GB418649A - Improvements in and relating to electric motor control systems - Google Patents

Abstract

418,649. Control of A.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Ewing, S. C. ; 1733, Avenue H, Schenectady, New York, U.S.A.) Jan. 23, 1933, No. 2190. Void [Abridged as open to inspection under Sect. 91 of the Acts, July 24, 1933]. [Class 38 (iii).] The Specification describes a system for closing the field excitation circuit of a synchronous motor when synchronous speed is reached in starting, and for opening the excitation circuit and permitting the motor to run synchronously when it falls out of synchronism. In the arrangement of Fig. 1, the motor armature 2 is fed through a full voltage starter 4 from a supply 5, and the circuit of the field 3 is controlled by switches 13, 14 having series connected operating coils 15, 16 for connecting the field across either a discharge resister 11 or a source of direct current 12. Speed responsive means comprising a relay 17 connected through a rectifier 18 across the resister 11 as described in Specification 384,567, and an overload relay 23 connected to the motor armature circuit for opening a locking circuit of the operating coils 15, 16 are also provided. In operation, closure of auxiliary contacts 29 on the starter 4 completes a control circuit through contacts 26 on a relay 22, and contacts 19 on relay 17, for the operating coil of a relay 21 which closes its contacts 30 included in an energizing circuit of switches 13, 14. When the motor attains approximately synchronous speed, relay 17 closes its contacts 20, and completes the energizing circuit of switches 12, 14 which then operate to connect the field across the supply 12, and to complete a locking circuit for coils 15, 16 through the relay 23. In order to maintain the energizing circuit of the coils 15, 16 in the event of relay 23 operating when the motor pulls into synchronism, relay 21 maintains its contacts 30 closed for a predetermined time after relay 17 opens its contacts 18, 19. Reconnection of the field 3 to its source 12, after the machine has fallen out of synchronism, is delayed by constructing the relay 22 so that it maintains its contacts 26 open for a predetermined time after its winding 22 is deenergized. Fig. 2 shows a modified arrangement in which the running connections of the field can still be made should contacts 20 close before the contacts 26. In both arrangements, a single two-position switch may replace switches 13, 14.