internal temperature. At
any speed the large clearance will reduce the likelihood of
mechanical damage to the rotor during installation caused by
bending of the outer housing, and also provide a measure of
tolerance to contaminant particles.
[0087] Furthermore, it will be found that the deliberately large
gap reduces the eddy current losses, and hence heating, induced in
the retaining sleeve 205 and magnetic material according to their
conductivity. These losses increase approximately as the square of
rotation speed, but diminish with distance from the lamination tips
210. The inter-magnet spaces 213 may be filled but, unless care is
taken to seal the cavity, particles of filler may dislodge over
time and damage the motor. If required the cavities may be left
unfilled. This is made possible by the sleeve 205, since it
presents a low drag rotating surface to the clearance gap while
making an enclosure to trap the fluid in the cavities. This trapped
fluid is limited to bodily rotation or axial flow and does not
contribute to friction in the clearance gap.
[0088] A further reduction in eddy current losses in the rotor can
be obtained by laminating the magnets axially. Rotor eddy current
losses originate from flux harmonics in the stator, the eddy
currents circulating on the face of the magnets and penetrating
through the depth of the magnets and then into the steel that the
magnets are bonded to. Most of the ohmic losses resulting from this
current flow are in the magnets, assuming the retaining sleeve is
non-conducting or very thin, and the current flow increases with
the face area of the magnets. Accordingly, in the same way that the
stator steel is laminated to reduce the effect of rotor flux, the
magnets can be laminated to reduce the effect of stator flux. For
an elongated motor the face area is the width of the magnet times
the continuous length of the magnet section. Therefore, by using a
series of short magnets to make a continuous length that are
electrically isolated from one another where they would otherwise
touch, the effect is to produce an axially laminated magnet.
Practically the magnet ends may be coated with epoxy or varnish
during assembly or spacers used. An approximately equal length and
width of each magnet will be found to give a good reduction in
losses while not unduly complicating manufacture. This method is
unlikely to work usefully in motors of conventional length to
diameter ratios as the magnet face area is already relatively
small.
[0089] Other types of PMSM construction are possible, while
maintaining the large gap. A slot-less construction in which the
laminations become a stack of rings, or are replaced with a
magnetically permeable tube, requires much more magnet material and
will normally be found uneconomic for submersible pumping.
[0090] It is also possible to design the PMSM so that the slots are
fully closed or almost fully closed in the vicinity of the
lamination tips 210. This ensures the retention of the winding
without use of insulating retaining wedges that may degrade. It
also reduces the cogging torque, that is the alternating
accelerating and retarding torque developed as the magnets come
into and out of maximum overlap with the teeth.
[0091] For the purpose of maximum power output at high efficiency
it is necessary to optimise the electromagnetic design. Unlike
conventional submersible pump induction motors, which invariably
have two poles for the reasons given above, it will be found that
the optimum number of poles is usually six or more for PMSM motors
up to approximately seven inches (17. 5 cm) in diameter. Four poles
will give an acceptable output for smaller motors but even more
poles are preferred for larger motors. The higher pole count allows
the flux density in the stator laminations to be better distributed
so that the amount of steel in the outer areas 211, may be reduced.
This permits the area of the slots 207, and hence the amount of
copper in the windings, to be increased. When the high frequency
restrictions discussed below on drive output are considered it will
be appreciated that, in larger motor sizes, higher pole counts are
more demanding of the drive. A limit may be reached where accepting
additional stator lamination outer material is appropriate to make
the drive practical. Conversely as taught in U.S. Pat. No.
6,388,353, with drives and step-up transformers typical of oilfield
induction motor technology, a high pole count motor permits
operation at low speed and high torque for progressive cavity
pumps. For example, a ten-pole motor driven at a frequency of 60 Hz
will rotate at 720 rpm.
[0092] FIG. 5 shows a representative cross-section of a PMSM motor
of the present invention constructed using known technology in the
field of submersible induction motors. A plurality of rotor
assemblies is used to achieve the desired output power, the
assemblies being rotationally locked to a common shaft 201 running
continuously through the electrical section of the motor. Shaft
stability is ensured by bearings 401 between each rotor assembly.
These bearings 401 are commonly made of two concentric rings
running freely one over the other, one keyed axially and
rotationally to the shaft 201 and the other locked to the stator
bore using thermal expansion caused by the motor's self-heating, or
pegged in some way.
[0093] U.S. Pat. No. 4,513,215 and U.S. Pat. No. 4,521,708 teach
means added to the bearing outer ring for pegging or gripping the
bearing outer ring to prevent rotation during motor start up,
before thermal expansion has taken effect. However the larger shaft
diameter made possible with PMSM motors necessarily reduces the
bearing outer ring wall thickness so that such known methods cannot
be used with such motors. FIG. 37 shows a bearing outer ring 3704
suitable for use with PMSM motors utilizing a spring clip 3702
fitted to the stator which engages in a shallow axial groove 3701
in the bearing outer ring 3704 (as may be best appreciated by
referring to the inset view showing a section taken normal to the
plane of the drawing). The spring clip 3702 is preferably an open
circular spring, such as a commercial circlip or steel wire, since
this provides a natural axial resilient lead-in as the spring clip
3702 engages the bearing outer ring 3704. When the bearing outer
ring 3704 is inserted into the stator bore, in all probability the
groove 3701 will not be opposite the spring clip 3702. The
resilient lead-in allows the spring clip 3702 to push back to allow
bearing insertion. When the motor starts, the bearing outer ring
3704 will rotate until the groove 3701 comes opposite the spring
clip 3702, allowing it to expand and engage the groove 3701,
thereby preventing further rotation. A spring loaded pin or
cantilever may also be used. In normal construction the stator is
made of brass or possibly steel laminations 403 at the bearing
sections. To make the stator-mounted spring clip 3702 practical,
these laminations are preferably replaced by a single thick block
3703, cut as if it were a very thick lamination (slots not shown in
FIG. 37). This may be a casting. The spring clip 3702 is then
mounted in a pocket in the block so that it cannot fall out during
assembly. A small peg 3705 prevents the spring clip 3702
rotating.
[0094] An improved method of assembling the rotor illustrated in
FIGS. 7 and 8 simplifies the bearing assembly and also the means of
affixing the rotor sleeves 203 to the motor shaft 201, using a
reduced number of parts.
[0095] The need to assemble the rotor with long solid sleeves 203
presents a problem in that a stable fit to the shaft 201 is
necessary, but the required shaft straightness for closely fitting
sleeves 203 to pass smoothly over the shaft 201 during assembly is
very demanding. The preferred means of assembly is to use support
rings 411 as shown in FIG. 7. These rings 411 are a close fit on
the shaft 201 but, being short in length, will slide easily over
it. Lands 415 provide a concentric fit for the sleeves 203 and
shoulders 416 provide an axial abutment. The bore of the sleeve 203
is only a loose fit on the shaft 201. As shown in FIG. 8, one or
more sleeves 203 and support rings 411 may be threaded onto the
shaft 201, and axially constrained by convenient means such as snap
rings 414. Each sleeve 203 must be rotationally fixed to the shaft
201 in order to transfer the motor torque, and each ring 411 must
be prevented from rotation on the shaft 201 in order to eliminate
wear. Referring again to FIG. 7, a key 413 may be provided to
accomplish this, in which case, during assembly, the ring 411 is
first slid onto the shaft 201, then the key 413 is inserted into a
groove 419 in the shaft 201 and within a locating notch 418 in the
ring 411. The sleeve 203 has an internal groove 417 so that, when
it is slid onto the shaft 201, it becomes rotationally locked to
the shaft 201 by the key 413 and also prevents the key 413 from
subsequently falling out.
[0096] The use of a relatively short key, such as the key 413,
ensures that the torsional stress in the sleeve 203 is limited to
that caused by the torque generated by the magnets on the same
sleeve 203. In a long motor, the portion of the shaft 201 under the
sleeve 203 nearest the output end of the motor will carry a high
torque accumulated from all the other sleeves. Particularly where
multiple motors are connected in series to increase power output,
this torque can be very high. If the accumulated stress in the
shaft 201 were to be shared with the sleeve 203 by way of a long
key, there would be a risk that the magnets, being brittle, would
fracture. (In submersible induction motors long keys are used to
maintain all the laminations of the rotor in alignment, as well as
to transfer torque.)
