GB733869A - Improvements relating to electric motor control systems - Google Patents

Abstract

733,869. Control of A.C. motors. ENGLISH ELECTRIC CO., Ltd. Sept. 25, 1953 [Sept. 30, 1952], No. 24426/52. Class 38 (3). A slip-ring induction motor 10 has one phase of the stator winding 10b supplied from a substantially constant voltage source 17 of directcurrent for dynamic braking whilst the other two phases are supplied from a source 18 the voltage of which is varied in accordance with the rotor current of the motor. The induction motor drives a mine winder, and, assuming the winder to be at rest with one cage at a bank, closure of a push-button SB energizes a forward contactor F to connect the stator of the motor to the three-phase supply 12 and release the mechanical brakes. If, during the wind, a negative torque is required to maintain the desired speed, contacts EBI are automatically operated to open the contactor F and contacts EB2 close after a time delay to energize the dynamic braking relay B (provided that the source 17 has not failed). On the other hand, if the direct-current supply has failed, a relay DBF opens contacts DBF1, and the automatic opening of contacts EB3 when negative torque is required causes the de-energization of relay X. Contacts XI open to isolate contactor B and contacts X2, X3 close to complete a circuit to reverse contactor R so introducing countercurrent braking in place of dynamic braking. Near the end of the wind, the limit switch LSE opens to de-energize contactor R which opens contacts R3 to isolate no-volt coil 14 and trip circuit-breaker 13. Manually controlled switch means DBS1, DBS2, are adapted to render the relay X and coil 14 independent of the operation of the relay DBF whereby countercurrent braking may always be introduced when the contacts EB3 open. If thesupply 17 fails with the motor already connected for dynamic braking, the relay X is de-energized by the opening of contacts DBF1 so that contactor B is opened and contactor R closed, as before. If negative torque is not required during the wind then, even if the supply 17 fails, the contactor F remains closed until the limit switch LSB opens. A winder-operated switch TS is arranged to open the circuit-breaker and apply emergency braking if, with dynamic braking in operation, the supply 17 should fail near the end of the wind, or, if the directcurrent has already failed, the contacts EB1 are operated near the end of the wind. Specification 715,583 is referred to.