Rotary electric machine and power conversion system using same

A power conversion system for converting between electrical power at different frequencies, including a rotary electrical machine including a rotor, a stator, first and second independently controllable field windings, and at least one armature winding for each phase; a switching circuit including a plurality of switching devices and having first terminal means interconnected with the armature winding which carries a high frequency signal established by the machine, and a second terminal for interconnection with an impedance establishing a lower frequency signal; the first field control circuit for monitoring the machine to sense phase difference between the optimum zero crossings and actual zero crossings of the higher frequency signal for driving the first field winding to adjust the phase of the higher frequency signal to minimize the phase difference; a second field control circuit for modulating the higher frequency signal carried by the armature winding with the lower frequency signal and for monitoring the second terminal means to sense amplitude difference between a selected one of the voltage and current parameters of the lower frequency signal and a reference level for driving the second field winding to adjust that parameter towards the reference level; and a switch firing circuit responsive to the machine voltage and one of the voltage and current parameters of the lower frequency signal at the second terminal means for selectively triggering to the on state and self-commutating to the off state the switching devices synchronously with the zero crossings of the higher frequency signal for transferring power between the higher and lower frequency signals through the switching circuit.

FIELD OF INVENTION 
This invention relates to a power conversion system for converting between 
electrical power at different frequencies, and to a rotary electrical 
machine for use therein. 
BACKGROUND OF INVENTION 
Investigation of power conversion systems applicable to high speed and 
variable speed shafts have received much attention recently because of 
their ability to generate power at higher speeds with reduced weight and 
size. They are of particular interest due to recent interest in their use 
with windmills and flywheels. In flywheel storage systems the conversion 
system, and particularly the motor generator machine, must be capable in 
one mode of generating power at higher and variable frequencies and 
presenting it at constant and conventional lower frequency and in another 
mode of accepting power at the constant lower frequency and converting it 
to a higher variable frequency to drive the flywheel. When used with 
windmills and other prime power sources the system must be capable of 
converting the generated higher varying frequency electrical signals to 
constant lower frequency signals. See Report R-960, Interim Report on 
Research Toward Improved Flywheel Suspension and Energy Conversion 
Systems, by David Eisenhaure, George Oberbeck, Stephen O'Dea, and William 
Stanton. 
In general, the variable speed (frequency) power must be converted to an 
existing fixed frequency for use. Many different combinations of 
conventional power conversion systems have been applied to this task, each 
with its own deficiencies. Typical systems have employed multiple machine 
configurations, variable mechanical speed reducers, A.C.- D.C.-A.C. 
converter-inverters, cycloconverters, field modulated down converters and 
many other arrangements. Many of these systems have suffered from large 
weight and size, inefficiency due to poor waveform quality and form 
factor, or unreliability due to complexity and switching losses. 
SUMMARY OF INVENTION 
It is therefore an object of this invention to provide an improved, 
efficient, integrated power conversion system for converting between 
electrical power at different frequencies, and an improved extremely 
efficient rotary electrical machine for use therein. 
It is a further object of this invention to provide such an improved system 
capable of transferring power either from the lower to the higher or the 
higher to the lower frequency whether the frequencies are fixed or 
varying. 
It is a further object of this invention to provide such an improved system 
which reduces harmonic content of the waveforms and the need for 
filtering. 
It is a further object of this invention to provide such an improved system 
which produces controlled voltage, frequency and power output despite 
varying frequency input. 
It is a further object of this invention to provide such an improved system 
in which the rotary electrical machine is a single unit. 
It is a further object of this invention to provide such an improved system 
which may be used as a stand-alone power supply or may be coupled with a 
power line. 
The invention results from the realization that an improved rotary 
electrical machine, for utilization with switching circuitry of an 
improved power conversion system, can be constructed using two 
independently controllable fields which induce a back EMF in the rotary 
machine compatible with self-commutating operation of the switching 
circuitry, and further that the controllable fields can also be used to 
control relative amplitudes and phases of the output voltage and current. 
