U.S. patent application number 10/172150 was filed with the patent office on 2002-10-17 for method and apparatus for controlling rotation of a magnetic rotor.
This patent application is currently assigned to CAPSTONE TURBINE CORPORATION. Invention is credited to Gilbreth, Mark G., Wacknov, Joel B..
Application Number | 20020149205 10/172150 |
Document ID | / |
Family ID | 23824718 |
Filed Date | 2002-10-17 |
United States Patent
Application |
20020149205 |
Kind Code |
A1 |
Gilbreth, Mark G. ; et
al. |
October 17, 2002 |
Method and apparatus for controlling rotation of a magnetic
rotor
Abstract
A method of capturing a sensorless magnetic rotor for
acceleration and rotation with a rotating magnetic field generated
by a stator is used in a turbogenerator including a compliant foil
fluid film radial bearing. The method includes energizing the
stator to generate a magnetic field and slowly rotating the
magnetic field approximately 360.degree. to capture the magnetic
rotor. The rotational speed of the magnetic field is quickly
accelerated to quickly accelerate the magnetic rotor, thereby
quickly reaching a liftoff speed associated with the compliant foil
fluid film radial bearing to prevent damage thereof.
Inventors: |
Gilbreth, Mark G.; (Woodland
Hills, CA) ; Wacknov, Joel B.; (Thousand Oaks,
CA) |
Correspondence
Address: |
Rachele Wittwer
IRELL & MANELLA LLP
Suite 900
1800 Avenue of the Stars
Los Angeles
CA
90067
US
|
Assignee: |
CAPSTONE TURBINE
CORPORATION
|
Family ID: |
23824718 |
Appl. No.: |
10/172150 |
Filed: |
June 14, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10172150 |
Jun 14, 2002 |
|
|
|
09459426 |
Dec 13, 1999 |
|
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Current U.S.
Class: |
290/1R ;
290/52 |
Current CPC
Class: |
H02P 9/06 20130101; F16C
17/024 20130101; F01D 25/16 20130101 |
Class at
Publication: |
290/1.00R ;
290/52 |
International
Class: |
H02P 009/04; F02C
006/00 |
Claims
What is claimed is:
1. A method of capturing a sensorless magnetic rotor for
acceleration and rotation with a rotating magnetic field generated
by a stator for use in a turbogenerator including a compliant foil
fluid film bearing, the method comprising: energizing the stator to
generate the magnetic field; slowly rotating the magnetic field
approximately 360.degree. to capture the magnetic rotor; and
quickly accelerating the rotational speed of the magnetic field to
quickly accelerate the magnetic rotor, thereby quickly reaching a
liftoff speed associated with the compliant foil fluid film bearing
to prevent damage thereof.
2. The method of claim 1, wherein said step of slowly rotating the
magnetic field comprises rotating the magnetic field approximately
to 270.degree. to 360.degree. in approximately 1 second.
3. The method of claim 2, wherein said step of quickly accelerating
the rotational speed of the magnetic field comprises accelerating
the rotational speed of the magnetic field to at least
approximately 10,000 rpm within approximately 1 second after said
slow rotation of approximately 270.degree. to 360.degree..
4. The method of claim 3, wherein said step of quickly accelerating
the rotational speed comprises quickly accelerating to
approximately 14,000 rpm within approximately 1 second.
5. The method of claim 1, wherein said steps of energizing the
rotor and slowly rotating the magnetic field comprises generating a
peak current in one of three inductors in a three-phase permanent
magnet rotor, and rotating the peak current through the three
inductors.
6. A turbogenerator comprising: a rotatable rotor including at
least one magnet, said rotor lacking means for determining its
rotational position; a compliant foil fluid film bearing positioned
for supporting the rotor; a stator configured for generating a
rotatable magnetic field to capture said magnet for rotating the
rotor; a computer system operatively connected with the stator, and
including a computer-readable storage medium having data thereon
operative to instruct the stator to perform the steps of:
energizing to generate the magnetic field; slowly rotating the
magnetic field approximately 360.degree. to capture the magnetic
rotor; and quickly accelerating the rotational speed of the
magnetic field to quickly accelerate the magnetic rotor, thereby
quickly reaching a liftoff speed associated with the compliant foil
fluid film bearing to prevent damage thereof.
