Pumps & Systems - September 2007

Improving Motor Life Cycle Costs
Preventing Drive Induced
Electrical Damage to
AC Motor Bearings
Adam Willwerth, Electro Static Technology
Currents are induced on the motor shafts when AC motors are employed with
VFDs to drive air pumps, chillwater pumps and compressors in HVAC/R systems.
Without a grounding device, these currents typically discharge through the
motor bearings, causing frosting, pitting, fusion craters, and fluting.

Due in large part to an increased focus on energy savings, the use of pulse width modulated (PWM) variable frequency drives (VFDs) to control AC motors has grown dramatically over the last few years. While they offer low operating costs and high performance, VFDs are not without their problems.

Shaft currents induced by VFDs can lead to motor failures. Without some form of mitigation, shaft currents travel to ground through bearings, causing pitting, fusion craters, fluting, excessive bearing noise, eventual bearing failure, and subsequent motor failure. This is not a small problem. Consider:

• Most motor bearings are designed to last for

100,000 hours, yet motors controlled by VFDs can fail within one month (720 hours).

• An HVAC contractor recently reported that of the VFD-controlled 30-hp to 60-hp vane axial fan motors he installed in a large building project, all failed within a year (two within six months). Repair costs totaled more than

$110,000.

• Several large pulp and paper companies surveyed noted that the VFD-controlled AC motors used in their plants typically fail due to bearing damage within six months.

• The largest U.S. motor manufacturer cited eliminating drive-related motor failures as its number one engineering challenge.

• Almost a dozen blogs now on the Internet focus on discussing the problems presented by VFD-induced shaft currents, sharing information and experiences, and suggesting solutions.

• Motor failures caused by VFD-induced shaft currents result in hundreds of thousands of hours of unplanned downtime in the U.S. each year. These failures affect the performance and mean time between failure (MTBF) of the OEM systems in which they are used.