[0097] A further consequence of very high torque is that twist in
the shaft 201 may cause sleeves 203 at opposite ends of the shaft
201 to come out of alignment with each other and hence with the
stator, with the result that the sleeves 203 cannot at the same
time produce maximum torque. U.S. Pat. No. 6,388,353 suggests
mounting the sleeves on the shaft with an angular skew relative to
one another so that, when twisted in use, the sleeves are brought
back into alignment. Alternative methods that can be used with
series-connected motors are (i) to stagger the angle of each shaft
to the next in line, such as by cutting the splines at the ends of
the shaft with a small angular offset relative to one another, or
(ii) to connect the housings to one another with a small angular
offset. Within a single stator, the simple expedient of twisting
the stator will effect a compensating correction to a twisted
shaft, and is similar to a well-known technique for reducing motor
cogging torque. The compensation of all these methods has variable
effectiveness as the skew is fixed at one angle to compensate the
angle of twist at one level of torque, and cannot therefore be
correct at other levels of torque. It should be noted that the
amounts of twist referred to are very small, typically less than a
degree, and the problem may not be significant if the motor is
designed to maximise shaft diameter and hence resistance to
torsion. Accordingly it is preferred to design elongated motors so
as not to suffer from excessive shaft twist.
[0098] The sleeve 403 carries, or is integral with, the shaft
bearing. A ring 407 of bearing quality material, such as that
marketed under the tradename Deva Metal, may be pressed onto the
ring 411. The outer ring 420 of the bearing runs on the ring 407,
and is axially captured by thrust washers 421 which themselves are
captured between the sleeves 203 and the support rings 411.
Alternative arrangements for the bearings, in which for example the
support ring 411 is made entirely of bearing material, eliminating
the ring 412, are possible within the scope of the invention.
Similarly the outermost rings 411 may be of modified shape as their
outermost ends do not mate to rotor sleeves.
[0099] Substantial heat, of the order of 100 W, will be generated
within the bearing. This heat is transferred to the laminations and
thence the motor housing by two main means, namely conduction
through the outer ring 420, and conduction through the support ring
411 and the rotor. In the latter case heat passes through the
magnets near each bearing and across the oil-filled rotor/stator
gap. This second path, though less direct than the first, will
significantly raise the magnet temperature. A thermal insulator in
this path between the bearing running surface and the magnet, such
as may be provided by making the support ring 411 of ceramic, will
increase the thermal resistance in this path, and thus reduce the
magnet temperature rise.
[0100] In a completed PMSM the rotor is centred by the bearings 401
and so is magnetically balanced. Particularly when the motor is
installed vertically the bearing loads will be very low, and the
bearings 401, which necessarily run hydrodynamically for maximising
lifespan, may become unstable, resulting in shaft whirl and other
vibrations. In the present invention therefore, the bearings 401
must be designed to create sufficient internal pressure to remain
stable and hydrodynamic at low shaft load. FIG. 9 shows a means of
achieving this in which a proportion of the length of each bearing
401 is etched or machined with spiral grooving 409. The grooving
409 swirls the oil within the bearing 401 at interface 407,
increasing bearing pressure and enforcing stability. The length of
grooving 409 is a means of varying the pressure. The grooving 409,
being inherently a miniature pump, controls and also promotes flow
of oil through the bearing 401, assisting in cooling and cleaning
it. Alternative bearings, such as known bearings with non-circular
bores, may also be used to achieve stability.
[0101] A further means of purging oil through the bearings 401 is
to bore the motor shaft 201 for the introduction of oil throughout
the length of the motor to cross bores 406 at each bearing 401 as
shown in FIG. 7. Utilising an impeller or cross drillings on the
shaft 201, preferably by way of an oil filter thereupon, oil is
forced into the shaft 201, through the bearings 401 and then
returned by way of the rotor-stator clearance 409. Pilot bores 410,
as shown in FIG. 7, or grooves in the bearing housings or the
stator bore may be provided to assist this return path, as will
unfilled interstices between the magnets on the rotor. The bearing
running clearances, being small, resist and thereby limit the flow
of oil from the shaft 201. This is particularly the case for plain
bearings. Because spiral groove bearings control their rate of oil
intake, it is preferable to arrange a copious supply of fresh oil
near the inlet end of the bearing 401, such as by making the cross
bores 406 near and at least partially beyond the inlet end of the
spirals. Then there is no need to force the oil through the
bearings 401. Instead the oil moves freely through the bores 406,
circulating past the bearing, while the bearing ingests the portion
of the oil that it requires. Alternatively or additionally, oil
that is flowing axially through the rotor-stator annulus would not
normally help to lubricate the bearings 401 as no pressure is
developed to force it into them. The spiral groove bearings will
benefit as they ingest from the flow. This method thereby separates
the general circulation of substantial rates of fresh and cooling
oil from the individual bearing lubrication process. It is
generally applicable to any type of elongated fluid-filled
motor.
[0102] If the stator bore is of constant, carefully controlled
diameter, then the rotor assembly, complete with bearings, may be
slid into the stator. The stator thereby provides the outer support
for the bearing outer rings 420. However this known arrangement
necessarily requires the bearing rings to rub the stator bore
during insertion, with the possible risk of abrasive damage.
[0103] Alternatively the stator bore may be made of smaller
diameter 403 in the axial sections opposite the bearings, such that
the reduced bore lies within the stator to rotor clearance, as
indicated by the broken lines 408 in FIG. 7. In the assembled
position shown in FIG. 5, the bearings 401 are shown in contact
with these specially reduced sections 403 of bore.
[0104] This means of assembly is not immediately suited to PMSMs
due to the extremely high side magnet forces between the rotor and
the stator caused when the rotor becomes slightly eccentred in the
stator bore. This is well known in small industrial motors where
external fixtures are used to handle the forces involved during
insertion. In an elongated motor the problem is very serious since
it is not effective to support the shaft from each end. It may be
found from electromagnetic calculations that a force of thousands
of Newtons per metre of rotor length and millimetre of deflection
may be produced. A much smaller force is sufficient to bend, or
deflect, a shaft held only at its ends. Any deflection increases
the side force leading to more deflection. Thus if the rotor
bearings mate only to a restricted stator bore, then for most of
the insertion process they will not provide support to the rotor,
and the rotor will deflect until the bearings touch the stator bore
between the restricted sections. It would be difficult to slide the
rotor into the motor, and also the bearing outer diameters will now
be offset from the restricted bore section 403, so that the
restrictions become obstructions.
[0105] FIGS. 24 to 29 show representative means of the invention
for overcoming these problems. More particularly FIG. 27 is a
cross-section of the stator bore in the vicinity of the bearing
restricted bore section 403. The restriction surface 2101 is
interrupted by three equiangularly spaced cutaways 2103 that take
the bore back to the normal diameter of the lamination tips 210.
FIG. 29 is a cross-section of the bearing outer ring incorporating
three equiangularly spaced rebates 2105 and intermediate lands 2104
corresponding in position to the cutaways 2103 of the restricted
bore section 403, the rebated surface 2102 having the same diameter
as the restriction surface 2101, and these surfaces 2101,2102
mating when the rotor is finally installed and the bearings are in
the restricted bore section 403, as shown in FIG. 24. The outer
surfaces 2113 of the bearing outer ring are a sliding fit with the
lamination tips 210. At interim positions during installation, as
shown in FIG. 25, the outer surfaces 2113 provide a good degree of
centralisation of the shaft 201 between each rotor assembly. This
mechanical support ensures the rotor side forces remain acceptably
low during rotor insertion, and make it possible to insert the
rotor assembly without damage. The external fixture will
necessarily constrain the axial movement of the rotor to prevent it
being pulled into the stator by the magnetic force. It will be
apparent to a skilled person that, by suitable shaping of the
leading edges of the rebates 2105, the alignment of the rotor
bearings to pass through the bore restrictions may be facilitated.
