General Electric Company

Leland C Tupper

Paul M Espelage

David L Lippitt

Loren H Walker

Herbert W Weiss

James W Sember

B Paul Chausse

A system for controlling the operation of an alternating current induction motor having windings supplied by electrical power from the polyphase alternating current source includes a load commutated inverter circuit which is connected between the power source and the induction motor for furnishing electrical power to the motor. Controllers responsive to a command signal and a feedback signal indicative of motor operating parameters control the operation of said load commutated inverter circuit. There is further provided a fixed capacitor circuit connected between the windings of the motor for supplying reactive volt amperes (VARs) to said load commutated inverter circuit and said motor. Finally, there is provided a variable VAR generator connected between the motor windings for supplying VARs to said load commutated inverter circuit and said motor in response to the output of a third feedback control path which is also responsive to a command signal and prescribed operating parameter(s) of said load and/or said commutated inverter.

Excitation current control for induction motor drive using load

A load commutated inverter synchronous motor drive system wherein a thyristor firing control of the inverter is determined by the amplitude of pesudo flux waveforms which are derived from the integral of the line voltages coupling the inverter to the motor being driven. A firing strategy is provided based on the premise that optimum thyristor firing in a load commutated inverter operating at a leading power factor occurs at a point just below the peak of the forthcoming pseudo flux waveform which point comprises an amount of volt-seconds necessary to effect current commutation plus a nominal safety margin. The determination and control is implemented, preferably, in a software phase lock loop but can, when desirable, be implemented in hardware.

Flux feedback firing control for a load commutated inverter

A control system for a load commutated inverter AC synchronous motor drive provides an operational shift in the event of an increase in motor terminal voltage V.sub.TERM above a preselected reference level. Under such an increase in terminal voltage, control is transferred from a condition where torque is controlled by current and the terminal voltage is uncontrolled to a condition where the load current, e.g., the DC link current I.sub.L flowing between a source side thyristor converter (AC to DC) 12 and a load side thyristor inverter (DC to AC) 14 is utilized to control the motor terminal voltage while the firing angle of the thyristor inverter 14 is used to control the motor torque. This involves increasing the minimum load or motor drive current for a desired torque while adjusting the inverter firing angle from a previous value, so that the desired torque is provided at a higher current I.sub.L and lower terminal voltage V.sub.TERM than before. Such an operational shift, in effect, uses a component of the load current to demagnetize the motor field in order to maintain the motor terminal voltage at an acceptable level.

Terminal voltage limit regulator for a load commutated inverter

The load side converter or inverter in a load commutated inverter motor drive, including a source side AC to DC converter coupled to a DC to AC converter via a DC link circuit, is adapted to have at least three and preferably four modes of operation for bringing an AC motor load, and more particularly a synchronous motor, up to speed. In the preferred embodiment, the first mode constitutes an idle mode wherein the converter thyristors are maintained in a non-conducting state until commanded for normal operation. The second mode constitutes an initial start-up mode which assumes no knowledge of initial rotor position of the motor and simply utilizes a fixed low frequency firing signal to sequentially gate the thyristors using forced commutation to commutate the off-going thyristor. Thirdly, a second type of forced commutation mode is next entered into where the inverter frequency is synchronized to the rotor of the motor by sensing pseudo-flux waveforms until 5% to 10% of rated motor speed is achieved whereupon a fourth mode is entered into which comprises a self-commutation mode utilizing a phase lock loop operation using pseudo-flux wave zero crossings as synchronizing signals. In the three mode embodiment, the third mode is eliminated and the second mode modified to vary the load side converter frequency on a time basis until reaching some preset value (e.g., five (5) to fifteen (15) percent of motor rated frequency) at which time transition is made to the self-commutation mode.

Transitioning between multiple modes of inverter control in a load

A current limiter for a load commutated inverter-synchronous motor drive including a source side AC to DC converter (12) and a load side DC to AC converter (14) coupled via a DC link circuit including an inductor (16) wherein a torque reference signal (TORQUE REF) generated for controlling both the converters is limited in response to the peak value .psi..sub.MAX of the pseudo flux waveform (.psi..sub.ab, .psi..sub.bc, .psi..sub.ca) which is derived from the integral (.intg.) of the motor terminal voltage (v.sub.ab, v.sub.bc, v.sub.ca) in order to limit the motor stator current I.sub.s to a value corresponding to the region of peak output torque T.sub.MAX obtainable for the motor field current I.sub.f applied. Control of both the source side converter (12) and the load side converter (14) by the torque reference signal operates to control the magnitude of the current I.sub.L in a DC link circuit which corresponds to the motor stator current I.sub.s as well as the frequency of the AC power applied to the motor.

Current limiter for a load commutated inverter

A control system for powering an AC load such as a polyphase synchronous motor having at least two sets of three phase (3.phi.) windings. Each three phase winding set is powered by an independently operable six pulse load commutated inverter drive including a source side converter and a load side converter coupled together by means of a DC link and wherein the source side converter and the load side converter are operated in accordance with respective current and load angle regulators which respond to torque command signals generated by speed regulators. The two motor drives operate concurrently in separate control channels which are substantially independent of one another with the exception that in one embodiment, which is the preferred embodiment, the speed regulator and the motor terminal overvoltage regulator are cross-coupled to effect a balanced torque command and motor terminal voltage from each channel while still providing for continued motor operation by one of the motor drive channels in the event that the other becomes inoperative for any reason. In a second embodiment, one of two redundant speed regulators is adapted to provide a common torque command to both channels with the other speed regulator taking over in the event of the failure of said one speed regulator.

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Schenectady, NY, United States of America

Leland C Tupper