1492861 Exciting dynamo electric machines; starting prime movers GENERAL ELECTRIC CO 7 Feb 1975 [7 Feb 1974] 5299/75 Heading H2A and H2H An excitation system for a dynamo-electric machine (D.E.M) comprises an exciter mounted on the same shaft as the D.E.M, the exciter having its stationary field windings selectively connected in a polyphase connection or a series connection and correspondingly energized by a polyphase A.C. signal or a D.C. signal, the exciter armature output being rectified to energize the rotary field winding of the D.E.M. According to the connection and supply, the exciter, and D.E.M. act as rotating transformer and D.C. brushless motor respectively or both as synchronous generators. The arrangement is used to drive an aircraft jet engine up to starting speed and thereafter as an A.C. power supply. Initially, with the common rotor shaft 15 (Fig. 1) stationary, switch 23 connects the exciter field windings in a star-connection and applies a three-phase A.C. input thereto, causing a rotating flux field to induce A.C. in the exciter armature winding. The output thereof is rectified at 19 to supply field excitation for the machine 14 which operates as a D.C. brushless motor, the stator winding being supplied with current depending on the rotor position. Thus, the shaft 15 rotates and accelerates up to the ignition speed of a jet engine mounted on the shaft 15. At this speed, the sensor 31 opens switch 24, closes switch 25 and operates switch 23 to reconnect the exciter field windings 20, 21, 22 in series. A permanent magnet generator 26 with its rotor 27 also mounted on the shaft 15 provides an A.C. signal which is rectified at 29 to provide D.C. excitation for the exciter field and the exciter field now operates as a synchronous generator. The D.E.M. 14 now also operates as a synchronous generator, the jet engine acting as prime mover. Fig. 3 shows the reconnection of the windings 20-22 and the input switches 24, 25. At start up, switch 60 is momentarily closed setting bistable 58 such that its Q output provides a positive gate voltage to thyristor pairs Q 1 -Q 2 , Q 3 -Q 4 , Q 5 -Q 6 . These interconnect the 3 phase supply terminals 42 and the windings 20-22. Bistable 58 also supplies a gate voltage to thyristors Q 7 -Q 8 , which ensures a star-connection of the windings since thyristors Q9, Q 10 are blocked. As the shaft speed reaches the predetermined ignition speed, control 63 produces a pulse which resets bistable 58, blocking thyristors Q 1 -Q 8 and setting bistable 59 thereby enabling Q 9 , Q 10 . This provides disconnection of the A.C. supply, and reconnection of the windings 20-22 in series between the D.C. supply terminal 64 and ground 55.
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