Zero-cross detection method, detection system, PG motor and air conditioner

The invention provides a zero-cross detection method and system, a PG motor and an air conditioner. The zero-cross detection method comprises the following steps: determining the power supply frequency of a current power supply; executing a conduction starting point selection step: determining a current conduction starting point by taking any moment as a starting point; controlling the solid-state relay to be conducted according to the power supply frequency; monitoring the rotating speed of the PG motor; judging whether the current conduction starting point is a zero crossing point or not according to the rotating speed of the PG motor; and if not, turning to a conduction starting point selection step. According to the zero-cross detection method provided by the invention, a zero-cross detection circuit is replaced by a core software algorithm, so that abnormality of a hardware circuit can be effectively avoided, the design cost is greatly reduced, the reliability of PG motor control is improved, and the ser