Patent GB-669924-A (GB)
Title: Improvements relating to electric lifts or elevators
IPC: B66B1/36
Filed: 1949-06-02
Abstract: 669,924. Control of D.C. motors: WADSWORTH & SONS, Ltd., W., KERSE, W. H., and RYDER, A. D. May 30, 1950. [June 2, 1949] No. 14811/49. Class 38 (iii). [Also in Group XXXV] A control system for an electric lift comprises means, for controlling the speed of the lift motor in accordance with a voltage applied to said means, a mechanically operated rheostat positively driven in unison with the lift car for accelerating and decelerating the car, means for providing a voltage depending on the speed of the lift motor, the difference between this voltage and the voltage derived from the rheostat being applied to the speed control means and means for applying to the speed control means when required, a voltage depending in an adjustable manner on the lift 'motor torque. As shown, the lift motor g is supplied by a generator i driven by a three phase A.C. motor k, the field for the motor g being supplied from a rectifier or rotary converter m. A rheostat a is connected across a constant voltage source b and has contacts actuated by a camshaft as. in Specification 402,111 to provide voltage dependent on the speed and/or position of the lift car in the shaft.. When the car is at rest the rheostat contacts and contact c are open, and contacts d, e closed, and when the "up" contactor is energized, contact d opens and the lift moves, closing contact c and the rheostat contacts in succession, a voltage equal to the difference between the voltage across the rheostat and that of a tachometer generator h driven by the lift motor, being applied to the grid of a valve V1, the rheostat voltage increasing as the contacts are closed. The anode current of Vl flows through the winding n of a saturable reactor and varies the inductance of winding o, which is connected to the'grid and cathode of a gas-filled valve V2, and also through a resistor p to the secondary of a transformer T1 whose primary is supplied with A.C. A further transformer T2 connected to the same A.C. source supplies the anode of V2, the rectified' anode current passing through the field winding of the generator i driving the lift motor. Winding o and resistor p form a circuit in which the phase of the alternating voltage applied to the grid of V2 from transformer T1 is varied in relation to the alternating voltage at the anode of V2, and is dependent, on the voltage at the grid of V1 In this way the mean excitation of the field of the generator i supplying the lift motor is varied, any depression of speed due to a load thrown on the motor resulting in an increase of voltage on the grid of V1 and a corresponding increase in the motor input, and on light load an increase of the motor speed resulting in a decrease in the motor'input the motor tending to maintain a speed at which the tachometer voltage almost equals the rheostat, voltage. When the car reaches a floor a a switch in the shaft causes the camshaft to open the theostat contacts successively to slow down the lift, and finally to open switch c, the voltage across a variable resistor f in the supply circuit to the armature of motor g then being added to the rehostat voltage to counteract the effect of the load when the brake is applied and the motor de-energized, and assist in levelling the lift. When the "up" contactor is de-energized the contact d, closes, and when the "down" contactor is energized contact e opens.