Electronic commutation for a moving magnetic field electric power converter

An apparatus and method for inverting direct current electrical power into alternating current electrical power by producing a moving magnetic field having substantially constant flux density when the direct current power is applied to a primary winding, magnetically coupling a secondary winding to the moving magnetic field and creating a substantially triangular-shaped current in each phase of the primary winding so as to produce substantially sinusoidal alternating current in each phase of the secondary winding.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a linear electronic commutation apparatus and 
method for using direct current to produce a substantially uniform 
rotating magnetic field and then using the rotating field in a direct 
current to alternating current inverter, in an electric motor and in a 
synchronous condenser. 
2. Description of the Background Art 
DC-AC Inverter 
Presently there exist many types of inverters, many of them electronic, for 
converting electrical power from DC to AC. Typically, electronic inverters 
produce a square wave output waveform which inherently contains harmonics 
that are usually undesirable but often tolerable. Improved inverters have 
been developed which attempt to approximate a sine wave through the use of 
six pairs of controlled rectifiers thereby producing an alternating square 
wave output crudely approximating a sine wave and whose frequency is 
determined by the firing of the controlled rectifiers. Disadvantageously, 
these six step inverters also draw DC discontinuously rather than 
continuously. 
In a previous invention of one of the inventors hereof, an improved DC to 
AC inverter was disclosed that employed a magnetic field rotating at a 
substantially constant angular velocity and having a substantially 
constant flux density. Specifically, as set forth in U.S. Pat. No. 
4,870,558 entitled "Moving Magnetic Field Electric Power Converter" issued 
to John W. Luce on Sep. 26, 1989, the disclosure of which is hereby 
incorporated by reference herein, this prior improved inverter comprised a 
pair of windings wound on fixedly positioned inner and outer ferromagnetic 
cores. The DC input winding was wound much in the same manner as the 
armature windings of a DC generator, with the winding preferably twelve 
phase star-connected with the neutral brought out. The polyphase winding 
output was wound much in the same manner as the stator winding of a three 
phase AC motor, either synchronous or induction (either squirrel cage or 
wound rotor). 
As shown in FIG. 1 hereof, the twelve star-connected input phases of this 
prior improved DC to AC inverter each included an electronic switch, such 
as a gate turn off (GTO) thyristor switch, having their outputs connected 
together and brought out as a negative terminal. The gates of the GTOs 
were intended to be controlled by means of a switch controller timed by an 
oscillator according to square waves of the timing diagram of FIG. 2A 
hereof. Notably, it was contemplated that by gating each of the GTOs in 
the sequence and with the timing reflected in FIG. 2A hereof, three 
adjacent GTOs would be successively gated (i.e. turned ON) so that during 
each cycle, the first GTO would be turned OFF, leaving the other two ON. 
Then another succeeding adjacent GTO would be turned ON for a repeated 
total of three GTOs. This alternating three-two-three-two sequence was to 
be continuously repeated so that at all times at least two GTOs were ON 
and such that at no time two GTOs would be turned ON (or OFF) 
simultaneously. The stepped output waveform of FIG. 2B hereof was intended 
to be obtained. 
Advantageously, the input direct current consumption of this prior improved 
DC to AC inverter was continuous. The magnetic field in the core never 
totally collapsed which would have otherwise created an inductive kick. 
Further, with the stepping interval significantly shorter than five 
electrical time constants of the inverter, the rotating magnetic field 
moved in discrete steps and at a rate sufficient to preclude steady-state 
conditions. Further experimentation of this inverter using N channel power 
MOSFETs for the switches and square wave generators achieved satisfactory 
three phase output voltage. However, the magnetic field did not move at a 
perfectly constant angular velocity nor was the magnetic field of a 
perfectly constant magnitude. 
An object of this invention to provide an improved DC to AC inverter that 
employs a magnetic field created by overlapping triangular-shaped coil 
currents such that the magnetic field rotates at a nearly perfect constant 
angular velocity and has a nearly perfect constant flux density, such that 
input direct current consumption is nearly perfectly continuous and such 
that three phase output voltage is obtained having nearly perfect 
sinusoidal waveforms. 
