GB733493A - Improvements in and relating to alternating current electric motor equipments - Google Patents

Abstract

733,493. Control of A.C. motors. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Sept. 30, 1953 [Oct. 8, 1952], No. 25253/52. Class 38 (3). An induction motor 1 is braked by connecting its stator winding to a direct-current source whilst the circuit of the slip-ring rotor winding 2 is closed through a variable resistance 8. A servo-motor 10 for moving the electrodes of the braking resistance is supplied from an amplifier 11. A transformer has its primary winding 12 energized in accordance with the rotor voltage, its secondary winding being connected through inductance 15 and rectifier 14 to a load resistor 16. A current transformer 17 is connected in one of the rotor leads and has its secondary winding connected by means of rectifier 18 with a load resistor 19. When the voltage across the resistor 16 exceeds that derived from the resistor 19, the input to the amplifier 11 is such that the braking resistance is reduced, whereas, when the rotor current increases with relation to the rotor voltage, the braking resistance is increased. The inductance 15 causes the current supplied to the resistor 16 also to vary inversely as the rotor frequency so that the braking resistance is automatically controlled to a value which is substantially proportional to the motor speed. The braking resistance may, however, be controlled as a non-linear function of the speed by means of appropriate impedance networks in the circuits associated with the voltage and current transformers. In another arrangement, the inductance 15 is omitted and a capacitor connected in series with the secondary winding of the current transformer. The voltage across the resistor 19 is thus proportional both to the rotor current and frequency, Fig. 2 (not shown). In a further modification, an inductance connected across the secondary winding of the current transformer provides a voltage proportional to the rotor current and speed, Fig. 3 (not shown). The resistor 16 may be connected as a potentiometer with a movable contact which is adjusted by a centrifugal governor driven by the motor, Fig. 4 (not shown). The amplifier 11 may be of the phase-sensitive type with its input circuit including means responsive to rotor voltage in series opposition with the secondary winding of a quadrature transformer having a primary winding energized in accordance with the braking current, the output of the quadrature transformer being proportional to the rotor frequency, Fig. 5 (not shown). The braking resistance may be of the contactor-controlled metallic type, the contactors being regulated by a sequence switch.