GB563842A - Improvements in and relating to electric relays - Google Patents

Abstract

563,842. Electromagnetic switches ; control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. May 14,1943, No. 7698. Convention date, May 16,1942. [Classes 38 (iii) and 38 (v)] A reverse-current relay, particularly for controlling an electric motor connected to an overhauling load, has two coils 19, 20, which act together to attract an armature 13, and a magnetic shunt 30 arranged so that when the current in the coil 19 reverses and the coils 19, 20 oppose each other so that the armature 13 is released it is held in the release position by the flux from the coil 19 traversing a closed path through the shunt 30 and the armature 13 even though the flux due to the reverse current overpowers considerably the flux due to the coil 20. In the form shown the coil 19 is a " series " coil connected in the armature circuit 35 of a motor having a field winding 36 and the coil 20 is a " shunt " coil. The motor may drive a crane hoist and normally the armature 13 will be attracted so that a resistance 37 is included in the field circuit and the motor speed increased. Should a heavy load that is being lowered overhaulthe motor, the current in coil will reverse, the relay will release, the resistance 37 be short circuited and the full braking effect of the motor brought into play.