An induction motor vector control apparatus calculates a slip factor ( omega s) using a secondary time ( tau 2) constant. A reference signal (i1n) is added (41) as noise to the excitation (io*) current command signal. The secondary time constant ( tau 2) is brought closer to its actual value by modifying (34) the secondary time constant in a direction zeroing the correlation between the motor speed feedback ( omega ) signal and the reference ( omega sr) signal.
