Sizing Up a Motor's Condition Situation

By James Piper, P.E.  
OTHER PARTS OF THIS ARTICLEPt. 1: Should You Repair, Rebuild or Replace That Motor?Pt. 2: This PagePt. 3: Operating Efficiency Important Factor in Motors' Repair or Replace Decision Pt. 4: Spotlight on NEMA

Even motors that are well-maintained and have been properly matched to the load they drive have a finite service life. A motor's time and operating hours, particularly if is subject to frequent starts, take their toll on components, and eventually something fails.

When failure does occur, managers need to review the motor's history to gauge its performance. What repair work have technicians performed on the motor? Does it have a record of reliability, or is it unreliable?

Before managers can decide whether to rebuild or replace the motor, they might need to determine exactly what failed. If the damage is extensive, the cost of rebuilding the motor might approach the cost of replacing it. If the manager determines the motor has undergone an extensive rebuilding, replacement might be the best option.

Sizing Up The Situation

About 80 percent of pump motors are incorrectly sized, according to some estimates, and similar estimates have been made for other motor applications. Most such motors are oversized. Oversizing wastes energy.

Undersizing, while less common, causes the motor to run hotter than normal, shortening its service life. Correcting an undersized motor requires replacement. Simply rebuilding an undersized motor will only lead to future replacements.

Managers have two options with an oversized motor — replacement with a properly sized motor and the installation of a variable frequency drive (VFD) for the motor. In the case of a failed, oversized motor, managers should examine the application's economics closely. In replacing an existing motor with one that is properly sized to be economical, the oversizing and the annual hours of operation must be sufficiently large. The greater the oversizing and the higher the annual run time, the quicker the recovery time for the cost differential between rebuilding and replacement.

Even if the motor has not failed, managers can improve the operating efficiency of oversized motors by installing a VFD, which alters the voltage and frequency of the power provided to a drive based on the load. Energy savings come from a characteristic of the induction motor where the power draw of the motor varies with the cube of the motor's speed. Installing a VFD on an oversized motor or on one where the load varies typically produces enough savings to recover the cost of the installation in one to two years.

VFDs also offer the advantage of initiating a soft start for the motor. When a motor start, the inrush current can be three-ten times the full-load current drawn by the motor. This inrush current results in heating within the motor's windings and can shorten motor life, particularly if the motor stops and starts frequently. VFDs limit the inrush current, thus limiting heating of the windings.

If motors are simply replaced in kind when they fail, then the undersizing or oversizing issue will never be identified or corrected, resulting in continued energy waste or future additional motor repair or replacement costs.

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  posted on 11/24/2014   Article Use Policy

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