Patent GB-1193235-A (GB)
Title: Dual Converter Electrical Drive System
IPC: H02P7/292
Filed: 1968-01-29
Abstract: 1,193,235. Control of D.C. motors. WESTINGHOUSE ELECTRIC CORP. 29 Jan 1968 [13 Feb., 1967], No. 4486/68. Heading H2J. [Also inDivision G3] A common gate pulse generator controls the firing of both the forward FO and reverse RE valves in circuit with a motor MO, the selection of the valves being effected by a relay RR included in the pulse generator. A speed controller compares reference and tachometer signals and its output acts as a current reference signal. The current feedback signal is passed through a current sensor, a voltage reference signal being derived from the current controller. The voltage loop comprises a voltage senser which obtains a signal from the motor terminals, the gate pulse generator receiving an output from the voltage controller to set the firing angle of the valve sleected for conduction, and selecting the valves which is to be connected in circuit with the motor. Depending on the polarity and magnitude of the voltage error signal, a reversing senser transmits a signal to the gate pulse generator which sets the latter to select the appropriate valve, this being effected by relay RR with the co-operation of relays ICR, ICRA. The speed, current, and voltage loops have descending values of time delay, respectively. The forward valve includes thyristors F-1TH to F-6TH, whilst the reverse valve comprises thyristors R-1TH to R-6TH, each having a firing transformer associated with a pulse shaping network. The thyristors are supplied from a three-phase source. The firing pulse generator comprises blocks 1-6 each adapted to fire a pair of forward thyristors or a pair of reverse thyristors in accordance with the position of the relay RR. A signal from the voltage controller is applied to an emitter follower which transmits a D.C. signal to transformer TPC connected to switching transistors STR1- STR6. The voltage senser, Fig. 6 (not shown), comprises an inverter formed by transistors (1TR, 2TR) and transformer (IT) delivering a square wave signal to a magnetic amplifier (MA) which rectifies this signal and selects the polarity corresponding to the change in motor voltage. This voltage is applied to winding (W3) which determines the saturation of cores (1X, 2X). The magnetic amplifier also comprises balancing windings (W1, W2) and feedback windings (W4). Any change of motor voltage from a set value unbalances the magnetic amplifier and produces an output signal which is applied to the voltage controller. In the voltage controller, Fig. 7 (not shown) the voltage derived from the voltage senser is compared with a reference derived from the current controller which is impressed on a base of an operational amplifier (A1) through a limiter bridge (11D-14D). The output of the amplifier is applied through a rectifier bridge to the emitter follower which determines the firing- instants of the main thyristors. The relay 1CR is actuated by the voltage sensor to select a feedback path for the amplifier (A1 which determines the polarity of the signals passed by the amplifier. In the reversing senser, Fig. 8 (not shown) the base of a transistor (4TR) receives a positive signal when the motor is to operate forwardly so that transistor (5TR) becomes non-conductive and transistor (3TR) conducts to actuate relays 1CRA, 1CR and RR, whereby the forward thyristors are energized. The current senser comprises a rectifier bridge supplied from current transformer UCT, VCT, WCT, and connected to a thermal overload device. The polarity of feedback signal which is fed to the current controller depends upon the position of the relay 1CRA. Excessive current causes a suppression signal to be applied to transformer SP due to the firing of thyristor (1TH). The voltage at which this thyristor fires is reduced when the smaller-rated forward thyristors are conducting. If, when the motor is operating forwardly, the reference signal applied to the speed controller is reversed, the motor current is reduced and the forward and reverse thyristors are changed-over. The reverse thyristors are gated while the motor is still rotating forwardly and act as inverters. There is then a period when no current flows in either direction, after which the current builds up in the reverse direction until the speed and reference signals are matched. The reverse current is then reduced to a value just sufficient to overcome the load.