Protective circuit for brushless synchronous motors

Claim 1

In a synchronous motor having a rotating field winding, an alternating current exciter having an armature winding rotatable with said motor field winding, and rectifier means connected to said exciter armature winding and rotatable therewith for supplying direct current excitation to the motor field winding, a control system for the motor field winding including solid-state exciter switch means connected between said rectifier means and the motor field winding to control said direct current excitation, said switch means normally being non-conductive during operation of the motor at sub-synchronous speeds, a discharge resistor connected across the motor field winding, solid state means connected in series with said resistor for permitting current to flow in the resistor only in response to voltage induced in the field winding during sub-synchronous operation of the motor, said exciter switch means being connected in the field winding circuit between the rectifier means and the discharge resistor, and short-circuiting means connected across the output of the rectifier means between the exciter switch means and the rectifier means, said short-circuiting means being adapted to shunt the rectifier means in response to conduction of the exciter switch means during sub-synchronous operation of the motor.