A control system for a permanent magnet motor driven by a multiphase line commutated inverter is provided with integration for integrating the back EMF of each phase of the motor. This is used in generating system control signals for an inverter gate logic using a sync and firing angle (alpha) control generator connected to the outputs of the integrators. A precision full wave rectifier provides a speed control feedback signal to a phase delay rectifier via a gain and loop compensation circuit and to the integrators for adaptive control of the attenuation of low frequencies by the integrators as a function of motor speed. As the motor speed increases, the attenuation of low frequency components by the integrators is increased to offset the gain of the integrators to spurious low frequencies

Bailey, D. A.

Dolland, C. R.

English

NASA Technical Reports Server

United States Patent [I91 
Dolland et al. 
[54] ADAPTIVE CONTROL SYSTEM FOR 
[75] Inventors: Carlisle R. Dolland; David A. Baifey, 
[733 Assignee: The United States of America as 
LINE-COMMUTATED INVERTERS 
both of Torrance, Calif. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 291,132 
[22] Filed: Aug. 7, 1981 
[51] Int. CI.3 .............................. 
3 I8/806 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,619,749 11/1971 Schieman ......... 318/798 
4,112,339 9/1978 Lip0 .................................... 318/798 
4,137.489 1/1979 Lip0 3 1 X/722 
4,264,853 4/1981 Morishita .......... 318/805 
4,335,343 6/1982 Dreiseitl et al. .................... 308/798 
.......................... 
[I11 4,401,934 
[45] Aug. 30, 1983 
Primory Exominer-David Smith, Jr. 
Afforney, ,4genf. or Firm-Joseph H. Beumer; John R. 
Manning: Leon D. Wofford, Jr. 
P71 ABSTRACT 
A control system for a permanent-magnet niotor (10) 
driven by a multiphase line-commutated inverter (12) is 
provided with integrators (24) for integrating the back 
EMF of each phase of the motor for use in generating 
system control signals for an inverter gate logic (30) 
using a sync and firing angle (a) control generator (26) 
connected to the outputs of the integrators. A precision 
full-wave rectifier (16) provides a speed control feed- 
back signal to a phase-delay rectifier (14) via a gain and 
loop compensation circuit (20) and to the integrators for 
adaptive control of the attenuation of low frequencies 
by the integrators as a function of motor speed, 
whereby as the motor speed increases, the attenuation 
of low frequency components by the integrators is in- 
creased to offset the gain of the integrators to spurious 
low frequencies. While the attenuation may be a contin- 
uous linear function of speed, a switch (Ql) is employed 
to provide a step change in attenuation at 40% of speed. 
4 Claims, 3 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19830026956 2020-03-22T01:10:12+00:00Z
U.S. Patent Aug. 30, 1983 Sheet 1 of 3 4,401,934 
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U.S. Patent Aug. 30,1983 Sheet 2 of 3 4,401,934 
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4,401,934 
1 
ADAPTIVE CONTROL SYSTEM FOR 
LINE-COMMUTATED INVERTERS 
ORIGIN O F  INVENTION 5 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC 2457). 
BACKGROUND O F  T H E  INVENTION 
This invention relates to a control system for a per- 
manent-magnet motor driven by line-commutated in- 
verter, and more particularly to an adaptive high-pass l 5  
filter network for use in such a control system to condi- 
tion back EMF signals utilized to generate inverter 
control signals. 
In some applications, a permanent-magnet (PM) 
motor is controlled by a line-commutated inverter 20 
driven by a dc source comprised of a phase delay recti- 
fier (PDR). The PDR rectifies the 3-phase power from 
a utility line and delivers the resulting dc current to the 
inverter over a dc link choke. The dc link choke is 
designed to provide a smooth current source for the 25 
line-commutated inverter. The inverter provides the 
10 
2 
system for a permanent-magnet motor driven by a line- 
commutated inverter to integrate the back EMF of each 
phase of the motor for use in generating system control 
signals. Each integrator is comprised of a high-pass 
filter, the corner frequency of which is shifted succes- 
sively away from the Jo axis as the speed of the motor 
increases, whereby the attenuation of low-frequency 
components by the integrators is increased as the motor 
speed increases to offset the gain of the integrators to 
spurious low frequencies so that at all speeds the desired 
minimum power factor is assured. In the simplest form 
of the invention, two or more filter networks are pro- 
vided for each back EMF phase and selectively 
switched into operation in the integration process as a 
