GB465435A - Improvements in and relating to motor control systems - Google Patents

Abstract

465,435. Control of A.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. May 20, 1936, No. 14323. Convention date, May 22, 1935. [Class 38 (iii)] In starting a synchronous motor 1 having a field winding 3 provided with a source of D.C. excitation 8, the D.C. source is connected to the field winding at the correct moment in the slip cycle of induced field current by means responsive to a half cycle of induced current through the field winding of a predetermined direction and predetermined duration, which effects the connection of the D.C. source to the field winding in response to the next half cycle of induced field current of the predetermined direction. In the full voltage starting arrangement shown, the armature 2 is connected directly across the supply 5 by closing a switch 4, the field winding 3 being at first shortcircuited through a discharge resistance 6. The connection of the winding 3 to the source 8 is controlled by a field switch 7 having an operating winding 9, the energization of which is controlled by relays 10 and 12. The relay 10 is connected in series with a rectifier 11 across part or all of the resistance 6. As long as the motor is operating well below synchronous speed, the induced current in the field winding 3 causes the relay to pick up and close its contacts 19. When a predetermined subsynchronous speed is reached, the frequency of the induced current becomes so low that the half cycle 'during which no current flows through the relay.winding is long enough to allow the relay to return to its normal position, in which contacts 14 are closed. Each time it does so, it initiates the operation of a time relay 12, which does not operate until its winding has been energized for a predetermined time. The relay 12 is provided with contacts 15 which are connected in an energizing circuit for the winding 9 and are closed when the relay 10 maintains its contacts 14 closed for a sufficient time interval, after the motor has been started, to allow the relay 12 to operate. In order to allow a sufficient time for the relay 10 to pick up after the motor has been started, a relay 16 is provided, with contacts 17 connected in series with the contacts 14 and winding of relay 12. Relay 16 does not operate until contacts 19 are closed, and on operation it closes a hold-on circuit for itself through contacts 24. When relay 12 operates it completes a hold-on circuit for itself through contacts 25 and auxiliary contacts 26 on the field switch 7, and at the same time opens the energizing circuit of relay 16 at contacts 23. Relay 16, however, does not fall out until after a predetermined interval, which is sufficient to allow the relay 10 to be picked up by the next half cycle of induced field current. By closing its contacts 15, relay 12 prepares an energizing circuit for the winding 9, including the contacts 18 of the relay 16, which are now closed, and the next time the relay 10 closes its contacts 14 the operating winding 9 is energized to move the switch 7 and connect the source of energization to the field winding. Auxiliary contacts 27 on the field switch 7 complete a maintaining circuit for the winding 9,the holding circuit for the relay 12 being at the same time opened at the contacts 26. Specification 384,567 is referred to.