GB722744A - Improvements relating to electric motor control systems - Google Patents

Abstract

722,744. Control of A.C. motors; controllers. ENGLISH ELECTRIC CO., Ltd. June 19, 1953 [June 20, 1952]. No. 15617/52. Class 38 (3). A three-phase slip-ring induction motor 10 for driving a winder has its rotor liquid resistance controller 16 operated by oil actuator 17 which is regulated by a torque motor 19 under the control of a magnetic amplifier 20. During motoring and reverse current braking operation, control winding 20a is supplied from current transformer 21 in the stator circuit of the winder motor, whilst, during dynamic braking, the control winding is connected to current transformer 25 in the rotor circuit, so that in either case the energization of the winding 20a is dependent on motor torque. On power operation, reference winding 20b is supplied from potentiometer 28 which is connected across a constant potential source and in series with a variable resistor 29. The reference winding is connected for dynamic braking to a potentiometer 34 which is reversed relative to potentiometer 28. The sliders of the two potentiometers are adjusted by a driver's lever 35 or by cams driven by the winder. The variable resistance 29 is controlled by a device 36 having two inputs proportional respectively to the rotor resistance and to the ratio between rotor resistance and rotor frequency. The first input is obtained by means of a torque mo'tor 37 which is biased to the minimum position of resistance 29 and is supplied from potential transformer 38 through a potentiometer 39 adjusted by torque motor 40. A current transformer 41 energizing the motor 40 which is biased to the position in which minimum voltage is derived from potentiometer 39. The torque exerted by the motor 37 is thus proportional to the rotor resistance. The signal dependent upon the ratio between rotor resistance and frequency is obtained from a three-phase torque motor 42 which is supplied from current transformer 43 to set up a rotating field and from a potential transformer 44 which provides an alternating field. The position of the rotor of the torque motor 42 is thus dependent upon the power factor of the winder motor and upon the said ratio. The torque motor 37 is coupled through rod 36g to a lever 36a having a pivot 36b which is movable by a slotted cam 36f driven by the torque motor 42. A radius arm on the resistor 29 is coupled to the lower end of the lever by means of rod 36h. Increase of speed during motoring is provided by moving the control lever 35 to reduce the voltage derived from potentiometer 28 and the current in winding 20b, so that the actuator 17 moves the controller 16 to reduce the rotor resistance. The variable resistance 29 is then automatically adjusted to increase the excitation of winding 20b until the windings 20a, 20b, reach a new state of balance. The product of motor torque and resistance, as represented by the voltage derived from the current transformer 21 and the setting of resistor 29, respectively, will be maintained substantially at a fixed value. A fall in load decreases the excitation of winding 20a, so that actuator 17 increases the rotor resistance, the resistor 29 being adjusted to reduce the excitation of winding 20b until a balanced condition is again reached. Thus, when the load torque falls to zero, the liquid resistance controller reaches its maximum position and may then automatically actuate time delay contacts for instituting dynamic braking conditions. During dynamic braking operation, the winder speed is kept at the values set by the controller 35, and the liquid resistance controller again reaches the maximum position when the negative load torque decreases to a low value so allowing the winder to be changed over to power operation. Should failure of the dynamic braking equipment occur during a part of the wind other than the deceleration period, reverse current braking is introduced. The change-over of the reference quantity is obtained by connecting winding 20b to potentiometer 47.