However, in a motor with many bearings, this will remain
troublesome. The larger rotor-stator gap in a PMSM permits a means
of alignment to be used in which each bearing ring is bored so that
a stout wire, such as a tempered steel wire, may be threaded
through each bearing. Keeping this, or a guide strip in a groove
cut in the surface 2103, taut will greatly facilitate the alignment
of all the rebated bearings as the rotor is inserted.
[0106] FIG. 4 shows a first step in the known, labour intensive
method of manufacturing an electric submersible induction motor
stator. The loose laminations 206 are threaded onto a mandrel 303
and inserted into the housing, being prevented from escape by an
internal ring 301. The shoulder 304 is then used to compress the
lamination stack, which is fastened in place by a second internal
ring 302. The housing length extends beyond the laminations
considerably further than the illustration shows, in order to leave
space for the stator winding end turns and for mechanical
components. Winding the stator is laborious as the conductors have
to be threaded axially through the slots back and forth, turned
around, wrapped in additional end insulation and inspected, all
taking place inside the housing ends. Furthermore, the laminations
necessarily are a relatively loose fit in the housing in order to
be able to slide them in. This leaves a significant thermal contact
resistance between the laminations and the housing, which impedes
heat transfer, raising the motor internal temperature and hence
reducing its reliability. During motor start up, the torque
reaction on the stator is transmitted to the housing by way of
rings 301 and 302, and, when the motor has warmed up, the stator
expands so that it grips the housing along its length. It is a
significant mode of failure during start up for the middle section
of the stator to twist between the ends portions, thereby damaging
the windings, and the arrangements described below are effective in
preventing such failure.
[0107] The preferred embodiments of the present invention radically
change the method of assembly of elongated motors. Firstly the more
conventional approach to making small non-submersible motors is
adopted, in which the stator is wound before insertion into its
housing.
[0108] A shrink fit of the finished stator is used to ensure high
contact pressure with the housing, reducing thermal contact
resistance and thereby minimising the internal temperature of the
motor. However the invention provides for the special circumstances
of an elongated motor that has internal bearing surfaces that may
be on a reduced stator bore diameter. FIGS. 26 and 28 illustrate a
means of aligning the laminations ready for winding utilising a
rebated mandrel 2107 having outer surfaces 2105 that are a close
but sliding fit in the stator bore. The rebates 2106 are clear of
the restriction surfaces 2101. This mandrel 2107 provides a
centring surface for all the laminations, including those used for
the bearings. A simple nut and shoulder on the mandrel 2107 is
sufficient to clamp the laminations ready for winding by threading
of the wire through the slots. The shoulders of the rebates 2106
may be partly tapered to bring the bearing laminations into
rotational alignment. However, in order to reduce sliding friction
when the mandrel 2107 is eventually removed, the surfaces 2105 are
preferably reduced to thin ribs or have ridges that reduce the
contact area. If the laminations are open to the stator bore, or if
the tips have notches on the internal bore, such features on the
mandrel 2107 or inserts mounted thereupon may be used to
rotationally align all the laminations.
[0109] The laminations may be welded together on their outer
diameter so as to maintain the close stacking of the laminations,
or some other known means may be used. After winding and possibly
varnish impregnating, the stator may be ground on the outside
diameter to make it a close fit in the housing. Preferably the.
housing will be pre-expanded in order that, after insertion of the
stator, the housing will relax to a shrink fit. The pressure of
contact will then greatly reduce the contact thermal resistance. A
means of expanding the housing for assembly is to pre-heat it.
Conversely, to repair a stator, or to recover the housing for
re-use, it will be necessary to expand the housing with the stator
in situ. The length of the stator renders the required force to be
too high for a press tool to be used. A preferred means is to use
an induction heater, which essentially comprises an electrical coil
that slides over the housing, the coil being connected to a power
generator of appropriate frequency. It is known from the theory of
induction heating that, by choosing a frequency such that the skin
depth of radiation in the housing does not penetrate through the
housing, energy may be rapidly and selectively imparted to it
without penetrating the stator. This provides a time window in
which the housing will release the stator and the stator may be
extracted before it becomes heated by diffusion and expands to
re-establish the lock. This method is suited even to elongated
motors.
[0110] A further method with great advantages in terms of
manufacturing and repair cost, as well as in terms of reliably
preventing the stator from rotating within its housing, is to
mechanically lock the stator to the housing with an anti-rotation
device or devices. One possibility is to provide an axial key 3603
between the stator and the housing 202, as illustrated in FIG. 36.
The housing 202 is formed with an axial groove 3601, such as by
milling or broaching, and the stator is formed with a corresponding
axial groove 3602. The axial key 3603 or series of keys fits in
both grooves 3601,3602 so as to prevent the stator from turning
relative to the housing 202. This technique avoids the need to
either press fit the stator with great force or to shrink fit the
housing over it, both of which are inevitably time consuming in
manufacturing terms and require specialist equipment. With the
keyed housing there is no possibility for breaking free. It will be
apparent that there are many other possible arrangements that can
be adopted using keying, such as leaving an integral raised feature
on the circumference of the lamination which locates in the groove
3601. Where the laminations are pre-bonded into shorter lengths the
anti-rotation device may be applied on a per length basis.
[0111] The embodiment disclosed has the further advantage over
conventional construction of induction submersible motors that, by
providing easy access to the end-windings, the highest quality
winding procedure may be followed and the results easily inspected.
This technique is applicable to all types of elongated motor.
[0112] FIG. 30 shows an improved reliability and improved
performance method of winding a PMSM. The winding is a known
short-pitched concentric winding, which is not normally suited to
industrial PMSMs or induction motors because the back-emf waveform
is far from sinusoidal. It may however be driven by a variable
speed drive in which the back-emf waveform is taken into account,
and it is particularly suited to the variable speed drive
embodiments of the present invention that are disclosed below. In
the example shown the motor has three phases A, B, C. The
laminations 2201 have six slots for an eight-pole rotor. For phase
A, coil 2202 is wound through adjacent slots so there is a single
tooth 2210 separation between the coil sides. Identical coils 2203
and 2204 are wound on alternate teeth for the other phases B and C,
where the hatching emphasises the extent of the completed coils.
The slots are shown closed at the stator bore as this is
preferable.
[0113] One advantage for reliability is that there are only three
coils, one per phase. Consequently there is no crossing of phase
windings at the end turns. The end turns 2205 possess further
advantages. The end turns 2205 fall naturally within-the radial
limits of the slots and are short. This minimises risks with
insulation chafing, provides a short path back to the stator for
conduction cooling, and has minimal energy waste in unproductive
copper. This in direct contrast to double-layer lap windings, as
used in induction submersible motors. In these the end turns need
to expand beyond the radial limit of the slots and/or become very
long in order to accommodate the wire crossings between layers and
phases. Since the winding area is constrained by the housing
internal diameter and the bearing/rotor outer diameter, the problem
is severe.
[0114] A winding with six slots and eight poles as described above
provides phase separation in the slots. However, at high speed, the
high pole count makes heavy demands of the variable speed drive as
described below. Furthermore the self-inductance of each winding is
high, requiring more drive voltage to overcome its reactance for a
given motor current, which is manifest as a poor power factor. A
compromise embodiment of six poles and nine slots for a motor
having three phases A, B and C, as shown in FIG. 31, is
satisfactory and preferred. In this embodiment three
series-connected coils are provided for each phase, and each slot
accommodates the coils of two phases. However, since these coils
are wound around separate teeth, they are naturally spaced apart by
a gap 3106 as indicated in FIG. 31 and can be well insulated from
each other.
[0115] A preferred improvement to the laminations where two coils
are adjacent in the same slot is to introduce partial teeth 3110
shown representatively in FIG. 31A. These teeth have little effect
on the motor magnetic circuit as they do not form a closed loop
around the coils. However they form an intermediate path for heat
transfer to the outside of the motor, and, if the motor body is
held near the potential of the neutral point, there is less strain
on the insulation between each of the coils and the tooth 3110 than
between the coils of different phases.