The invention features a power conversion sytem for converting electrical 
power at different frequencies. It includes a rotary electrical machine 
including a rotor, a stator, first and second independently controllable 
field windings, and at least one armature winding for each phase. There is 
a switching circuit including a plurality of switching devices and having 
first terminal means interconnected with the armature winding which 
carries a higher frequency signal established by the machine and a second 
terminal means for interconnection with an impedance which establishes a 
lower frequency signal. The lower frequency signal can be a zero frequency 
or D.C. signal. A first field control circuit monitors the machine to 
sense phase difference between the optimum zero crossings and actual zero 
crossings of the higher frequency signal for driving the first field 
winding to adjust the phase of the higher frequency signal to minimize the 
phase difference. A second field control circuit modulates the higher 
frequency signal carried by the armature winding with the lower frequency 
signal and monitors the second terminal means to sense amplitude 
difference between the selected one of the voltage and current parameters 
of the lower frequency signal and a reference level for driving the second 
field winding to adjust that parameter towards the reference level. A 
switch firing circuit responsive to the machine and to the voltage and 
current parameter of the lower frequency signal at the second terminal 
means selectively triggers the on state and self-commutates to the off 
state the switching devices synchronously with the zero crossings of the 
higher frequency signal, for transferring power between the higher and 
lower frequency through the switching circuit. 
The invention also features a rotary electrical machine, such as a motor 
generator, motor, or generator, having a rotor, a stator, an armature, and 
dual independently controllable field windings in spaced quadrature for 
selectively shaping the back EMF waveform of the machine to control 
amplitude and phase relations between the voltage and current outputs of 
the machine. 
In a preferred embodiment the rotary machine may be a motor-generator or a 
motor or a generator. If it is a motor-generator capable of operating in a 
motor mode and a generator mode, its rotor is adapted for mechanical load 
and/or drive. The impedance is an electrical load and/or power source and 
the power transfer through the switching circuit occurs in the generator 
mode by detecting the lower frequency signal from the modulated higher 
frequency signal and presenting the lower frequency signal at the second 
terminal means, and in the motor mode by chopping the lower frequency 
signal at the frequency of the higher frequency signal and presenting it 
at the first terminal means. 
When the machine is a motor, the impedance is an electrical power source 
and the power transfer through the switching circuit occurs by the 
chopping of the lower frequency signal at the frequency of the higher 
frequency signal and presenting it at the first terminal means. When the 
machine is a generator, the impedance is an electrical load and the power 
transfer through the switching circuit occurs by detecting the lower 
frequency signal from the higher frequency signal and presenting it at the 
second terminal means. 
When the impedance includes an electrical power source such as a utility 
company power line, the second field control circuit monitors the current 
parameter of the lower frequency signal at the second terminal means and 
when the system is operating as a stand-alone system, the second field 
control circuit monitors the voltage parameter of the lower frequency 
signal at the second terminal means.

The power conversion system according to this invention may be accomplished 
using a rotary electrical machine including a rotor, a stator, first and 
second independently controlled field windings, and at least one armature 
winding for each phase. There may be one or more phases, the field 
windings may be either on the stator or the rotor. The system includes a 
switching circuit including a plurality of switching devices such as 
SCR's, and the switching circuit includes a first terminal interconnected 
with the armature winding which carries a higher frequency signal 
inherently established by the machine operation and a second terminal for 
interconnection with an impedance, either a load impedance or a power 
source impedance, which establishes a lower frequency signal at that 
terminal. The lower frequency signal may be zero frequency or D.C., as 
well as A.C. There is a first field control circuit for monitoring the 
machine voltage to sense the phase difference between the optimum zero 
crossings and the actual zero crossings of the higher frequency signal for 
driving the first field winding to adjust the phase of the higher 
frequency signal and minimize that phase difference. In effect, the phase 
difference between the output voltage and output current of the machine is 
being monitored in this way. The internal voltage of the machine which 
determines the optimum zero crossings may be monitored using an encoder 
which indicates the relative position of the rotor and stator. The actual 
zero crossings indicative of the output current zero crossings may be 
determined by a polarity indicator. Any phase difference between the two 
generates an error signal which is fed to the first field which drives the 
machine to correct that error. 
There is a second field control circuit for modulating the higher frequency 
signal carried by the armature winding with the lower frequency signal, 
and for monitoring the second terminal of the switching circuit to sense 
the amplitude difference between the selected one of the voltage and 
current parameters of the lower frequency signal and a reference level, 
for driving the second field winding to adjust that parameter towards the 
reference level. If the second terminal means of the switching circuit is 
coupled to an external power source, then it is the current parameter that 
is sensed, and the reference is a current level. The two are combined in a 
comparator and an error signal is used to drive the second field to adjust 
the amplitude and phase of the output current relative to the output 
voltage. 
