U.S. patent application number 10/161893 was filed with the patent office on 2002-10-10 for rotary electrical machines.
Invention is credited to Pullen, Keith Robert.
Application Number | 20020145360 10/161893 |
Document ID | / |
Family ID | 26313524 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020145360 |
Kind Code |
A1 |
Pullen, Keith Robert |
October 10, 2002 |
Rotary electrical machines
Abstract
A rotary electrical machine comprising a stator (10) and at
least one rotor (12) having a plurality of permanent magnets (14)
The rotor consists of a rotor disc, at the outer edge of which the
permanent magnets are mounted. The rotor disc (12) is provided with
airgap varying means (19) which are angled towards the stator (10)
and mounted on the rotor hub (24) for rotation therewith. When the
rotor is stationary, the airgap (30) between the magnets (14) and
the stator (10) is at a minimum. In operation, as the speed of
rotation of the rotor (12) increases, a centrifugal force is
generated which acts to bend the airgap varying means (19) and,
therefore, the rotor disc (12) back, away from the stator (10),
thereby drawing the magnets (14) away from the stator and
increasing the size of the airgap (30). The increase in size of the
airgap results in a corresponding decrease in flux and therefore a
decrease in the maximum output voltage for that rotor speed. In
this way, the output voltage of the machine is kept substantially
constant. A stationary iron ring (32) may be provided on the stator
(10) to assist in drawing away flux as the size of the airgap is
increased.
Inventors: |
Pullen, Keith Robert;
(Acton, GB) |
Correspondence
Address: |
KAPLAN & GILMAN , L.L.P.
900NROUTE 9 NORTH
WOODBRIDGE
NJ
07095
US
|
Family ID: |
26313524 |
Appl. No.: |
10/161893 |
Filed: |
June 4, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10161893 |
Jun 4, 2002 |
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09673857 |
Feb 1, 2001 |
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6404097 |
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09673857 |
Feb 1, 2001 |
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PCT/GB99/01254 |
Apr 23, 1999 |
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Current U.S.
Class: |
310/268 |
Current CPC
Class: |
H02K 21/026 20130101;
H02K 21/24 20130101; H02K 1/2793 20130101; H02K 3/26 20130101 |
Class at
Publication: |
310/268 |
International
Class: |
H02K 001/22 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 1998 |
GB |
9808721.6 |
Dec 1, 1998 |
GB |
9826365.0 |
Claims
1. An electrical machine having a rotor and a stator, the rotor
comprising at least one magnet which is located adjacent to the
stator with an airgap therebetween, the machine comprising means
for varying the size of said airgap in response to a variation in
the speed of rotation of said rotor.
2. A machine according to claim 1, wherein the means for varying
the size of the airgap comprises mechanical means.
3. A machine according to claim 2, wherein said mechanical means is
a resiliently flexible member formed integrally with or mounted on
the rotor, the resiliently flexible member co-operating with the at
least one magnet so that, as the speed of rotation of the rotor
increases, the centrifugal force generated thereby causes movement
of the resiliently flexible member and thereby draws the at least
one magnet away from the stator to increase the airgap.
4. A machine according to claim 3, comprising a plurality of said
resiliently flexible members, each co-operating with a respective
magnet of the rotor.
5. A machine according to claim 4, wherein the rotor comprises a
plurality of equi-angularly spaced magnets, said plurality of
resiliently flexible members being substantially equi-angularly
spaced.
6. A machine according to any preceding claim, further comprising a
stationary metal ring which is mounted concentrically with the
stator, wherein when the airgap is increased in response to an
increase in speed of rotation of the rotor, the at least one magnet
is moved closer to the stationary metal ring
7. A machine according to any one of claims 1 to 5, further
comprising a stationary metal ring which is formed integrally with
the stator, wherein when the airgap is increased in response to an
increase in speed of rotation of the rotor, the at least one magnet
is moved closer to the stationary metal ring
8. A machine according to claim 6 or claim 7, wherein said
stationary metal ring is formed of iron.
9. A machine according to any one of claims 6 to 8, wherein the
diameter of the metal ring is less than that of the stator.
10. A machine according to any one of claims 6 to 8, wherein the
diameter of the metal ring is greater than that of the stator.
11. A machine according to any preceding claim, wherein the means
for varying the airgap is preferably mounted or biased such that
when the rotor is stationary or at its lowest operating speed the
airgap is at a minimum.
12. A machine according to claim 11 when dependent on claim 2,
wherein the means for varying the airgap comprises one or more
members supported on a rotor drive shaft, the member or members
being angled to be progressively closer to the stator with
increasing radial distance from the drive shaft.
