GB724661A - Improvements in or relating to arrangements for the operation of three-phase motors with an auxiliary frequency lower than the normal operating frequency - Google Patents

Abstract

724,661. Automatic speed-control systems. SIEMENS-SCHUCKERTWERKE AKT.-GES. Jan. 22, 1953 [Jan. 22, 1952], No. 1893/53. Class 38 (4). [Also in Group XXXVII] In a three-phase-hoist motor-control system of the kind using a low auxiliary frequency during, e.g., braking and stopping, the excitation of the motor at the auxiliary frequency is automatically controlled to maintain a predetermined speed. As shown, a three-phase wound rotor induction motor 1 may be supplied from a main supply at, e.g., 50 cycles through " forward " or " reverse " switches 2, 3, or it may be supplied from an auxiliary supply at, e.g., 2 c/s. through " forward " or " reverse " switches 4, 5, and a motor-operated resistor 6 for varying the excitation. The hoist-motor 1 drives a tachometer-generator 14 having a three-phase field rotating in the opposite direction to the rotor, whereby the rotor output voltage varies with the speed, and there is a high output at synchronous speed. The output of the tachometer 14 is rectified at 21 and applied to the resistance-operating servomotor 28, in opposition to a reference voltage, from rectifier 22, the magnitude of which is preset by a contact arm 23 co-operating with resistors 24, 25 and mechanically coupled to the hand-control lever (not shown). The direction of rotation of the servomotor is controlled by a relay 20 according to whether the motor is hoisting, or braking an overhauling load (i.e. according to the tachometer output indicating that the motor is below or above synchronous speed) so that resistor 6 is varied in the correct direction to maintain constant speed.