The rotary electrical machine may be a motor-generator if bi-directional 
power conversion is desired, or may be simply a motor or simply a 
generator. If it is a motor-generator, the shaft of the motor-generator is 
typically connected to a device such as a flywheel, which may either 
provide mechanical force to drive the generator in the generator mode or 
be driven by the motor in the motor mode. In such a case, the switching 
circuit would be connected to an impedance which would operate in the 
motor mode as a power source and in the generator mode as a load. Similar 
arrangements would adhere in the motor and in the generator modes. When 
the system is operating in the stand-alone mode, that is, it is not 
coupled to an external power source, the comparator in the second field 
control circuit monitors the voltage parameter on the output from the 
switching circuit, and the reference parameter is a reference voltage. 
Report R-960, Interim Report on Research Toward Improved Flywheel 
Suspension and Energy Conversion Systems, by David Eisenhaure, George 
Oberbeck, Stephen O'Dea, and William Stanton, is incorporated here by 
reference. 
There is shown in FIG. 1 a power conversion system 10 according to this 
invention including a rotary electrical machine 12 having a first field, 
field A 14, and a second field, field B 16. Field A is fed by field A 
control circuit 18, and field B by field B control circuit 20. Switching 
circuit 22 having a first terminal 24 connected to the output of rotary 
machine 12 and a second terminal 26 connected to the output line 128, 
chops the lower frequency signal at terminal 26 at a frequency equal to 
the higher frequency provided in machine 12 in the motor mode, and detects 
the lower frequency envelope of the higher frequency signal produced in 
machine 12 in the generator mode. The switching circuit is controlled by 
firing circuit 28, which synchronously switches circuit 22 in a sequence 
dependent upon the mode of operation of machine 12, motor or generator, 
and the optimum zero crossover point of the voltage output from machine 
12. 
Field control circuit 18 includes a polarity indicator 32 which provides a 
signal each time the higher frequency signal at terminal 24 crosses zero, 
i.e. the actual time that the output current crosses zero, and a 
rotor/stator position encoder 34 which, by reference to the position of 
the rotor relative to the stator, provides a signal indicating the optimum 
zero crossover time, or the time that the voltage crosses zero. The 
outputs of polarity indicator 32 and encoder 34 are submitted to a 
phase-sensitive detector 36, which provides an error to amplifier 38 if 
there is a phase difference detected. Amplifier 38, in response to an 
error signal, drives field A to adjust the phase of the output current 
relative to the output voltage in terminal 24 and eliminate the error. 
Field control circuit 20 includes a comparator 40 which senses either the 
voltage or the current parameter on output line 128 and compares it with a 
like reference parameter, voltage or current respectively, to provide an 
error signal if there is a difference between the two parameters. That 
error signal is provided to amplifier 42, which responds by driving field 
B 16 to adjust the amplitude and phase relationship of the output current 
and voltage to minimize that error. If output line 128 is coupled to an 
external power source, then the relevant parameter is current. 
Throughout the specification, similar parts have been given similar numbers 
accompanied by a lower case letter. 
Alternatively, as shown in FIG. 2, rotary electrical machine 12a may be a 
motor-generator having a flywheel 44 connected to a shaft. System 10a is 
coupled with an external power source, such as power company 46, which 
together with system 10a supplies user load 48, such as a residence. Such 
a system may be used to maintain constant loading on power company 46, so 
that during peak periods system 10a operates as a generator to take energy 
from flywheel 44 and supply power to user load 48 in conjunction with that 
supplied by power company 46, while during low power drain periods, when 
user load 48 is below that constant set for power company 46, the extra 
power is supplied through switching circuit 22a to motor-generator 12a 
operating as a motor, which drives flywheel 44 to store energy therein. 
