Synchronous motors

Claim 1

A synchronous motor comprising: a stator winding; a rotor; an a.c. exciter winding mounted on the rotor; a field winding mounted on the rotor and arranged for excitation by the rectified output of the a.c. exciter, and a synchronizing arrangement for delaying energization of the field winding by the exciter until the rotor is rotating approximately at the same speed as the rotating field produced by the stator winding of the motor, and the rotor is correctly positioned relative to this rotating field, the synchronizing arrangement comprising: a pair of rectifying devices connected in parallel, oppositely poled, across the motor field winding, one of the devices being controllable and poled so as to be forward biassed by the rectified output of the a.c. exciter; trigger means for the controllable device whereby the controllable device is rendered conducting when the voltage across the field winding foward biasses said controllable device and exceeds a predetermined value; means for deriving two discrete signals respectively continuously representative of current flow in the two rectifying devices; and control means responsive to said two signals to connect the field winding with the rectified output of the a.c. exciter when either (a) no current has passed through the controllable rectifying device for a predetermined time, or (b) the motor field winding current, as indicated by the current in either rectifying device, has a frequency below a predetermined value, the controllable rectifying device is not conducting, and the uncontrolled rectifying device has started to conduct.