13. A machine according to claim 12, wherein the means for varying
the airgap is arranged such that an increase in rotor speed which
results in a centrifugal force which draws the angled portion of
the airgap varying means back to a substantially vertical position
at maximum rotor speed.
14. A machine according to claim 2, comprising electronic means for
additional voltage regulation.
15. A machine according to claim 1, wherein the means for varying
the airgap comprises electronic means.
16. A machine according to claim 15, comprising additional
mechanical means for assisting in drawing the at least one magnet
away from the stator to increase the airgap.
17. A machine according to claim 16, wherein said additional
mechanical means comprises flyweights.
18. A machine according to any preceding claim, wherein said rotor
includes a plurality of equi-angularly spaced magnets.
19. A machine according to any preceding claim, further comprising
feedback means for feeding back at least a portion of the output
current to means which operate in response to said output current
to push the at least one magnet back towards the stator and
decrease the size of the airgap by an amount corresponding to the
value of output current drawn by a load.
20. A machine according to claim 19, wherein said means for pushing
the at least one magnet back towards the stator includes a
solenoid.
21. A machine according to any preceding claim, wherein the stator
includes inserts formed of a ferromagnetic material.
22. A machine according to claim 21, wherein said inserts are
formed of soft iron.
23. A machine substantially as herein described with reference to
the accompanying drawings.
24. A method of regulating the output voltage of a machine having a
rotor and a stator, the rotor comprising at least one magnet which
is located adjacent to the stator with an airgap therebetween, the
method including the step of varying the size of said airgap in
response to a variation in the speed of rotation of the rotor.
25. A method according to claim 24, including the step of varying
the size of the airgap by mechanical means.
26. A method according to claim 25, including the step of
additionally regulating the output voltage of the machine by
electronic means.
27. A method according to any one of claims 24 to 26, including the
step of feeding back at least a portion of output current drawing
from the machine by a load to means which operate in response to
said output current to push the at least one magnet back towards
the stator, thereby decreasing the size of the airgap by an amount
corresponding to the value of output current drawn by the load.
28. A method of regulating the output voltage of a machine
substantially as herein described with reference to the
accompanying drawing.
29. A stator for an electrical machine, the stator comprising
electrical windings arranged as coil sectors disposed substantially
equi-angularly in a generally circular pattern on two opposing
sides of the stator, wherein at least some of the coil sectors are
wound in a generally spiral fashion when viewed in the direction of
the axis of symmetry of said generally circular pattern,
characterised in that at least two of the coil sectors, one on each
of the two opposing sides of the stator, are formed of a continuous
electrical winding which passes through the stator from one side to
the other side.
30. A stator as claimed in claim 29, wherein each of the coil
sectors is wound in a generally spiral fashion.
31. A stator as claimed in claim 30, wherein all of the coil
sectors are formed of a single electrical winding which passes back
an forth through the stator from one side of the stator to the
other side.
32. A stator substantially as herein described with reference to
the drawings.
33. An electrical machine comprising at least one rotor and a
stator according to any one of claims 29 to 32.
Description
[0001] The present invention relates to a rotary electrical machine
of the kind in which a plurality of permanent magnets are arranged
around a rotor and a stator is provided with appropriate electrical
windings. Such machines can act as electrical motors, i.e. produce
rotary motion upon application of electricity to the stator
windings. Alternatively, they can perform as electrical generators,
i.e. alternators or dynamos, wherein rotary motion imparted to the
rotor can produce an electrical output from the stator
windings.
[0002] Machines of the aforementioned kind can be embodied in
relatively compact efficient units. One application where small
size, and high efficiency is called for is in automotive
generators, for example automobile alternators. Many different
alternator designs have been proposed since the inception of the
internal combustion engine. One such proposal is disclosed in UK
patent specification number GB-A-2 174 252.
[0003] However, the constant demand for reduction in manufacturing
costs, less consumption of raw materials, lightweight components,
etc., means that there is a need for even smaller and more
efficient, lightweight alternators.
[0004] A number of arrangements have been proposed for generating
the field using permanent magnets instead of rotor field windings.
