Patent GB-1389261-A (GB)
Title: Method and means for controlling the output  conditions  of a self-excited alternating current motor
IPC: H02P23/06
Filed: 1972-03-21
Abstract: 1389261 Control of A.C. motors LITTON INDUSTRIAL PRODUCTS Inc 21 March 1972 13082/72 Heading H2J [Also in Division G3]  Relates to control circuit arrangements for obtaining a desired torque condition in a squirrel cage induction motor. Arrangements for controlling the motor speed to a desired value by making a torque command signal proportional to a speed error, are also disclosed (Fig. 16). To obtain desired torque, the stator may be supplied with current I s  the magnitude of which is regulated as a predetermined function of the torque command signal and the phase angle of which with respect to its "no torque" position when "slip" is zero (i.e. with respect to its magnetizing current component I m ), may be adjusted so that the magnetizing current I m  is maintained constant. In addition, the slip frequency is adjusted to a required value by varying the frequency of the stator current I s  or by varying the motor load. Induction motor control (Fig. 11).-In the control of an induction motor 22 a torque command signal is set up on a potentiometer 60. A function generator 94 relates this desired torque to the required stator current I s  for both driving and braking torque, and feeds a desired current signal to a summing junction 90. The stator current is derived from A.C. source 62 via a rectifier unit 66 and a half-wave inverter 96. The summing junction 90 compares the required current value with the actual current value of the rectifier output fed back via a line 86 and adjusts the rectifier firing circuit 80 accordingly, whereby to maintain the correct magnitude of direct current to the inverter 96, via a smoothing choke 92.  The inverter output frequency is determined by an oscillator circuit 126 which provides firing pulses to CR's 104, 108, 112 via a ring counter 124. The oscillator pulse frequency is controlled by both a tacho-signal in a line 138 as a function of rotor speed or frequency f r  and the torque command signal. The tacho-signal causes the oscillator to produce pulses at a frequency corresponding to the rotor frequency f r  and the torque command signal increases or decreases the pulse rate by an amount corresponding to the necessary slip frequency. A second function generator 130 may be connected between the torque command and the oscillator and which will advance or retard the firing pulses. Thereby the stator current I s  may be phase shifted by an angle 0 with respect to its no torque position, i.e. with respect to I m  as shown Fig. 6 (I s  being assumed to be in phase with I m  when slip is zero). The counter e.m.f. across one stator winding is used as a reference for advance or retard of the firing pulses and I s . Thus the frequency, phase angle and magnitude of stator current I s  are adjusted to produce the desired torque. It is stated that braking torque is also controlled when motor is in the regenerative mode. Speed control of induction motor.-Speed may also be controlled by making the torque command signal proportional to a speed error signal (Fig. 16). The concurrance of the required magnitude of stator current and necessary slip frequency provides an output torque condition tending to reduce speed error. A limit circuit limits the maximum output torque. In an alternative embodiment (Fig. 20, not shown) the control circuit is energized from a D.C. source via a current regulated D.C. chopper the output of which is fed to a full-wave three-phase inverter to energize an induction motor.