U.S. patent application number 13/450047 was filed with the patent office on 2012-11-15 for variable speed generator.
This patent application is currently assigned to ROLLS-ROYCE PLC. Invention is credited to Ellis FH CHONG, Andrew MACKAY.
Application Number | 20120286516 13/450047 |
Document ID | / |
Family ID | 44243917 |
Filed Date | 2012-11-15 |
United States Patent
Application |
20120286516 |
Kind Code |
A1 |
CHONG; Ellis FH ; et
al. |
November 15, 2012 |
VARIABLE SPEED GENERATOR
Abstract
A variable speed generator for producing AC electrical power
includes an alternator to generate a first AC current, and a first
rectifier which rectifies the first AC current from the alternator;
a main exciter having a first field winding which receives the
rectified first AC current and a first armature which produces a
second AC current in response; a second rectifier which rectifies
the second AC current from the first armature; a main generator
having a second field winding which receives the rectified second
AC current and a second armature which produces an output AC
current in response; and a control arrangement for activating a
selected pole configuration. The second field winding provides a
plurality of selectively activatable pole configurations which
differ in the number of their poles, such that the frequency of the
output AC current can be varied by switching between the pole
configurations.
Inventors: |
CHONG; Ellis FH; (Derby,
GB) ; MACKAY; Andrew; (Derby, GB) |
Assignee: |
ROLLS-ROYCE PLC
London
GB
|
Family ID: |
44243917 |
Appl. No.: |
13/450047 |
Filed: |
April 18, 2012 |
Current U.S.
Class: |
290/52 |
Current CPC
Class: |
H02K 7/1823 20130101;
H02K 19/32 20130101; H02P 9/48 20130101 |
Class at
Publication: |
290/52 |
International
Class: |
H02P 9/48 20060101
H02P009/48 |
Foreign Application Data
Date |
Code |
Application Number |
May 11, 2011 |
GB |
1107833.4 |
Claims
1. A variable speed generator for producing AC electrical power,
the variable speed generator including: an alternator powerable by
rotational action to generate a first AC current, a first rectifier
which rectifies the first AC current from the alternator, a main
exciter having a first field winding which receives the rectified
first AC current, and having a first armature which produces in
response a second AC current, a second rectifier which, rectifies
the second AC current from the first armature, and a main generator
having a second field winding which, receives the rectified second
AC current, and having a second armature which produces in response
an output AC current; wherein: the second field winding is
configured to provide a plurality of selectively activatable pole
configurations which differ in the number of their poles, such that
the frequency of the output AC current can be varied by switching
between the pole configurations, and the variable speed generator
further includes a control arrangement for activating the selected
pole configuration.
2. A variable speed generator according to claim 1, wherein the
second field winding includes a plurality of pole windings, the
selectively activatable pole configurations being produced by
switching on or off and/or reversing the polarities of selected of
the pole windings.
3. A variable speed generator according to claim 2, wherein the
control arrangement includes: a control switch for changing the
frequency of the distribution current, a secondary exciter having a
third field winding which on operation of the control switch
receives the rectified first AC current, and a third armature which
produces in response a third AC current, a third rectifier which
rectifies the third AC current from the third armature, and a
switching circuit which is activated by the rectified third AC
current to switch on or off and/or reverse the polarities of
selected of the pole windings of the second field winding and
thereby activates a different pole configuration.
4. A variable speed generator according to claim 3, wherein the
switching circuit includes a plurality of field effect transistors,
the rectified third AC current providing gate currents for the
transistors, and the conducting channels between the sources and
drains of the transistors transmitting the rectified second AC
current from the second rectifier to the second field winding.
5. A variable speed generator according to claim 4, wherein the
polarity of each selected pole winding is changed by an arrangement
of depletion mode and enhancement mode field effect transistors in
the switching circuit.
6. A variable speed generator according to claim 1, wherein the
second field winding includes first and second independently
activatable winding formations, the first winding formation
providing a pole configuration having a first number of poles, and
the second winding formation providing a pole configuration having
a different second number of poles, such that the frequency of the
output AC current can be varied by switching between the first and
second winding formations.
7. A variable speed generator according to claim 6, wherein the
variable speed generator further includes: a second main exciter
having a third field winding which receives the rectified first AC
current, and having a third armature which produces in response a
third AC current, and a third rectifier which rectifies the third
AC current from the third armature; and wherein: the first winding
formation receives the rectified second AC current, the second
winding formation receives the rectified third AC current, and the
control arrangement includes a control switch which directs the
rectified first AC current to either the first field winding or the
third field winding to change the frequency of the distribution
current.
