Repairing vs. replacing electric motors

The key is to plan ahead

By Chuck Yung, Technical Support Specialist, Electrical Apparatus Service Association (EASA), St. Louis, Missouri

The best way to avoid making hurried decisions when a motor fails is to plan ahead. Analyze the economics of repair and replacement for the motors in your facility and develop a comprehensive motor repair/replacement policy.

Whether you are developing a repair/replace policy or deciding whether to repair or replace a failed motor, the most important factor to consider is usually downtime. Other variables include the type, size, age and maintenance history of the motor, special electrical or mechanical features, the cost to repair or replace the motor and its operating costs.

Sometimes the best decision is fairly obvious, based on the cost of a new motor versus that of repair. At other times, it makes sense to perform a simple payback analysis, factoring in costs, variables and other considerations discussed below. The important thing to remember is that an investment in repair or replacement is not just a one-time cost. Other expenses should also be considered, including installation, maintenance and energy costs for the life of the motor.

Downtime

Production downtime generally is the key factor considered when a motor system breaks down. Lost production means idle workers, missed deadlines and disappointed customers.

The only way to deal effectively with the reality of motor failure is to take a proactive approach. Develop information on the types and sizes of motors that power your most crucial applications. If possible, build an inventory of spare motors that can serve as replacements for these critical production applications.

Many companies cannot afford to keep an inventory of spare motors. Even so, it is important to maintain detailed information about the motors in the facility. Such information is useful when contacting service centers and other motor vendors about repair services or to determine if they commonly stock particular models.

Whether repair or replacement will be faster depends on the size and type of motor, stock availability, and the service center’s production schedule. It usually takes three to five workdays to repair a motor in the 1- to 200-hp range, but on fast turnaround jobs, the time often can be shortened.

Replacements for most general-purpose open drip-proof (ODP) and totally enclosed fan-cooled (TEFC) motors under 100 hp are stock items at many service centers and other vendors. Rush orders for general-purpose motors in larger sizes (100 to 500 hp) can often be delivered within two to four days, whereas special motors and motors larger than 500 hp could take several weeks to obtain.

When time is critical — as it almost always is — check with the service center to determine a realistic delivery schedule for repair and replacement. Keep in mind that workloads at service centers may vary with the season and other business factors.

Above all, let the service center know if fast turnaround is essential. Most are sensitive to customers’ needs and will work hard to get the repair completed as quickly as possible. Most also pride themselves on doing the job right the first time, so avoid asking them to “cut corners” to get the motor running sooner. Remember, any compromises made in the quality of the repair could lead to higher operating costs or even premature failure.

Reliability of repaired/rewound motors

 Before deciding to have a motor repaired or rewound, ask if the repaired motor will be as reliable as a new one. While no studies to date directly address this issue, the quality of materials and workmanship that go into properly repaired/rewound motors often surpass manufacturers’ design specifications.

 

Consider this... The Electrical Apparatus Service Association, St. Louis, Mo., suggests the following factors to consider when making the decision to repair or replace your motor:
  • Importance of having the motor back in service immediately
  • Type and size of the motor (e.g., horsepower, efficiency rating)
  • Application (speed/torque requirements), electrical operating costs and hours operated annually
  • Simple payback analysis
  • Cost and availability of repair service as compared with those of a new motor
  • Age and repair history of the motor
  • Maintenance and capital budgets

As an example, to improve reliability of rewound motors, many service centers routinely upgrade insulation system components to a higher temperature class than that used by the original manufacturer (e.g., from Class B to Class F or H). This significantly improves the motor’s ability to withstand higher temperatures, thereby reducing the possibility of premature failure. Insulation upgrades of this kind often equal or exceed the temperature class of even today’s highly reliable energy-efficient motors.

Many service centers also offer a range of other repair options aimed at improving reliability, including use of premium grades of magnet wire, improved methods of applying insulating varnish, epoxy treatments, precision balancing of rotating elements, proper bearings and higher temperature lubricant. Of course, well-documented quality assurance systems, repair practices designed to maintain motor efficiency, and extensive testing protocols are among the other ways that many service centers ensure that the repaired motor will operate as efficiently and at least as reliably as it did before it failed.

Efficiency of repaired/rewound motors

Does repairing or rewinding affect motor efficiency? Studies show that the motors that are rewound or repaired using industry best practices maintain their original operating efficiencies.

To ensure that the repaired motor will retain its original efficiency, use a service center that adheres to EASA’s Recommended Practice (or equivalent repair specifications) and follows the repair procedures from EASA Tech Notes 16 & 17.

Application considerations

To assure proper performance, analyze the speed/torque requirements of the application before replacing a motor. Generally speaking, the replacement should have torque equivalent to that of the old motor. For centrifugal applications, the replacement should also have a full-load speed equal to or less than that of the original motor. Finally, be sure to consider whether any special couplings, mounts or adaptors will be needed for a replacement motor or a spare from inventory.

Repair and replacement costs

Motor failures often result in seized bearings, burned-out windings, or both. Qualified service centers routinely repair such damage, so repair estimates are relatively straightforward. For more complex repairs (e.g., a bent or broken shaft or a cracked frame), get a repair quote before going further — especially if the economics of the repair/replace decision are in doubt.

This is also the time to check on the cost and availability of a replacement and to reevaluate the application and operating conditions. In some cases, there may be opportunities to improve performance or reduce operating costs.

Of course, if the failed motor has special electrical or mechanical features, making frequent repairs is the best option. Such motors normally are well suited for their applications and will continue to give good service after repair or rewinding.

Operating costs and motor efficiency

When making a repair/replace decision, consider the energy efficiency of the present and replacement models, the utility rates and the hours of operation. Where utility rates and usage are low, the payback may be longer. Be sure to calculate payback based on the difference between repair and replacement costs, taking into account any modification costs associated with the new motor.

It makes sense to know as much as possible about your motor system application requirements and operating environments. Communicate this information when shopping for a service center or other motor supplier. Learn as much as possible about the services and products offered by prospective vendors. Develop a motor repair/replacement policy. By planning ahead, you will be assured of making the right decision each time.

This information is part of Electrical Apparatus Service Association’s booklet, “A Guide to AC Motor Repair and Replacement.” 

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