Patent GB-726669-A (GB)
Title: Improvements relating to electric motor control systems
Inventor: DOLAN HARRY
IPC: B66B1/12
Filed: 1952-07-18
Abstract: 726,669. Automatic acceleration control systems. ENGLISH ELECTRIC CO., Ltd. July 17, 1953 [July 18, 1952], No. 18210/52. Class 38 (4). [Also in Group XXXVII] A three-phase winder motor of the slip-ring type is supplied for power operation through reversing contactors 12, 13 and for dynamic braking through contactors 14a, 14b from directcurrent sources 15a, 15b. Source 15a is of substantially constant voltage, whilst the voltage of source 15b varies with the rotor current of the motor. The rotor liquid resistance controller 16 is operated by an hydraulic actuator 17, the control valves 18 and the piston of which are coupled to a torque motor 19 which is biased to a neutral position. The torque motor is supplied from a magnetic amplifier 22 having a reference winding 22a and an opposing control winding 22b which is excited in accordance with the rate of change of speed of the winder from a direct-current tachometer generator 27 through a differentiating circuit comprising capacitor 28 and resistor 30. Actuation of a push-button to initiate forward operation closes contactor 12 and connects winding 22a to voltage divider 24 for energization in the appropriate direction. Since the winder is held at rest by the mechanical brakes, the output of the amplifier 22 is determined solely by the reference voltage so that the torque motor is energized to reduce the rotor resistance. At a predetermined value of load current, a relay 32, energized through current transformer 33, operates to release the mechanical brakes. The winder thus accelerates to full speed, as determined by the load torque, at a rate dependent upon the current in the reference winding of the magnetic amplifier, the rate of movement of the actuator 17 being such that the excitation of the winding 22b substantially balances that due to the winding 22a. When the winder approaches full speed, the current in the control winding 22b will decrease so that the controller 16 is held in the minimum resistance position, the full speed part of the wind being effected with no control of speed. When contacts 34 are opened by a winder-driven cam, a relay 35 is connected in circuit with the current transformer 33, but this relay picks up only if the load current of the winder motor is above a predetermined value. Assuming that the relay 35 does operate, the forward contactor 12 remains closed and the reference winding 22a is reversed, so that the torque motor 19 moves to increase the rotor resistance. Deceleration of the winder then follows at a controlled rate, dependent upon the relative excitation of the windings 22a, 22b. Towards the end of the wind, the rotor resistance controller 16 reaches its maximum position, a track-operated limit switch finally opening its contacts to apply the brakes and disconnect the motor. If, on the other hand, the relay 35 does not operate when connected in circuit with the current transformer, the forward contactor 12 is de-energized. As before, the winding 22a is reversed so that the controller 16 is moved towards the maximum resistance position, and, when this position is reached, a limit switch operates to energize dynamic braking contactors 14a, 14b. The input to the torque motor 19 is reversed relative to the magnetic amplifier 22 so that the controller 16 is moved to reduce the rotor resistance, the control winding 22b being again energized to maintain the rate of deceleration at the desired value. Graded rates of acceleration and deceleration may be obtained by means of a movable slider for the potential divider 24 operated by a winder-driven cam. The moment at which deceleration is initiated may be automatically varied in accordance with the load. For manual operation of the winder, the magnetic amplifier is disconnected from the torque motor and connected to solenoid 41 coupled to restriction valves 20; the winding 22a of the magnetic amplifier being rendered ineffective. A control lever 42 is coupled to the link 19b. The solenoid 41 exerts a pull on restriction valves 20 in accordance with the energization of winding 22b so that, when the acceleration limit is reached, the valves 20 block the exhaust passages of actuator 17 and further reduction of rotor resistance is prevented. In the case of winders driven by a direct-current motor, the speed regulator may comprise a field regulator in circuit with an exciter for a variable voltage generator which supplies the winder motor.