7. The turbogenerator assembly of claim 6, wherein said step of
slowly rotating the magnetic field comprises rotating the magnetic
field approximately to 270.degree. to 360.degree. in approximately
1 second.
8. The turbogenerator assembly of claim 7, wherein said step of
quickly accelerating the rotational speed of the magnetic field
comprises accelerating the rotational speed of the magnetic field
to at least approximately 10,000 rpm within approximately 1 second
after said slow rotation of approximately 270.degree. to
360.degree..
9. The turbogenerator assembly of claim 8, wherein said step of
quickly accelerating the rotational speed comprises quickly
accelerating to approximately 14,000 rpm within approximately 1
second.
10. The turbogenerator assembly of claim 6, wherein said stator
comprises a three-phase permanent magnet rotor including three
inductors, and wherein said steps of energizing the rotor to
generate the magnetic field and slowly rotating the magnetic field
comprises generating a peak current in one of the inductors and
rotating the peak current through the other two inductors.
11. The turbogenerator assembly of claim 9, wherein the stator is
further operative as a generator for generating power from the
rotating rotor.
12. A method of capturing a sensorless magnetic rotor for
acceleration and rotation with a rotating magnetic field generated
by a stator, the method comprising: energizing the stator to
generate the magnetic field; slowly rotating the magnetic field
approximately 360.degree. to capture the magnetic rotor; and
quickly accelerating the rotational speed of the magnetic field to
quickly accelerate the magnetic rotor.
13. The method of claim 12, wherein said step of slowly rotating
the magnetic field comprises rotating the magnetic field
approximately 270.degree. to 360.degree. in approximately 1
second.
14. The method of claim 13, wherein said step of quickly
accelerating the rotational speed of the magnetic field comprises
accelerating the rotational speed of the magnetic field to at least
approximately 10,000 rpm within approximately 1 second after said
slow rotation of approximately 270.degree. to 360.degree..
15. The method of claim 14, wherein said step of quickly
accelerating the rotational speed comprises quickly accelerating to
approximately 14,000 rpm within approximately 1 second.
16. The method of claim 12, wherein said steps of energizing the
rotor and slowly rotating the magnetic field comprise generating a
peak current in one of three inductors in a three-phase permanent
magnet rotor, and rotating the peak current through the three
inductors.
17. The method of claim 12, further comprising quickly decelerating
the rotational speed of the magnetic field to quickly decelerate
the magnetic rotor.
18. The method of claim 17, wherein said step of quickly
decelerating the rotational speed of the magnetic field comprises
decelerating from approximately 14,000 rpm to 0 rpm in
approximately 1 second.
19. A method of capturing a sensorless magnetic rotor for
acceleration, rotation and deceleration with a rotating magnetic
field generated by a stator for use in a turbogenerator including a
compliant foil fluid film radial bearing, the method comprising:
energizing the stator to generate the magnetic field; slowly
rotating the magnetic field approximately 360.degree. to capture
the magnetic rotor; quickly accelerating the rotational speed of
the magnetic field to quickly accelerate the magnetic rotor to its
operating speed, thereby quickly reaching a liftoff speed
associated with the compliant foil fluid film radial bearing to
prevent damage thereof; and quickly decelerating the rotational
speed of the magnetic field from said operating speed to zero to
quickly decelerate and stop rotation of the magnetic rotor, thereby
preventing prolonged contact of the rotor with the compliant foil
fluid film radial bearing to further prevent damage thereof.
20. The method of claim 19, wherein said step of quickly
decelerating comprises quickly decelerating the magnetic field from
its operating speed to zero within approximately 1 second by
decelerating a peak current of approximately 80 amps as it rotates
through three inductors of a three-phase permanent magnet
rotor.
