GB669901A - Improvements relating to the control of electric motors for operating long-travel mechanism of travelling cranes - Google Patents

Abstract

669,901. Control of D.C. motors. KUKA, K. S. Feb. 22, 1949. No. 4790/49. Class 38 (iii). Relates to a control system for a series crane motor comprising a speed-responsive relay subjected to the armature voltage of the motor, and another relay which is of the fixed timedelay type, the two relays in combination operating an electromagnetic contactor which completes a circuit comprising a currentlimiting resistor in parallel with the motor armature thereby strengthening the motor field and limiting the speed when the latter tends to exceed a pre-determined maximum value. As shown, a direct-current motor which may drive the operating mechanism of a crane bridge, has an armature A and a series field winding SF, a series brake coil SB being also provided. A main contactor coil M1 is energized to close main contacts M and holding contacts Ma when a master controller is in the "off" position. Movement of the controller in the appropriate direction closes contacts 1F, 2F, for forward operation of the motor. Thereafter the motor is accelerated by removing resistor-sections R1, R2, R3, R4, in turn by closure of contacts P, 1A, 2A, 3A. The coil of a time-delay relay, such as 1AR, is connected across each of resistor-sections R1-R3; these relays preventing removal of the next resistorsection until a fixed interval has elapsed after short-circuiting of each coil. During reversecurrent braking operation, the voltage across relay-coil PFR1 is substantially zero since the coil is subjected to the combined voltage of the armature and that across resistor-sections R1, R2, The contacts PFR therefore open to deenergize relays P, 1A, 2A, 3A, and cause the insertion of the total resistance R. A contactor TF is provided to complete a circuit through resistor R5 in parallel with the armature for separate energization of the series field winding and brake coil. The energization of this relay on the higher notches of the controller is dependent upon relays c e m f and 3AR. The coil of the relay c e m f is energized in accordance with the back-electromotive force of the motor which varies with the acceleration and therefore the load. The relay 3AR is of the time-delay type and is designed to time out a fixed interval after accelerating relay 2A has short-circuited resistor-section R3. If there is a light load on the hook the relay c e m f closes contacts c e m f a, c e m f b, before relay 3AR opens contacts 3ARa, the relay TF is closed, and the field winding SF strengthened to prevent overspeeding of the motor. For coasting operation, the controller is brought to the "off" position to disconnect the motor from the line, the relay TF remains energized over contact b of the controller and the coil SB remains energized to keep the brake lifted. The series brake coil also remains energized during the change-over to reverse-current braking.