function of motor speed. 
The novel features that are considered characteristic 
of this invention are set forth with particularity in the 
appended claims. The invention will best be understood 
from the following description when read in connection 
with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a block diagram of a control system for a 
permanent-magnet motor driven by a line-commutated 
inverter in which the present invention is utilized. 
FIG. 2 is a schematic diagram of exemplary integra- 
tors in the system of FIG. l. 
FIG. 3 is a plot of the gain and phase of each integra- 
necessary current to the motor windings. 
A motor speed loop is closed around the inverter 
and a change in required speed in a tor of FIG. 2 before and after switchover from one 
This in turn changes 30 high-pass filter configuration to another as a function of change in the PDR firing the dc link current, and hence the motor speed. It is 
possible to vary the current in the dc link from zero to 
a maximum by adjusting the firing angle of the PDR in 
response to a gain and loop compensated error signal, 
where the error is the difference between a speed com- 35 
mand signal and a signal proportional to the speed of the Of the drawings9 a permanent- 
motor generated by precision full-wave rectification of magnet motor '' 's shown contro'led by a line- 
commutated inverter 12 that is driven by a phase delay the motor back EMF. 
The back EMF is also utilized to generate the in- rectifier (PDR) 14. The latter rectifies power from a 
verter control signals. D~~ to commutation overlap in 40 3-phase utility line and delivers the resulting dc current 
the inverter, the back EMF is distorted. B~ integrating to the inverter through a dc link choke 15. While the 
the back EMF, constant control sinusoidal waveforms motor speed is brought up to about 15% of its maxi- 
devoid of distortion and in phase with the motor line-to- mum, the necessary Current to the motor windings is 
neutral voltage is obtained. However, the integrators synchronized by conventional means (not shown). 
provide much more gain to spurioils low frequencies 45 Once 15% speed is reached, sufficient back EMF can be 
than the actual operating frequency. These spurious sensed on the inverter output lines. These back EMF 
low-frequency fluctuations are caused by modulation signals are rectified and combined in a precision full- 
between the PDR ripple and the back EMF frequency. wave rectifier 16 to provide a dc signal proportional to 
At high motor speeds, these low-frequency fluctuations the motor speed. This speed signal is then compared 
modulate the motor speed and are further reinforced by 50 with a dc speed command signal at a comparator 18 to 
the integrators. Thus, in order to operate a line-corn- provide an error signal that is applied to the PDR 
mutated permanent-magnet motor at high speed, these through a gain and loop compensation circuit 20. In that 
low-frequency components must be filtered. A single manner, a motor speed loop is closed around the line- 
high-pass filter, however, introduces undesirable phase commutated control system for the motor which em- 
shifts which result in poor motor power factors. The 55 bodies the invention that will now be described. 
filter break (corner) frequency must be selected such The back EMF is also utilized to generate the control 
that the phase shift does not cause excessive change in signals for the line-commutated inverter 12, but due to 
the power factor. It has been found that any single zero commutation overlap, the back EMF is distorted. To 
location that results in acceptable poer factors will not obtain constant control sinusoidal waveforms devoid of 
adequately eliminate low-frequency noise at all speeds. 60 distortion, and in phase with the motor line-to-neutral 
Consequently, there is a need for a high-pass filter voltage, the back EMF signals BC4, CA+ and ABcb 
whose break frequency can be changed as a function of from differential amplifiers 22 are conditioned by a set 
speed SO that adequate low-frequency filtering and SYS- of three integrators represented by a single block 24. 
tern power factors can be obtained at all motor speeds. The differential amplifiers produce sinusoidal signals 
65 that are equal to the difference between the back EMFs 
A, B and C of the phases indicated in pairs as follows: 
speed. 
DESCRIPTION O F  PREFERRED 
EMBODIMENTS 
Referring to 
SUMMARY OF THE INVENTION 
In accordance with the present invention, adaptive 
high-pass filters are provided for integrators in a control AB6 -= A - H 
4,401,934 
3 
BC+ = B --C 
CA4=C - A 
The outputs of these integrators are utilized to obtain 
information for generating the firing angles for all three 