21. The method of claim 19, wherein said decelerating step further
comprises dissipating inertial energy of the rotor by converting
the inertial energy to a DC bus voltage and dissipating the DC bus
voltage with an off-load device including an off-load resistor
connected in series with an off-load switching device.
22. A rotor assembly comprising: a rotatable rotor including at
least one magnet, said rotor lacking means for determining its
rotational position; a stator configured for generating a rotatable
magnetic field to capture said magnet for rotating the rotor; a
computer system operatively connected with the stator, and
including a computer-readable storage medium having data thereon
operative to instruct the stator to perform the steps of:
energizing to generate the magnetic field; slowing rotating the
magnetic field approximately 360.degree. to capture the magnetic
rotor; and quickly accelerating the rotational speed of the
magnetic field to quickly accelerate the magnetic rotor, thereby
quickly reaching a liftoff speed associated with the compliant foil
fluid film radial bearing to prevent damage thereof.
23. The rotor assembly of claim 22, wherein said stator comprises a
three-phase permanent magnet rotor including three inductors, and
wherein said steps of energizing the rotor to generate the magnetic
field and slowly rotating the magnetic field comprise generating a
peak current in one of the inductors and rotating the peak current
through the other two inductors.
24. The rotor assembly of claim 22, wherein said data on the
computer-readable storage medium is further operative to instruct
the stator to decelerate from an operating speed reached after the
accelerating step to zero within approximately 1 second.
25. A method of decelerating a magnetic rotor by controlling a
rotating magnetic field generated by a stator for use in a
turbogenerator including a compliant foil fluid film radial
bearing, the method comprising: increasing current in the stator
using an open loop rotor position command to generate a peak
rotating magnetic field to assure retention of the magnetic rotor;
and quickly decelerating the rotational speed of the magnetic field
from an operating speed to zero to quickly decelerate and stop
rotation of the rotor, thereby preventing prolonged contact of the
rotor with the compliant foil fluid film radial bearing to prevent
damage thereof; and dissipating inertial energy of the rotor during
said decelerating step by converting the inertial energy to a DC
bus voltage and dissipating the DC bus voltage with an offload
device including an offload resistor connected in series with an
offload switching device.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method and apparatus for
capturing a magnetic rotor for acceleration, rotation and
deceleration with a rotating magnetic field generated by a stator
for use in a turbogenerator including a compliant foil fluid film
bearing.
BACKGROUND ART
[0002] Compliant foil fluid film radial bearings are currently
being utilized in a variety of high speed rotor applications. These
bearings are generally comprised of a bushing, a rotating element
such as a rotor or a shaft adapted to rotate within the bushing,
non-rotating compliant fluid foil members mounted within the
bushing and enclosing the rotating element, and non-rotating
compliant spring foil members mounted within the bushing underneath
the non-rotating compliant fluid foil members. The space between
the rotating element and the bushing is filled with fluid (usually
air) which envelops the foils. Conventionally, the compliant fluid
foil elements are divided into a plurality of individual compliant
foils to form a plurality of wedge-shaped channels which converge
in thickness in the direction of the rotation of the rotor.
[0003] The motion of the rotating element applies viscous drag
forces to the fluid in the converging wedge channels. This results
in increases in fluid pressure, especially near the trailing edge
of the wedge channels. If the rotating element moves toward the
non-rotating element, the convergence angle of the wedge channel
increases, causing the fluid pressure rise along the channel to
increase. Conversely, if the rotating element moves away, the
pressure rise along the wedge channel decreases. Thus, the fluid in
the wedge channel exerts restoring forces on the rotating element
that vary with and stabilize running clearances and prevent contact
between the rotating and non-rotating elements of the bearing.
Flexing and sliding of the foils causes coulomb damping of any
axial or overturning motion of the rotating element of the
bearing.
