Evaluation of an alternate soft charge circuit for diode front end variable frequency drives

Variable frequency drives (VFDs) with diode rectifier front end are typically equipped with a resistor-contactor arrangement to limit the inrush current into the DC bus capacitors, thereby providing a means for soft charging the DC bus capacitors. Because of the mechanical nature of the magnetic contactor typically used in VFDs, there exists a concern for fatigue. In addition, during a brown out condition, typically the contactor remains closed and when the voltage recovers, the ensuing transient is often large enough to possibly cause unfavorable influence to surrounding components in the VFD. Many researchers and application engineers have thought about this issue and many are actively seeking non-mechanical solutions in a cost effective manner. In this paper, a new topology to soft charge the DC bus capacitor is proposed. Other techniques that have been evaluated are also introduced. The relative advantages and disadvantages are discussed. Experimental tests to show the feasibility of the proposed idea is also provided.