Motor starting method, and starting and synchronous operation method of motors powered by different power supplies

The invention belongs to a motor starting and operating method, and aims to solve the problems that when two motors needing different power supplies to supply power jointly drive the same load at present, an existing power supply unit cannot meet the synchronous control requirement, shaft torque is generated due to the difference of power grid parameters such as voltage and frequency of the two power supplies, unit faults are caused and units cannot operate normally. According to the motor starting method and the starting and synchronous operation method for the motors powered by the different power sources, during starting, a first motor running at power frequency is started firstly, and then a second motor running under the control of a variable-frequency torque loop is started. The second motor is started through rotating speed closed-loop control and stator current closed-loop control, after the rotating speed of the second motor is synchronous with the rotating speed of the first motor, the synchronous c