[0004] Owing to preload spring forces or gravity forces, the
rotating element of the bearing is typically in physical contact
with the fluid foil members of the bearing at low rotational
speeds. This physical contact results in bearing wear. It is only
when the rotor speed is above what is termed the liftoff/touchdown
speed that the fluid dynamic forces generated in the wedge channels
assure a running gap between the rotating and non-rotating
elements.
[0005] Compliant foil fluid film radial bearings typically rely on
backing springs to preload the fluid foils against the relatively
movable rotating element so as to control foil position/nesting and
to establish foil dynamic stability. The bearing starting torque
(which should ideally be zero) is directly proportional to these
preload forces. These preload forces also significantly increase
the rotor speed at which the hydrodynamic effects in the wedge
channels are strong enough to lift the rotating element of the
bearing out of physical contact with the non-rotating members of
the bearing. These preload forces and the high liftoff/touchdown
speeds result in significant bearing wear each time the rotor is
started or stopped.
[0006] These compliant foil fluid film radial bearings (air
bearings) may be positioned at multiple locations along the rotor
in a turbogenerator assembly, such as between the stator,
compressor, and turbine wheel. These air bearings are operative to
support the shaft at rotational speeds above approximately 8,000
rpms. Rubbing of the rotor against the foil occurs prior to liftoff
and after touchdown, which is generally under approximately 8,000
rpms. This rubbing is undesirable because it may cause wear on the
foil.
[0007] While compliant foil fluid film radial bearings have been
specifically described above, much the same considerations apply to
compliant foil fluid film thrust bearings which are also currently
being utilized in a variety of high speed rotor applications.
[0008] The least expensive turbogenerator design includes a
sensorless rotor. By "sensorless" it is meant that the system
includes no means for determining the rotational position of the
rotor because the rotor has no sensors to provide such information.
A problem inherent in such a low cost generator is that it may be
difficult to capture the rotor for rotation with the stator if the
rotational position of the rotor is unknown. The challenge is to
not allow the rotor to rotate in contact with the air bearings for
prolonged periods of time before liftoff has been reached. Such
prolonged contact causes damage to the air bearings.
[0009] In order to achieve minimal contact between the shaft and
the air bearings, quick acceleration is necessary, which requires
efficient capture of the rotor for rotation with the stator. If the
stator is simply accelerated quickly, it may fly by the rotor
without capturing the rotor, in which case the stator must be
decelerated again to capture the rotor, and then accelerated again.
Obviously, this provides an inefficient system. Therefore, it is
desirable to provide a method and apparatus for capturing,
accelerating, and decelerating a sensorless magnetic rotor in a
turbogenerator in a manner which minimizes the duration of contact
between the rotor and the air bearings in order to prevent damage
to the air bearings.
DISCLOSURE OF INVENTION
[0010] The present invention provides a method and apparatus for
capturing a sensorless magnetic rotor for acceleration and rotation
by slowly rotating the stator magnetic field approximately
360.degree. to capture the magnetic rotor, and quickly accelerating
the magnetic field to quickly accelerate the magnetic rotor. In
this manner, the rotor is efficiently captured and liftoff speed is
quickly reached to minimize damage to the air bearings. Also, the
magnetic rotor is quickly decelerated from its operational speed
down to zero after the touchdown speed has been reached.
[0011] More specifically, the present invention provides a method
of capturing a sensorless magnetic rotor for acceleration and
rotation with a rotating magnetic field generated by a stator for
use in a turbogenerator including a compliant foil fluid film
bearing. The method includes: a) energizing the stator to generate
the magnetic field; b) slowing rotating the magnetic field
approximately 360.degree. to capture the magnetic rotor; and c)
quickly accelerating the rotational speed of the magnetic field to
quickly accelerate the magnetic rotor, thereby quickly reaching a
liftoff speed associated with the compliant foil fluid film bearing
to prevent damage thereof.
