Patent GB-647535-A (GB)
Title: Improvements in or relating to control systems for alternating current motors
IPC: H02P27/02
Filed: 1947-09-12
Abstract: 647,535. Automatic speed control systems. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Sept. 12, 1947, No. 25078. Convention date, Dec. 14, 1946. [A Specification was laid open to inspection under Sect. 91 of the Acts, April 8, 1948.] [Class 38 (iv)] [Also in Group XXXVII] A control system for a polyphase motor of the type in which a desired speed-torque characteristic of the motor is obtained by imparting a controlled degree of unbalance of the primary polyphase energization circuit of the motor, comprises a reactive impedance device having an impedance winding or windings and a control winding or windings inductively associated with said impedance winding or windings for controlling the impedance value of the latter, said impedance winding or windings being connected in said energization circuit so as to form a controlling part thereof, at least one electron discharge tube plate-connected to said control winding or windings, and a control circuit for progressively varying the effective conductance of said tube in response to the speed of the motor. A wound rotor induction hoist motor has its stator terminals T1, T2, T3 connected to line terminals L1, L2, L3, through a circuit comprising an impedance bridge network N which includes transformer primary windings 304, 305, 306, 307, serving as variable impedances and controlled by secondary windings 308, 309, 310, 311. When the network N is balanced the motor is energized under approximate single phase conditions, the relative impedance-values of the pairs of primary windings 304, 306, and 305, 307, determining the degree of unbalance and the phase rotation of the voltages applied to the stator windings of the motor. Secondary windings 308, 310, are connected in the anode circuit of a discharge valve E1 which may be a thyratron, secondary windings 309, 311, being similarly connected to valve E2. The grids of the valves are interconnected through resistors 318, 319, 320, 321, the electrical mid-point C of which is connected to the cathodes through a tapped portion of a rheostat 322 across which a constant grid-bias component is developed by means of rectifier 323 and transformer 324. Opposing voltages, derived from a motor-driven tachometer generator 326 and a potentiometer 328, are applied to tappings in the resistors 318-321, the potentiometer 328 having a constant voltage applied across it by means of rectifier 329 and transformer 330. When the opposing voltages are equal the motor runs at a speed determined by the setting of the potentiometer 328, and, if the speed deviates from this value, a resultant grid-bias component, the polarity of which is dependent on whether the speed is above or below the set value, is impressed across points A, B, such component being additive with respect to that derived from the rheostat 322 in the control circuit of one of the valves, and is subtractive in the other. Consequently the firing-period of one valve is increased whilst that of the other is decreased and the impedance distribution of the network N is changed to control the speed-torque performance of the motor. For lowering operation, the terminals D1<SP>1</SP> and A are connected so excluding the potentiometer 328, and, since at zero speed the voltage of generator 326 is zero, the grid bias component from the rheostat 322 is alone effective. This component may be of such value that the valves E1, E2 are both conductive with medium firing-period so that the windings of network N have medium effective impedance, are balanced and the motor has approximate single-phase energization. With an overhauling load, the motor speed and the voltage of the generator 326 increases, the grid bias component derived from the latter opposing that of the rheostat 322 in the grid circuit of the valve E1 which is rendered more conductive. The valve E2 is simultaneously rendered less conductive so that the impedance distribution of the network N is changed in such manner that the motor stator voltage approaches progressively a balanced three-phase energization with a phase rotation enabling the motor to develop a hoisting torque to oppose the load. When terminals A and D2<SP>1</SP> are connected, a small voltage derived from rectifier 329 is effective in the grid circuits of the valves. so that, at standstill, the valves E1, E2, are conditioned to afford an unbalanced three-phase energization of the motor which develops a downward torque. As the motor speed increases the voltage due to generator 326 becomes effective so that, at one stage, the motor develops zero torque, and then reverses its torque to oppose the load. Terminals A and H2<SP>1</SP> are connected for hoisting operation and the valves E1, E2 are so biassed that a substantially balanced phase distribution of the motor stator voltage provides a heavy hoisting torque at zero speed. As the hoist speed increases the voltage of generator 326 becomes effective to reduce the motor torque and then reverse it. In the modification of Fig. 2 the transformers of network N are replaced by saturable reactors. The anode voltages of valves E1, E2 are supplied from transformer TR, a grid voltage transformer 433 being provided. The grid voltage component dependent on the speed of the motor is derived from the rotor circuit in which is connected transformers 436, 437, 438, in series with capacitors 439, 440, 441. The secondary windings of the transformers are connected to rectifiers 442, 443, 444, the outputs of which are applied in parallel to rheostat 435. The transformers operate with a high degree of saturation, the capacitors serving as peaking devices. The voltage applied to the rheostat 435 is substantially proportional to the rotor frequency and hence to the speed of the motor. Valves 446, 447, are connected across reactor control windings and do not pass current when the respective valves E1, E2 conduct. When the main valves are cut off, the reverse current due to the self-induction of the control windings flows through the valves 446, 447 and extends the intervals of energization of these windings beyond the periods of conductance of the main valves. For hoisting operation, a contactor RC is actuated to disconnect the network N and the control of the motor is effected by varying its rotor resistance. Specification 647,534 is referred to.