Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2002-09-27
2003-12-23
Masih, Karen (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S808000, C318S801000, C318S807000
Reexamination Certificate
active
06667597
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The technical field of this invention is motor speed control and particularly using rotor flux based MRAS (Model Reference Adaptive System) speed observers.
BACKGROUND OF THE INVENTION
Electronic speed control of AC induction motors is well known in the art. Such AC induction motors are typically driven by selectively switching a DC voltage formed by rectifying an AC power source across the motor phase windings. An electronic controller including a microcontroller or digital signal processor (DSP) controls three phase switching of the DC voltage to produce a pulse width modulated three-phase AC input to the motor. It is feasible to employ speed feedback in these systems. Thus the electronic controller would compare an actual motor speed from a sensor with the command speed. This electronic controller would then continuously modify the drive to the AC induction motor to minimize any difference between the actual motor speed and the commanded motor speed.
A modification of this technique is the subject of this invention. Sensors that generate an electrical indication of motor speed such as tachometers are expensive and unreliable when compared with sensors for measuring electrical quantities such as voltage and current. Thus, speed observers such as rotor flux based model reference adaptive system speed observers are widely used. Rotor flux based model reference adaptive system speed observers employ electrical measurements of currents and voltages in the stator windings of the AC induction motor. The electronic controller employs these measures in a rotor flux estimator. The rotor flux estimate is employed both in generation of the motor drive control signals and in the rotor flux estimation.
A typical prior art system is illustrated in
FIGS. 1 and 2
.
FIG. 1
illustrates the typical AC induction motor drive hardware
100
in block diagram form. AC induction motor drive hardware
100
includes high voltage module
110
, electronic control module
120
, AC induction motor
130
coupled to high voltage module
110
via three phase AC lines
135
and load
140
coupled to AC induction motor
130
. High voltage module
110
includes all electronic parts that must handle high voltages. Alternating current (AC) line power supplied to rectifier/doubler
111
enables production of a direct current (DC) voltage used for induction motor drive. Switching module
113
is shown schematically in FIG.
1
. Switching module
113
includes six high voltage semiconductor switches connected in three series pairs between the DC voltage and ground. The junction between each pair of semiconductor switches drives one phase of the three phase inputs
135
to induction motor
130
. Predriver module
117
supplies switching signals to the six semiconductor switches. Current shunt module
115
includes a current shunt sensing resistor of resistance R
sense
in the ground path of each series pair of semiconductor switches. Signal conditioning module
119
receives the voltage across each of these resistors. The voltage across these sensing resistors corresponds to the current to ground from the corresponding semiconductor switch pair.
Electronic control module
120
provides the control function for AC induction motor drive hardware
100
. Electronic control module
120
receives three analog input signals, ADC
1
, ADC
2
and ADC
3
from signal conditioning module
119
. These three signals correspond to the voltage across the respective current shunt sensing resistor. Electronic control module
120
employs these input signals together with a speed command or other command input (not shown) to produce six switching signals PWM
1
, PWM
2
, PWM
3
, PWM
4
, PWM
5
and PWM
6
. These six switching signals PWM
1
, PWM
2
, PWM
3
, PWM
4
, PWM
5
and PWM
6
are supplied to predriver module
117
. Each of the six switching signals PWM
1
, PWM
2
, PWM
3
, PWM
4
, PWM
5
and PWM
6
provides control of the ON/OFF state of one of the six semiconductor switches of switching module
113
. As known in the art, each of these signals provides a pulse width modulated drive to a corresponding one of the three phase drive lines
135
to induction motor
130
.
FIG. 2
illustrates speed estimator algorithm
200
typically used in such rotor flux based model reference adaptive system (MRAS) speed observers. Speed estimator algorithm
200
forms two rotor flux estimates. Reference rotor flux estimator
201
forms rotor flux reference estimate &lgr;
r
from stator voltage vector v
s
and stator current vector i
s
. These two dimensional vectors having direct and quadrature components are derived from the stator voltages and currents of all three phases using the well-known Clarke transformation. It is understood that the rotor flux estimate is based upon the respective inputs for all three phases. Adaptive rotor flux estimator
202
forms adaptive rotor flux estimate &lgr;
a
from the stator current i
s
and a motor speed estimate &ohgr;. Error calculator
203
determines the difference between the reference rotor flux estimate &lgr;
r
and the adaptive rotor flux estimate &lgr;
a
. The error signal e supplies a proportional/integral controller
204
which forms the motor speed estimate &ohgr;. Motor speed estimate &ohgr; is compared with a commanded speed in a feedback loop. This drives a conventional pulse width modulation algorithm generating the six switching signals PWM
1
, PWM
2
, PWM
3
, PWM
4
, PWM
5
and PWM
6
.
This rotor flux estimator method offers many advantages over a classic open loop speed observers. It does not involve any open loop integrators or differentiators. It is computationally simple. It does not require a separate slip speed calculation. However, this rotor flux estimator method is prone to stability problems, especially at high speeds.
FIG. 3
illustrates a typical adaptive rotor flux estimator algorithm
202
. Gain blocks
301
and
302
receive respective stator currents i
sd
and i
sq
. The output of gain block
301
supplies an additive input of summing junction
305
. The output of summing junction
305
supplies low pass filter
309
, which produces rotor flux estimate &lgr;
ad
. Similarly, the output of gain block
303
supplies an additive input of summing junction
307
. The output of summing junction
307
supplies low pass filter
311
, which produces rotor flux estimate &lgr;
aq
. Adaptive rotor flux estimator
202
includes two feedback paths. The rotor flux estimate &lgr;
ad
supplies one input of multiplier
313
. The other input of multiplier
313
receives the motor speed estimate &ohgr;. The product output of multiplier
313
supplies a second, additive input of summing junction
307
. The rotor flux estimate &lgr;
aq
supplies one input of multiplier
315
. The other input of multiplier
315
receives the motor speed estimate &ohgr;. The product output of multiplier
315
supplies a second, subtractive input of summing junction
305
.
Adaptive rotor flux estimator
202
is stable at low speeds where the feedback is near zero. At higher speeds, the cross-coupling becomes significant. The estimator poles become more and more lightly damped at higher speeds. At a sufficiently high speed, the additional phase lag in a digital implementation due to zero order hold reconstruction and processor time delay will cause instability.
SUMMARY OF THE INVENTION
A method of AC induction motor control known as rotor flux based model reference adaptive system. Model reference adaptive systems develop two estimates of rotor flux. The reference flux estimate is based on the voltages and currents in the stator windings. The so-called adaptive estimate is based on the stator currents and the measured or estimated operating speed. These two estimates are compared by taking the cross product between the reference and adaptive rotor fluxes and the estimated speed is adjusted by a proportional-integral controller until the estimator outputs agree. In this invention, the upper speed range of the rotor flux-based MRAS speed observer is extended by discretely or continuously
Cole Charles P.
Fedigan Stephen J.
Brady III W. James
Marshall, Jr. Robert D.
Masih Karen
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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