Patent GB-482463-A (GB)
Title: Improvements in and relating to synchronous electric motors
IPC: H02P1/46
Filed: 1936-11-05
Abstract: 482,463. Master control of A.C. motors ; electricity supply. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 5, 1936, No. 30161. Convention date, Nov. 6, 1935. [Class 38 (iii)] [Also in Group XXXV] A synchronous motor is brought to synchronous speed by operating the motor with its field winding short-circuited through a discharge resistor, then increasing the effective resistance of the field winding discharge circuit at a predetermined motor speed and at a predetermined point in the cycle of slip current and then supplying direct current to the field winding at a subsequent predetermined point in the cycle of slip current. The maximum pull-in torque is obtained when the load angle is substantially zero and the alternating current flux in the field poles of the motor is at its maximum value and is in the same direction as the flux produced therein by the direct current flowing through the associated field winding during synchronous operation, load angle being defined as the electrical angle between the axis of any pair of field poles of the motor and the corresponding pair of a synchronous machine generating a no-load voltage equal to and in phase with the terminal voltage applied to the motor. To obtain the best induction motor torque as well as the best pull-in torque, a field shortcircuiting resistance 6 of five to twenty times the resistance of the field winding 3 of the motor 1 is employed so proportioned that the reactance and resistance of the field discharge circuit are equal at the maximum sub-synchronous speed obtainable by induction motor action. When the motor 1 is started, the field winding 3 is short circuited by the low resistance 6 and the high resistance 24, and the relay 12, in series with the half-wave rectifier 13 and connected across the resistance 6, is energized. This energizes the relays 14, 15 which energize the relay 20, but the relay 12 remains energized through its hold-in circuit at contacts 23. When a predetermined sub-synchronous speed is reached, the resistance 6 is cut out of circuit, leaving across the winding 3 the resistance 24 which is of high value and has an inverse potential impedance characteristic. This is effected while the flux in the field poles is building up in the positive direction and the flux increases rapidly to substantially the maximum possible value by the time the load angle is zero, whereupon the direct current source is connected to the field winding. When the frequency of the induced current in the winding 3 becomes sufficiently low, the relay 12 which has a timedelayed return is de-energized and the contacts 23 are broken and contacts 16 made by the time the induced current has decreased to zero from its positive value. After a time determined by the setting of time-delay of the relay 14, this relay falls and closes contacts 17 thereby energizing relay 10 which cuts out resistance 6 and at the end of a further time determined by setting of relay 15, this relay also falls to close contacts 18 and energize relay 11 whereby the field winding 3 is connected to the supply 8. The contacts 21 are normally closed but the circuit of the relay 20 is interrupted by the relays 14, 15 and after a time delay the relay falls to close the contacts 22 so as to re-establish the original condition for the circuit of the relay 12. Specifications 292,110, [Class 37], and 384,567 are referred to.