8. A variable speed generator according claim 1, which is an
aircraft engine variable frequency starter generator, the
alternator being powerable by rotational action extracted from the
aircraft engine, and the output current being for use within the
aircraft.
Description
[0001] The present invention relates to a variable speed generator
for producing AC electrical power, and particularly, but not
exclusively, to an aircraft variable frequency starter
generator.
[0002] With reference to FIG. 1, a ducted fan gas turbine engine
generally indicated at 10 has a principal and rotational axis X-X.
The engine comprises, in axial flow series, an air intake 11, a
propulsive fan 12, an intermediate-pressure compressor 13, a
high-pressure compressor 14, combustion equipment 15, a
high-pressure turbine 16, and intermediate-pressure turbine 17, a
low-pressure turbine 18 and a core engine exhaust nozzle 19. A
nacelle 21 generally surrounds the engine 10 and defines the intake
11, a bypass duct 22 and a bypass exhaust nozzle 23.
[0003] The gas turbine engine 10 works in a conventional manner so
that air entering the intake 11 is accelerated by the fan 12 to
produce two air flows: a first air flow A into the
intermediate-pressure compressor 13 and a second air flow B which
passes through the bypass duct 22 to provide propulsive thrust. The
intermediate-pressure compressor 13 compresses the air flow A
directed into it before delivering that air to the high pressure
compressor 14 where further compression takes place.
[0004] The compressed air exhausted from the high-pressure
compressor 14 is directed into the combustion equipment 15 where it
is mixed with fuel and the mixture combusted. The resultant hot
combustion products then expand through, and thereby drive the
high, intermediate and low-pressure turbines 16, 17, 18 before
being exhausted through the nozzle 19 to provide additional
propulsive thrust. The high, intermediate and low-pressure turbines
respectively drive the high and intermediate-pressure compressors
14, 13 and the fan 12 by suitable interconnecting shafts.
[0005] Electrical power is usually extracted from such an engine
for use within the aircraft by a wound-field synchronous generator.
The generator can be mechanically connected to either the
high-pressure shaft or to the intermediate-pressure shaft, via a
transmission drive and accessory gearbox. DC current is applied to
the rotor of the so generator in the field winding. The frequency
of the current produced in the generator stator winding is thus
directly proportional to the speed of the shaft to which the
generator is connected, the gear ratio between the engine shaft and
the generator, and the number of pole pairs in the generator.
[0006] In contemporary aircraft platforms, the output frequency
range of the generator typically varies over a frequency range of
400 to 800 Hz; the exact numbers depending upon the platform, and
corresponding directly to an acceptable speed range for the shaft
to which the generator is connected. The generator frequency range
is provided to the suppliers of electrical equipment within the
aircraft, so that their equipment can be configured to receive
voltage in this frequency range.
[0007] Due to its variable output frequency, this type of generator
is known as variable frequency starter generator (VFSG). FIG. 2
shows a schematic diagram of a VFSG, which includes a permanent
magnet alternator (PMA) 30, main exciter 31 and main generator 32.
The rotating parts of the PMA, exciter and generator are physically
all mounted on the same shaft 33 and rotate at the same speed. The
DC current injected into the field winding of the main generator
comes from a rotating diode rectifier which is powered from the
main exciter, which in turn is powered from the PMA.
[0008] The PMA 30 has permanent magnets mounted on its rotor 34. As
the rotor spins, an AC main exciter voltage is induced across the
stationary armature winding 35 of the PMA. This winding is
connected to a voltage regulator circuit 36 which rectifies a
controlled amount of AC current from the PMA stator winding and
injects DC current into the stationary field winding 37 of the
exciter 31. This in turn induces an AC voltage across the rotating
armature winding 38 of the exciter (the exciter is referred to as
inside out, with a stationary field winding and a rotating armature
winding). A rotating diode rectifier circuit 39, producing a DC
current, is connected to the armature winding of the exciter. The
output of this rotating rectifier is then connected to the rotating
field winding 40 of the main generator 32, inducing a controlled,
AC voltage across the generator's stationary armature winding 41.
Due to the high speed of rotation of the shaft 33 in aerospace
generators, a brushed system for applying field current to the
rotor of the main generator is not desirable.