[0116] A further, preferred feature, that may be used where the
coils are pre-fabricated and accommodated in a common slot
including partial teeth 3110 for separating adjacent coils as shown
in FIG. 31A, is the provision of slots that are substantially
shaped to conform to the cross-section of the coils in order to
provide close thermal contact and mechanical support between the
coils and the surrounding laminations. Such an arrangement is shown
in FIG. 38 in which only one of the coils is shown within the slot,
and the coil comprises four coil sections 3801 preformed from
rectangular wire, each individual coil section being encapsulated
within a respective layer of insulation 3803 and the coil sections
being fitted together to form a rectangular bar which is itself
encapsulated within an overall layer of insulation 3802.
[0117] A further, and preferred, aspect of the present invention is
shown in FIG. 32, which, for convenience in drawing, is again shown
based on six-slot laminations. In this case the laminations are
made in two parts, as if split in the vicinity of the root of the
slots so that a multi-tooth inner part 2207 and a circular outer
part 2208 are obtained, as shown separately on the right hand side
of the figure and fitted together on the left hand side of the
figure. These parts 2207,2208 are preferably made from the same
piece of material and are thus of closely similar thickness. In the
final assembly this will reduce bridging of the tooth tips between
laminations due to thickness mismatches. In the assembly process
the lamination parts 2207 are assembled onto a mandrel, and
separately wound and formed coils, preferably vacuum pressure
impregnated and over-wound with protective insulation, are then
simply slid over the lamination teeth 2209 of the parts 2207.
Sometimes varnish is considered unsuitable in the face of
hydrolysis from moisture ingress into the motor. In these cases,
wire insulation such as polyetheretherketone may be used in loose
coils, whilst still being wrapped to resist chafing in the
electromagnetic fields of the motor. Conveniently stacks of outer
laminations 2208 bonded together or welded along their exterior are
slid over the wound core to complete the stator. While it is
possible to heat-shrink these onto the wound core, the housing
shrink fit disclosed above will also apply the light compression
necessary to ensure good mechanical stability of the stator.
[0118] The outer lamination stacks may instead be made as
magnetically permeable tubes of cast insulated iron powder, with
the advantages of offering a smooth surface where the inner and
outer parts of the stator come together, and of economy of
materials. Each tube may be made by combining smaller arcuate
segments, to reduce the size and cost of the casting, such an
arcuate construction being unfeasible with laminations which would
effectively be small steel fragments. The partial teeth disclosed
above may be incorporated into the outer ring of the split
lamination. The disclosed multi-part stator must also be multipart
or open-slotted outwards at the bearing sections 202 to permit
loading of the winding from the outside.
[0119] This method of assembly translates the known advantages of
form-wound coils used in physically large industrial motors to the
difficult elongated small diameter geometry of submersible
motors.
[0120] Form-wound coils for lap wound large motors are manufactured
separately from the laminations and are then inserted into
rectangular slots open to the stator bore. In submersible motors
there is insufficient working space in the bore of the laminations
to load the formed-coils ready to insert into the slots, and for
high speed motors the open slots would cause substantial
losses.
[0121] By opening the lamination slots from the outside, the
invention permits formed coils to be used while not incurring these
problems.-The particular advantage of the concentrated winding is
that the formed coils are very simple and that the large winding
slots for the small size of motor facilitates the use of semi-rigid
rectangular or wedge-shaped copper wire.
[0122] With formed coils the wire is bent once at any position as
it is wrapped to form the coil, unlike the conventional process for
winding elongated motors in which the wire is threaded back and
forth through the slots. Consequently a much more rigid wire may be
used, as there is no work hardening and insulation damage that
would occur if it was attempted to wind conventionally, with
repeated bending. Rigidity solves the known problem in elongated
motors of wires crossing within a coil deep inside the stator. It
provides a non-rubbing and stiff end turn assembly. Round wire is
known to give a very poor copper fill factor in a slot compared to
rectangular wire, essentially because the latter packs together
better. Typically the thermal conductivity from the copper through
its insulation back to the lamination is improved also. With a high
copper fill the motor will have much reduced internal heating
compared to a conventionally round wire wound motor. This is a
source of improved reliability, or alternatively of higher torque
for the same temperature rise.
[0123] While rectangular wire is preferred, it will be appreciated
that formed coils made from round wire will nevertheless be
superior to round wire conventionally wound by threading through a
stator. For a lower pole count motor, such as the commonly used
two-pole induction motor, the winding pitch is necessarily
substantially half the circumference of the motor. This means that
many end turn crossings are unavoidable.
[0124] The flexibility of round wires is beneficial in this case,
whilst retaining the key advantage disclosed above of
prefabricating the coils.
[0125] Formed coils that are fully encased in insulation over the
portions that enter the stator slots do not require insulating slot
liners. Furthermore it is not necessary to impregnate the coils
after insertion to complete the insulation and secure them into the
slots, provided that the insulation is impregnated or encapsulated
prior to insertion and the slots are shaped to retain the coils as
disclosed above. The means that the present invention not only
allows the stator to be removed from the motor housing, but permits
the winding to be disassembled from the stator. Axial movement of
the coils can be prevented by inserting insulating blocks between
the end turn loops and the stator.
[0126] Despite the many advantages of the split lamination
construction, it does require careful design and attention to
manufacturing to ensure satisfactory engagement of the stator
parts. All the aforementioned advantages for windings may be
obtained when the number of coil turns is not too large, and
especially for short-pitched coils, by using one-part laminations
as will now be described with reference to FIGS. 39a and 39b. A
four-turn coil is made first from four U-shaped coil sections 3901
bound together by a layer of insulating material 3902 over at least
the straight parts of the coil sections 3901 as shown in FIG. 39a,
each such coil section being termed a hairpin coil. During
manufacture the open ends of the U-shaped coil sections 3901 are
inserted directly into the laminations and bearing sections from
one end without requiring them to be divided into two or more
parts. Once this has been done the coil loops may be completed by
joining appropriate ends together 3903 as shown in FIG. 39b, for
example by brazing directly or using bridging pieces. These joints
may then be covered with insulation, --such as insulating tape, and
impregnated or encapsulated. It will be appreciated that the
details of the hairpin structure may be varied in a number of
different ways within the scope of the invention. It is not
necessary in any of the coils for the start and finish of a coil to
be at the same end of the structure.
[0127] Referring again to FIG. 5, the disclosure of the described
embodiments has been based on a single wound stator. Commonly, for
higher power, multiple housed motors are combined in series. FIG. 6
illustrates an embodiment in which two or more stator sections are
accommodated within a single housing 202, in this case with one
stator section per rotor section. The corresponding phase windings
214 of each stator section are connected in series while integrity
of synchronicity of the sections is obtained by rotationally
aligning the stator and rotor sections. The rotor sections are
easily aligned on the shaft by keys. Disadvantageously, in a lap
wound motor, the end turns will consume a large proportion of the
overall motor length and the reliability will diminish in
proportion to the extra end turns. Also, for a high speed motor,
the distance between bearings will become large and possibly
necessitate a larger number for shorter stages to maintain
mechanical stability of the rotor. However, in the preferred
embodiments disclosed above in which concentrated windings are
used, the penalty for additional end turns is much reduced and the
motor is feasible from reliability and performance points of view.
In this case the practical advantages are the possibility of
manufacturing a large variety of motor powers from a basic stator
length and the relative ease of winding shorter stators.
[0128] The stator sections may be carried and aligned on a common
mandrel for insertion in the motor housing 202, similarly to the
foregoing descriptions for a single stator. The stator bore
restrictions in which to house the bearings 401 are replaced by
housings 404 concentric with the individual stator sections.
Concentricity is maintained by the motor housing 202 when the
entire assembly of bearing housings 404 and stator sections are
inserted. The series connection 405 of the windings 214 of the
stator sections is preferably achieved by permanent means such as
brazing. The use of connectors, while possible, reduces
reliability. It is a feature of the invention that winding before
insertion permits these connections to be made and inspected
beforehand.
