GB456729A - Improvements in or relating to systems of control for electrically operated lifts - Google Patents

Abstract

456,729. Control of A.C. motors. EXPRESS LIFT CO., Ltd., Greycoat Street, Westminster, and STEVENS, R. J., 22, Millway, Duston, Northamptonshire. May 8, 1935, No. 13587. [Class 38 (iii)] The degree of energization of an eddy-current brake for decelerating an electrically driven lift, is controlled in accordance with the current supplied to the lift-driving motor during decleration. Upon operation of a switch SU, self maintaining relays B, U, are energized, the former shortcircuiting secondary circuit resistances ZF, ZG, ZH, of the driving motor ST, RO, and opening the circuit of one coil of an eddy-current brake X, and the latter connecting the motor to the supply. The relay B also shortcircuits a resistance ZA which is in one phase of the motor supply, and across which a potential is developed controlling the grid of a triode TR through which a second coil of the eddy-current brake is energized. A friction brake K is released and the motor starts. As it gains speed, a relay T, energized from a transformer Q connected to the motor secondary circuit, operates to energize a relay R. This relay inserts resistance ZJ in the circuit of the coil of the relay T and energizes a relay V which short-circuits further secondary existances ZC, ZD, ZE. The driving motor now runs up to full speed. As the lift approaches a floor at which stoppage is required the switch SU is opened, and, when an inductor relay W is operated by a zoning plate, the relay B is de-energized. Thus relays V, R, are de-energized, all secondary resistance is inserted, a coil of the eddy-current brake is energized and the impedance ZA is inserted in the primary circuit of the motor. The lift begins to slow down, and its primary current, which is a measure of the load on the motor, produces a corresponding controlling voltage on the grid of the triode TR. Thus the current flowing through the triode to the second coil of the eddy-current brake is dependent on the load of the motor so that the braking effect is modified and the deceleration-time curve of the lift rendered independent of the motor load. The lift speed reaches a low value, and, when the floor is reached, an inductor relay S is operated to de-energize the relay U to disconnect the motor and cause a friction brake K to be applied bringing the motor to standstill. In a modification, Fig. 4, not shown the impedance ZA and triode TR are replaced by a current transformer supplying an eddycurrent brake coil through a rectifier. The system is applicable to automatically-started lifts operating on the " collector " principle.