GB1246970A - Improvements in and relating to the control of synchronous machines - Google Patents

Abstract

1,246,970. Electric motors. ENGLISH ELECTRIC CO. Ltd. 13 Dec., 1968 [15 Dec., 1967], No.57210/67. Heading H2A. A control system, for a synchronous machine 11, comprises a rectifying thyristor bridge network 7 connected to an A.C. supply 9 and controlled by an input signal 15 and feeding an inverting thyristor bridge network 8 connected to the machine windings 10, the inverter being controlled by a logic circuit 21 itself controlled in dependence on the position of the machine shaft 18, the logic circuit being adapted during starting and low speed running periods firstly to override the input signal and interrupt the current flow in one half of the rectifier bridge, e.g. 1, 3, 5, while commutation is effected between two thyristors, e.g. 2, 4, in the corresponding half of the inverting bridge, and secondly, to activate during commutation a switch 28, 29 which completes, through the upper halves of the two bridges, an auxiliary conductive path including that conducting thyristor e.g. 3 in the inverter, not involved in the commutation. The logic circuit is controlled by a shaft position encoder 19 and amplifier 20, and during low-speed operation the logic circuit output feeds a memory circuit 22 (comprising two bi-stable circuits 22A, 22B) and a two-part servo control circuit 16 controlling the firing angles of the rectifying thyristors. One or the other of the two halves of the circuits 22 and 16 are rendered operational by the logic circuit depending on which half of the rectifying bridge has to be switched off. The operational half of memory 22 also opens one or the other of two thyristors 28, 29, thus completing an auxiliary conductive path through corresponding halves of the two bridges. The auxiliary path prevents notching in the current waveform of thyristor 3, say, while commutation is effected between 2 and 4. A zero detector 24 or 25 detects zero current in the appropriate half (1, 3, 5 in this case) of the rectifying bridge and resets the operational half of the memory circuit. Commutation is completed and thyristor 28 or 29 switched off. Above a specified shaft speed the output of a digital to analogue converter 31 causes a level detector 32 to block the bi-stables 22A, 22B. Natural commutation then occurs in the inverting bridge and shaft speed is controlled by the input signal 15 controlling the D.C. output of the rectifying bridge or by varying the excitation of field winding 12. If the shaft speed falls below a specified value the level detector 32 becomes ineffective and forced commutation of the inverting thyristors is again effected using circuits 21, 22 and 16. It is stated that provision may be made for producing a constant D.C. link current or for regenerative action in the two bridges or for bi-directional running of the machine.