[0129] High speed multi-pole PMSMs present. a variable speed drive
problem that the present invention addresses as described below.
The origin of the problem is that the base electrical frequency
that the drive must generate is the product of the number of motor
pole pairs and the number of shaft revolutions per second. A
standard induction motor having two poles and turning at 3600 rpm
therefore has an electrical frequency of 60 Hz. A PMSM in
accordance with the invention rotating above 4500 rpm has a much
higher frequency. At 7200 rpm and six poles, the electrical
frequency is 360 Hz. This six-fold increase is a step change in
operating conditions for electric submersible pumping systems and
well beyond the range of general industrial drives.
[0130] FIG. 10 shows a known electrical representation of a
balanced PMSM with three phases a, b, c and isolated neutral.
Referring to phase a, reference numeral 701 is a motor terminal to
which the voltage Va is applied. Current Ia indicated by the
reference numeral 702 flows into the motor winding which has
resistance R indicated at 703 and an effective inductance L
indicated at 704. The effect of the permanent magnets rotating past
the stator winding is to induce an electromotive force (EMF) Ea
indicated at 705. The other phases b and c may be described in the
same way with appropriate substitution of indices. The three phases
are joined together at the neutral point N indicated by the
reference numeral 706.
[0131] It will be appreciated that multiple motors, or stators, may
be connected electrically in series so that the resistances,
inductances and EMFs add to make a single equivalent larger motor
with a common shaft. Placing the terminals in parallel is also
possible but poses difficulties in controlling currents between all
the windings. More realistic motor models in which for example the
EMF source and inductance are lumped together as an element that
calculates the time rate of change of the internal flux linkage,
and in which magnetic saturation is taken into account, are all
refinements which do not affect the present invention. The number
of phases, three, is well suited to the task of electric
submersible pumping motors, but is not limiting.
[0132] An idealised PMSM as described with reference to FIG. 10
produces sinusoidal EMF, with each phase 120 degrees apart, and is
driven by a three-phase sinusoidal voltage source. FIG. 11 shows
graphically how a sinusoidal voltage V, 802, applied to the motor
with suitable amplitude and in the presence of the motor EMF E,
801, will result in a phase current I, 803. The source may also be
current-controlled in which case V is the consequence of I and
E.
[0133] The sinusoidal nature of the electrical quantities is
ideally suited to the task of electric submersible pumping. This is
because the smoothly varying waveforms do not cause damaging
transients at the motor terminals, and because the motor torque can
be shown to be constant with rotation, which reduces the likelihood
of torsional vibration.
[0134] The construction of such a motor requires careful attention
to the distribution of the turns of the windings within the stator
slots. To produce a sinusoidal EMF with a reasonable number of
slots cut in the laminations requires the turns from different
phases to share slots and to be distributed among many slots. This
immediately causes a reduction in reliability due to the potential
for insulation failure in the many end-turn crossings and due to
the mixed phases. There is also the loss of useful copper due to
increased insulation between the phases.
[0135] When the windings are made so that the phases are kept in
separate slots, the back EMF will be more similar in form to E in
FIG. 13. This is often referred to as a trapezoidal EMF. If the
motor is driven with sinusoidal voltage or current the performance
will not be as good as the ideal sinusoidal PMSM made with the same
amount of copper in the windings.
[0136] FIG. 14 shows how a motor with trapezoidal EMF is driven,
compared to the sinusoidal motor waveforms of FIG. 11. The key
feature is that voltage is applied to the motor across two phases
only at a time whereas in a sinusoidally driven motor voltage is
applied to three phases at a time. The two-phase driven trapezoidal
wound permanent magnet motor is commonly termed a brushless DC
motor. The two phases are changed cyclically, as in AB, BC, CA, AB
. . . . Whenever the phase pair is changed, one phase is
electrically disconnected. Since there will be current in the phase
winding, the terminal voltage exhibits a voltage flyback spike
1002, known as a commutation spike. These spikes occur twice per
electrical cycle, on each phase. They present a serious limitation
for the successful use of brushless DC motors in electric
submersible pumping, since the voltage spikes lead to damaging
electrical conditions on long cables, as hereinbefore described.
The electric submersible pumping system of the present invention
drives all three motor phases continuously such that damaging
transients will not arise, without requiring the motor emf to be
sinusoidal. It is convenient nevertheless to explain the principles
in terms of sinusoidal waveforms, as the fundamental frequency
component of the drive and motor electrical quantities predominate
in a detailed analysis.
[0137] FIG. 1A shows a block diagram of a drive circuit 111
comprising an adjustable voltage converter 113 and an inverter 114
for supplying drive currents at output terminals 901 at the surface
for supplying the three phases A, B and C of the motor via the
power cable extending down the borehole. The inverter 114 is
supplied with an upper voltage at 904 and a lower voltage at 905,
the difference between the upper and lower voltages being commonly
termed the link voltage.
[0138] FIG. 12 shows a schematic circuit diagram for the inverter
114 which is well known. For each phase output there is an upper
switch and a lower switch, representatively shown for terminal AA
at 906 as 902 and 903 respectively. By alternately turning on these
switches, upper switch 902 on and lower switch 903 off or vice
versa, this terminal may be sensibly connected to either the upper
voltage 904 or the lower voltage 905. This arrangement is termed a
two-level inverter. It will be appreciated by one versed in the art
of inverter design that multi-level inverters may be made in which
the terminals may be switched to voltage levels intermediate
between the upper voltage and the lower voltage, such multi-level
inverters being usable in alternative embodiments in accordance
with the present invention. A filter is connected between the
switch terminals AA, BB and CC and the drive terminals A, B, C at
901, representatively comprising inductors 907 and capacitors 908.
The purpose of the filter is to smooth out the rapid switching
transitions, and thereby present a smooth voltage to terminals A,
B, C. It will be appreciated that other filters, for example for
the removal of radio interference, may be added.
[0139] By contrast, in a brushless DC motor inverter, the filter is
not present and the motor is connected directly to the terminals
AA, BB and CC. Only two phases are active, that is only one switch
is turned on, at a time, whilst the switches for the third phase
both remain turned off as described above.
[0140] In driving of the motor in accordance with the present
invention all three phase outputs are active at all times. In a
sinusoidal variable speed drive it is necessary to use pulse-width
modulation (PWM) or other switching modulation scheme known in the
art, e.g. hysteretic, space vector, switching table, to create the
effect of a sinusoidal output current. In the following
description. PWM drive is referred to by way of non-limiting
illustration.
[0141] FIG. 15 shows one phase of the output of a PWM drive
according to which the upper and lower switches of a phase leg are
alternated with a variable mark-space ratio. The voltage curve
shows the switching at terminal AA, whereas the superimposed phase
current curve is seen be sinusoidal with only a little ripple.
Fourier analysis of the voltage would show it to have a predominant
fundamental component at the phase frequency. Filter 907,908
filters the voltage output of the drive circuit so that only the
fundamental smooth voltage is passed to the power cable and thence
to the downhole motor. This is therefore a suitable transient-free
approach, in principle, for the PMSM submersible pump
application
[0142] However, to produce a high-power high-speed variable speed
drive with sinusoidal output presents severe difficulties, as will
now be described. The method is best suited to trapezoidal or
similar EMF but is also applicable to sinusoidal driving, the
difference being in the harmonic content of the waveforms and hence
the best use of available power capacity.
[0143] The majority of variable speed drive circuits operate at
typical utility supply voltages of 380 V AC-690 V AC, since the
power semiconductors that they use for switches are well proven and
efficient. However, just as in utility power transmission, for
efficient motor operation using long power cables, it is necessary
to use Medium Voltages, commonly in the range 1000 V AC-4000 V AC.
Such voltages reduce the motor current and hence the ohmic losses
in the cable. The majority of variable speed drive circuits for use
with submersible pumps are therefore installed with a step-up
transformer on the output. These transformers are a source of
additional power loss, direct cost, and are often large and
oil-filled, requiring special environmental precautions and
substantial space. A wide speed range requires expensive core
material for high speed but also a very large core to prevent
magnetic saturation if operation at low speed is also required.
