Patent GB-568549-A (GB)
Title: Improvements in or relating to controllers for synchronous dynamo-electric machines
IPC: H02J3/42
Filed: 1943-06-29
Abstract: 568,549. Electricity supply. IGRANIC ELECTRIC CO., Ltd. June 29, 1943. No. 10519. Convention date, July 3, 1942. [Class 3S (iv)] [Also in Group XXXVII] Synchronizing.-A synchronous machine 1 having a salient pole field and a damper winding is started as an induction motor with its field winding 1b shunted by a discharge resistance 17, and as synchronous speed is approached a frequency-responsive relay 11. disconnects the discharge resistance and applies D.C. excitation to the field winding. The energizing circuit of the relay comprises transformers 5, 6, the primary windings 5a, 6a of which are either connected directly in series with the A.C. windings 1a of the motor and the phases L2, L1, or supplied, as shown, through current transformers 3, 4 so as to carry currents proportional to the phase currents. The secondary windings 5b, 6b feed the energizing coil 11a of the relay in series, with phase reversal of one winding. Winding 6b is shunted by a resistance 10, and winding 5b by condenser 7, choke 8 and resistance 9 in series, the resistance 9 being one-half the value of resistance 10 which is preferably of the same value as the impedance of the circuit 7, 8, 9 at line frequency. The machine is started by depressing a button 13 which energizes the coil 2a of a line contactor 2, thereby connecting the machine to the phases L1-L3 over contacts 2b, 2c, 2d and establishing a, holding circuit over contacts 2e ; the flow of line current causes relay 11 to actuate, thereby opening at contacts 11b, 28 the energizing coil circuit of a field contactor 16 having back contacts 16d which connect the discharge resistance 17 across the field winding 16. As synchronous speed is noared, the current through relay coil 11a falls, the relay eventually dropping out. at a setting determined by a variable resistance 12 shunting the coil, and closing contacts 11b, 28. whereupon the field contactor becomes energized, disconnects the discharge resistance and connects the field winding to D.C. supply lines L11, L12 over contacts 16b, 16c. Should the machine pull out, relay 11 picks up again and attempts re-synchronization, but continued attempts are prevented by a thermal switch 15. The relay comprises a U-shaped laminated core 20. Fig. 6, having the coil 11a, mounted on one limb 21 and a main armature 22 pivoted at 23 to the other limb 24 and normally biassed away from limb 21 by a spring 25, this armature carrying the movable contact 28 which bridges the fixed contacts 11b when the armature is unattracted. A shortcircuited secondary winding 29 embraces the core and acts upon an auxiliary armature 30 pivoted at 31 and normally biassed into the dotted position by springs 32, in which position an abutment 34 of the auxiliary armature engages an abutment 35 on an extension 26 of the main armature and prevents release bf the latter from its attracted position. Armature 30 has a very low natural period of vibration to prevent it oscillating when the frequency of the energizing current in coil 11a is low. The action of the relay is as follows : on depressing the starting button, the heavy line current of the machine results in a large E.M.F. of line frequency across coil 11a and a heavy induced current in the secondary winding 29 ; both armatures are, therefore, attractedopening contacts 11b, 28-and remain so until one-half synchronous speed is reached when the current supplied by the energizing circuit of coil 11a becomes small and of zero frequency. Armature 30 then drops out, but the main armature is prevented from doing so by the abutment 34 now engaging abutment 35. As the machine nears synchronous speed, the frequency of the coil - exciting current approaches line frequency, causing re-attraction of the auxiliary armature and unlatching of the main armature which then drops out and closes contacts 11b, 28.