When it is desirable to have a varying reference to comparator 40 in field 
control circuit 20a, a load levelling programmer 50, which varies the 
reference level over a period of time, may be used. Load levelling 
programmer 50 may be a fixed, cyclical program or it may respond to real 
time inputs, such as the drain on the power company 46 output, as 
indicated by dashed line 52. In addition to these applications a prime 
mover 54, such as a windmill, may be used to drive flywheel 44 and 
motor-generator 12a in the generator mode. System 10a is not restricted to 
stationary uses, as it may be used to power vehicles as well. In a 
vehicle, flywheel 44 may be used to drive motor-generator 12a as a 
generator, to drive another motor-generator set which drives the wheels of 
an automobile and, during downhill runs when the automobile is coasting, 
the second motor-generator set may be used to supply power to switching 
circuit 22a to drive motor-generator 12a as a motor and put energy into 
flywheel 44. 
Switching circuit 22 may include a plurality of switching devices SCR's 
1-8, circuit 22c, FIG. 3. SCR's 1-8 are connected in pairs, in parallel, 
and oppositely polarized. SCR's 1 and 2 are connected directly between 
terminals 24 and 26. Terminals 24 and 26 may include two busses 24c and 
24cc and 26c and 26cc, respectively. The first pair of SCR's 1 and 2 are 
connected between terminals 24c and 26c; SCR's 7 and 8 are connected 
between terminals 24cc and 26cc; SCR's 3 and 4 are cross-connected from 
terminal 24cc to terminal 26c; and SCR's 5 and 6 are connected from 
terminals 24c to 26cc. There may be one such group of SCR's or other 
semiconductor devices for each output phase of the system. 
Switching circuit 22 is fired in a synchronous pattern with the 
high-frequency signal from rotary machine 12 by means of firing circuit 
28a, FIG. 4, which includes a one of eight decoder 60 having eight outputs 
labelled 0 through 7, and having three inputs: one from exclusive OR gate 
62 and one from polarity sensor 64, and one from rotor/stator position 
encoder 34. The inputs to exclusive OR gate 62 are from polarity sensors 
64 and 66. Polarity sensor 64 senses the output voltage V.sub.O on line 
28, while polarity sensor 66 senses the polarity of output current I.sub.O 
on line 128, through lines 68 and 70 respectively. When the polarities of 
the output voltage and current are different, exclusive OR gate 62 
indicates operation in the motor mode, and when they are the same, in the 
generator mode. The eight outputs labelled 0-7 of decoder 60 are fed 
through OR gates 70, 72, 74, and 76, which respectively operate SCR 
switches 1 and 8; 4 and 5; 2 and 7; and 3 and 6; FIG. 3. A truth table 80 
for decoder 60 is shown in FIG. 4A. A 1 indicates motor operation and 0 
indicates generator operation in the motor-generator column. In the 
V.sub.O column, 1 indicates positive output voltage and 0 negative output 
voltage; and in the rotor column, 1 represents that the rotor position 
indicates positive machine voltage and a 0 negative machine voltage. Thus 
in the motor mode, when V.sub.O is positive, first SCR'1 and 8 will be 
fired when the back EMF is positive, and SCR's 3 and 6 will be fired when 
the back EMF is negative. In the generator mode, SCR's 2 and 7 will be 
fired for positive back EMF, and then SCR's 4 and 5 will be fired for 
negative back EMF. For negative V.sub.O, in the generator mode SCR's 3 and 
6 are fired for positive back EMF and SCR's 1 and 8 for negative back EMF. 
In the motor mode for negative V.sub.O, SCR's 4 and 5 are fired for 
positive back EMF and SCR's 7 and 2 for negative back EMF. In each of the 
two modes, motor and generator, the sequence repeats itself. Decoder 60 
may be implemented by a group of AND gates 82, 84, 86, 88, 90, 92, 94, and 
96, FIG. 5 with inverters 99. 
The rotary electrical machine 12, and more specifically a motor-generator 
12a, may be implemented by a two-phase field-modulated inductor 
motor-generator 100, FIG. 6, and includes an inductor rotor 101 surrounded 
by an eight tooth stator including stator teeth 102, 104, 106, 108, 110, 
112, 114, and 116. The stator structure is omitted for clarity. Armature 
or input-output winding 118 is wound on the stator and has access through 
two terminals 120. Field A and field B are also wound on the stator, 
windings 121, 123 respectively, and have access to their terminals 122 and 
124 respectively. The structure shown in FIG. 6 represents but one phase 
of the two-phase motor-generator. The second phase is simply a duplicate 
of the first, axially stacked with the portion shown and with the teeth 
102-116 shifted to provide a second phase at 90.degree. with respect to 
the first phase. The field A winding 121 makes teeth 102, 104, 106, and 
108 assume one magnetic polarity, and teeth 110, 112, 114 and 116 assume 
the opposite polarity. Field B winding 123 for the generator mode causes 
teeth 102, 106, 110, and 114 to add flux in the same direction as field A 
and to oppose field A in teeth 104, 108, 112 and 116. 
