Patent GB-663776-A (GB)
Title: Improvements relating to electrical control circuits
Inventor: GHALIB SELCHOUK AHMED
IPC: H02P7/30
Filed: 1948-10-12
Abstract: 663,776. Control of D.C. motors. METROPOLITAN-VICKERS ELECTRICAL CO. Ltd. July 12, 1949. [Oct. 12, 1948] No. 26546/48. Class 38 (iii). (Also in Group XXXV] In order to control the speed of a D.C. motor M to a required value, the armature voltage of the motor being supplied through a pair of thyratrons, a voltage proportional to the speed of the motor is connected in series opposition to a reference voltage across the control winding of a speed control transductor connected as a variable inductance T2 in a phase shifting bridge circuit comprising resistors R1, R2, R3 and transformer T1, so as to control the phase of the grid voltage of the thyratons and thereby the armature voltage of the motor in such a way as to maintain the speed of the motor M at the required value. As shown in Fig. 1 the reference voltage is developed across a potentiometer P1, a rectifier K1, giving the error signal unidirectional characteristics. In order to obtain a voltage truly proportional to motor speed an IR compensation voltage proportional to the voltage drop across the armature is subtracted from the armature voltage thereby obtaining a voltage proportional to the back e.m.f. of the motor and to speed. The IR compensation voltage is obtained by a transformer T4 (Fig. 2) in the anode circuits of the thyratrons, the secondary of which feeds a potentiometer P2 via a full wave rectifier K2, K3. In order to avoid hunting the IR voltage is made slightly larger than the voltage drop across the armature and in order to avoid instability of operation part of the voltage arm the potentiometer P2 i.e. y.z. is fed direct to the motor circuit whilst the remainder y.x. is fed via a delay network R5, C1. The part y.z. is made as large as is possible while still maintaining stable operation. An armature current limiting device is shewn in Fig. 3 and comprises a high speed polarised relay HSR one coil of which is fed via a potentiometer P3 from the potentiometer P2 and the other from a constant voltage source. The relay contacts are normally held closed by the action of the constant voltage coil but if the armature current, and hence the IR compensation voltage become excessive the first coil overcomes the effect of the second (constant) coil, the relay contacts open and impedances R7, R8 and C2 are introduced in the motor control circuit reducing the current through the central winding of the impedance T2 causing the thyratrons to phase off and the motor armature current to fall. The relay contacts vibrate so as to maintain the thyratron currents substantially constant. In another embodiment (Fig. 4) the magnitude of the error signal is limited and its effect overriden by a second control winding B fed with the current limitation signal. A is the normal control winding. B is similar and is connected via a rectifier K5 to A in such a way as to tend to cancel out the flux produced by A. The rectifier K5 acts as a limit setter to the voltage across A. Resistance R9 prevents excessive current from flowing through A and B when the current limit is operative. An overriding winding C is fed from a potentiometer P4 the voltage across which is derived from the back polarising voltage of a rectifier K4. The voltage across P4 is limited by the action of resistance R15 and rectifier K8. In a modification (Fig. 4A) the current in the winding A may be limited by a resistance R9 and rectifier K10 connected across A in a manner similar to R15 and K8 across P4 in Fig. 4. In another modification (Fig. 5 not shown) the system of Fig. 4A is employed together with an additional variable inductance in series with T2, and normally run with saturated control current, to retard the phasing of the thyratron grids to effect the current limitation proper. In a further modification, Fig. 6 (not shown), greater phase shift is achieved by replacing the transformer T1 (Fig. 1) by a second similar phase shifting bridge circuit including the imput transformer, whilst in a still further embodiment the rectifier K10 of Fig. 4A is replaced by two rectifiers connected back to back, Fig. 7 (not shown).