Exciter arrangement for synchronous motors

Claim 1

A synchronous machine of the alternating current exciter type comprising a stator having a polyphase winding thereon for connection to a polyphase power supply source, a rotor, said rotor including thereon a field winding, a resistor connected in parallel with said field winding, and a polyphase rectifier bridge of the semiconductor type having the direct current output thereof connected to said field winding, the semiconductors in one half of said bridge being constituted by diodes and the semiconductors in the other half of said bridge being constituted by thyristors, an exciter alternator comprising a stationary field coil and a polyphase exciter winding on said rotor connected to the input of said rectifier bridge, said thyristors being blocked during asynchronous speeds of said machine such as during starting to prevent flow of any output current from said bridge to said field winding, and means to unblock said thyristors including an alternating current generator producing a voltage having a frequency substantially higher than the frequency of the voltage generated by said exciter alternator, a transformer, a gate connected between the output of said higher frequency generator and the primary oF said transformer, said transformer including a secondary on said rotor and which is connected to the control electrodes of all of said thyristors, and means responsive upon attainment of near synchronous speed to actuate said gate and thereby connect the output of said higher frequency generator to said transformer and application of the output from the secondary thereof continuously to said control electrodes of said thyristors to effect a corresponding continuous unblocking thereof and hence an unblocking of said bridge thereby to establish a flow of direct current to said field winding powered by the output from said exciter alternator.