GB175815A - Improvements in and relating to electric motor control - Google Patents

Abstract

175,815. British Thomson - Houston Co., Ltd., (General Electric Co.). Dec. 9, 1920. Systems for continuous-current motors.-In starting or otherwise controlling electric motors, the successive short-circuiting of resistances is made to depend on automatic electromagnetic contactors or relays having two windings, one of which is short-circuited in a previous stage of the control, while the other is left connected across some of the resistances so as to respond when the current passing through the motor armature rises or falls to a designed value as the motor slows down or speeds up. As applied to the starting of a series motor, Fig. 8, three electromagnetic contacts 71, 72, 74 are mechanically biassed to short-circuit three resistances R<11>, R<12>, R<13>. One winding of each contactor is connected between the supply side of a starting-switch 70 and one end of the corresponding resistance, while the second winding of each contactor is connected across the two ends of the corresponding resistance. When the switch 70 is closed, the first winding of the contactor 71 is short-circuited and the second winding remains excited proportionally to the potential difference across the resistance R", so that the first contactor closes when the motor current falls off sufficiently as the armature speeds up. The closure of the first contactor short-circuits one winding of the contactor 72 and leaves the second winding to act responsively to a further increment of speed. A similar process is carried on automatically to the end of the sequence. In a modification, the responsive windings energize relays which control contactors of ordinary type. Another modification employs relays mechanically biassed to open and contacts biassed to close. Further modifications provide a pair of initially acting contactors or relays, corresponding to 71 in Fig. 8, but arranged to come into action alternatively according to the direction of running. Provision may also be made for ensuring that resistances are in the motor circuit when current restarts after a failure. A further modification shown in Fig. 4 relates to the lowering sequence of a hoist motor. An automatic relay 48 having two windings respectively connected across resistances R<5>, R<6>, so long as it is energized, keeps a contactor 46 closed and another contactor 47 open. The closed circuit through 46 and R<6> contains a coil B which holds a brake out of action, and this circuit remains closed after power has been turned off from the motor, owing to the relay having its active coil excited proportionally to the potential difference across R<6> from the motor working as a generator. When the speed falls off sufficiently, the relay opens, the brake is allowed to act, and the contactor 47 completes a rheostatic brake circuit through R<7>.