[0012] Preferably, the magnetic field is slowly rotated for
approximately 1 second and then quickly accelerated to its
operating speed in 1 second.
[0013] Also, the magnetic field is preferably decelerated from a
touchdown speed to zero in approximately 1 second to prevent damage
to the compliant foil fluid film bearing when stopping rotation of
the rotor. A dynamic brake is used to dissipate inertial energy of
the rotor during deceleration.
[0014] Another aspect of the invention provides a turbogenerator
assembly including a rotatable rotor having at least one magnet,
wherein the rotor lacks means for determining its rotational
position. A compliant foil fluid film radial bearing is positioned
for supporting the rotor. A stator is configured for generating a
rotatable magnetic field to capture the magnet for rotating the
rotor. A computer system is operatively connected with the stator,
and includes a computer-readable storage medium having data thereon
operative to instruct the stator to perform the steps of: a)
energizing to generate the magnetic field; b) slowing rotating the
magnetic field approximately 360.degree. to capture the magnetic
rotor; and c) quickly accelerating the rotational speed of the
magnetic field to quickly accelerate the magnetic rotor, thereby
quickly reaching a liftoff speed associated with the compliant foil
fluid film radial bearing to prevent damage thereof.
[0015] Preferably, the stator is a three-phase permanent magnetic
rotor including three inductors, and the steps of energizing the
rotor to generate the magnetic field and slowly rotating the
magnetic field include generating a peak current in one of the
inductors and rotating the peak current through the three
inductors.
[0016] Accordingly, an object of the invention is to provide a
method and apparatus for capturing a sensorless magnetic rotor for
acceleration, rotation, and deceleration with a rotating magnetic
field generated by a stator for use in a turbogenerator including a
compliant foil fluid film bearing (or journal bearing) in a manner
in which damage to the compliant foil fluid film bearing is reduced
by quickly accelerating and decelerating the sensorless rotor.
[0017] The above objects and other objects, features, and
advantages of the present invention are readily apparent from the
following detailed description of the best mode for carrying out
the invention when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 shows a partial cut-away perspective view of a
turbogenerator for use with the present invention;
[0019] FIG. 2 shows a schematic diagram of a stator control system
implementing the present invention; and
[0020] FIG. 3 shows a schematic block diagram of a preferred energy
discharge system for use with the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0021] A permanent magnet turbine generator/motor 10 is illustrated
in FIG. 1 as an example of a turbogenerator in which the methods
and apparatus of the present invention could be implemented. The
permanent magnet turbine generator/motor 10 generally comprises a
permanent magnet generator 12, a power head 13, a combustor 14, and
a recuperator (or heat exchanger) 15.
[0022] The permanent magnet generator 12 includes a permanent
magnet rotor 16 having a permanent magnet embedded therein, such as
a samarium cobalt magnet. The rotor 16 is rotatably supported
within a permanent magnet generator stator 18 by air bearings
19,21. Radial permanent magnet stator cooling fins 25 are enclosed
in an outer cylindrical sleeve 27 to form an annular air flow
passage which cools the stator 18 with the air passing through on
its way to the power head 13.
[0023] The power head 13 of the permanent magnet
turbogenerator/motor 10 includes compressor 30, turbine 31, and
bearing rotor 36 through which the tie rod 29 passes. The turbine
31 drives the compressor 30, which includes a compressor impeller
or wheel 32 which receives preheated air from the annular air flow
passage in cylindrical sleeve 27 around the permanent magnet stator
18. The turbine 31 includes a turbine wheel 33 which receives
heated exhaust gases from the combustor 14 supplied with air from
recuperator 15. The compressor wheel 32 and turbine wheel 33 are
rotatably supported by the rotor 36 having a radially extending
bearing rotor thrust disk 37. The bearing rotor 36 is rotatably
supported by a single air bearing within the center bearing housing
38 while the bearing rotor thrust disk 37 at the compressor end of
the bearing rotor 36 is rotatably supported by a bilateral thrust
bearing. The bearing rotor thrust disk 37 is adjacent to the thrust
face at the compressor end of the center bearing housing while a
bearing thrust plate is disposed on the opposite side of the
bearing rotor thrust disk 37 relative to the center housing thrust
face.