DC Motor 
A conventional polyphase AC motor comprises a polyphase multipole input or 
primary winding that is powered by three-phase AC electrical power. There 
exists many advantages to polyphase AC motors; however, three-phase AC 
power is required to drive the motor. 
Another object of this invention is to provide an improved DC motor having 
the advantages of a polyphase AC motor, but that employs a magnetic field 
created by overlapping triangular-shaped coil currents such that the 
magnetic field rotates at a nearly perfect constant angular velocity and 
has a nearly perfect constant flux density and such that input DC current 
consumption is nearly perfectly continuous. 
DC Synchronous Condenser 
A synchronous condenser comprises basically a synchronous motor connected 
in parallel with a distribution system that requires low power factor. The 
synchronous condenser is overexcited to act like a capacitor to take 
leading KVA, thereby improving the distribution system's power factor. The 
synchronous condenser is typically operated with no mechanical load. 
Often, the synchronous condenser is hermetically sealed and operated in an 
atmosphere of hydrogen to reduce the windage loss and increase the coding 
effect. However, significant maintenance is still required due to wear. 
Hence, a synchronous condenser is typically justified when the investment 
and cost of maintenance is less than the electrical power charged by the 
power company that may apply a surcharge for low power factor. 
Another object of this invention is to provide an improved synchronous 
condenser with no moving parts that employs a magnetic field created by 
overlapping triangular-shaped coil currents such that the magnetic field 
rotates at a nearly perfect constant angular velocity and has a nearly 
perfect constant flux density and such that input DC current consumption 
is nearly perfectly continuous. 
The foregoing has outlined some of the pertinent objects of the invention. 
These objects should be construed to merely illustrative of some of the 
more prominent features and applications of the intended invention. Many 
other beneficial results can be attained by applying the disclosed 
invention in a different manner or modifying the invention within the 
scope of the disclosure. Accordingly, other objects and a fuller 
understanding of the invention and the detailed description of the 
preferred embodiment in addition to the scope of the invention defined by 
the claims taken in conjunction with the accompanying drawings. 
SUMMARY OF THE INVENTION 
DC-AC INVERTER 
For the purpose of summarizing this invention, this invention comprises a 
DC to AC inverter further improved from that disclosed in U.S. Pat. No. 
4,870,558. Specifically, in embarking on a new approach to further improve 
the DC to AC invertor, it was again concluded that in order to obtain the 
desired output of polyphase (e.g. three phase) sinusoidal AC, a magnetic 
field rotating at a substantially constant angular velocity and having a 
substantially constant flux density should be employed. However, contrary 
to the teachings of U.S. Pat. No. 4,870,558, it was discovered that at one 
instant of time, the desired field could be furnished by an electrical 
current in just one of the twelve coils and then thirty electrical degrees 
later, the desired field could be furnished by the same amount of current 
in an adjacent coil. In between these two points, the desired field would 
be produced by the vector sum of these two adjacent coils. It was also 
discovered that if the current in each coil had a triangular shape, then a 
nearly perfect sinusoidal AC output would be obtained. Upon still further 
experimentation, an electronic circuit was invented that accurately 
produces overlapping triangular waveforms for driving the electronic 
switches, thereby obtaining the nearly perfect sinusoidal AC output from 
DC input. 
DC MOTOR 
A DC motor built according to this invention is identical to any particular 
known polyphase AC motor except for the primary or AC input windings. 
Instead of the usual polyphase multipole input or primary winding, a 
winding of the same number of poles is installed, but arranged as the 
armature winding of a DC shunt motor, having a small odd number of 
commutator bars, preferably 5 to 9 bars. Actual commutator bars are not 
necessary, but their connections are brought out to transistor pairs which 
would be controlled by the same type of control circuit used for the 
inverter described above. The armature winding is wound on the same core 
in the same slots that would have been used for the corresponding AC 
motor. An important factor of this DC motor is the fact that it operates 
from DC power rather than from 3 phase AC power. 
