Abstract
V/f ratio constant control has been widely utilized for induction motor drives as a general purpose inverter. Then, notwithstanding that the inverter is required to meet the load drive of various styles of which torque characteristics or inertias are largely different, it does not always satisfy all sorts of the specification required from the load sides. Particularly, in the low speed range, generation of large torque, stable operation at light loads and excellent dynamic response have hardly been achieved yet.
As a countermeasure to solve these problems, this paper proposes equivalently realize the idealized induction motor which has no stator winding resistance and leakage inductance. This is done by introducing such a kind of vectorial feedback control focussing our attention on the resultant rotating vector by three phase instantaneous voltage over the equivalent magnetizing inductance. The proposed control is that the ratio obtained by dividing the magnitude of the vector by the instantaneous frequency is kept constant in any instant. As a result, the electromagnetic torque is almost precisely proportional to the slip frequency and the torque slip characteristics are given as a group of parallel straight lines which have no breakdown torque. Torque generation over 150% is feasible even at the frequency 1.5Hz. Moreover, the torque response is quite fast to the change of stator frequency and rotor speed compared with the conventional V/f constant control. Therefore, the system is exactly stabilized. There often occurs the problem on wave distortions in stator voltage and current, which are caused by the dead time accompanied by the switching, as an inherent phenomenon in the conventional PWM type voltage controlled inverter and its suppressing control has to be additionally equipped. However, the proposed control does not need such a equipment. Nevertheless, the waveform of the stator voltage and current is kept sinusoidal in any states.
