May2007 Electrical Apparatus

May2007 Electrical Apparatus

This is a summary of the Electrical Apparatus May 2007 featured technical article,  by Richard L. Nailen, P.E.   

With introduction of higher efficiency a-c motor designs (under Energy Policy Act legislation in the United States, and the “EuroDEEM” program in Europe), some motor users feel these newer products may be less reliable than earlier models. On the other hand, some motor manufacturers have implied that higher efficiency motors should be more reliable. Neither view is justified.

Reliability is usually measured by mean time between failures or MTBF. Failure is whatever requires a motor to be removed from service. Comparing reliability of one group of motors with another is difficult. Comparison requires that all motors be subject to the same operating conditions (applied voltage, environment, and loading), and all maintained in the same way (lubrication, cleaning, etc.).

Even then, whatever their efficiency, motors of different ages are subject to different failure rates as built-in defects are revealed and corrected. Equally important: industrial motor users seldom keep complete or accurate failure history records.

For those reasons, reliability is best compared by examining those components most subject to failure to see how a new design differs from the old. Such studies have concentrated first on bearings, which account for more than half of all motor breakdowns. Newer, higher efficiency motors generally use the same bearings as in earlier designs. Lubrication systems have not changed. Hence, failure rates should be unaffected.

The second leading failure cause is insulation breakdown. Thermal aging alone is seldom responsible. However, the more efficient motors of today use Class F (155 C) insulation. Operating within the standard Class B (130 C) limit will greatly increase the thermal life, indicating far higher reliability. That presumes, of course, that the user minimizes damaging conditions other than heat.

Maintenance is always a concern. Modern practices such as infrared thermography, laser alignment, and vibration spectrum analysis may lead some users to neglect routine surveillance, especially when coupled with the widespread tendency to cut maintenance budgets and staff. Any consequent increase in failure rate should not be blamed on the motor design.

The conclusion: higher efficiency motors should be neither more nor less reliable than earlier designs.

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