In the differential protection device of piezoelectric motor, the current with the same direction and equal size at both ends of the generator is called through current, while the current with opposite direction is called non through current. As the main protection, the ratio restraint differential protection of generator takes the non through current as the action quantity and the through current as the braking value to distinguish the normal state, fault state and abnormal operation state of the protected components. Under normal operation condition, the through current is the load current, and the non through current theory is zero. In the state of internal phase to phase short circuit, the non through current increases sharply.