Minimizing electric bearing currents in adjustable speed drive systems

Electrical bearing currents have become of increasing interest in recent years due in part to the introduction of high speed switching technology employed in state-of-the-art adjustable speed drives (ASDs). High rates of change of ASD output voltage (dv/dt's), will cause transient currents to flow from motor windings to ground through stray capacitances. Under certain circumstances, these currents can flow through motor or gearbox/machinery bearings, and if large enough in amplitude, lead to reduced bearing life. This paper offers a view, from an industrial environment perspective, of the most common reasons that damaging electrical bearing currents can occur in an ASD-fed motor or in connected machinery, and how installation practices can affect the generation of bearing currents. Technology is discussed that facilitates easy measurements of the common mode current, which can be used to quantify the risk a particular installation may have, as well as offer measurable proof of bearing current reduction after the implementation of corrective measures. Practical suggestions are given to optimize installation practices, which if followed, can reduce or eliminate the major risks of electrical bearing damage.