Patent GB-807642-A (GB)
Title: Improvements in or relating to the control of electric motors
IPC: B66C13/26
Filed: 1954-03-16
Abstract: 807,642. Control of A.C. motors. WEST & CO. Ltd., and STACEY, J.R. June 8, 1955 [March 16, 1954], No. 7602/54. Class 38 (3). A hoist is driven by a slipring induction motor M having a control drum D with a setting for driving the load in the lowering direction and other positions in which dynamic braking circuits for controlling the descent of the load are established. An electro-mechanical brake Bw is adapted to be applied a time interval after the drum has been moved from one setting to the other if the power or braking circuits should fail to be operative. A relay VR1w is energized when the power circuit L1, L2, L3 for the motor is operative, a relay CRw being effective if the dynamic braking circuit is closed, and these relays have respective contacts VR1C2, CRC2 in series with a contactor coil IVw controlling the brake Bw. Thus, when the controller is moved from one setting to the other, one of the relays VR1w, CRw will fall out before the other is pulled in thereby interrupting the normal energization circuit of the contactor IVw which is maintained energized, however, to hold off the brake for a period determined by the rate of discharge of a capacitor C2. Normally, when the controller is returned from a power lowering notch to the " off position, the circuit of winding IVw is opened at contact 12, the power contactor II is opened and simultaneously the contactor III is closed, thereby applying direct-current braking with the first section only of the rotor resistance short-circuited by closure of contactor V. When the speed has fallen to a predetermined value (as detected by the holding coil VR2W2 of a rotor current or voltage-responsive relay) contactor VI closes to remove another section of rotor resistance. A further fall of speed is detected by relay coil VR3W2 to close contactor VII. As soon as this contactor closes, a second circuit is established across capacitor C2 through contact VIIC3, thus expediting the application of the electro-mechanical brake. The dynamic braking circuit is maintained for a given time interval after the application of brake Bw, this interval being dependent upon the dropping out of a relay the coil 1R1w of which is shunted by capacitor Cl. Whilst the electro-mechanical brake is not normally applied until the motor speed has been reduced, if the direct braking current should fail, the application of the brake will still be effective after a short delay, when the capacitor C2 has finished discharging. During the early part of the return movement of the controller, contactor XI is closed to increase the direct braking current. When the controller is moved quikly from the lowering side to the hoisting side for plugging, a short delay elapses while the relay coil 1R1w) is maintained energized by capacitor C1 before contactors VI-X are closed to remove resistance from the rotor circuit of the motor, so preventing excessive torque and current during the plugging operation. In a modification counter-current braking operation replaces the direct-current braking operation referred to previously. The sequential removal of rotor resistance is controlled in some positions of the control drum by a timing relay TR comprising upper and lower rows of contacts and a bar TRB which is driven by a pilot motor (not shown) so as alternatively, at timed intervals, to bridge the upper and lower rows of contacts. The pilot motor is effective when one of the power contactors I, II is closed. Specification 807,641 is referred to.