Patent GB-784103-A (GB)
Title: Improvements in or relating to electrical control systems
IPC: H02P29/00
Filed: 1955-06-09
Abstract: 784,103. Control of D.C. motors. BROWN, W. J. June 9, 1955 [July 12, 1954], No. 16669/55. Class 38 (3). [Also in Group XXXV] A series D.C. motor is supplied from an A.C. source through a half-wave grid-controlled rectifier which is controlled by a voltage dependent upon the difference between a feedback voltage derived from the motor current and a reference voltage derived from a half-wave rectifier supplied from the same A.C. source, the reference voltage pulses being delayed or extended in duration so as to embrace the feedback voltage pulses. In one embodiment, a load such as the armature 62, Fig. 4, of a D.C. motor is supplied from an A.C. source through a converter 1. The converter is controlled by applying a voltage to its grid 11 from a phaseshifting network 12 energized from a transformer 17 and including a saturable reactor 58. In order to limit the load current, a pulsed D.C. voltage representing the load current is compared with a pulsed D.C. reference in a control winding 61 of the reactor. If the pulsed D.C. voltage derived from a resistor 8 and representing the load current exceeds a pulsed D.C. reference derived from the supply transformer 17 over a rectifier 20, a rectifier 23 conducts to allow a signal to pass in the control winding 61 of the reactor whereby its impedance is varied to shift the phase of the voltage applied to the converter grid 11 and thus limit the output current to the load. Since as shown in Fig. 3, the load current pulses 42, 43 are phase-retarded with respect to the A.C. supply 44, the reference pulses are also retarded by deriving the reference from the junction between a condenser 64 and a resistor 65 connected across the secondary of transformer 17. The reference pulse 41 is thus substantially in phase with the load current pulses as shown, so that a true comparison of their amplitudes may be made for the control of the load current. In practice the load current pulses may be " tailed " due to eddy current effects in an inductive load or to the back E.M.F. in a motor armature load. A condenser 25 may be connected across the reference resistor 21 in order to give a corresponding " tailing " of the reference pulses. The reference may be derived from point 55 in the phase shifting network or, if condenser 25 gives sufficient " tailing ", directly from one end of the transformer secondary. The Specification states that regulation of the load current to a constant value may be obtained in the absence of rectifier 23. The system is applicable as a current-limit device for D.C. motor speed control systems such as described in Specifications 686,799, 697,609, 746,213 and 749,220, a second control winding 60 on the reactor carrying the speed error signal. Condenser 25 permits sufficient tailing of the current reference pulses to compensate for the wider phase angle spread of the load current pulses due to the additional control of the con - verter grid in order to maintain a desired speed. An application to a complete D.C. motor speed control system is described, Fig. 7 (not shown) in which the motor armature has a reversible series field for rotation in both directions. As described in Specification 749,220 the speed error resulting from a comparison of a reference set on a potentiometer with the motor back E.M.F. is applied to a control winding of the reactor. To bring the motor rapidly to a standstill the armature connections are reversed and the reference potentiometer is short-circuited by a switch. Specification 656,816, [Group XXXV], also is referred to.