This is particularly attractive now that high flux rare earth
Neodymium Boron Iron magnets are available at a relatively low
cost. One such arrangement is described in International patent
application number PCT/GB96/01293, which relates to a rotor for an
electrical machine, the rotor comprising a plurality of
equi-angularly spaced magnets. The elimination of the field
windings reduces the complexity of the machine in removing the need
for slip rings, thereby also increasing the reliability of the
machine. Since no field current is required, losses are reduced and
efficiency is increased to up to twice that of a conventional claw
pole device under typical operating conditions. Furthermore, in
general, the power-to-weight ratio of a permanent magnet excited
machine is much greater than that of the conventional claw pole
machine, thereby enabling a much smaller, lower weight machine to
be used for similar applications.
[0005] However, some difficulties have been encountered in the
regulation of the voltage output of permanent magnet machines.
Since the voltage is proportional to the speed of rotation of the
rotor, the speed ratio often being as high as 10:1, any regulation
means must be able to cope with a high voltage as well as having to
regulate the full output current. The output voltage must be over
the minimum supply voltage at the lowest speed, so that the voltage
at the highest speed will be at least ten times this value, which
can also have an adverse effect on the safety of the machine. In
the arrangement described in International patent application
number PCT/GB96/01293, the voltage is regulated by electronic
means.
[0006] In the case of a conventional claw pole machine, voltage
regulation is achieved by varying the field current (flowing in the
rotor field windings) which causes a corresponding variation in the
field strength. This has two main benefits. Firstly, since the
field current is a fraction of the full output current, a
relatively low cost regulator can be used.
[0007] Another problem associated with permanent magnet machines
which may arise is the generation of excessive eddy currents, which
in turn could cause unacceptably high levels of heat loss in the
machine. The level of eddy currents is approximately proportional
to the square of speed and the square of flux density. For a
permanent magnet machine where the flux is at a constant value, the
eddy current losses increase with the square of speed, hence finely
stranded conductors are necessary to avoid excessive losses at high
speeds. The use of finely stranded stator conductors substantially
reduces the conductor copper density and, therefore, the power
output at a given speed. Thus, because the copper packing density
of the stator is reduced, a larger less efficient machine is
required for a given power output. Furthermore, the use of finely
stranded stator conductors results in poor heat transfer in the
stator, such that the stator temperature is relatively high
resulting in an unacceptably low power output, especially at high
temperatures.
[0008] However, in the case of the claw pole machine, eddy currents
are not substantially increased by an increase in speed because the
flux density in a claw pole machine falls in response to an
increase in speed.
[0009] In summary, therefore, although conventional permanent
magnet machines are significantly more efficient and of lower
weight than the claw pole machine, the difficulty in regulation
often outweighs the benefits gained, particularly in high speed
applications. This difficulty is quantified in terms of the cost of
the regulation electronics and the weight and size of the
machine.
[0010] It is an object of the present invention to provide an
electrical machine having a rotor and a stator, the rotor
comprising at least one magnet which is located adjacent to the
stator with an air gap therebetween, the machine comprising means
for varying the size of said air gap in response to a variation in
the speed of rotation of said rotor.
[0011] At the lowest speed of rotation of the rotor, the air gap is
of minimum size and the magnet is located as close as possible to
the stator. Thus, the flux density is at a maximum level and a
maximum voltage is generated. As the speed of rotation increases,
the voltage generated should increase. However, as the speed of
rotation increases, the magnet is moved away from the stator,
thereby increasing the size of the air gap and decreasing the flux
density at that speed. Thus, the otherwise increased voltage is
compensated by increasing the size of the air gap, and the output
voltage remains substantially constant. Thus, the need for complex
and expensive circuitry to regulate the output voltage is
substantially eliminated. Even if additional circuitry is required
to assist in the voltage regulation, such regulating electronics
would be relatively simple and less expensive than the circuitry
required for voltage regulation in conventional permanent magnet
machines.
[0012] Furthermore, as stated above, the level of eddy currents
generated in the stator is proportional to the square of the
frequency and the square of the flux density. As the speed of
rotation increases, obviously the frequency increases accordingly.
However, the flux density is reduced by the increase in size of the
air gap, thereby compensating for the increase in flux density. In
this way, any increase in eddy currents with the increased speed of
rotation is substantially eliminated. In fact, because of this it
is not necessary to use finely stranded conductors in the stator.
Instead, the stator can be formed of copper sheeting, onto which
the wiring pattern is etched, stamped out, or cut by other means,
e.g. laser or water jet. As a result, the heat transfer
characteristics of the stator are good because the copper packing
density of the stator is high. Thus, the size of the machine
required to give a particular power output can be reduced, and the
cost of manufacture is also reduced. At higher speeds, high eddy
current losses do not occur since the drop in field strength due to
the increased air gap would compensate for the increase in
frequency.
