PURPOSE: To start at a low frequency with a large load torque by variably setting a voltage/frequency, starting an induction machine at a low frequency, accelerating in the vicinity of a synchronizing speed, then completing synchronization in further lower frequency, then maintaining the voltage/frequency constant, and gradually raising it up to a rated frequency in a degree not falling out of phase.

CONSTITUTION: A dotted chain line starting from '0' indicates torque characteristic of a load. A folded line indicates voltage/frequency characteristic. A motor is started from a low frequency f1 (3-5Hz). A voltage/frequency is varied in response to a load torque. When the load torque is large, the voltage/frequency is regulated like a voltage V1. When it is accelerated to the vicinity of a synchronizing speed, it is synchronized in further lower frequency f3 (1-3Hz). The voltage/frequency is maintained constant, and an operating frequency f2 is gradually raised up to a rated frequency f22 in a degree not falling out of phase. Thus, the generated torques of the motor in frequencies f1, f3 become larger than the load torque to smoothly start the motor.