Patent ID: 8258732

Claim:
A drive system of a synchronous motor comprising: a three-phase synchronous motor; an inverter connected to the three-phase synchronous motor and having a plurality of switching elements; and a controller which selects two conducting phases from a three-phase winding of the three-phase synchronous motor and performs conduction control of the inverter in six conduction modes by a pulse width modulation operation, wherein the controller is provided with means of detecting at least one of a terminal potential of a non-conduction phase and a connecting point potential (neutral point potential) of a stator winding of the three-phase synchronous motor, sampling the detected value in synchronization with a conduction pulse of a conduction phase, and computing an absolute value of the sampled value and means of comparing the absolute value of the sampled value and a reference voltage in terms of level and sequentially switching the conduction mode in accordance with a result of the level comparison.