The chart in FIG. 6A shows the combined effect of fields A and B on the 
stator teeth. 
In the generator mode, when the lower frequency is zero or D.C. at terminal 
26 and output line 28, the idealized waveforms for one phase of a 
two-phase system are as shown in FIG. 7, assuming that the machine has an 
inductive output impedance, which is normally the case. V.sub.O 130, FIG. 
7, is a square wave, and the generator output, or the back EMF of machine 
12, V.sub.E, has a step shape as shown at 132 with V.sub.O 130 
superimposed on it. V.sub.E 132 is formed by the combination of the 
voltage due to field A, V.sub.A 134, FIG. 8, the voltage due to field B, 
V.sub.B 136, FIG. 8. The combination of these two waveforms results in 
V.sub.E 132, shown in full lines in FIG. 8 and in dashed lines in FIG. 7, 
superimposed on V.sub.O 130. It is the control over the relative 
amplitudes of these two voltages, V.sub.A and V.sub.B, which enable this 
motor-generator to vary the phase relationship and the amplitude 
relationship of the voltages and currents. 
V.sub.E 132, in the form shown in FIGS. 7 and 8, produces an output 
current, I.sub.O 138, in FIG. 7. This I.sub.O is actually I.sub.O1 or the 
output current for phase 1. The same wave shapes shifted 90.degree. apply 
to the second phase of the system, and the two output currents I.sub.O1 
138 and I.sub.O2 140 are shown in solid lines after rectification in FIG. 
9. The sum of the rectified solid line waveforms 138 and 140 closely 
approximates the desired zero frequency or D.C. output. The portion of 
V.sub.E which induces the current 138 is that voltage difference 133 
between the salient portion 135 of V.sub.E 132 and the top of V.sub.O. 
When the system is operated as a motor, FIG. 10, V.sub.O 130a appears the 
same as V.sub.O 130, but each portion of V.sub.E 132a appears as a mirror 
image of the similar portion of V.sub.E 132, due to the fact that in the 
motor operation the current is moving in the opposite direction, or is 
180.degree. out of phase with respect to that same current in the 
generator mode. Thus the output current I.sub.O1 138a, FIG. 10, is 
similarly shifted by 180.degree. with respect to the output current 
I.sub.01 138, FIG. 7. 
The dual field system controls the phase relationship of the output current 
I.sub.O1 with respect to the machine back EMF V.sub.E. If, for example, 
the field voltage V.sub.A, FIG. 8, should increase relative to V.sub.O, 
FIG. 7, the portion 140 of V.sub.E 132 would increase, causing a resulting 
increase in the positive slope 142 of the associated portion of I.sub.O 
138, FIG. 7. The negative slope 144 of current I.sub.O would not be as 
steep, due to raised level 141, and therefore not reach zero before the 
voltage V.sub.E switched from positive to negative. However, at that point 
the slope would become much steeper as at 146, and would soon after pass 
through zero. Because of this occurrence, there would be a virtual phase 
shift between I.sub.O and V.sub.E. This would be detected by field control 
circuit 13, FIG. 1, by means of phase-sensitive detector 36, which 
compares the phase of I.sub.O from polarity indicator 32 and that of 
V.sub.E from encoder 34. The resulting error output from phase-sensitive 
detector 36 drives amplifier 38 to cause field A 14 to decrease the field 
voltage and thereby decrease voltage level V.sub.A so that levels 140 and 
141 are symmetrically disposed about the output voltage V.sub.O, FIG. 7, 
and thus restore I.sub.O waveform 138 to its normal form. 
