PURPOSE:To cut out an accident current when cutting out the current of a short-circuit accident occurred at the primary side of a wound-rotor induction machine by turning on/off a switching element in response to the value of secondary current, superimposing a DC component to the secondary current and creating a zero cross point for the primary current. CONSTITUTION:When a U-phase secondary current becomes larger than zero, the output of an AND circuit 46 becomes '1' and GTO 15 is turned on. At this time, the secondary current flows from the secondary U-phase to GTO15 diode 10 returning to the secondary V-phase thereby acting in the direction for increasing the secondary current, so that U-phase current is shifted to the positive direction. Then, when the U-phase secondary current reaches the value of GTO controllable current, then U-phase GTO ON signal becomes '0' and GTO 15 is turned off. The U-phase secondary current and W-phase secondary current are shifted to the positive side, and the V-phase secondary current is shifted to the negative side. In this way, a zero cross point can be created in the primary current by superimposing a DC component to the secondary current of a wound-rotor induction machine 1.
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