Starting circuit for single phase electric motors

Claim 1

A time-responsive switching circuit for use with a single phase alternating current electric motor having a start winding and a run winding connected across a pair of first and second supply terminals, comprising: a solid state bilateral switching device adapted to be connected in series with said start winding, said device having first and second main terminals for connection respectively to said start winding and to said second supply terminal, and a control terminal connected through an impedance to said second main terminal, a full wave rectifier bridge having a pair of a.c. terminals and a pair of d.c. terminals, one a.c. terminal being connected to said control terminal of the solid state bilateral switching device, circuit means for connecting the other a.c. bridge terminal to said first supply terminal, an SCR connected across said d.c. bridge terminals to conduct current supplied by the rectifier bridge, the SCR having a gate, a charging circuit consisting of a capacitor and a resistor connected in series across said d.c. bridge terminals, the charging circuit having a time constant defining a predetermined time delay, a two-state trigger device connected between the common junction of the capacitor and resistor of the charging circuit and the gate of the SCR, said trigger device applying a trigger voltage to the gate of the SCR, and said trigger device being responsive to charging of the capacitor of said charging circuit to a predetermined level for disconnecting the trigger voltage from the gate of the SCR, thereby to render the SCR non-conducting and so switch off the solid state bilateral switching device.