[0024] Intake air is drawn through the permanent magnet generator
by the compressor 30 which increases the pressure of the air and
forces it into the recuperator 15. In the recuperator 15, exhaust
heat from the turbine 31 is used to preheat the air before it
enters the combustor 14 where the preheated air is mixed with fuel
and burned. The combustion gases are then expanded in the turbine
31 which drives the compressor 30 and the permanent magnet rotor 16
of the permanent magnet generator 12 which is mounted on the same
shaft as the turbine 31. The expanded turbine gases are then passed
through the recuperator 15 before being discharged from the
turbogenerator/motor 10.
[0025] The air bearings 19,21 are preferably compliant foil fluid
film radial bearings, as described in U.S. Pat. No. 5,915,841,
which is hereby incorporated by reference in its entirety. The air
bearing inside the center bearing housing 36 is also preferably a
compliant foil fluid film radial bearing. The bilateral thrust
bearing is also preferably a compliant foil fluid film thrust
bearing, as described in U.S. Pat. No. 5,529,398, which is hereby
incorporated by reference in its entirety.
[0026] As described in the background section of the present
application, the challenge is to not allow the rotor 16 to rotate
in contact with the air bearings for prolonged periods of time
before liftoff has been reached, which is typically around 8,000
rpm. This must be achieved by quickly capturing the magnetic rotor
16 with the magnetic field generated by the stator 18, and
accelerating the rotor quickly beyond the liftoff speed. This is
achieved by the control system 50, shown in FIG. 2, which
implements the methods of the present invention by properly
energizing the stator 18 in a manner to achieve the stated objects
of the present invention.
[0027] The control system 50 includes a digital signal processor
52. This is only a preferred processor, and any microcontroller
could be used to implement the present invention.
[0028] The digital signal processor 52 is in electrical
communication with the integrated gate bipolar transistor (IGBT)
54, which receives signals from the DSP 52 for operating switches
56,58 for selectively providing 760 volts of DC power from source
60 through selected motor inductance 62, 64, 66 of the
stator/generator/motor 18.
[0029] The stator 18 is preferably a three-phase synchronous
generator/motor. The stator 18 includes three inductors associated
with phases A, B and C as shown. The synchronous permanent magnet
generator 18 is preferably a model 330 Capstone microturbine,
available from Capstone Turbine Corporation, the assignee of the
present application. Two of the three inductors A, B and C have
current sensors 68 thereon for control purposes.
[0030] The digital signal processor 52 includes space vector
control algorithms which are operative to control the peak current
associated with the three sine waves in the inductors A, B, C,
thereby controlling the rotating magnetic field generated by the
stator 18. A description of a space vector control algorithm may be
found in Electric Machines And Drives, Chapter 10, "Induction Motor
Drives", Slemon, Addison-Wesley Publishing Company, Inc., which is
hereby incorporated by reference. This type of control is known in
the art. Space vector control uses a current-driven induction
motor. The stator current in the induction motor should have a
magnetizing component of magnitude which is in space phase with the
rotor flux linkage space vector. If the current magnitude is held
constant, the flux linkage will be constant in magnitude at the
desired value. The stator current now can be forced to have a
further component which is 90.degree. in angular space behind the
rotor flux linkage. The magnitude of this component can be made
proportional to the demand torque.
[0031] The required stator current is the vector sum of these two
components. If this stator current could be supplied from an
essentially ideal three-phase current source in which both the
magnitude and the space angle can be instantaneously established,
the drive theoretically would be capable of producing instantaneous
torque response to a command signal. Such a controlled stator
current can be produced approximately by use of a hysteresis
control scheme.
