METHOD FOR DETECTING ROTATIONAL SPEED OF INDUCTION MOTOR IN FREE ROTATION, AND METHOD FOR RESTARTING INDUCTION MOTOR

摘要

PROBLEM TO BE SOLVED: To restart operation with no impact by detecting the rotational speed of an induction motor under free rotation, without coupling a motor with a speed detecting means and supplying the motor with an AC power, having a frequency and a voltage corresponding to a detected rotational speed. SOLUTION: Current and voltage of an induction motor 5 are divided into a d-axis component and a q-axis component which are orthogonal to each other, a q-axis voltage command value to the induction motor 5 in free rotation is brought to zero and a stepwise trigger voltage is applied in the d-axis direction. A q-axis current, having a phase shift of 90 deg. from the trigger voltage, is detected and a control operation for bringing the detection value of q-axis current to zero is conducted, thus obtaining the rotational speed information of the induction motor. Alternatively, the product of a d-axis current command value and the primary resistance of the induction motor is employed as the trigger voltage.