Electrical Apparatus, February 2014

Electrical Apparatus, February 2014

This is a summary of the Electrical Apparatus February 2014 featured technical article,  by Richard L. Nailen, P.E. 

In both Great Britain and the U.S., extensive studies of electric motor repair processes have led to standards aimed at maintaining existing motor efficiency. The service industry has since called attention to the possibility of increasing the lower efficiency of some older motors. A 2005 paper from the Institute for Electrical and Electronics Engineers offered suggestions for doing so.

Some of these suggestions are of obvious benefit. Others can be troublesome. None can be fully quantified. Their costs cannot be weighed against future energy savings.

The paper considered only medium-voltage a-c motors. Most of these are in the “large” category as defined by the National Electrical Manufacturers Association. Sleeve bearings are most common. In totally enclosed construction, ribbed or finned frames (IC411) are used in the lower power ratings; air-to-air heat exchangers (IC611) for larger units. The paper suggested replacing cooling fans with more efficient types, including use of airfoil blades.

This is not good advice, because of the need for careful balance between fan power and noise, developed pressure, air flow, and heat transfer. Such analysis isn’t feasible in a service center (nor are airfoil blades a practical alternative for the service center). Moreover, fan changes alone do not affect windage loss elsewhere in the motor, or the bearing friction.

Increasing stator copper may be an option. Slot space originally provided cannot be filled arbitrarily, however, because of the need to minimize pressure points between coil sides, facilitate wedge driving, and provide for temperature detectors. For high-speed machines, increasing coil pitch to reduce slot harmonics can increase mean turn length by several percent, directly increasing stator copper loss.

Replacing slot wedges with the magnetic type tends to reduce stray loss, but the difference isn’t readily calculated or measured, and mechanical weakness of most magnetic wedges is a drawback.

A change to oil mist lubrication can enhance bearing reliability, but influences efficiency only when the bearings were originally grease lubricated—rare for large medium-voltage ratings.

The results of most attempts to increase motor efficiency during repairs, then, are either insignificant, not measurable, or counterproductive.

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