[0009] The voltage regulator circuit 36 responds to changes in the
load on the generator to maintain its output voltage at the
required magnitude. It does not affect the generator output
frequency. In some arrangements, for example during starting, the
PMA 30 may not be used and electrical power can be provided
directly to the exciter 31 from an alternative source.
[0010] The use of a VFSG and direct mechanical coupling between the
engine shaft and the generator means that a restriction on the
frequency range of the generator electrical output maps directly to
a speed range restriction on the engine shaft. The ratio of maximum
to minimum speed is typically around 2.2:1 (producing a frequency
range of e.g. 800 Hz to 360 Hz).
[0011] If the VFSG and gearbox are configured to produce maximum
frequency when the engine shaft is at its maximum speed, the
minimum frequency condition effectively imposes a minimum speed and
therefore a minimum thrust condition on the engine. During idling
conditions, such as descent and taxiing, it is desired that the
engine should produce as little thrust as possible, in order to
save fuel. However, in order to remain within the electrical
frequency range of the generator, the idle thrust of the engine may
have to be set artificially high. Therefore it is desirable to have
some degree of freedom between the electrical frequency and the
mechanical speed.
[0012] It would be desirable to provide a generator which can vary
the frequency of its output current independently of engine
speed.
[0013] Accordingly, the present invention provides a variable speed
generator for producing AC electrical power, the variable speed
generator including:
[0014] an alternator powerable by rotational action to generate a
first AC current,
[0015] a first rectifier which rectifies the first AC current from
the alternator,
[0016] a main exciter having a first field winding which receives
the rectified first AC current, and having a first armature which
produces in response a second AC current,
[0017] a second rectifier which rectifies the second AC current
from the first armature, and
[0018] a main generator having a second field winding which
receives the rectified second AC current, and having a second
armature which produces in response an output AC current;
[0019] wherein:
[0020] the second field winding is configured to provide a
plurality of selectively activatable pole configurations which
differ in the number of their poles, such that the frequency of the
output AC current can be varied by switching between the pole
configurations, and
[0021] the variable speed generator further includes a control
arrangement for activating the selected pole configuration.
[0022] Advantageously, by switching between the pole configurations
to vary the frequency of the output AC current, the frequency can
be changed independently of the speed of the rotational action,
such that the allowable rotational speed range can be increased.
Furthermore, the variable speed generator can be implemented
without using brushes and slip rings.
[0023] The generator may have any one or, to the extent that they
are compatible, any combination of the following optional
features.
[0024] Preferably, the pole configurations of the second armature
are reconfigurable to match the number of poles in the second field
winding, e.g. to make the number of poles on the second armature
the same as the number of poles on the second field winding. For
example, the control arrangement can activate reconfiguration of
the second armature at the same time as activating the selected
pole configuration of the second field winding.
[0025] Typically, the alternator has a rotor carrying one or more
field magnets, and a stator carrying an alternator armature in
which the first AC current is generated. Typically, the main
exciter has a stator carrying the first field winding, and a rotor
carrying the first armature. Typically, the main generator has a
rotor carrying the second field winding, and a stator carrying the
second armature. Typically, the main exciter and main generator are
powered by the same rotational action as the alternator.
Conveniently, the rotors of the alternator, main exciter and main
generator may be coaxially mounted in the variable speed generator,
and, in use, rotate at the same speed. For example, they may be
mounted on the same rotatable shaft.
[0026] Preferably, the second rectifier is a diode rectifier. Such
rectifiers are passive and generally reliable, particularly at the
high rotational speeds which the rectifier may experience.
[0027] Preferably, the first rectifier is a voltage regulator
circuit.
[0028] The second field winding may include a plurality of pole
windings, the selectively activatable pole configurations being
produced by switching on or off and/or reversing the polarities of
selected of the pole windings. For example, the control arrangement
can include:
[0029] a control switch for changing the frequency of the
distribution current,
[0030] a secondary exciter having a third field winding which on
operation of the control switch receives the rectified first AC
current, and a third armature which produces in response a third AC
current,
[0031] a third rectifier which rectifies the third AC current from
the third armature, and
[0032] a switching circuit which is activated by the rectified
third AC current to switch on or off and/or reverse the polarities
of selected of the pole windings of the second field winding and
thereby activates a different pole configuration. The switching
circuit can then include a plurality of field effect transistors
(e.g. MOSFETS), the rectified third AC current providing gate
currents for the transistors, and the conducting channels between
the sources and drains of the transistors transmitting the
rectified second AC current from the second rectifier to the second
field winding. For example, the polarity of each selected pole
winding may be changed by an arrangement of depletion mode and
enhancement mode field effect transistors in the switching circuit.