phases of the inverter, A+I, B+I and C+I. This inforrna- 
tion is obtained through a sync and a-control generator 
26 which then generates the gate firing pulses for in- 
verter gate drivers 28 through inverter gate logic 30. 
The integrators 24 provide lower gain to spurious 
low frequencies than the actual operating frequency. 
These spurious low-frequency fluctuations are caused 
by modulation of the PDR ripple frequency with the 
back E M F  frequency. These low-frequency compo- 
nents at high motor speds are reinforced by the integra- 
tors, consequently low frequency filtering is required. 
This is accomplished, according to this invention (with- 
out introducing undesirable phase shifts, which result in 
poor power factors for the motor) by adaptive control 
of the break (corner) frequency of high-pass filters at 
the inputs of the integrators as a function of speed so 
that adequate low-frequency filtering and system power 
factors can be obtained at all motor speeds. 
FIG. 2 illustrates an embodiment of this adaptive 
control in its simplest form. It is comprised of three 
integrators 31, 32 and 33 for the respective back EMF 
signals BC+, CA+ and AB+. Each integrator is com- 
prised of an operational amplifier with a feedback (inte- 
grating) capacitor C, and resistor R, in parallel, such as 
an operational amplifier 31a utilized to condition back 
EMF signal BC+. The noninverting input terminal is 
connected to circuit ground through a resistor Re The 
outputs of the operational amplifiers 31,32 and 33 are ac 
coupled to amplifiers 34, 35 and 36 to remove the dc  
component caused by the offset voltage of the amplifi- 
ers 31,32 and 33 and the high dc  gain of these integrator 
stages. These amplifiers also invert the integrator out- 
puts to provide the signals A+, B+ and C+ which are in 
phase with the line-to-neutral back EMF waveforms. 
The back EMF inputs to the integrators are essen- 
tially sinusoidal. Distortions in the inputs due to com- 
mutation overlap are eli,minated by the filtering action 
of the integrators, and at low motor speed, spurious 
low-frequency fluctuations (caused by modulation be- 
tween the PDR ripple frequency present in the inverter 
outputs and the back E M F  frequency) are eliminated by 
including a high-pass filter at the input of each of the 
amplifiers 31a. 32a and 33a. In this embodiment of the 
invention in its simplest form, these filters are comprised 
of two parallel branches, one comprised of a resistor 
Rfl in series with a capacitor Cfl is fixed, and the other 
comprised of a resistor Rf2 in series with a capacitor 
Cf2 is connected in parallel with the fixed branch by a 
conducting FET switch Ql. 
As noted hereinbefore, the integrators provide unde- 
sired gain to spurious low frequencies which, unless 
filered, will cause low-frequency fluctuations in the 
motor speed. Because of the closed loop control of the 
line-commutated inverter, these low-frequency fluctua- 
tions increase in amplitude, resulting in regenerative 
feedback of the spurious low frequencies in the back 
EMF. Consequently, for low speeds, these spurious low 
frequencies are filtered out by the two parallel high-pass 
filter branches functioning with the operational ampli- 
fier and integrating capacitor as a single high-pass filter 
with a break frequency so selected that phase shift does 
not cause any excessive reduction in the power factor. 
4 
However, a single zero location that results in accept- 
able power factors at low speeds will not adequately 
eliminate low-frequency noise at high speeds. The fil- 
ters are therefore made adaptive by the inclusion of the 
5 FET switches in one branch so that, at high speeds, that 
branch may be eliminated by simply turning off the 
transistor. The break or corner frequency of the filters 
are shifted away from the j w  axis when the switches are 
turned off. The result is that gain as a function of phase 
10 is decreased at high speed once the transistors are 
turned off. FIG. 3 shows a graph of gain as a function of 
frequency with the FET switch on at low speeds and a 
plot of gain with the FET switch off at high speeds. 
Also plotted is a graph of phase shift as a function of 
The FET switches are controlled by the output of a 
comparator 37 which compares the motor speed signal 
with a reference. When the reference is exceeded, the 
output of the comparator switches from a positive volt- 
20 age to a negative voltage to turn the FET switches off. 
In that manner, as the speed increases above some pre- 
determined reference, such as 40%, the low-frequency 
components of the back EMF signals are subjected to 
high attenuation by switching out one of the parallel 
25 branches of the filter. It is evident that more than two 