This description includes linear and rotary motors (but not stepper 
motors), regular or inside out loom motors, and all types of rotors 
including squirrel cage and wound rotor induction, electromagnetic, 
permanent magnet and reluctance synchronous motors. It should be noted 
that if the controller is designed to gradually ramp the frequency from 
zero up to operating speed, amortisseur or damper windings would not be 
required for starting such synchronous motors. Induction motors are 
inherently self starting at operating frequency however, if their 
frequency is ramped up gradually the usual high surge of starting current 
can be avoided. 
DC SYNCHRONOUS CONDENSER 
A synchronous condenser built in accordance with this invention would be 
essentially identical to any existing synchronous condenser, except for 
the field winding. Previously the field consisted of a DC winding on a 
rotating core. In this invention, the field winding is similar to that of 
a DC shunt motor armature but it does not rotate. The leads that would 
normally be connected to the commutator bars are connected to the 
transistor pairs and are controlled by a circuit similar to that used for 
the inverter described above. An important feature of this synchronous 
condenser of this invention is the fact that nothing physically rotates, 
thus permitting the condenser to be oil filled for improved insulation and 
cooling. 
The foregoing has outlined rather broadly the more pertinent and important 
features of the present invention in order that the detailed description 
of the invention that follows may be better understood so that the present 
contribution to the art can be more fully appreciated. Additional features 
of the invention will be described hereinafter which form the subject of 
the claims of the invention. It should be appreciated by those skilled in 
the art that the conception and the specific embodiment disclosed may be 
readily utilized as a basis for modifying or designing other structures 
for carrying out the same purposes of the present invention. It should 
also be realized by those skilled in the art that such equivalent 
constructions do not depart from the spirit and scope of the invention as 
set forth in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
DC-AC INVERTER 
As shown in FIG. 3, similar to the prior improved DC to AC inverter of U.S. 
Pat. No. 4,870,558, this further improved inverter 10 includes a pair of 
windings 12 and 14 wound on fixedly positioned inner and outer 
ferromagnetic 16 and 18 cores, respectively. The DC input winding 12 is 
wound on the inner core 16 much in the same manner as the armature 
windings of a DC motor. The AC output winding 14 is delta wound on the 
outer core 18 much in the same manner as the stator winding of a three 
phase AC generator. However, as more fully described in U.S. Pat. No. 
4,870,558, other winding arrangements are feasible. 
Star-Connected DC Input Winding 
As shown in FIG. 4, in one embodiment, the DC input winding 12 comprises 
twelve coils 12A-12L star-connected, with the neutral brought out as the 
DC positive input. Transistors 20A-20L are respectively connected in the 
twelve coils 12A-12L of the input winding 12, with each of their outputs 
connected together and brought out as a DC negative input. The transistors 
20A-20L preferably comprise N or P channel power MOSFETs, or power 
Darlington bipolar junction or insulated gate bipolar transistors, of 
either polarity. Transistors 20A-20L may include any other control devices 
operating in the linear mode (rather than in the switching mode) such as 
bipolar junction transistors, field effect transistors, Darlington 
transistors, or other semiconductor devices and also vacuum or other 
electron tubes. 
The electronic switches 20A-20L are operated so as to produce 
triangular-shaped currents in their respective coils 12A-12L, with the 
timing indicated in FIG. 5, such that as the current in one coil is 
decreasing, the current in the next coil is correspondingly increasing. 
The total current from the DC input therefore remains nearly constant. 
Further, the succession of overlapping triangular-shaped current pulses 
produces a rotating magnetic field having a practically constant magnitude 
and angular velocity, which produces the sinusoidal output voltage at 
output winding 14. Specifically, the following table lists computed coil 
currents necessary to produce a perfect magnetic field rotating at a 
perfect angular velocity and at a perfect constant magnitude: 
______________________________________ 
CURRENT CURRENT CURRENT 
POSITION Coil #1 Coil #2 Total 
(degrees) (amp) (amp) (amp) 
______________________________________ 
0.00 1.0000 0.0000 1.0000 
3.00 0.9080 0.1047 1.0127 
6.00 0.8135 0.2091 1.0225 
9.00 0.7167 0.3129 1.0296 
12.00 0.6180 0.4158 1.0339 
15.00 0.5176 0.5176 1.0353 
18.00 0.4158 0.6180 1.0339 
21.00 0.3129 0.7167 1.0296 
24.00 0.2091 0.8135 1.0225 
27.00 0.1047 0.9080 1.0127 
30.00 0.0000 1.0000 1.0000 
______________________________________ 
FIG. 6 graphically illustrates the above-tabulated coil currents. It is 
seen that the falling current in the first coil 12A and the rising current 
in the second coil 12B are nearly perfect straight lines. Thus, it is seen 
the triangular-shaped coil currents of FIG. 5 will produce nearly perfect 
sinusoidal AC output voltage. 
