Patent GB-802708-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: B66C13/22
Filed: 1956-07-02
Abstract: 802,708. Automatic speed control systems. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. July 2, 1956 [July 11, 1955], No. 20397/56. Class 38 (4). [Also in Groups XXXVII and XXXIX] The stator terminals T1 . . . T3 of a polyphase induction motor for driving a hoist are supplied from lines L1 . . . L3 through saturable reactors 1-If . . . 3-If and 1-Ir ... 3-Ir for forward and reverse operation respectively. The impedance of the reactors is determined by the output of magnetic amplifiers Ar, Af, the arrangement being such that, with current flowing in one direction through windings 115, 135, an increase of the input current increases the output of amplifier Af, thereby increasing the current supplied through conductors DL3, DL4, to the control windings of the forward reactors. Similarly, when the excitation of the windings 115, 135, is reversed, an increase in the input current results in an increase of the output current of the amplifier Ar. The setting of a voltage divider V determines the polarity of the current in the windings 115, 135 and, for any setting of the arm 165 on either side of the centre tap 163, there is in series with each motor terminal, a reactor of low impedance and a reactor of high impedance. The magnitude and phase rotation of the stator voltage is determined essentially by the reactors having a low impedance but, whatever the magnitude, the voltage applied to the motor is balanced. When the motor is running, a voltage derived from a motor-driven tachometer generator T opposes the output of divider D. In addition, a damping potential is impressed through transformer TD. If the load overhauls the motor during lowering operation, excessive speed is prevented by counter-torque braking. In another arrangement, two motors are used for the actuation of a vertical lift bridge, each motor being supplied through two groups of reactors controlled by magnetic amplifiers. A control unit common to the two motors includes a tachometer generator and voltage divider, as in the previous embodiment. In addition, a synchronous receiver, responsive to the difference in outputs of two synchronous transmitters driven by the respective motors, is adapted to vary the current supplied to control windings of the magnetic amplifiers, Fig. 6 (not shown). In this manner, the differential receiver provides a compensating voltage when the speed of one motor exceeds that of the other. In a further modification, the motor is supplied directly through two groups of magnetic amplifiers for reversing, and an additional amplifier is provided, common to the two groups, for balancing the supply. Each of the reversing amplifiers has a pair of output windings, and four input windings comprising a speed control winding, two damping windings, and a biasing winding. The balancing amplifier has a pair of output windings, and a pair of input windings which are so wound that current-flow through each reduces the impedance of the amplifier, Fig. 7 (not shown). The control unit is similar to that shown except that one set of damping windings is supplied through a transformer connected across motor-terminals T1, T2. A control system for two motors applied to a vertical lift bridge may also employ magnetic amplifiers in the motor circuits, Fig. 9 (not shown). A master controller with contacts for regulating a mechanical brake and the rotor resistance may be provided. This controller may be combined with a relay for overload and under-voltage protection, Fig. 10 (not shown).