parallel branches may be included in each filter, with 
more than two comparators, to switch successive 
branches out at successively higher motor speeds, or 
that a plurality of filters be otherwise provided with 
30 break frequencies shifted sequentially away from the jo 
axis, with means for switching into operation the suc- 
cessive filters as the speed of the motor increases. Still 
other ways of achieving a speed-adaptive filter for each 
integrator will occur to those skilled in the art. 
It should be noted that as the motor speed increases, 
and the gain of the integrators decreases, the output 
amplitude of the integrators also changes. Conse- 
quently, this concept of an adaptive filter for the inte- 
gfators cannot be utilized in a conventional line-com- 
40 mutated inverter since the outputs of the integrators are 
compared to a constant voltage for the purpose of gen- 
erating firing angle control signals, so that any change 
in amplitude represents a change in the operating power 
factor of the system. Thus, the line-commutated in- 
45 verter must be made immune to any amplitude fluctua- 
tions on the back EMF or the filtered back EMF. One 
technique for providing such immunity is disclosed in 
the a copending application filed concurrently entitled 
50 ATOR FOR FIRING ANGLE CONTROL OF 
LINE-COMMUTATED INVERTERS” which is in- 
corporated herein by reference for the purpose of show- 
ing utility of the present invention. Briefly, if the inte- 
55 grated line-to-neutral back EMF waveforms A+, B+ 
and C+ are compared in pairs to generate square sync 
waveforms as follows: 
15 frequency. 
35 
“ADAPTIVE REFERENCE VOLTAGE GENER- 
A+ SYNC=A+>B+ 
60 B+ SYNC=B+>C+ 
C+ SYNC=C+>A+ 
The leading edges of these waveforms occur at the time 
65 where positive halves of the integrated back EMF 
waveforms are maximum. These leading edges are 
therefore used to sample and hold the amplitude of the 
highest back EMF waveform. The sample is used over 
4,40 
5 
the next 120" to control the reference voltage with 
which the integrator outputs are compared in generat- 
ing the firing angle control signals for the inverter. 
Although particular embodiments of the invention 
have been described and illustrated herein, it is recog- 
nized that modifications and variations may readily 
occur to those skilled in the art. Consequently, it is 
intended that the claims be interpreted to cover such 
modifications and equivalents. 
,934 
6 
so alter filter configurations as to shift the corner fre- 
quency of said integrating means away from the jo axis 
thereof as the speed of the motor increases. 
3. An improvement as defined in claim 2 wherein said 
filter circuits for each EMF signal are comprised of an 
operational amplifier with an integrating capacitor for 
each EMF signal and at least two high-pass filters, a 
first one permanently connected to couple said EMF 
signal to said operational amplifier for integration and a 
What is claimed is: 10 second one connected in parallel with said first one by 
1. In a control system for a permanent-magnet motor one of said electronic switches. 
driven by a multiphase line-commutated inverter, said 4. In a control system for a permanent-magnet motor 
control system having means for integrating the back driven by a line-commutated inverter, a separate inte- 
EMF of each phase of said motor for use in generating grator having an adaptive high-pass filter for use in 
system control signals for said inverter, an improvement I5  generating system control signals for said inverter from 
comprising back EMF of said motor, and means for generating a 
means for generating a signal proportional to motor signal proportional to motor speed, and means respon- 
speed, and sive to said signal for shifting the corner frequency of 
adaptive means responsive to said motor speed signal the high-pass filter of each integrator away from the jo 
for increasing the attenuation of low frequencies in 20 axis as the speed of the motor increases, whereby the 
the back EMF of each phase of said motor by said attenuation of low-frequency components in said back 
integrating means as said motor speed increases. EMF by the integrators is increased as the motor speed 
2. An improvement as defined in claim 1 wherein said increases to offset the gain of the integrators to spurious 
adaptive means is comprised of electronic switches and low frequencies, thereby assuring a desired minimum 
filter circuits at the input of said integrating means for 25 power factor at all speeds. 
selectively altering connections of said filter circuits to * * * * *  
30 
35 
40 
45 
50 
55 
60 
65 


Adaptive control system for line-commutated inverters

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