More specifically, the following table lists the deviation from a perfect 
rotating field that results from using perfectly triangular-shaped coil 
currents: 
______________________________________ 
COIL COIL 
12A 12B RESULT. FLUX FLUX ERROR 
TIME MMF MMF MAG. AXIS MAG AXIS 
Deg. P.U. P.U. P.U. Deg. % Deg. 
______________________________________ 
0.0 1.0 0.0 1.0000 0.000 
0.00 0.000 
3.0 0.9 0.1 0.9879 2.901 
1.21 0.099 
6.0 0.8 0.2 0.9783 5.867 
2.17 0.133 
9.0 0.7 0.3 0.9715 8.882 
2.85 0.118 
12.0 0.6 0.4 0.9673 11.933 
3.23 0.067 
15.0 0.5 0.5 0.9659 15.000 
3.41 0.000 
18.0 0.4 0.6 0.9673 18.068 
3.27 0.068 
21.0 0.3 0.7 0.9715 21.117 
2.85 0.117 
24.0 0.2 0.8 0.9783 24.134 
2.70 0.134 
27.0 0.1 0.9 0.9879 27.098 
1.21 0.098 
30.0 0.0 1.0 1.0000 30.000 
0.00 0.000 
______________________________________ 
As can been seen from the foregoing table, the deviation amounts to less 
than 3.5% fluctuation. 
Mesh-Connected DC Input Winding 
As shown in FIG. 10, in its preferred embodiment, the DC input winding 12 
comprises seven mesh-connected coils 12M-12S. Transistor pairs 20M-20S are 
respectively connected to the taps between the coils 12M-12S. Each 
transistor pair 20M-20S comprises reverse polarity transistor pairs 
connected to a positive bus and a negative bus to which the DC input is 
connected. All of the transistors of like polarity are controlled by a 
separate control circuit in the manner described below. For example, an 
NPN transistor is connected to the negative bus and a PNP transistor is 
connected to the positive bus. Likewise, when using MOSFETs, the N channel 
should be connected to the negative bus and the P channel should be 
connected to the positive bus. 
It is noted that in a mesh winding, like a DC machine with carbon brushes 
on a commutator, as the current is being transferred from one coil tap to 
the next, the coil between those taps undergoes a reversal of current 
direction. This results in an L di/dt voltage being developed across that 
coil and applied to the transistors 20M-20S. Hence, the reverse polarity 
transistor pairs 20M-20S are necessary. However, the mesh windings have 
the advantage that in the course of transferring the current from one tap 
to the next, the total magnetic field is significantly less disturbed. For 
example, in a twelve coil mesh-connected winding, the total magnetic field 
is disturbed only by one-fifth as much as the twelve coil star-connected 
winding. 
Preferably, the mesh-connected winding 12 includes an odd number of taps so 
that there will be one ripple per transistor per cycle rather than one 
ripple per transistor pair that would occur with an even number of taps 
(i.e. diametrically opposing transistors turning ON at the same time). 
For optimum sine wave output, the flux magnitude and rotational velocity of 
the magnetic field should be perfectly constant. To achieve this 
condition, the current fed into the stator should have the following 
functional form. 
When conducting current from the positive DC bus to the stator, the current 
designated I1 in FIG. 10 should be: 
##EQU1## 
units of angle, then 
##EQU2## 
for a like number of units of angle. When a branch conducts current from 
the stator to the negative DC bus, the corresponding equations for the 
current shape should be: 
##EQU3## 
followed by 
##EQU4## 
where 
EQU N.sub.Total =2N.sub.coils 
and N.sub.start is the subdivision of the period in which node #2 first 
conducts current where there are N.sub.Total subdivisions. The other node 
currents are merely phase shifted versions of this current where the phase 
shift is an appropriate integral multiple of .alpha..sub.R increments. 