[0013] The size of the stator, and therefore the overall machine,
can be further reduced by the addition of ferromagnetic material
typically laminated soft iron therein, to assist in drawing more of
the flux into the stator coils, thereby increasing the voltage
generated. Furthermore, as the airgap increases, the flux going
through the stator can be reduced by increasing the flux leakage.
One or more rings of additional stationary iron of a particular
shape may be placed in such a way as to assist the leakage of flux
going away the stator coils. This has two main benefits: (i) the
maximum movement of the rotor(s) is reduced; and (ii) the design of
the means for varying the airgap is easier, since the iron ring
shape can be tuned to give the required voltage regulation.
[0014] The use of iron in the stator used in the present invention
is permissible, because iron losses are not significantly increased
at high speeds due to the resultant drop in field strength.
[0015] Thus, the machine of the present invention may comprise one
or more stationary metal rings, preferably formed of iron, which
is/are mounted concentrically with the stator with substantially no
space therebetween. Alternatively, a stationary iron ring may be
formed integrally with the stator. In a preferred embodiment, as
the at least one magnet is drawn away from the stator and the
airgap is increased, the at least one magnet is drawn closer to the
stationary iron ring which assists in drawing away flux, thereby
increasing the effectiveness of the regulation process.
Furthermore, since the maximum airgap does not have to be so large,
the compactness of the machine is also improved. The diameter of
the iron ring(s) may be less than that of the stator. However, in a
preferred embodiment the diameter thereof is greater than that of
the stator. Soft iron inserts may also be placed in the stator to
increase the voltage generated in the stator coils.
[0016] The means for varying the airgap preferably comprises one or
more resiliently flexible members formed integrally with or mounted
on the rotor, the resiliently flexible member co-operating with a
respective magnet of the rotor so that, as the speed of rotation of
the rotor increases. the centrifugal force generated thereby causes
movement of the resiliently flexible member or members and thereby
draws the or each respective magnet away from the stator to
increase the airgap. In a preferred embodiment, the rotor comprises
a plurality of equi-angularly spaced magnets.
[0017] The means for varying the airgap is preferably mounted or
biased such that when the rotor is stationary or at its lowest
operating speed, the airgap is at a minimum. In one embodiment, the
means for varying the airgap may comprise one or more members
supported on a rotor drive shaft, the member or members being
angled to be progressively closer to the stator with increasing
radial distance from the drive shaft. In this case, an increase in
rotor speed results in a centrifugal force to draw the angled
portion of the airgap varying means back to a substantially
vertical position at maximum rotor speed. For the avoidance of
doubt, the term "progressively" is not intended to mean only
linear. The angled portion of the airgap varying means may, for
example, be curved or stepped.
[0018] The machine may further comprise electronic means for
additional voltage regulation. Because a large proportion of the
voltage regulation is achieved by mechanical means, the electronic
circuitry required for any additional voltage regulation is
substantially simplified. Additional mechanical means, for example
flyweights, may also be provided for assisting in drawing the
magnets away from the stator to increase the airgap.
[0019] When current is drawn from the machine by a load, there is a
corresponding drop in output voltage for a particular rotor speed.
As the rotor speed itself is unchanged, such a voltage drop is not
compensated for by the airgap varying means. Thus, in a preferred
embodiment of the present invention, feedback means are provided
for feeding back a portion of the output current to, for example, a
solenoid which operates to push the at least one magnet back
towards the stator and decrease the airgap by an amount
corresponding to the drop in output voltage at that rotor
speed.
[0020] According to another aspect of the present invention there
is provided a stator for an electrical machine, the stator
comprising electrical windings arranged as coil sectors disposed
substantially equi-angularly in a generally circular pattern on two
opposing sides of the stator, wherein at least some of the coil
sectors are wound in a generally spiral fashion when viewed in the
direction of the axis of symmetry of said generally circular
pattern, characterised in that at least two of the coil sectors,
one on each of the two opposing sides of the stator, are formed of
a continuous electrical winding which passes through the stator
from one side to the other side.
[0021] Alternatively, regulation which is necessary in order to
compensate for changes in voltage according to changes in load may
be carried out entirely by electronic means, such as, for example,
a DC to DC converter placed after the voltage rectifier or a
controlled rectifier.
