A voltage detecting circuit includes a rectifying circuit, a voltage dividing circuit, and a comparing circuit. The rectifying circuit is configured to rectify a plurality of AC phase voltages to output a plurality of rectified voltages respectively. The voltage dividing circuit is configured to divide the plurality of rectified voltages respectively to output a plurality of sampling voltages. The comparing circuit is configured to compare the plurality of sampling voltages with a reference voltage respectively to provide a plurality of corresponding phase failure detecting voltages. On the condition that the AC phase voltages are unbalanced, the phase failure detecting voltage switches between a high level and a low level.