Control circuits for electrically driven vehicles

Claim 1

A control circuit for an electrically driven vehicle comprising in combination a traction motor driving the vehicle, a thyristor chopper circuit controlling the speed of the traction motor, said chopper circuit including a main thyristor in series with the motor, and a commutating thyristor which when fired turns off the main thyristor, demand means controlling the instants of firing of the thyristors to regulate the mean current flow in the motor, said demand means setting a required current through said motor and firing the commutating thyristor when said required current is attained, said demand means including brake selection means arranged to operate the motor in a braking mode as well as in a normal drive mode, an overriding control permitting firing of said commutating thyristor when the main thyristor has been on for a predetermined time, timing means operable when the overriding control is operative for setting the minimum off period of the main thyristor, said timing means serving when the overriding control is operative to increase the normal minimum off period of said main thyristor, means for setting the normal minimum on and off period of the main thyristor, said timing means operating to increase the normal minimum off period only when the motor is being used to provide electrical braking.