Mechanical testing revealed a catastrophic winding failure that was suspected to have initiated in the field and was the most likely reason for replacing the motor. A review of field electrical signature analysis (ESA) data indicated that the unfiltered drive output and drive settings resulted in bearing and winding failures. An overly tight bearing journal (Figure 2) caused the opposite drive-end bearing to fail due to a loss of clearance.

The drive-end bearing, shown in Figure 3, exhibited bearing fluting. The 75-hp, 1,800-rpm electric motor used a mounted shaft brush. How can a motor with a shaft brush have bearing fluting?

There are several causes. One is that thrust causes occasional bearing contact when the potential voltage on the shaft is much higher than expected. In this case, a combination of high ground currents and high potential on the shaft generated enough voltage to saturate the resistive path.

The circuit includes capacitance from the outer race to the ball, through oil, then from the ball to inner race through oil. Along with resistance through the bearing components, the shaft brush creates a low-resistance resistor in parallel with the capacitance, generating a resistor-capacitor (RC) circuit. The result is that, while shaft brushes reduce the potential across the bearing, they do not eliminate it and some level of energy remains. The goal is to drop this to a harmless level. If the potential is high enough and high-frequency harmonics are present, then current will still pass through the bearings at a potential high enough to damage them.

Solutions to this condition include filtering the variable-frequency drive output to reduce harmonic current, checking and correcting ground potential and currents, and adding an insulated opposite drive end bearing. If potentials are high enough, add two insulated bearings and shaft brush(es) on one or both ends of the motor.

By Howard W. Penrose, PhD, CMRP, CEM, MotorDoc LLC

Howard W. Penrose, PhD, CMRP, is president of MotorDoc LLC, Lombard, IL (motordoc.com). He chairs the wind-power standards and government relations participation for American Clean Power (ACP/AWEA), is the USA representative to CIGRE for high-voltage electric machinery, holds various IEEE standards positions, and is a past chair of SMRP.  He holds certifications from SMRP as a Certified Maintenance and Reliability Professional and the Association of Energy Engineers (AEE) as a Certified Energy Manager. Reach him at [email protected].

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