Rotational speed detection device and detection method of permanent-magnet synchronous motor

The invention discloses a rotational speed detection device and detection method of a permanent-magnet synchronous motor. The device comprises a DC voltage source, a three-phase full-bridge inverter, a high-resistance resistor and a permanent-magnet synchronous motor, wherein the DC voltage source is used for supplying power to the three-phase full-bridge inverter, the three-phase full-bridge inverter is connected with the permanent-magnet synchronous motor, a middle point of the DC voltage source is grounded, and a neutral point of the permanent-magnet synchronous motor is grounded via the high-resistance resistor. The method comprises the following steps of detecting to obtain voltages at the two ends of the high-resistance resistor as the voltage of a neutral point of the permanent-magnet synchronous motor; determining an end voltage of the permanent-magnet synchronous motor, and subtracting the voltage of the neutral point from the end voltage to obtain a phase voltage; calculating to obtain a three-phase