In this way, a reliable means of switching on or off and/or
reversing the polarities of selected of the pole windings can be
achieved which can operate at high rotational speeds and does not
require the use of brushes and slip rings. Typically, the secondary
exciter is powered by the same rotational action as the alternator,
main exciter and main generator. Typically, the secondary exciter
has a stator carrying the third field winding, and a rotor carrying
the third armature. Conveniently, the rotor of the secondary
exciter may be coaxially mounted (e.g. on the same shaft) with the
rotors of the alternator, main exciter and main generator.
[0033] Alternatively, the second field winding may include first
and second independently activatable winding formations, the first
winding formation providing a pole configuration having a first
number of poles, and the second winding formation providing a pole
configuration having a different second number of poles, such that
the frequency of the output AC current can be varied by switching
between the first and second winding formations. For example, the
variable speed generator can further include:
[0034] a second main exciter having a third field winding which
receives the rectified first AC current, and having a third
armature which produces in response a third AC current, and
[0035] a third rectifier which rectifies the third AC current from
the third armature;
[0036] wherein:
[0037] the first winding formation receives the rectified second AC
current,
[0038] the second winding formation receives the rectified third AC
current, and
[0039] the control arrangement can include a control switch which
directs the rectified first AC current to either the first field
winding or the third field winding to change the frequency of the
distribution current. Typically, the second main exciter is powered
by the same rotational action as the alternator, main exciter and
main generator. Typically, the second main exciter has a stator
carrying the third field winding, and a rotor carrying the third
armature. Conveniently, the rotor of the second main exciter may be
coaxially mounted (e.g. on the same shaft) with the rotors of the
alternator, main exciter and main generator.
[0040] The variable speed generator can be a variable frequency
starter generator. More particularly, the variable speed generator
can be an aircraft engine variable frequency starter generator, the
alternator being powerable by rotational action extracted from the
aircraft engine, and the output current being for use within the
aircraft.
[0041] Embodiments of the invention will now be described by way of
example with reference to the accompanying drawings in which:
[0042] FIG. 1 shows a schematic longitudinal section through a
ducted fan gas turbine engine;
[0043] FIG. 2 shows a schematic diagram of a variable frequency
starter generator;
[0044] FIG. 3 shows a schematic diagram of a variable frequency
starter generator according to a first embodiment of the present
invention;
[0045] FIGS. 4(a) to (c) show respectively 8-pole, 4-pole and
4-pole salient pole configurations for the rotating field winding
of the main generator of the variable frequency starter generator
of FIG. 3;
[0046] FIGS. 5(a) and (b) show respectively 8-pole and 4-pole
configurations for the stationary armature winding of the main
generator for use with the pole configurations of FIG. 4;
[0047] FIGS. 6(a) and (b) show respectively 6-pole and 4-pole
salient pole configurations for the rotating field winding of the
main generator of the variable frequency starter generator of FIG.
3;
[0048] FIGS. 8(a) and (b) show respectively 6-pole and 4-pole
configurations for the stationary armature winding of the main
generator for use with the pole configurations of FIG. 6 or 7;
[0049] FIG. 9 shows an arrangement of depletion mode (D) and
enhancement (E) mode switches to switch the polarity of a pole
winding; and
[0050] FIG. 10 shows a schematic diagram of a variable frequency
starter generator according to a second embodiment of the present
invention.
[0051] FIG. 3 shows a schematic diagram of a VFSG according to a
first embodiment of the present invention. Like the VFSG of FIG. 2,
the VFSG of the first embodiment includes a PMA 50, a main exciter
51 and a main generator 52. The rotating parts of the PMA, main
exciter and main generator are all mounted on the same shaft 53 and
rotate at the same speed. The PMA has permanent magnets mounted on
its rotor 54. As the rotor spins, an AC main exciter voltage is
induced across the stationary armature winding 55 of the PMA. This
winding is connected to a voltage regulator circuit 56 which
rectifies a controlled amount of AC current from the PMA stator
winding and injects DC current into the stationary field winding 57
of the main exciter. This in turn induces e.g. a 3 phase AC voltage
across the rotating armature winding 58 of the main exciter. A
rotating diode rectifier circuit 59, producing a DC current, is
connected to the armature winding of the exciter. The output of
this rotating rectifier is then connected via a control arrangement
(discussed below) to the rotating field winding 60 of the main
generator, inducing an output AC voltage across the generator's
stationary armature winding 61. The field winding 60 is configured
to provide a plurality of selectively activatable pole
configurations which differ in the number of their poles, such that
the frequency of the output AC current can be varied by switching
between the pole configurations.