[0032] Direct measurement of the angular position of the net rotor
flux is not normally feasible because the path of the desired flux
is deep in the rotor winding and the rotor is rotating. However,
indirect methods may be employed to evaluate the angular position
of the rotor flux linkage space vector. In one approach, two stator
voltages and two stator currents are measured and manipulated to
produce two space vectors, each as a complex number. The stator
voltages may, alternatively, be inferred from the duty cycle that
must be applied to the switches 56,58 to maintain current
regulation. The resistance drop is subtracted from the stator
voltage to give the induced voltage vector. This is then integrated
to obtain the stator flux linkage from which the leakage flux
linkage may be subtracted to derive the rotor flux linkage space
vector. The angle of this space vector is now multiplied by the
desired magnitude to obtain the space vector of the required
magnetizing current. The same angle delayed by .pi./2 rad is then
multiplied by the desired rotor current magnitude which is
proportional to the demand torque. The two vector components are
now added to produce the required space vector of the stator
current. The hysteresis controller of the current source inverter
then proceeds to produce the required instantaneous phase currents
to inject into the stator windings. Normally, these operations are
performed digitally.
[0033] Other versions of vector control use a shaft position sensor
to obtain the angular position of the rotor and then add to this a
computed angle derived from integration of the predicted rotor
frequency to obtain a prediction of the angle of the rotor flux
linkage. However, the present invention addresses the particular
situation in which no shaft position sensor is provided on the
rotor.
[0034] Referring again to FIG. 2, when the inductors A, B and C are
energized, a peak amperage of 80 amps is generated in one of the
inductors A, B, C, which results in -40 amps at the other two
inductors B and C. When the inductors A, B, C are energized, if the
magnetic rotor 16 rests in a position such that the north pole of
its magnetic field is within 90.degree. of the peak position of the
current applied to stator 18, then the rotor 18 will rotate into a
position of alignment with the peak current of the stator 18, and
be captured by the stator 18. However, if the north pole of the
magnetic rotor 16 is more than 90.degree. out of phase with the
peak current applied in the inductors A, B, C, then the peak sine
wave currents must be rotated 360.degree. through the phases A, B
and C to capture the magnetic field produced by the magnet within
the rotor 16 so that the rotor 16 is captured for rotation with the
peak sine wave current rotated through the inductors A, B and C. In
other words, three sine waves of current 120.degree. out of phase
are put into inductors A, B and C, and the peak magnitude resulting
from the combination thereof captures the north pole of the magnet
embedded within the rotor 16, and thus rotation of the phase
position of the sine waves through the inductors A, B, C is used to
cause rotation of the rotor 16.
[0035] In order to capture the rotor every time, in the preferred
embodiment, the initial frequency applied to the stator 18 is one
revolution per second for 1 second to capture the rotor 16. Once
one revolution has been completed, the rotor 18 has been captured
and the frequency is linearly increased to 14,000 rpms over the
next second. This linear increase is actuated using an open loop
position command. At this time, there is enough voltage applied to
the inductors to begin using the commanded voltage feedback to
ascertain the rotor position for closed loop position control.
[0036] Because the magnetic rotor 16 will be captured by the
energized stator 18 if the north pole of the magnetic field
generated by the magnetic rotor 16 is less than 90.degree. out of
phase with respect to the peak current of the inductors A, B, C, it
is not necessary to slowly rotate the magnetic field of the
inductors A, B, C the full 360.degree.. For example, it could
rotate 270.degree. and would likely capture the rotor every time.
However, to assure capture, the preferred method slowly rotates the
magnetic field a full 360.degree. for approximately 1 second to be
sure that the rotor 16 is captured by the stator 18.
[0037] In this manner, the rotor 16 is quickly caused to reach the
liftoff frequency associated with the compliant foil fluid film
radial bearings 19,21, thereby minimizing damage caused to the foil
of such bearings 19,21.