FIGS. 12, 13 and 14 illustrate the resultant MMF magnitude and angle, 
normalized node currents and coil currents as a function of .omega.t for 
the seven coil mesh-connected inverter of FIG. 10 operated with the 
optimum sine function node currents. FIGS. 15, 16 and 17 illustrate the 
resultant MMF magnitude and angle, normalized node currents and coil 
currents as a function of .omega.t when the first two terms of a Taylor 
series expansion for the time varying sine function of its argument are 
employed. And FIGS. 18, 19 and 20 illustrate the resultant MMF magnitude 
and angle, normalized node currents and coil currents as a function of 
.omega.t when triangular-shaped current approximation is employed. 
Analog/Digital Control Circuit 
As shown in FIGS. 21-24, the control circuit that drives the 14 transistors 
of FIG. 10 comprises an analog portion and a digital portion which 
produces the triangular-shaped waveforms as shown in the timing diagram of 
FIG. 11. It is noted that the digital portion of the control circuit 
serves only to provide timing signal to the four demux chips used to 
distribute base current drive signals. Hence, the analog portion of the 
control circuit is described first and then the digital circuit is 
described. 
As shown in FIGS. 21 and 22, a reference triangle wave is generated by an 
oscillator with a frequency f.sub.osc =f.sub.output (N.sub.coils /2) With 
standard 60 hertz and a seven coil primary, 210 hertz is obtained from the 
oscillator. 
As noted above, control circuits must be provided for each of the like 
polarity transistors. Hence, there is a dual control circuit, one for the 
NPN transistors connected to the negative DC bus and another for the PNP 
transistors connected to the positive bus. The operation of the dual 
control circuits is essentially the same with the exception that the 
control circuit for the negative DC bus is phase shifted 90 electrical 
degrees as 210 hertz from the positive control circuit. Also, the polarity 
is reversed since PNP transistors are employed. With this understanding, 
the following only describes the control circuit for the positive bus. 
Referring now to FIGS. 21 and 23A-23D, it is seen that the input triangle 
of FIG. 22 is input to three separate linear operational amplifiers to 
provide the four waveforms of FIGS. 23A-23D. Specifically, 23A illustrates 
an input triangle wave with a DC offset so that it will always be above 
ground; FIG. 23D illustrates the inversion of the input triangle wave to 
produce a new triangle wave 180 electrical degrees out of phase from the 
reference triangle wave of FIG. 22; FIG. 23C illustrates the derivative of 
the input triangular wave which is a square with a positive transition 
when the triangle begins a positive slope and a negative transition when 
the slope changes to a negative; and FIG. 23D illustrates the output of 
the op amp inverter pass through another operational amplifier to produce 
an offset pulse train. 
Referring now to FIG. 24, the digital portion of the control circuit 
comprises a pair of demultiplexers (demuxs), each of which having eight 
outputs; however, only seven of which are used to drive the bases of 
transistors. It is noted that the waveforms illustrated in FIG. 23A is fed 
into the input of demux #1 and the waveform illustrated in FIG. 23D is fed 
into the input of demux #2. 
The demux steer the input to a particular output line at the command of a 
three bit control word. A digital network is therefore needed to count 
pulses and provide the address signals to the demux control line. With 
this purpose, a counter and shift register are employed. The counter is 
reset after counting up to seven. The three outputs of the counter provide 
direct control to demux #1 and also the input line to a shift register 
which give the ninety degree phase shift needed to drive demux #2. The 
counter triggers on a positive transition of the clock pulse while the 
shift register triggers on a falling pulse. Therefore, a single a square 
wave generated by the analog differentiator (FIG. 23C) is sufficient to 
provide clocks for both the counter and the shift register. 