[0052] Unlike an induction machine, where the rotor poles are
induced by the stator field, the rotating field winding 60 are
directly supplied by the DC current from the rectifier circuit 59.
However, the number of poles on the rotating field winding 60 can
be varied without the use of brushes and slip rings, which would be
undesirable due to the speed of rotation of the shaft 53. More
particularly, the field winding 60 has individual pole windings,
and the number of poles on the field winding can be varied by
switching on or off selected of the pole windings, or reversing
their polarities.
[0053] The rotating field winding 60 can be of round rotor design
or salient pole design. FIG. 4(a) shows, for example, a salient
8-pole rotor configuration for the winding. Each saliency comprises
an iron core and a pole winding, which produces a magnetic field
when direct current is injected into the pole winding. Depending on
the polarity of the injected current, the poles will be either
North (N) or South (S) polarity. Typically, the poles will be
arranged in alternating polarities to give an even distribution of
flux.
[0054] Switching to a 4-pole configuration can be achieved by
switching off half the pole windings and reversing the polarity on
2 of the remaining poles, as shown in FIG. 4(b). To provide the
same voltage on the terminals of the main generator 52, the amount
of field current in the rotating field winding 60 can be increased,
increasing the flux density in the iron cores of the active poles.
The size of the poles is determined by the configuration with the
lowest number of poles as the maximum allowable flux density is
limited by the iron core material.
[0055] An alternative approach is to have all the poles active and
to change the polarity such that each magnetic pole is shared
between 2 adjacent saliencies, as shown in the configuration of
FIG. 4(c). This allows the flux density to remain the same as the
8-pole configuration of FIG. 4(a), but may incur higher harmonic
content than the configuration shown in FIG. 4(b).
[0056] The poles on the stationary armature winding 61 can be
reconfigured by switching armature winding connectors to match the
number of poles on the field winding 60. For example, FIGS. 5(a)
and (b) show respectively 8-pole and 4-pole configurations of the
armature winding achieved by switching the polarity of every second
pole winding. In FIGS. 5(a) and (b), for convenience the armature
is represented as a linear armature and the pole windings are
represented by single coils. The North (N) and South (S) poles are
shown between the flux paths.
[0057] Depending on the specific application, it may be desirable
to switch to a different ratio of poles, rather than the 2:1 ratio
of FIGS. 4(a) to (c). For example, if switching from a 6-pole
configuration to a 4-pole configuration, the rotating field winding
60 must contain the least common multiple of saliencies i.e. 12.
FIG. 6(a) shows a 6-pole configuration on such a 12 saliency pole
rotor. To switch to a 4-pole configuration, four pole windings can
be switched off, two pole windings can be switched on, and the
polarity of one pole winding can be reversed, as shown in FIG.
6(b). FIG. 7(a) shows an alternative 6-pole configuration on a 12
saliency pole rotor. To switch to a 4-pole configuration, the
polarities of six pole winding can be reversed, as shown in FIG.
7(b). Advantageously, this configuration uses all the available
saliencies to form the poles. FIGS. 8(a) and (b) show respectively
the corresponding 6-pole and 4-pole configurations of the
stationary armature winding 61 achieved by placing, at the ends of
the windings, switches or contactors that connect different wires
within the armature together, to give the desired pole
configuration.
[0058] Returning to FIG. 3, as mentioned above, the rotating diode
rectifier circuit 59, which produces a DC current, is connected via
a control arrangement to the rotating field winding 60 of the main
generator 52. The control arrangement includes a secondary exciter
62, a passive (e.g. diode-based) rectifier circuit 65, and a
rotating switching circuit 66, which are all located on the shaft
53 between the main exciter 51 and the main generator. The
secondary exciter is an inside-out machine, similar to the main
exciter, in which the field winding 63 is located on the stator and
the armature winding 64 on the rotor. The secondary exciter
typically provides either single phase or 3-phase AC current on the
rotor, which is rectified to DC by the rectifier circuit 65. The DC
current is then fed to the switching circuit, which switches on or
off and/or reverses the polarities, of selected of the pole
windings of the rotating field winding 60 to activate a different
pole configuration in the main generator 52.