They are in addition to input transformers, commonly required as
described below to reduce harmonic distortion of the supply and to
match to the available supply voltage.
[0144] A Medium Voltage drive circuit operates from a supply
voltage directly at the voltage which is required for the motors.
It therefore eliminates the undesirable output step-up transformer
but has certain limitations for the purpose of high speed
pumping.
[0145] Medium Voltage power semiconductors when used for the
switches 902 and 903 of the drive circuit of FIG. 12 have large
switching losses, i.e. unlike ideal switches they carry both
current and voltage during the time it takes to open or close the
path to current. The losses are inherently proportional to the
number of switching operations per second. As an example, to turn
on a switch at 3000 VDC assuming a current of 200 A might cause a
loss of 1 J (Joule). If repeated 1000 times per second, the heat
created would be 1000 W. It is easy to see that, once accumulated
across all the switches of the drive, there would be a substantial
cooling problem and loss of efficiency.
[0146] To produce the quality sinusoidal waveform in FIG. 15,
thirty switching cycles per fundamental motor frequency cycle were
used. FIG. 16 shows the effect of reducing this to ten switching
cycles per fundamental motor frequency cycle. The waveform is
already of poor quality and difficult to filter.
[0147] With high speed multi-pole motors the switching speed
becomes too high to be economic with Medium Voltage semiconductors.
For example, a high speed motor with six poles operating at 7200
rpm has a fundamental frequency of 360 Hz, so that the drive should
operate with a switching frequency of at least 3600 Hz just to
achieve the quality of the response of FIG. 16, and preferably at
least twice that. The normal range for Medium Voltage power
semiconductors is 500-1000 Hz. This is why Medium Voltage drives
for two-pole induction motors, which need a fundamental of 60 Hz at
3450 rpm, are typically specified at an upper fundamental of less
than 90 Hz, far short of that needed for the high speed motors
referred to above. If a lower voltage drive is considered as an
alternative, then despite more efficient semiconductors, it too
will reach a switching limit at high power. Moreover the step-up
transformer has to be a more costly design as mentioned above.
[0148] The present invention overcomes these problems by
deliberately over-modulating (non-linearly modulating) the output
stage of FIG. 12 in conjunction with a variable voltage source such
as shown in FIG. 19. Normally the PWM waveform in FIG. 15 may be
used to produce a good sinusoidal waveform until the peak value
exceeds 4/7c times the internal drive voltage. If the depth of
modulation is increased beyond this the PWM output will become
distorted, that is the modulation will become non-linear, as shown
in FIG. 17. This non-linearity is characteristic of any modulation
scheme used for driving a motor in accordance with the present
invention where the output voltage is unable to follow the peak of
the sine wave or other waveshape that is demanded, in simple
proportion. A particular case of over-modulation is to consider the
peak voltage as fixed to the upper and lower levels of the internal
drive voltage for substantial parts of each cycle, with pulse width
or other modulation used to progressively vary the output between
the upper and lower levels of the internal drive voltage for the
remaining parts of the cycle. It is also possible, within the scope
of the invention, to generate the waveform within the linear range
of modulation, particularly at lower power levels.
[0149] The distorted switching waveform shown in FIG. 17 has
several features. There are far fewer switching cycles than at
lower modulation, and these are at the lowest-current intervals of
the fundamental cycle. Switching losses will therefore be much
reduced even if the switching frequency is kept high to facilitate
filtering. The filtered output voltage, obtained, for example, by
filtering the single phase in isolation, is quite similar to
trapezoidal, and is transient free as required. When the filter is
as shown in FIG. 12, it becomes a three-phase filter and the
phase-phase voltage applied to the motor will be found to be even
more smooth. The output is usable for sinusoidal motors, with some
unwanted harmonics, and is well adapted to the non-sinusoidal
windings of the preferred motors of the present invention.
[0150] FIG. 21 shows the benefits of over-modulation more clearly.
Horizontal axis 1702 is modulation depth normalised to 4/n, and
vertical axis 1701 is the heat loss of a typical switch. Curve 1703
shows the switch conduction loss, which is typically low for a
submersible motor running at 100 A. Curve 1704 shows the switching
loss. It is high for normal modulation levels, but reduces rapidly
by a factor of three as over-modulation is increased. This
represents a dramatic improvement and makes Medium Voltage drives
of the present invention suited to high speed motors.
[0151] Since the amplitude of the drive voltage is fixed once
over-modulation is employed, the only way of varying the motor
voltage and hence the speed is to vary the internal drive voltage
Vjick applied between the terminals 904 and 905 in FIG. 12 by means
of the adjustable voltage converter 113. There are many known
circuits to do this, including phase-controlled rectifiers and
choppers.
[0152] However, the present invention seeks to make use of the
special characteristics of the pumps it is powering, in order to
further improve drive performance. In accordance with the power
characteristic of centrifugal pumps as mentioned earlier and as
depicted. in FIG. 18, the power output at half speed is only 12.5%
or so of the full speed power, and therefore of little interest in
the well for which the motor and pump are specified. Similarly,
though less dramatic, in a positive displacement pumping system the
power will be proportional to speed and more than half power is
normally required.
[0153] Therefore a properly specified drive can be assumed to be
run most of the time above half speed.
[0154] A first embodiment of adjustable voltage converter 113
particular to the present invention incorporates a specially
adapted variable voltage chopper source shown in FIG. 19 to
provides an efficient means of regulating the internal drive
voltage, and hence the motor speed, over the power range of
interest.
[0155] In this circuit a first fixed supply voltage source 1401 is
connected in series with a second fixed supply voltage source 1402,
and a chopper, comprising a switch 1403, a diode 1404, an inductor
1405 and a capacitor 1406, is connected across the source 1402. By
varying the duty cycle of the switch 1403, the voltage across the
capacitor 1406 may be varied between zero and the fixed voltage of
the source 1402. Since the voltage across the capacitor 1406 is in
series with the fixed voltage of the source 1401, the voltage
across the output terminals 904 and 905 may be varied from the
fixed voltage of the source 1401 to the sum of the voltages of the
sources 1401 and 1402.
[0156] When the motor is operating at low speed, as when starting
and stopping, the power level and motor frequency will be low.
Consequently conventional pulse width modulation by the drive
output may be used with little penalty, with the chopper turned
off, leaving the drive voltage fixed at the level of 1401. At full
power output the chopper may be left permanently on. Therefore it
has no switching losses in either case.
[0157] The switching losses of a chopper are proportional to
switching frequency, input voltage and output current. The
advantages of the arrangement shown are that the input voltage is
only half that of the full supply, and that the frequency of
chopping may be set independently of the motor speed since it is
used to produce the link voltage and not the modulated drive output
to the motor. For example, with a pump load and high speed
corresponding to high power, the chopper might be operated at 500
Hz to limit switch losses, whereas the output stage in FIG. 12 may
be switching, except when saturated, at 3600 Hz or more to produce
a fundamental frequency of 360 Hz. If the conventional modulated
PWM approach with a fixed internal supply were used, the output
might have to be limited to 500 Hz, that is there would not even be
one pulse per half cycle, resulting in an ineffective drive.
[0158] If a chopper were used across the full available supply
voltage, the losses would be doubled as the switching voltage would
be doubled while the switched current remained the same. This may
be acceptable for lower power low cost drives where the dual fixed
voltage supply 1401,1402 is not implemented.
[0159] A further feature to optimise the drive, based on the
characteristics of the electric submersible pump. is to vary the
chopper frequency. Inductor 1405 is heavy, costly and has losses
proportional to ripple current and average output current. As such
it is an undesirable addition by comparison with conventional
drives.
