Abstract: The present invention provides a digital circuit for controlling application of DC excitation to the rotor field winding of a synchronous electric motor. The circuit detects the analog slip frequency of the motor and converts the analog signal to a digital signal. A digital timer produces a timing signal after a predetermined time interval. After the frequency of the slip signal has slowed sufficiently, as indicated by the timing signal, a control signal is supplied to a thyristor to gate the thyristor and provide DC excitation to the rotor field winding.