[0059] The switching circuit 66 contains an arrangement of
semiconductor-based switches. The conducting channels of the
switches receive the output of the rotating rectifier 59. The
secondary exciter 62 provides a gate current to the switches. Each
switch either switches on or off individual pole windings of the
field winding 60 or reverses the polarity of the pole windings, to
give a desired new pole configuration on the rotor of the main
generator 52.
[0060] As the switches are typically required to carry
approximately 10-100 A, they are generally high-powered devices
such as power MOSFETs (metal-oxide-semiconductor field-effect
transistors). Both depletion mode and enhancement mode MOSFETs can
be used to switch from one pole configuration to another. The
depletion mode MOSFETs conduct when there is no gate current and
open when gate current is applied, while the enhancement mode
MOSFETs open when there is no gate current and conduct when gate
current is applied. For example, to switch the polarity of a pole
winding, an arrangement of depletion mode (D) and enhancement (E)
mode switches can be used as shown in FIG. 9. In this arrangement
the depletion mode switches will conduct and the enhancement
switches will open when no gate current is applied, i.e. the
secondary exciter 62 is not providing power. When the secondary
exciter is switched on it provides gate current, opening the
depletion mode switches and closing the enhancement mode switches
and thus reversing the polarity of the winding.
[0061] This secondary exciter 62 generally has a lower rating than
the main exciter 51, as it only needs to provide gate current to
the switches.
[0062] The control arrangement also includes a switch 67 which
determines whether or not the field current provided by the voltage
regulator circuit 56 is sent to the secondary exciter 62. When the
switch 67 is closed, the secondary exciter 62 is activated and gate
current flows to the switches of the switching circuit 66. When the
switch 67 is open, the secondary exciter 62 is disabled and no gate
current flows. The switch 67 can also be operatively connected
(dashed line) to the stationary armature winding 61 so that its
poles are reconfigured to match the number of poles on the field
winding 60 when the switch 67 is operated.
[0063] The VFSG can switch poles on the main generator 52 during
operation thus providing a wider rotational speed range or the
shaft 53 while remaining within a predetermined electrical
frequency range of the output AC current. The VFSG can provide and
control its own reactive power and terminal voltage. As it does not
require brushes and slip rings, it is suitable for high-speed
applications and should not require high levels of maintenance.
[0064] FIG. 10 shows a schematic diagram of a VFSG according to a
second embodiment of the present invention. Corresponding features
have the same reference numbers in FIGS. 3 and 10. Like the VFSG of
the first embodiment, the VFSG of the second embodiment includes a
PMA 50, a main exciter 51 and a main generator 152. However, an
alternative arrangement used by the second embodiment for changing
the poles on the rotor of the main generator 152 without the use of
brushes and slip rings is to have a second main exciter 151
operating at the same power level as the first main exciter 51,
along with a second rotating diode rectifier 159 for the second
main exciter. In addition, the rotating field winding 160 of the
main generator 152 has first and second independently activatable
winding formations, each with a different number of poles. The
outputs of the rotating rectifiers 59, 159 are then connected to
respectively the first and second winding formations, inducing an
output AC voltage across the main generator's stationary armature
winding 161. For example, first and second winding formations can
be provided by respectively a first set and a second set of pole
windings on each saliency of the rotor of the main generator.
[0065] In the second embodiment, the control arrangement is simply
a switch 167 which switches the output of the voltage regulator
circuit 56 from one main exciter to the other. Again, the switch
167 can also be operatively connected to the stationary armature
winding 161 so that its poles are reconfigured to match the number
of poles on the field winding 160 when the switch 167 is
operated.
[0066] However, a drawback of this the second embodiment is that it
tends to increase the overall size of the VFSG. Additionally, as
there are two separate winding formations on the rotating field
winding 160, undesirable voltage may be induced in the inactive
formation.
[0067] Although the VFSG of the first and second embodiments is
intended for aircraft use, a variable speed generator according to
the present invention may also find use in other applications,
particularly where it is desired to extend the rotational speed
range of is the shaft and to avoid the use of brushes and slip
rings.
[0068] While the invention has been described in conjunction with
the exemplary embodiments described above, many equivalent
modifications and variations will be apparent to those skilled in
the art when given this disclosure. Accordingly, the exemplary
embodiments of the invention set forth above are considered to be
illustrative and not limiting. Various changes to the described
embodiments may be made without departing from the spirit and scope
of the invention.
* * * * *