[0022] An embodiment of the present invention will now be described
by way of example only and with reference to the accompanying
drawings in which:
[0023] FIG. 1 is a schematic cross-sectional view of a single stage
of a generator according to an embodiment of the invention;
[0024] FIG. 2A is a front view of a stator for use in the generator
of FIG. 1;
[0025] FIG. 2B is a rear view of the stator of FIG. 2A;
[0026] FIG. 3 is a side view of the rotor disc of the generator of
FIG. 1;
[0027] FIG. 4 is a side view of airgap varying means of the
generator of FIG. 1;
[0028] FIG. 5A shows a plan view from one side,
[0029] FIG. 5B shows an axial cross-section, and
[0030] FIG. 5C shows a plan view from the other side of the coil
windings of an alternative stator construction formed using
stranded wire (soft iron inserts not shown);
[0031] FIG. 6A shows a plan view from one side, and
[0032] FIG. 6B shows a plan view from the other side of the coil
windings of another stator suitable for use in a generator
according to the present invention (soft iron inserts not
shown).
[0033] Referring to FIG. 1 of the drawings, a generator according
to an embodiment of the invention comprises a generally disc-shaped
stator 10 mounted between two concentric rotor discs 12. The stator
10 and the rotor discs 12 each have an opening to receive a central
spindle or shaft 14. The stator 10 and the rotor discs 12 are
mounted in a casing 16 formed of two halves 16a, 16b. The spindle
or shaft 14 as it emerges from the casing 16 is provided with a
pulley 18 for a drive belt (not shown).
[0034] Each rotor disc 12 is provided with a "spider"-shaped airgap
varying means 19 which is formed integrally with or fixed to the
rotor disc 12. Each airgap varying means 19 is formed of a
resiliently flexible material, and is bolted or otherwise fixed via
a flange-like mounting portion 22 to a rotor hub 24 which
co-operates with the spindle or shaft 14. A side view of the airgap
varying means is shown in FIG. 4 of the drawings.
[0035] The outer edge of each airgap varying means 19 is bent or
curved inwards so as to form a generally flat, flange-like portion
26. A plurality of equi-angularly spaced permanent magnets 28 are
mounted, by means of adhesive or otherwise, close to the outer
edges of the rotor discs 12, facing the stator 12. A side view of
the rotor disc and magnets is shown in FIG. 3 of the drawings.
Referring back to FIG. 1, when the rotor is stationary, the air gap
30 between the magnets 28 and the stator 10 is at a minimum, and
the plane of the magnets is substantially parallel with that of the
stator 10. A stop 5 may be provided, if required, to prevent the
rotor going too close to the stator. Cooling airflow shown by the
arrows is drawn into the machine through vents V and is pumped
outwards by the rotor before leaving through a finned disc D.
[0036] The stator 10 comprises a generally disc-shaped solid copper
sheet, onto which stator windings 29 have been etched using known
techniques. Soft iron inserts 31 are provided to increase the
voltage generated in the stator coils, as shown in FIGS. 2A and 2B
of the drawings.
[0037] Referring back to FIG. 1, the generator further comprises a
stationary iron ring 32 which is mounted within the casing 16,
concentric with the stator 10 and with no gap therebetween. In FIG.
1, the iron ring 32 is shown as having a diameter greater than that
of the stator 10 for optimum efficiency. However, it is envisaged
that its diameter could be less than that of the stator 10.
[0038] In use, at minimum rotor speed, the air gap 30 between the
rotor magnets 28 and the stator 10 is at a minimum of, for example,
10 mm. Thus, the magnetic flux density is at a maximum for a
minimum allowed rotor/stator clearance and the output voltage,
which is proportional to the flux density, is at a maximum for that
rotor speed.
[0039] As the speed of rotation of the rotor increases, the
centrifugal force generated causes the airgap varying means 19 and,
therefore, the rotor discs 12 to bend from the mounting portion 22
outwards so that the magnets 28 are drawn away from the stator 10,
thereby increasing the air gap 30, as shown by the broken lines in
FIG. 1.
[0040] As the size of the air gap so increases, the flux density
decreases which causes a compensating decrease in output voltage
for that rotor speed. As an example, if the air gap 30 is increased
from 10 mm to 30 mm, the flux typically falls by a factor of about
10. Thus, by varying the size of the air gap as a function of rotor
speed, the output voltage is substantially constant.