Circuit Control Feedback Circuit 
Experimentation with both the star-connected and the mesh-connected 
revealed that while the transistors were being fed the desired 
triangular-shaped phase current, the emitter current did not have the same 
shape as the phase current due to the non-linear response of the 
transistors. Optimally, the transistors should behave as a linear 
amplifier of-the input base signal since any added distortion caused by 
non-linearity in the power stage is translated to the output of the 
inverter as a distortion to the desired sine wave output. 
To remove the effect of the power amplifier as a source of distortion, a 
conventional feedback technique may be employed to force the emitter 
current to be a faithful amplification of the base drive signal. As shown 
in FIG. 25, the reference current generated in the control circuit of the 
correct triangular-shape is fed into the non-inverting input of an 
operational amplifier. A small resistance (usually just a piece of wire) 
is connected in series with the emitter of the transistors so as to 
generate a signal that is proportional to the actual emitter output 
current. The output signal is then fed into the inverting input of the 
operational amplifier. The operational amplifier therefore acts as a 
differential summing amplifier which compares the reference signal with 
the output feedback signal and produces an error signal that is an 
amplified version of the referenced signal minus the feedback signal. The 
amplified error signal is fed into the base of the respective transistors 
to drive the coils to produce the desired AC output. 
Pulse Width Modulation Control Circuit 
In the context of the present invention, a pulse width modulation (PWM) 
control circuit may alternatively be used for generation of 
triangular-shaped coil or node currents needed to drive the inverter to 
generate sinusoidal AC output waveforms. The primary method discussed 
above has been the linear control of current from a DC voltage source. For 
any input voltage the current in a resistor/inductor loop is described by: 
##EQU5## 
and the current will approximately follow the voltage wave shape as 
indicated in FIG. 26. In this technique, with minor feedback corrections, 
the triangular shaped current wave is generated through the coil. 
DC MOTOR 
It is noted that the DC windings may be located on the outer core or stator 
and then controlled with the above-described control circuit so as to 
produce the rotating magnetic field. A DC induction motor is created by 
positioning a motor rotor in the stator. The rotor may be a squirrel cage 
rotor, a wound rotor, a permanent magnet rotor, an electromagnetic rotor, 
a salient pole rotor, a reluctance rotor or the like. The motor will 
therefore operate as an ordinary motor with that particular type of rotor. 
The frequency of the triangular-shaped currents applied to the DC winding 
on the stator may be varied to thereby vary the speed of the motor. It is 
noted that a motor of this type that operates on direct current has the 
ruggedness and low maintenance of the squirrel cage induction motor. The 
motor speed can be adjusted by simply varying the frequency of the control 
circuit. There are no torque pulsations in this motor as there otherwise 
exists in all other solid state AC motor drives. 
STATIC SYNCHRONOUS CONDENSER 
A synchronous condenser with no moving parts may be constructed by 
utilizing the above-referenced techniques to produce a rotating magnetic 
field that is applied to the field of a synchronous condenser. The rotor 
will no longer rotate, but will be fixed to carry a multicoil winding 
controlled by one of the methods described above. Adjusting the strength 
of the magnetic field will vary the value of reactance that the condenser 
presents to the distribution system to which it is connected. With no 
moving parts, the condenser may be immersed in oil to improve its 
electrical insulation and also its cooling capability. The KVAR rating of 
the condenser will therefore substantially increase. Even though there are 
no moving parts in this improved condenser, the air gap should not be 
reduced to zero because the air gap establishes a large exciting current 
that produces the large lagging KVAR capacity. However, if the lagging 
KVAR capacity is not needed, the air gap could be reduced or eliminated. 
Then, the leading KVARs can be obtained with a smaller field from the DC 
windings. It is believed that this synchronous condenser will have a 
greater KVAR rating for the same size and without any moving parts, 
thereby reducing maintenance and eliminating the need of hydrogen cooling. 
The present disclosure includes that contained in the appended claims, as 
well as that of the foregoing description. Although this invention has 
been described in its preferred form with a certain degree of 
particularity, it is understood that the present disclosure of the 
preferred form has been made only by way of example and that numerous 
changes in the details of construction and the combination and arrangement 
of parts may be resorted to without departing from the spirit and scope of 
the invention. 
Now that the invention has been described,