Patent ID: 8045309

Claim:
A magnetic energy recovery switch having a protective circuit and interposed between an AC power supply and a load to protect, against an overvoltage or an overcurrent, a magnetic energy recovery switch with a variable reactance function for turning ON/OFF of the AC current and changing a phase thereof, the magnetic energy recovery switch comprising: a bridge circuit constructed from four reverse-conductive type semiconductor switches; a capacitor connected between DC terminals of the bridge circuit, recovering and storing magnetic energy at cut off of a current by repetitiously changing voltage from substantially zero to a peak voltage at twice as much frequency of the AC current in normal condition; and control means; wherein the protective circuit comprises: a voltage detection unit connected in parallel with the capacitor and detecting a voltage of the capacitor; a discharge circuit connected in parallel with the capacitor and having a discharge resistor and a discharge switch connected in series; and a current detection unit interposed between the AC power supply and the load and detecting a current flowing to the load; wherein the control means controls a phase of a gate control signal such that each of the four reverse-conductive type semiconductor switches is controlled so as to be paired with diagonally opposed one, such that the diagonally opposed pair of reverse-conductive type semiconductor switches are simultaneously turned ON/OFF in synchronization with the voltage of the AC power supply; controls the gate of the discharge switch so as to short-circuit the discharge switch to discharge the electric charge of the capacitor through the discharge resistor when an output of the voltage detection unit exceeds a predetermined value; and controls to limit a current by making the duty ratio of the ON/OFF of the pulse of the control signals smaller than 0.5 when the output of the current detection unit exceeds a predetermined value.