Static Scherbius system of induction-motor speed control

The paper describes a control system in which a cycloconvertor is connected between the secondary winding of a 3-phase induction motor and the a.c. mains supply to give variable-speed subsynchronous and supersynchronous operation. Considerations of power flow in the secondary circuit show that for stable operation the cycloconvertor must be controlled from a frequency signal source synchronised to the secondary e.m.f. A secondary e.m.f. signal generator is described which produces the necessary control signal and provides a means of power-factor control. A simple model is proposed in which the cycloconvertor appears as a current source in the secondary circuit of the machine. Experimental results are presented showing smooth operation below, through and above synchronous speed with either driving or braking torques.