[0160] The inductance value is usually chosen to limit the chopper
ripple current to a reasonable level. The ripple current is at a
maximum when the chopper output is half its input voltage (50% duty
cycle). At the same time, because of the nature of the pump load,
the output power will be significantly reduced. Therefore at this
condition the chopper frequency can be relatively high, permitting
a lower inductance value. As the voltage increases the output power
will increase, and the chopper frequency must be reduced to limit
the switching losses. Since the ripple reduces as the output
voltage increases (higher duty cycle) but increases as the
frequency is reduced, it can be. seen that a compromise profile of
frequency versus power output can be found which allows a much
smaller value of inductance than would otherwise be the case,
reducing the adverse factors of weight, cost and power loss. It is
quite reasonable to reduce the value by a factor of two, or four if
the chopper is connected across the full supply and not a portion
of it. Thus variation of the internal frequency of the adjustable
voltage converter with output serves to improve efficiency and/or
reduce the size of components.
[0161] FIG. 20 shows a suitable circuit for the fixed voltage
sources 1401 and 1402. In this circuit the utility supply is first
transformed by a transformer 1501 having two secondary windings
1502, the output of each of which is fed to a three-phase rectifier
and smoothing capacitor. The resulting DC supplies are connected in
series. By. altering the relative turns ratios so as to change the
relative sizes of the voltages of the sources 1401 and 1402, the
variable speed range of the supply may be adapted to particular
requirements.
[0162] A particularly beneficial choice is when one secondary
winding is wye-connected and the other secondary winding is
delta-connected, and the turns ratios are adjusted such that the
rectified outputs are equal. In this case the current pulses taken
from the supply from one capacitor are displaced in time with
respect to the current pulses taken from the other capacitor. This
known arrangement is beneficial to the supply as the current pulses
taken from the supply by the assembly occur twelve times per supply
cycle and not six as when using a single rectifier. This
substantially reduces the harmonic distortion imposed by the drive
circuit on the utility supply.
[0163] A further embodiment of adjustable voltage converter 113 in
accordance with the present invention incorporates a three-phase
boost converter as shown in FIG. 35. This arrangement uses a
two-level inverter as in FIG. 9, but operated so as to absorb power
from the input terminals 3501, U, V, W which in this case are
connected to the utility supply or a generator, and produce a
voltage output between the terminals 905 and 904. This circuit
arrangement and variations thereof can use a greater or lesser
number of switches. Their use in industrial drives is mainly for
four purposes, namely (a) to draw a sinusoidal current from the
utility supply which improves the harmonic content, (b) to adjust
the utility supply power factor, (c) to permit the regeneration of
power from an over-running motor load to be returned to the utility
supply, and (d) to provide a constant link voltage just above the
maximum that would normally be obtained from an ordinary rectifier.
The latter is also the minimum link voltage that can be used for
the circuit to function with these purposes. In this embodiment the
three-phase boost converter is used to produce a variable voltage
between the minimum voltage level and a maximum voltage level that
is above twice this level, and therefore provides a suitable
adjustable range covering the main power levels of interest. A
particular benefit for smaller submersible motors is that ordinary
380-480 VAC utility supplies can be boosted into the low Medium
Voltage range, and therefore-the Medium Voltage drives of this
invention may be made without the cost of input transformers.
[0164] Other arrangements for efficient variable voltage are
possible. If the fixed voltage sources 1401 and 1402 are kept
separate and a chopper is placed across each, and the chopper
outputs are connected in series, one chopper may be kept fully on
or fully off which the other chopper varies its output. In this way
the entire voltage range may be covered with only one chopper
operating at less than the sum of the available supply voltage. It
is also possible to connect the chopper across the fixed voltage
source 1401 and to connect the fixed voltage source 1402 separately
to the chopper output so as to add to it. Where the interference to
the utility could be tolerated, one or both of the rectifiers in
FIG. 20 could be rendered controllable by means of a thyristor
bridge. Three-phase chopper circuit arrangements are known and can
also be used.
[0165] It is desirable to dynamically control the drive, cable and
motor arrangements to realise optimum efficiency. Unlike an
induction motor a PMSM must be driven synchronously with its shaft
rotation. With a brushless DC motor, for which two phases are
driven at a time as hereinbefore described, there are numerous
schemes to determine the shaft rotation without the use of rotation
sensors (such as "Sensorless Vector and Direct Torque Control",
1998, P Vas, Oxford University Press) based on observing the effect
of the voltage on the undriven phase. With a PMSM, because all
three phases are continuously driven, the shaft rotation must be
determined in another way. In a factory, a means is required to
directly measure the instantaneous angular position of the rotor
using a shaft sensor, such as a resolver or encoder, and to use the
result to control the phase of the voltage or current output of the
variable speed drive circuit. However, apart from the uncertain
reliability of such transducers, the additional cabling or other
means needed to transmit the position information from deep in the
borehole to the variable speed drive circuit at the surface makes a
sensorless technique almost certain to be required.
[0166] It is possible to control AC PMSM motors without sensors
utilising a computerised or discrete component model of the motor,
based on an electrical equivalent circuit as in FIG. 10 or a
physics-based description, incorporating intimate knowledge of the
motor's electromagnetic design. The model is kept supplied with
terminal voltage, current and frequency information, which allows
it to estimate the motor's internal variables such as rotor
position. In turn these allow the control algorithm to decide how
to adjust the drive output. These methods depend on an accurate
model. Substantial effort is devoted to measuring the model
parameters for a given motor before use, or by various monitoring
means during operation. In the case of submersible motors, or in
other applications where long cables are required, the cable
resistance and reactance parameters must be incorporated into the
model. Furthermore cable and motor parameters are subject to change
with operating temperature and age. In the present invention it is
shown how qualitative knowledge of the motor load characteristics
may be introduced so as to refine the rotor position estimate for a
PMSM without having to measure these uncertain system parameters. A
general purpose drive is designed to cover a wide range of loads
and dynamically varying conditions, as in a servo, and cannot
assume particular properties of the load.
[0167] The characteristic of the load that is required for the
feature of the invention now to be disclosed is that its power be
steady for a steady speed. By averaging over a sufficiently long
period random or short term load fluctuations can be accommodated.
A submersible centrifugal pump, or a turbine, meets this
condition.
[0168] Therefore, if. the PMSM motor control is changed while
keeping the frequency, and hence synchronous speed, constant, the
load power will remain unchanged. The optimum control condition
will be when the drive output power, which is measurable, is
minimised. For example if a pump is turning at a fixed speed and
takes 300 kW from the drive at one control condition, and 298 kW at
another, then the second condition is more efficient as there is
less power supplied, regardless of what the motor and cable
parameters are thought to be and regardless of what the actual pump
power is, since it has not changed.
[0169] Suitable means of effecting these control changes to find
the most efficient system operating point are now disclosed. At
constant speed the internal mechanical friction losses will be
fixed, so that the dominant variable loss that needs to be reduced
to a minimum by the control is the ohmic loss in the windings.
[0170] FIG. 22 shows the phasor diagram for the PMSM schematic
shown in FIG. 10 and corresponding to the waveforms shown in FIG.
11. This diagram, which is known to those skilled in the art,
refers to one phase of an ideal balanced motor with isolated
neutral. The motor EMF amplitude E, denoted by the reference
numeral 1801, is taken as reference. The phase current I lags
behind this by an angle. The voltage drop due to the winding
resistance 1802 and the voltage across the internal inductance 1804
sum as vectors to equal the driving voltage V denoted by the
reference numeral 1805. The motor power output, ignoring internal
mechanical and iron losses, is given by: P=3/2 E I cos (.phi.).
[0171] The EMF E depends to a good approximation only on the motor
speed, so that, for fixed speed operation and since the output
power is fixed by the load, the quantity I cos (.phi.) will be
constant. The broken line 1806 shows the locus of constant output
power. It is evident that minimum current, and hence least loss in
the resistance of the copper winding, .apprxeq.I.sup.2R, occurs
when .phi. is zero.
