Electric AC motors are widely used in various fields of national economy, especially the synchronous motor with its high energy efficiency and to provide reactive power to the grid, people of all ages. However, considerable asynchronous motor starting current, as well as synchronous motor does not have the self-starting capability of the defect has been plagued by people.

The frequency converter has successfully resolved these two issues, soft-start from zero speed, completely eliminates the problem of starting current and synchronous motor start difficult. Frequency converter as the starting power has brought new problems, that is, when the motor soft start increases the speed to close to the rated speed implementation of the motor power switch, power supply switching from the frequency converter to the power grid. Switching has long been used to cause the motor often subject to great impact, and even induce motor protection action to make the switch failure. The reason is that people often ignore the power switch must follow the same period of the AC motor (or synchronization) three criteria, which must be in power supply and grid work frequency power supply voltage, frequency similar to capture the supply voltage phase angle difference is zero the instantaneous power switching. Popular way to switch but they ignore the most important phase angle conditions, this is an important cause of the damage to the motor.

Switch modes between variable frequency and industrial frequency power supply:
(1) AC motor by the inverter power supply soft start switch to the frequency of power supply;
(2) AC motors driven by variable frequency power supply for a given load, switch to the frequency of power supply;
(3) switch to the variable frequency power failure when running AC motor variable frequency power supply, of course, this must be in working condition to allow the case.

On the above three switching modes (1) for a more stable load motor starting, to avoid starting current on the grid disturbance. Multi-mode (2) for a variable frequency power supply for multiple motor starting and running the control when the motor drive start and enter the operating conditions close to full load or a given load value, the motor switch to the frequency power on the run, and then start the original stop the standby motor, into the operating conditions, and so on. Means (3) used for that are higher power frequency power supply to run the motor on the motor continuity requirements.
Frequency power supply and variable frequency power supply switching frequency and phase angle, will lead to impaired motor and variable frequency power supply, frequent switching shocks pose a threat to run the equipment and product quality.