Patent GB-636210-A (GB)
Title: Improvements relating to systems of electric motor control
IPC: H02P7/292
Filed: 1947-04-15
Abstract: 636,210. Automatic speed control systems. ENGLISH ELECTRIC CO., Ltd., MITCHELL, H. W., and BENT, R. A. April 15, 1947, No. 9999. [Class 38 (iv)] [Also in Group XXXVII] An electric motor control system comprises a D.C. motor having its armature fed from an A.C. source through one or more rectifiers wherein speed control is effected by varying the phase of the instant at which the rectifiers become conductive, the phase of the instant being controlled in response to the output of a direct coupled amplifier, in the input circuit of which an adjustable voltage for setting the motor speed is balanced against the armature voltage and a part of the difference balanced against the output of a network having applied to its input a voltage proportional to armature current to set the compounding characteristic. The control grids of two thyratrons T1, T2 are fed with alternating voltages which are in quadrature with the respective anode voltages upon which is superimposed a D.C. voltage component, variation of which component controls the instant at which the rectifiers become conductive, thereby altering the current in, and the speed of, the armature A. The variable D.C. voltage is obtained from a direct coupled thermionic amplifier T7, T8 connected to a stablized D.C. voltage source L1, L2, L3. The output of this amplifier is controlled by the voltage difference between the armature voltage and a manually adjustable reference voltage to set the motor speed, potentiometers P2, P7 setting the upper and lower speed limits respectively and potentiometer P5 serving as a speed controller between these limits. A further voltage, proportional to the armature current and adjustable by potentiometer P9, is also injected into the input circuit of the amplifier to effect a desired compounding characteristic. A valve T9 in parallel with the output valve T8 of the amplifier and normally biassed to cut-off, is arranged to take over control from valve T8 to limit the maximum value of armature current and, in conjunction with condenser C9 and resistor R30, to limit a sudden rush of current at starting. Dynamic braking is provided by valve T14 having a relay in its anode circuit, energization of the relay being governed by the grid of the valve which is biassed to cutoff except when the armature current falls substantially to zero when the valve will conduct, allowing contacts DB1, to close and apply dynamic braking. The arrangement also allows dynamic braking whenever the speed is reduced. Excessive voltages upon reversal of the motor are prevented by means of a relay RR. The field F1, Fig. 2, is supplied in a manner similar to the armature, the field circuit being supplied with a relay FFR to interrupt the armature supply upon failure of the field. The field current is controlled by the potentiometer P6, which may be ganged with potentiometer P5, Fig. 1, to form a single-speed controller. A valve T10 is provided to allow the field to be boosted above that set by potentiometer P6, if desired, and also to ensure that the field is fully energized before starting the motor while valve T13 is effective to reduce the field if excessive voltages occur in the armature circuit.