Experimental case studies on dead-time induced oscillation and its mitigation in 5-HP and 10-HP induction motor drives

Inverter dead-time, which is meant for safe commutation of devices is known to induce sub-harmonic oscillations in lightly-loaded induction motor drives operating in open-loop. This paper presents an experimental procedure to determine whether a given motor drive would exhibit such oscillations, and if yes, the region of such oscillatory behaviour. In particular, this paper presents case studies on oscillations on account of dead-time in inverter-fed 5-HP and 10-HP induction motor drives. Experimental measurements are made at different fundamental frequencies and different flux levels (or V/f ratios) to identify if the motor drive exhibits oscillatory behaviour at these various operating points. The regions of oscillatory behaviour of the two motors under no-load are identified experimentally in terms of applied voltage and modulating frequency. A simple dead-time compensation scheme is employed to mitigate these low-frequency oscillations. The experimental waveforms and corresponding Fast Fourier Transform (FFT) analysis of the motor currents demonstrate the sub-harmonic oscillations and their mitigation.