AC UNINTERRUPTIBLE POWER SUPPLY

PURPOSE:To continue feeding without momentary interruption and without generation of a cross current between power sources, by outputting a control signal being synchronous with an output voltage of either a phase difference detecting circuit or a current state detecting circuit when the latter detects a discontinuous state of a load current even when the former detects a phase difference. CONSTITUTION:When a rectifier load is connected, a phase difference detecting circuit 102 judges that there is a phase difference between a voltage V and a current i0, Since a duty ratio is smaller than 50%, however, a made switching circuit continues to output a voltage mode to a gate signal generating circuit on the basis of a detection signal from a duty ratio detecting circuit 104. When a power source switching instruction is outputted from a power source switching determination circuit 103 at t1, therefore, a disadvantageous cross current is not generated between a commercial AC power source 1 and an inverter AC power source 2 during a period T4. Since thyristors A to D of an AC switch are controlled by gate signals Sa to Sd being synchronous with a voltage phase signal of a source voltage, besides, the waveform of an output voltage E is not distorted.