With an alternating current load or power source at terminal 26, line 28, 
FIG. 1, V.sub.O appears as sine wave 130b and V.sub.E, the back EMF of the 
machine, appears as waveforms 132b. With a two-phase system supplying an 
A.C. load instead of a D.C. load, the wave shapes for varying power 
factors are shown in FIGS. 11A, B, and C. In FIG. 11A, output voltage 
V.sub.O 130b, is sinusoidal, while the back EMF of the machine V.sub.E 
132b, is similar to V.sub.O 132 for the direct current output, FIG. 7, but 
varying in amplitude. This is so because as V.sub.O on line 28 varies, so 
too does the input to comparator 40, amplifier 42. Thus as V.sub.O 
increases, so too does the field voltage V.sub.E. This causes the salient 
portions 135 of V.sub.E 132b, FIG. 11A, to increase, thereby increasing 
the voltage difference 133a, which causes the current to increase, so that 
the current I.sub.O follows the voltage V.sub.O, ideally exactly, in FIG. 
11A, since the system is operating at a unity power factor. 
If, however, the power factor is zero as shown in FIG. 11B, the voltage 
difference 133b becomes minimum at the higher values of voltage and 
maximum at the minimum values of voltage, so that the current is out of 
phase by 90.degree. with the voltage. 
With reference to FIG. 2, if the power factor is to be controlled, the 
phase of the voltage V.sub.O and current I.sub.P, sensed on line 52, are 
compared in phase-sensitive detector 51. If a phase difference is detected 
an error signal is generated to phase shift the current reference supplied 
to control circuit 20 via load levelling programmer 50, so that the 
modulation of field B 16 adjusts the voltage differences 133 and thereby 
shifts the current I.sub.O toward the desired phase relationship or power 
factor with respect to the voltage V.sub.O. 
In the motor mode, FIG. 11C, the wave shapes are similar but those of 
V.sub.E are reversed. 
In a three-phase system, the waveforms for one phase with a zero frequency 
of D.C. load are shown in FIG. 12. There, V.sub.A FIG. 12, appears 
generally the same as V.sub.B, and has a more symmetrical shape, and 
V.sub.E has three steps instead of two relative to counterpart waveforms 
in FIGS. 7-10. The resulting output currents, I.sub.O1, I.sub.O2, and 
I.sub.O3, the three different stator/armature currents from the three 
different phases, have truncated triangular waveforms which are phased at 
120.degree. to one another and more nearly approximate uniform D.C. output 
per phase than the two-phase system. 
FIGS. 13A and B depict V.sub.O and V.sub.E for one phase of a three-phase 
system during an A.C. load for unity and zero power factors in the 
generator mode, and FIG. 13C depicts them for unity power factor in the 
motor mode. 
The rotary electrical machine 12 according to this invention may be 
structured as shown in FIG. 14, where one phase of a three-phase generator 
100a is shown, and the second and third phases are axially stacked with 
the one shown. Machine 100a includes twelve poles 202-226, in contrast to 
the eight poles of machine 100, FIG. 6. Winding 120a of field A is wound 
so that stator teeth 208, 210, 212, 214, 216, 218 are baised with one 
magnetic polarity, and stator teeth 220, 222, 224, 202, 204, and 206 with 
the opposite magnetic polarity. Winding 123a of field B, for the generator 
mode, makes the flux in 206, 212, 218 and 224 aid field A, and the flux in 
teeth 202, 208, 214, and 220 oppose field A. 
Machine 12 may also be constructed as a wound rotor-generator design 100b, 
FIG. 15, in which both the field A winding 250 and field B winding 252 are 
wound in slots on rotor 254 to interact with armature windings 256, 258, 
and 260, accessible through terminals 262, 264, and 266, respectively, 
located on stator 270. With this wound rotor design, all three windings 
for a three-phase output may be provided in the same stator portion. Rotor 
254 includes six poles 280, 282, 284, 286, 288, and 290, in which the 
fields A and B combine to produce a resulting field +A; +A -B; -A -B; -A; 
-A +B; A +B; respectively. 
Although a specific switching circuit, firing circuit and field control 
circuits have been shown in the embodiment of the power conversion system 
for use with the dual field rotary machine, this is not a necessary 
limitation of the machine invention. The dual field machine is useful in 
other applications and with other switching and control circuits. In 
addition, the specific techniques for determining amplitude, phase, zero 
crossovers, polarity and other control criteria are illustrative only and 
not limitations on either the dual-field machine feature of the invention 
or the power conversion system feature of this invention. 
Other embodiments will occur to those skilled in the art and are within the 
following claims: