A Unique Way to Address Component Breakdown Guidelines Set in UL 61800-5-1 for Variable Frequency Drives

The testing protocol detailed in UL 61800-5-1 requires all Variable Frequency Drives (VFDs) with DC bus capacitors to undergo a capacitor short circuit test. A systematic approach has been adopted in this paper to offer potential solutions to meet this new UL requirement, as well as improve the overall cost, size, and reliability of the VFD. To start, it is recognized that all VFDs are typically equipped with a resistor contactor arrangement to limit the inrush current into the DC bus capacitors. Attempts to open this contactor on sensing DC bus capacitor fault condition has not been met with success because the current flowing through the contactor during fault is inductive and is difficult to interrupt. Even if the contactor is opened, the fault current will then flow through the soft charge resistor creating heat and potentially causing the cotton wrapped around the drive to catch fire. The electrical position of this contactor is recognized to be a potential area for improvement. An idea to limit the energy involved during a capacitor short is also presented here. Most importantly, this paper provides new and concrete steps that can be taken to successfully meet the new UL testing requirement. The idea is implementable, cost effective and independent of the source short circuit current. The relative advantages and disadvantages of the idea proposed here are discussed. Experimental tests supporting the proposed idea are provided.