GB596671A - Improvements in the ward leonard system of electric motor control - Google Patents

Abstract

596,671. Control of D.C. motors. ENGLISH ELECTRIC CO., Ltd., and WILSON, G. R. April 19, 1945, No. 9832. [Class 38(iii)] [Also in Group XXXV] A Ward Leonard system controlling a reversible D.C. motor 10, such as a hoist motor, is arranged to limit the maximum values of the armature current by control means responsive to the excess of a control quantity, representing the value of the armature current, over a reference quantity, namely a biassing voltage, arranged to be pre-set to alternative values, one of which is brought into effect when the armature current is in one direction and the motor is motoring, and another of lower value which is brought into effect when the current is in the same direction and the motor is regenerating. The adjustable biassing voltage is provided by a potentiometer 20 connected across a D.C. source, and control is obtained by varying the excitation of the generator field 11b which is fed from an amplifying exciter 12 with one field winding 12g excited by a voltage which is the difference between the tapped bias voltage and a voltage proportional to the armature current, the latter voltage being derived from tappings 25, 24a on a potentiometer 24 shunted across the interpole windings 10a and 11a of the motor and generator. The system is provided with a combined speed controller and reversing switch 16 associated with four tappings 26-29 on potentiometer 20, of which tappings 28, 29 are connected to oppositely disposed rectifiers 22, 21 connected to the exciter field winding 12g. The arrangement is such that assuming the current in the main circuit flows anti-clockwise when the motor 10 is driving in the forward direction, the point 25 is positive relative to tapping 24a and, only if this voltage exceeds that which is tapped from potentiometer 20 between the tappings 26 and 28 will current flow through the exciter field winding 12g. This current will pass rectifier 22, and act differentially with respect to the current in exciter field winding 12e, thereby tending to diminish the voltage of generator 11 and reduce the main armature current. If the load on the motor becomes negative and regeneration takes place, the tapping 24a becomes positive relative to the point 25, and if the regenerated current exceeds the predetermined limit pre-set by the tapping 29 on potentiometer 20, current will flow in the reverse direction through exciter field 12g, and through tappings 29, 26 of the potentiometer. When the controller 16 is operated for reversal of the motor, the tappings 17a, 17b of the associated potentiometer 17 are disposed in positions opposite to those shown, and the contacts 18a, 19a are moved downwardly onto contacts 18e, 19e, respectively. The generator 11 is thereby excited in the opposite sense and the main armature current flows clockwise in the circuit shown. The contact 27 of potentiometer 20 is now effective, and the bias for motoring is determined by the position of tapping 29 relative to that of tapping 27, whilst the bias for regenerating is determined by the setting of tapping 28 relative to that of tapping 27. Tapping 26 is always placed nearer to tapping 29 than it is to tapping 28, and tapping 27 is placed nearer to tapping 28 than it is to tapping 29, so that with either sense of motor rotation the main armature current is limited to a lower value for regeneration than for motoring. The exciter 12 is preferably of the kind described in Specification 579,570, [Group XXXV]. Specification 560,531 also is referred to.