[0041] As shown in FIG. 1, due to the angled shape of the rotor
discs 12, as they are drawn backwards, the main angled section of
the discs 12 tends towards a vertical position, thereby drawing the
magnets 28 away from the stator 10 and upwards towards the
stationary iron ring 32. As the magnets 28 approach the iron ring
32, some of the flux is drawn away by the iron ring 32, thereby
assisting in the corresponding decrease in output voltage. There
are two main advantages of this. Firstly, of course, voltage
regulation in response to an increase in rotor speed is quicker,
thereby providing a more constant output voltage. Secondly, the
maximum air gap required can be smaller, thereby improving the
compactness of the machine. It has previously been less
advantageous to use iron in the stator because of the resulting
iron losses. However, iron losses are approximately proportional to
speed and field strength. In the machine of the present invention,
the field strength decreases as the speed increases, thereby
cancelling out any additional iron losses which would otherwise
occur at high rotor speeds.
[0042] A subsequent decrease in rotor speed reduces the magnitude
of the centrifugal force generated and allows the magnets to be
released accordingly back towards the stator 10 to decrease the
size of the air gap 30.
[0043] When current is drawn from the generator by a load (not
shown), there is a corresponding decrease in output voltage. In
order to compensate for this, a solenoid (not shown) may be
provided through which a small portion of the output current is fed
back, in response to which the magnets 28 are pushed back towards
the stator to decrease the air gap by an amount proportional to the
load current, and provide a corresponding increase in output
voltage to compensate for the voltage drop caused by the load.
[0044] It is also envisaged to provide additional mechanical means
to assist in moving the magnets, or electronic means for additional
voltage regulation. Such electronic voltage regulation means would
be simple and less expensive relative to a fixed rotor device
because most of the voltage regulation is carried out by variation
of the air gap.
[0045] The air gap 30 between the rotor 12 and the stator 10 should
be in the range of 8 to 12 mm for best power output performance, in
the case where no iron is present in the stator. If iron is
present, the optimum gap size would be greater. If the airgap size
is increased, the flux goes down sharply but more current can be
drawn from the resulting thicker stator. If the gap is smaller than
the minimum, the resulting low stator thickness does not allow much
current to be drawn and hence output is low. If the gap is larger
than the maximum, the resulting higher stator thickness allows a
high current to be drawn, but the voltage will be low since the
flux is low. Power is of course the product of current and
voltage.
[0046] A suitable stator winding assembly, particularly suitable
for high speed machines, is shown in FIGS. 5A to 5C.
[0047] The windings are made in three plans 81, 83, 85 overlapping
relative to the axis and each is provided with a respective
connection 87, 89, 91 (with respective counter-connections 93, 95,
97) for a three-phase electrical output (denoted X, Y, Z).
[0048] The windings are each formed into eight respective coil
sectors (99, 101, 103, 105, 107, 109, 111, 113). It would be
appreciated that each set of windings in the respective plans 81,
83, 85 are formed on or within respective laminar supports 115,
117, 119.
[0049] Each coil sector 99, etc. is the same shape. For
convenience, this will be described here only with reference to one
such sector 99. The sector is generally spiral in shape with the
wiring spiralling from the middle 121 to the periphery 123 thereof.
The radially outermost part 125 adjacent the stator periphery 127
is generally rounded. The innermost part 129 adjacent the opening
131 is inwardly pinched.
[0050] This type of stator winding assembly is described and
claimed in PCT application no. PCT/GB96/01292.
[0051] Obviously, the stator winding assembly shown in FIGS. 5A to
5C use a cross-over wire which links the inner spiral coil to the
outer spiral coil. This design is generally one layer of wire
thick, but is two layers thick at the cross-over. Therefore, when
the windings are compacted, the cross-over wire is crushed, which
weakens the design at that point. There is also the problem of
possible short-circuit.
[0052] An alternative design can be considered, particularly but
not necessarily for lower speed machines. Referring to FIGS. 2A and
2B, and 6A and 6B. In this design, a coil is formed on one side of
the stator, and the wire is then passed through an aperture in the
centre of the coil to the other side of the stator where it is
formed into a second coil this is repeated for each set of two
coils, as shown, the final arrangement of 8 coils on each side of
the stator being formed of a single winding. This design has
obvious advantages over the cross-over design, in that there is no
cross-over wire, thereby eliminating the problems caused by
compaction of the cross-over design.
[0053] In the case of both stator winding assemblies, the coils can
be formed of litz wire, which can be compacted. However, the
assembly shown in FIGS. 6A and 6B could also be formed of etched
copper windings, particularly in lower speed machines having
relatively low frequencies. Copper cannot be used for the
cross-over design because it is solid and cannot be sufficiently
compacted.
[0054] In the light of this disclosure, modifications of the
described embodiment, as well as other embodiments, all within the
scope of the invention, will now become apparent to persons skilled
in the art.
* * * * *