Patent US 4,134,048

A wheel slip correction system of the type in which boost current is supplied to the field winding of a d-c electric traction motor driving a slipping wheel from an a-c source in order to correct the slip is provided with a control system which automatically inhibits the application of boost current if control power fails. A current transformer connected to provide power to the field winding from the a-c source has a secondary winding normally short-circuited by inversely paralleled thyristors thus inhibiting power transfer to the field winding. The thyristors are maintained short-circuited by a current amplifier connected across the transformer secondary and adapted to normally supply current to the gate terminals of the thyristors. The current amplifier is inhibited by receipt of a wheel-slip signal so that the thyristors remain non-conductive and boost current is supplied to the motor field. In a preferred embodiment, low voltage wheel slip detection circuits are isolated from the current amplifier through the use of optical couplers.