GB706379A - Improvements in and relating to electric motor control systems - Google Patents

Abstract

706,379. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. May 15, 1952 [May 15, 1951], No. 12305/52. Class 38(3) A direct current motor control system comprises field reversing means for plugging the motor when reversing the direction of rotation thereof, further means being incorporated to limit the motor current during plugging, these latter means also serving to.introduce a time delay between the steps of cutting out resistance from the armature circuit during acceleration of the motor. To start the motor switch 17 is closed, manual operated switch 8 being moved to the forward position to energize forward contactor 6, operation of which causes contactor 11 and relay 14 to be energized. Energization of contactor 6 connects the armature la and the portion 1c of the series field across a D.C. supply 2 whilst energization of contactor 11 removes the short circuit from the portion 1b of the series field. Thus the motor starts under full field with maximum resistance in the armature circuit. The energization of relay 14 places the system ready for further speed control so that when the master controller 16 is moved to position " 3 " contactor 9 is energized so removing part 5a of the resistor 5 from the armature circuit and at the same time de-energizing relay 14. This relay does not immediately drop out due to the presence of the resistor 15 across the operating coil 14a. When relay 14 finally drops out contactor 10 becomes energized to remove the portion 5b of resistor 5 from circuit and to de-energize the contactor 11 thereby to replace the short circuit on the portion 1b of the series field. The motor is now running at full speed with reduced field. To reverse the motor rotation, switch 8 is moved to the reverse position whilst master controller 16 is moved to the " off " position and then returned to position " 3 " again. All the contactors thus drop out and at the same time reverse contactor 7 and contactor 11 become energized. Thus the direction of current in the portion 1b of the series field is reserved and since the armature is still rotating in the forward direction, the motor counter voltage is additive to the supply voltage. Thus a heavy current will flow in the armature circuit so exerting a heavy braking or " plugging " effect on the motor. Resistor 5 cannot be removed from circuit (so as to cause excessive current) since relay 14 cannot be energized because the potential difference between its connecting points, viz., the armature left hand terminal and the point 5c on resistor 5 is practically zero. Relay 14 does not become energized until the speed of the armature is low enough for a potential difference to exist across the operating coil 14a. After energization of the relay 14 contactors 9, 10 operate as before to accelerate the motor in the reverse direction.