[0172] In open-loop operation of the PMSM motor a given three-phase
voltage or current is applied at a given frequency. The motor
operates in accordance with the phasor diagram at an angle,
satisfying the relationships between the parameters. Operation is
at some non-zero 0, and as a result the motor is never optimally
efficient. If+ increases beyond approximately 45 degrees, the motor
operation becomes unreliable since fluctuations in the load may
increase the angle to the point where there is insufficient current
available from the drive circuit to maintain the output power. At
this point the motor will lose synchronism and stall. If the
conditions are as shown in FIG. 22, then increasing the motor
voltage will force 0 to reduce and hence reduce the current and
ohmic losses. It will eventually reach zero, the most efficient
point, and then become negative, when the current and ohmic losses
will increase again.
[0173] FIG. 23 shows this in terms of the input power 1902 of a
representative motor plotted against terminal voltage 1901,
normalised to the most efficient point of operation. The curve 1903
represents the average (real) input power and is the sum of the
fixed motor output power and the copper losses. The curve 1904
represents the volt-ampere power, which includes a power factor. It
can be seen that, by varying the input voltage, a point may be
found which minimises whichever power quantity is desired.
Increasing the normalised input voltage from below unity
changes<from positive through zero to negative.
[0174] Therefore the optimum operating point of the system at a
given speed may be found by varying the input voltage independently
of particular knowledge of the motor or cable parameters or actual
load power. Since the motor output power demanded by the pump for
constant pump speed and fluid type is constant, the control
observable is the input power measurable at the surface. Parameters
corresponding to the age or nature of the motor and/or cable are
not required.
[0175] This method has broader applicability. For example, if a
current-controlled drive is used, then current may be used as the
control variable and the drive voltage and output power will vary
as a consequence. Alternatively, for a fixed amplitude of voltage
or current, the phase of this quantity relative to the estimated
rotor angle could be slightly changed. In this case the optimum
condition will be when the speed is maximised, since, from the
characteristics of the load, increased speed corresponds to
increased output power, and the maximum output power always occurs
when reaches zero. (In FIG. 22, changing speed changes the
reactance X and the emf E in proportion.)
[0176] Practically the above method is applicable to PMSMs without
the simplified assumptions of FIG. 10, since the goal is simply
least drive power for a fixed output power. It is preferably
implemented as a long time average correction to an established
closed-loop control, or as a slow adjustment for open-loop control,
ensuring that load fluctuations do not cause false corrections. It
will be evident to those skilled in the art of control theory that
it is possible to merge the input power minimisation into an inner
control loop, by weighting its importance to other error terms or
using it as a constraint. In this way the control loop keeps
primary control over the range of .phi. that is permitted, while
accepting a safe level of correction from the power-optimised
control of the present invention. It should also be noted that, for
real non-sinusoidal motors and drives, the issue of torque ripple
can be important, and a lesser or greater amount of correction
might be empirically added to keep this to an acceptable level.
Such an arrangement is applicable to any synchronous motor with
suitable load and thereby includes brushless DC motors and
drives.
[0177] The improved winding and construction methods described
above make it possible to further extend the reliability of the
entire pumping system by means of cooperative duplication of
failure prone electrical elements. The cost of replacing a failed
system, and the loss of fluid production until the repair is
complete, will far exceed the cost of the duplicate parts to be
described.
[0178] FIG. 33 shows a variant installation according to which a
single submerged motor 108 is connected to two electric power
cables 110 and 110', each connected at the surface to a
corresponding motor drive 111 or 111'. FIG. 34 diagrammatically
shows the construction of the motor in the installation of FIG. 33
in which two sets of motor windings are provided, namely a first
winding set 3001 connected to the cable 110 and a second winding
set 3001' connected to the cable 110'. The motor is wound as a
six-phase motor, divided into two sets 3001 and 3001' of three
phases, each with its own neutral connection. The motor may either
be driven as a six-phase motor to its full power capability, or
alternatively as a three-phase motor using either set of windings
at reduced power.
[0179] The advantage of such an arrangement is that failure of the
cable 110 or its splices or connections, or of the drive circuit
111 or of any of the coils of the winding set 3001 will not affect
any of the corresponding parts associated with the winding set
3001'. By using concentrated windings the windings of the phase
sets may more easily be kept well insulated from one another than
with lap windings. In the simplest case one coil per phase is wound
over alternate teeth in twelve slots, the corresponding phases of
each set being adjacent to one another. In this way the six motor
leads exit the motor directly from their coils without
crossing.
[0180] The above arrangement of six phases split into two sets of
three phases, though having practical advantages, is not limiting.
However two-phase motors still require three conductors in a cable,
whereas motors with a larger number of phases require further cable
conductors, which is undesirable.
[0181] A six-phase, drive output circuit may be constructed by
adding three extra pairs of switches to FIG. 9. The phases operate
at a 60-degree separation, rather than 120 degrees. However, such a
drive sold as a unit is complex and is substantially only useful
for a fault tolerant application. The preferred embodiment uses two
adapted three-phase drive circuits 111 and 111', with the
adaptation being made by means of suitable signal and power
connections 3002 between the drive circuits. The signal connections
must ensure that the corresponding output phases of the drive
circuits are 60 degrees apart. One drive circuit makes the rotor
angle calculation to produce the master phase signal used by both
drive circuits. To ensure smooth running across all of the phases,
the amplitude of the drive output voltage must be the same on each
phase. In the case of the high-speed drives disclosed in the
present description, the terminals 904 and 905 of the drive
circuits may be connected together so that their voltages are the
same. One of many known power supply sharing methods may be used to
equalise the power supplied by each chopper circuit.
[0182] In an alternative embodiment of the fault-tolerant pumping
system, two motors are connected mechanically in series on a common
shaft, but powered by separate cables and drives as before. The two
motors may be operated individually or simultaneously. In the
latter case the drive circuits must again be arranged to cooperate.
This method is not applicable in the majority of wells as usually
the motor diameter is the largest that can be fitted within the
casing 103, and the motor cable 110 is fed past the pump and into
the top of the motor 108 immediately below it. In such cases it is
not possible to pass a second cable past this first motor to a
further motor arranged at a deeper level.
[0183] The foregoing has assumed two duplicate motor sections that
are identical, as that is the simplest fault tolerant arrangement.
However, with suitable changes to the control and drive levels, a
plurality of motor sections cables and drives of different
characteristics may be used within the scope of the invention, so
long as they are controlled to the same shaft speed.
[0184] The electric submersible pump system of the present
invention has broad application, particularly in the field of
downhole wellbore operations. Drilling for wellbore fluid at large
depths is typically restricted to relatively narrow boreholes, so
the facility of the present invention to provide the same motor
power in a smaller overall package is immediately advantageous.
[0185] A further application of the present invention is to
compress wellbore fluid in situ. It may sometimes not be required
to immediately transport the wellbore fluid to the surface from its
underground reservoir, but to compress it either for later recovery
or merely to facilitate further exploration. Alternatively it may
be required to transport the wellbore fluid from a first
subterranean location to a second subterranean location, for the
above reasons amongst others.
[0186] A recent development in mining operations is the application
of multi-lateral wellbore systems in which a number of small
diameter wellbores are drilled substantially horizontally from a
central subterranean sump. Currently known pumping systems have
significant difficulties in pumping from lateral wellbores, whereas
the pump of the present invention can still maintain a high output
in such environments. In this case, wellbore fluid is transported
from the multiple lateral wellbores to the central sump, where it
may be recovered to the surface or compressed as described
above.
[0187] As hereinbefore described, an objective of the present
invention is to provide a high-speed electric submersible pump,
capable of operating at speeds above the current maximum of
approximately 4,000 rpm. The standard operating speed of
embodiments of the invention intended for the above applications is
above 4,500 rpm, and an optimal speed, providing a marked
improvement over current systems, is approximately 7,200 rpm.
[0188] The present invention discloses a permanent magnet
synchronous motor submersible pumping system. It will be
appreciated by the person skilled in the art that various
modifications may be made to the above embodiments without
departing from the scope of the invention.
[0189] Reference should also be made to "The Technology of
Artificial Lift Methods", Vol. 2b, K. E. Brown, Penwell Publishing
1980, the contents of which are incorporated herein by
reference.
* * * * *