[0038] It is also important to decelerate the rotor as quickly as
it was accelerated in order to prevent prolonged contact of the
rotor with the foil of the air bearings as the rotor rotates during
deceleration. During the set down process, the stator 18 is brought
to 14,000 rpms from its operating speed. Once the stator reaches
14,000 rpms, it is brought to 0 rpms in 1 second to minimize
contact with the foil bearings. This is accomplished by increasing
the current to 80 amps peak and providing an open loop rotor
position command which drags the rotor to a rest position. With a
(position) sensorless control algorithm, it is difficult to
actually sense the rotor position at low speeds, so applying the
open loop command is critical. If commanded voltages were used as
at high speed, the algorithm might miscalculate the position of the
rotor, lose lock, and not be able to perform this rapid
deceleration.
[0039] A description of FIG. 3 is provided below to show the
integration of a dynamic brake 202 into the previously described
system for use in dissipating rotor inertia in the above-described
braking process.
[0040] FIG. 3 illustrates a preferred energy discharge system 102
incorporating the off-load device 73. The energy discharge system
102 includes an off-load device 73 having an off-load resistor 202
and off-load switching device 204. Off-load switching device 2045
may be an IGBT or similar device. The off-load device 73 is
connected across the DC voltage bus 206 and will turn on
proportionately to the amount of off-load required, thus providing
a load for the gas turbine engine while the fuel is being cut back
to stabilize operation at a reduced level.
[0041] A preferred off-load device control loop 208, which is
implemented in the DSP 52, is shown in FIG. 3 for controlling
energy absorption of the off-load device 73. A voltage sensor 210
in communication with the voltage bus 206 generates a DC bus
voltage feedback signal 212. Voltage feedback signal 212 is
compared in summer or comparator 214 with a DC bus voltage limit
signal 216. The voltage limit signal 216 is adjustable by the DSP
52 and is the maximum voltage that may exist on voltage bus 206
before the off-load device 73 is turned on and starts absorbing
energy. The difference between the voltage feedback signal 212 and
the voltage limit signal 216 is voltage bus error signal 218.
Voltage bus error signal 218 is utilized to control the off-load
device 73 to increase or decrease the amount of energy being
absorbed by the off-load resistor 202. A compensator 220 utilizing
proportional integral control is used to convert the error signal
218 to a recommended control signal 222. Recommended control signal
222 is limited by a limiter 224 and a limited recommended control
signal 228 is produced. Limited 224 receives an off-load resistor
temperature feedback signal 226 to set a maximum control value for
the recommended control signal 222. The maximum control value is
set to prevent thermal breakdown of the off-load resistor 202.
Limited recommended control signal 228 communicates a desired
conduction duration for the off-load switch 204 to the pulse width
modulator 230. The pulse width modulator 230, which is in
communication with a gate drive 232, commands the off-load switch
204 "on" (into conduction) for a specified time duration as
dictated by the pulse width modulator. The off-load switch 204,
when in conduction, allows current to flow through the off-load
resistor 202 to absorb energy on the voltage bus 206. The off-load
switch is modulated in the manner described until the energy
produced by a load disconnection or reduction is absorbed by the
off-load resistor 202 and the subsequent rise in voltage on the
voltage bus is stabilized.
[0042] Using the above-described dynamic braking system, when the
rotor is set down, the digital process controller decelerates the
rotor at the same rate at which it is accelerated. When the rotor
is set down in this manner, a significant amount of power is
generated as the energy stored in the inertia of the shaft is
reduced. At a rate of 20 k rpms, the rotor will generate a peak of
about 2.5 kw.
[0043] Without the dynamic brake, the DC bus voltage in the DC bus
206 would rise to unacceptably high levels during the set down
procedure. The dynamic brake is controlled to dissipate this energy
and thus allows the rotor to be set down rapidly, minimizing wear
and increasing life expectancy of the air bearings.
[0044] While the best mode for carrying out the invention has been
described in detail, those familiar with the art to which this
invention relates will recognize various alternative designs and
embodiments for practicing the invention within the scope of the
appended claims.
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