GB704700A - Improvements in or relating to control systems for electric motors - Google Patents

Abstract

704,700. Control of A.C. motors. IGRANIC ELECTRIC CO., Ltd. Oct. 15, 1951 [Oct. 16, 1950], No. 23968/51. Class 38(3) A hoist motor 10 of the slip-ring induction type has a torque-resisting means 13 (which may be an eddy-current or powder metal clutch) coupled to its shaft, an exciting winding 13a being provided for the torque-resisting means. The motor is controlled by means of a drum 15 which regulates reversing contactors H and L,. and contactors 1A- 1D for varying the rotor resistance 16. The wind ing 13a is energized from supply-lines L2, L3, through transformer PT1, and, combined with the current-component so derived is one dependent upon the current in one of the motor primary phases which is obtained by means of transformer CT. The respective current-components applied to the winding 13a may be arranged to balance at zero speed, and at speeds higher than standstill (including speeds above synchronism) the energization of the coil will be substantially proportional to motor speed. A magnetic amplifier having a tuning resistor R2 may be placed between the rectifier 17 and coil 13a On the first lowering position of the master controller the entire output of the rectifier 17 is supplied to the winding 13a, and, as the controller is advanced in the lowering direction, relays 2L-5L are closed in succession to by-pass progressively increasing amount of the current through the brake coil. The brake may be used during hoisting operation to obtain creeping speeds, but at the higher speed notches contactor 1B closes contacts 18 to render the torque-resisting means ineffective. If the motor has a comparatively low power factor it may be necessary to provide an additional compensating component for the brake coil in order that the resultant speed responsive current supplied thereto may be zero when the motor is at standstill. Such a component can be derived through a transformer having its primary winding connected across supply-lines LI, L3, and its secondary winding connected in series with the secondary winding of the transformer PT1, Fig. 5 (not shown).