Stability analysis in induction motor driven by V/f controlled general purpose inverter

For an induction motor driven by a sinusoidal voltage source of adjustable amplitude and frequency, such as a pulse-width-modulation (PWM) inverter, the occurrence of sustained oscillations has often been observed when the drive is underloaded. In order to solve this problem, a linearized model is considered. The conditions under which the oscillations originate and how to determine the oscillating range in the stator voltage versus frequency are clarified. A physical explanation is also given for the analytical results. It is found that the motor becomes unstable when the rotor circuit time constant becomes larger than the mechanical time constant in rotor motion: however, the existence of transient inductance works so as to suppress the instability and stabilize the motor. There exist two asymptotes which restrict the oscillating range in the plane composed of the axis of the equivalent magnetizating current magnitude and that of stator frequency.<
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