Resonant frequencies in naturally-commutated, converter-fed induction motor drive systems

Naturally-commutated, cage motor drive systems incorporate a capacitor bank connected to the motor terminals. The capacitor bank introduces resonant frequencies to the system and can represent a problem to the overall system control. A simple equivalent circuit can be used to determine standstill natural frequency and synchronous resonant frequency. The latter occurs close to zero slip and would only represent a hazard if the drive were operating at a critical rotor speed and low load. The standstill resonant frequency occurs at a higher fixed frequency (typically around 100 Hz) and is essentially load independent. It thus represents a more significant problem to the system control. The presence of the rotor deep bar effect introduces a further two resonant frequencies but these are found to be essentially indistinguishable from the usual synchronous resonant frequencies. Rotor skin effect however can significantly alter the dominant standstill resonant frequency. A simple modification to the normal equivalent circuit to incorporate rotor skin effect has been described and used to illustrate the influence on the resonant frequencies.