Drive system for synchronous motor

Provided is a position-sensorless drive method, wherein control of the rotational speed and torque of a permanent-magnet motor is executed by driving the motor with an ideal sinusoidal-wave current generated by a minimum necessary number of switching, using an inverter, and wherein driving from an extremely low speed range in the vicinity of zero is possible. A neutral point electric potential of a permanent-magnet motor (4) is detected by synchronizing with the PWM waveform of an inverter. The position of a rotor of the permanent-magnet motor (4) is surmised from changes in the neutral point electric potential. The rotor position of a three-phase synchronous motor is surmised by making, upon detecting the neutral point electric potential, three or four types of switching states wherein the output voltage of the inverter is not a zero vector, by shifting the timings of each of the phases of the PWM waveform, and sampling the neutral point electric potential in at least two types of switching states among those switching states.