GB843524A - Improvements in or relating to electric motor control systems - Google Patents

Abstract

843,524. Control of A.C. motors; controllers. CANADIAN WESTINGHOUSE CO. Ltd. April 2, 1958 [April 13, 1957], No. 10584/58. Class 38 (3). [Also in Group XXX] An induction motor driving a hoist is connected to sources of normal or low frequency in accordance with the position of a controller, the said position representing the hoist vehicle position modified in accordance with the velocity of the vehicle. The controller comprises carriages 10, 11 which are driven in opposite directions by sprocket 19 coupled to hoist drum 20 so that the position of a carriage corresponds to the position of the skip or cage, control devices for the driving motor being placed in the path of the carriage rather than in the hoist-shaft. A sprocket 22 is driven by a pilot motor which determines the deceleration programme of the vehicle whereby the positions of the carriages are advanced in proportion to the velocity of the hoist to compensate for changes in the velocity. Bars representing stopping-points of the vehicle are attached to the frame of the controller and, when the cage or skip is to be stopped at a particular level, a latch 27 prevents further motion of the carriage whereby sprocket 22 is driven by chain 16 for controlling the deceleration of the hoist. Contacts 28 are operated by the carriage to permit load-weighing so that the deceleration programme can be predetermined in accordance with the load. The controller is duplicated for co-operation with both the right-hand and lefthand hoist drums. The motor 41 driving the hoist is of the three-phase wound rotor induction type and is reversible when supplied at both normal and low frequency. A low-frequency generator 45 is driven by induction motor 46, and is provided with an exciter 47 which is rotatable at different speeds to afford a fourcycle or three-cycle output. A tapped transformer 50 enables the magnitude of the voltage derived from the generator 45 to be varied. A power meter 51 in the normal-frequency supply causes contacts LM1 to vibrate at a frequency determined by the load on the hoist motor, so that the number of pulses produced by the meter in a given period indicates the magnitude of the load. Before the hoist-brakes are released, torque is applied gradually, starting with a low-frequency, lowvoltage supply with maximum resistance in the rotor circuit of the motor 41, then shortcircuiting the rotor and increasing the voltage, whereupon the brakes are released. After a predetermined period, the pilot motor is started to operate contacts for removing the low-frequency supply from the hoist motor and connecting the normal-frequency supply. The short-circuit connection for the rotor is opened, and the rotor resistance is then reduced in timed steps until full speed is reached. Just before the point at which deceleration is due to commence is reached, contacts CCLL1 or CCLR1, are closed to measure the load and the final position of a stepping relay is determined by the number of pulses received during the load-measuring period. At the same time, the pilot motor, and a cam-shaft controller coupled thereto, is driven backwards by the sprocket 22, contactors for controlling the rotor resistance being under the joint control of the cam-shaft controller and the stepping relay. When the cam-shaft controller reaches a predetermined position, the low-frequency supply is substituted for the normal-frequency supply and all the rotor resistance is inserted. As the hoist travels further, the rotor resistance is reduced, until the rotor is short-circuited, whereupon the low-frequency voltage is increased. The hoist now continues at creep speed until limit switches open to apply the brakes and remove power from the driving motor.