Abstract: A plugging control circuit in a silicon controlled rectifier (SCR) control for a series-connected direct-current vehicle-propulsion motor. The plugging control prevents the main SCR from pulsing when plug current is high, restarts the SCR pulsing when the plug current decays but with a reduced pulse width, and delays application of power through the main SCR for a time to allow the vehicle to first come to a halt before power is applied to move the vehicle in the opposite direction.

Abstract: An oscillator for use in a silicon controlled rectifier (SCR) control for direct current powered loads and for repeatedly gating on the main SCR of the control at an operator-selected frequency. Operation of the oscillator is delayed on start-up to avoid erratic operation due to contact bounce. The pulse frequency of the oscillator increases and decreases linearly at different settable rates even though the accelerator control is abruptly changed. The oscillator is automatically turned off in the event the load is to be disconnected from the power source so that the disconnection will not break current.

Abstract: A fault detector circuit in a silicon controlled rectifier (SCR) control for series-connected direct-current motors in which current to the motor flows in pulses through a main SCR as the main SCR is repeatedly turned on and off. If during operation the main SCR shorts, the field winding is disconnected from the armature and maintained disconnected. If during operation the main SCR is not defective but fails to commutate, the field winding is disconnected from the armature and then reconnected thereto. If the main SCR again fails to commutate, the field winding is again disconnected from the armature and is maintained disconnected therefrom.