Patent GB-729996-A (GB)
Title: Improvements in or relating to alternating current motor control systems
IPC: H02P25/28
Filed: 1953-11-11
Abstract: 729,996. Automatic speed control systems. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Nov. 11, 1953 [Nov. 9, 1952], No. 31206/53. Class 38 (4). [Also in Groups XXXVII and XXXIX] A polyphase motor M has a magnetic amplifier MA, MB, MC, connected in series with each supply conductor L1, L2, L3, an additional magnetic amplifier MD having its main winding circuit connected to supply-conductor L2 through a winding T1b of a transformer T1, the other winding of which is connected across supply-conductors L1, L3. Windings AB-DB bias each magnetic amplifier to minimum output, and the amplifiers MC, MD respond in opposite senses to control voltages of the same polarity, so that one produces an output voltage and the other remains at minimum output. The amplifiers MA, MB produce output voltages in response to control voltages of either polarity. These control voltages are applied to the amplifiers MA-MD by means of forward and reverse magnetic amplifiers MF, MR, which are selectively operated in dependence upon reversible input voltages, one control winding AF-DF of each amplifier being energized by the output of the amplifier MF, and the other control windings AR-DR being connected in the output circuit of the amplifier MR. The forward and reverse magnetic amplifiers have control windings FC, RC, which are adapted to be excited in accordance with the difference between an adjustable reference voltage and a voltage which is proportional to the speed of the motor. The reference voltage, is derived from a rheostat RH and thei-speed-responsive voltage from a motor-driven tachometer generator PG, a change-over switch FW1-RE2 being provided for removing the reference voltage so that the speed-responsive voltage acts alone, reversing the excitation of the windings FC, RC, and thereby plugging the motor. In addition to the control windings FC, RC, the amplifiers MF, MR have damping windings FD, RD, and bias windings FB, RB. The ampere-turns of the windings FB, RB bias the respective magnetic amplifiers to minimum output. The damping windings are energized from one phase of the motor supply circuit through rectifier R7 and transformer T3. In Fig. 8, the motor M acts as a pilot motor driving the slip regulator SR of an induction motor IM. Magnetic amplifiers ML1, ME1 are responsive to the current supplied to the motor IM and are adapted for selectively applying reversible input voltages to the forward and reverse amplifiers ML, ME. Biasing windings LB, EB, are excited so that amplifier ML1 is normally conducting and amplifier ME1 has minimum output, and control windings L17, E16, are energized through a current transformer CT so that an increase in load current reduces the output of amplifier ML1 and increases that of amplifier ME1. The motor IM is started with the slip regulator in the full resistance position, and the output of amplifier ML1 is applied to pattern winding LP of magnetic amplifier ML. This output is sufficient to overcome bias winding LB so that amplifier ML excites the parallel connected control windings AL-DL of the pilot motor magnetic amplifiers. The amplifier MC is cut off and amplifiers MA, MB, MD are effective, so that the pilot motor is energized for lowering the moving electrodes of the slip regulator. The resultant increase in the load current of the motor IM has the effect of reversing the pilot motor which therefore stops in a position corresponding to a selected magnitude of the load current about which regulation is to take place. The magnetic amplifiers ML, ME are additionally excited by a voltage responsive to the speed of the pilot motor in such a manner that rapid electrode-positioning is obtained when the difference between the actual and desired load currents is large. When limit switches ELS, LLS, of the slip regulator are actuated, the speed feedback voltage has the effect of introducing plugging operation of the pilot motor.