A structured approach to motor management

Use a structured approach to the rewind/replace decision.

By Thomas H. Bishop, P.E.

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When production drops in a weak economy, plant managers often try to control costs by delaying replacement or repair of failed electric motors. While this might save money in the short term, it could put the plant at a competitive disadvantage when the economy revives itself. A better approach to motor management during slack periods is to target mission-critical equipment with a sound motor repair/replacement policy.

A good motor repair/replace policy is based on scientific facts, not opinions or assumptions.

– Thomas H. Bishop, P.E.

It’s easy to find so-called expert opinions in the trade press or on the Internet that explain which motors to repair or replace. So, it shouldn’t be too hard to draft a good repair/replace policy, right? Unfortunately, much of this free advice ignores motor type and size, as well as differences in applications, hours of operation and load conditions that detail how, where and when the motor is used.

Often such advice fails to consider modifications that might be needed or the time it might take to procure a new motor. Worse still, some of these expert opinions recommend replacing every motor that fails, based on the questionable assumption that repair inherently degrades motor efficiency, or that energy-efficient and premium-efficient motors are impossible to repair or rewind.

Energy-efficient motors — the facts

A good motor repair/replace policy is based on scientific facts, not opinions or assumptions. For starters, nothing about energy-efficient or premium-efficient motors is magical or mysterious. Manufacturers simply improved the efficiency of these models by minimizing the amount of input electrical energy that’s lost to heat, friction and windage. Simply put, there are no technological breakthroughs associated with these motors.

To reduce core losses, for example, some energy-efficient models have stator and rotor cores that are longer than those on standard motors. More copper in the windings decreases copper losses, and open or shielded bearings (lubricated with a premeasured quantity of grease) reduce friction. To minimize the input power needed for cooling, the totally enclosed, fan-cooled (TEFC) designs use the smallest fan that can handle the job.

Effect of repair and rewinding — the facts

A 2003 rewind study commissioned by the Electrical Apparatus Service Association (EASA, U.S.A.) and the Association of Electrical and Mechanical Trades (AEMT, U.K.) proved scientifically that following the best practices detailed in the study maintains the energy efficiency of high-efficiency NEMA and IEC motors. The motors covered by the study ranged from the EPAct level (original U.S. federal law enacted in 1992) to the NEMA Premium and IEC EFF1 levels.

The study, performed at the University of Nottingham (U.K.), tested 22 motors ranging in size from 50 hp to 200 hp (37 kW to 150 kW), before and after multiple winding burnouts and rewinds. A 1998 study by AEMT also proved that the efficiency of lower-horsepower motors can be maintained during repair, dispelling the notion that, of themselves, winding burnout and removal will damage the core beyond repair.

According to these studies, following best practices during repair is critical to maintaining motor efficiency. Controlling bearing friction loss, for example, requires using the original bearing type, maintaining or restoring the proper bearing journal and housing fits, using the correct quantity of lubricant and “running in” the bearings before testing the motor efficiency. Copies of the most recent study are available from the “Industry Info” button at www.easa.com.

To retain the original efficiency of repaired or rewound motors, use a service center that follows ANSI/EASA AR100-2006, “Recommended Practice for the Repair of Rotating Electrical Apparatus,” and the Rewind Study’s “Good Practice Guide.”

Crafting a repair/replace policy

 Figure 1. This flowchart lists the major decision points for specifying either a rewind or a complete motor replacement.
Figure 1. This flowchart lists the major decision points for specifying either a rewind or a complete motor replacement.

A motor repair/replace policy is useful for determining the best course of action when a motor fails. But, each application is unique. Although the extensive flowchart in Figure 1 lists key decision points, it doesn’t cover every possibility.

Application review: The first step is to determine if the failed motor is suitable for the application. A motor with an open enclosure, for instance, might not be practical for a sawmill application and its airborne dust and debris. A better choice might be a TEFC replacement. Processes and duty cycles often change over time, so it always pays to reexamine the application when deciding whether to repair or replace a failed motor.

If the failed motor was a good fit for the application, evaluate the condition of the stator core. Has it sustained significant damage? Did the motor exceed its rated temperature rise (i.e., high core losses)? Absent special features that might affect price or availability, it might cost less to buy a new motor than to repair a badly damaged stator core. Next, consider these decision points simultaneously:

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