GB1206259A - Limit control for an alternating current motor - Google Patents

Abstract

1,206,259. Automatic speech control. SQUARED. CO. June 19, 1969, No.30999/69. Heading G3R. [Also in Division H2] In a crane control system, movement of slider 38 in the clockwise direction from it central position, provides a reference voltage to turn on lowering control unit LX and common control unit CX as well as energizing relay 22. The impedance of static switches 1L, 2L, C, is reduced whereby induction motor 10 exerts a torque in the lowering direction. Feedback voltage from tachometer generator 35 opposes the reference voltage and tends to increase the impedance of the static switches. Under overhauling load conditions the net signal reverses whereby control unit LX is turned off and control unit HX becomes effective. The static switches 1H, 2H, are then turned off and the motor provides a counter-torque to stabilize the lowering speed. Upon return of the slider 38 to the central positon, a brake 28 is set and the motor exerts a hoisting torque for quick stoppage. When a limit switch 18 is untripped, full line voltage is impressed transformer winding 24p so that brake 28 is released. This is because transformer windings 21p, 21s have the same number of turns and have low impedance when interconnected through contacts 18b. When the motor enters the limit zone while operating normally in the hoisting direction, contacts 18a, 18b, open to de-energize the motor and set the brake. The load is lowered out of the limit zone by adjusting slider 38 to turn on switches 1L, 2L, and C. The winding 24p, is now connected across conductors L1, L2, through contact 18c, and switch 1L. Upon the switch 1L reaching a low impedance, the brake 28 releases and the load is lowered, after which the limit switch is reset to establish normal operation. If the motor is plugged when entering the upper limit zone, the brake is not set, but the motor then exerts lowering torque so that the load is brought to a standstill and lowered out of the zone. An additional safety feature prevents overspeeding, when lowering out of a limit zone, if the limit switch 18 fails to reset. The feedback voltage from the tachometer generator increases the impedance of switches 1L, 2L, C, so reducing the energization of winding 28w and resetting the brake. The motor is then alternately accelerated and stopped for safe lowering of the load, even if the master switch is held in a lowering position.