PURPOSE: To protect a motor ahainst a flashover by breaking a motor circuit when the direction of a current is inverted due to the variation in the voltage of a trolley line.

CONSTITUTION: Since the output VO of a current detector CD becomes negative when the voltage ES of a trolley line abruptly drops at a power driving time so as to become lower than the induced voltage EM so that an armature current IM becomes negative to become a current flowed out to the trolley line, the first discriminating means 1 detects the inverted current and outputs a breakdown signal. Since the output VO of the detector DC becomes negative when the voltage ES of the trolley line abruptly rises at the braking time to become higher than the induced voltage EM so that the armature current IM becomes positive and becomes a current flowed in from the trolley line, the second discriminating means 2 detects the inverted current and outputs a breakdown signal. In this manner, the motor can be protected from a flashover.