Motor Rewinding: Best Practices

looking into the center of a large electric motor

For engineers and mechanical experts, electric motors are relatively simple devices. However, rewinding the coils on these motors is a highly precise task that requires the utmost expertise.

If an EASA-accredited individual rewinds a motor, the technological advancements and best engineering practices can lead to increases in efficiency. This makes rewinding an appealing and cost-effective solution for motor longevity. On the other hand, a poorly performed rewind job could result in a motor that performs poorly or doesn’t work at all. At that stage, the only option would be to purchase a new motor.

It’s essential that you’re prepared before you begin rewinding, because, once you start removing coils, there’s no going back. Read on for a step-by-step overview of motor rewinding best practices.

Examine the Stator Core

Core integrity is crucial to the reliability and efficiency of motors. Any damage to the stator core is something that an expert should immediately check, both through magnetic saturation testing to determine losses and visual inspection. Winding failures might create enough heat to damage core lamination insulation which will increase cores losses generating excessive heating, while a bearing failure could cause the rotor to hit the stator.

If an inspection reveals damage to the core, you will need to repair or replace it before rewinding. Erosion within the laminated material could be re-stacked to fill out the damaged area, although lamination that is badly damaged may need to be completely replaced.

Repair or Replace Defective Cores

When you start to take a stator core apart, you’ll see a lamination stack. These are thin pieces of steel with insulation to reduce eddy currents (hysteresis losses) within the core. You might notice evidence of damage, like holes or fused laminations, when dismantling the core. Once you have repaired the motor laminations and verified magnetic suitability for rebuilding, you can examine the windings.

Removing old windings often involves the use of special ovens that remove insulation coatings of epoxy or polyester resins. This process when performed by an Accredited facility works to maintain the appropriate temperature throughout the heating process to protect the core’s insulation. Newer core insulations can often withstand temperatures that are far higher than their older counterparts.

Look for Ways to Improve Efficiency

Next, an expert in motor rewinding will examine any possibilities for improving motor efficiency. For instance, a service provider could pack more copper into each slot. This will help reduce the amount of overheating within a motor and effectively minimize the risk of damage.

Ideally, during an assessment of the winding, a rewinding expert should consider all aspects of the coil. Reducing the length of the coil loop when it exits one slot in the stator core and enters another can minimize resistance and boost efficiency. The shorter the turn area of the loop, coupled with more magnet wire, can improve power density in a motor.

Consider Other Issues

Though the core and the winding are important elements in any rewind process, it may be worth examining other areas of the motor for mechanical needs before putting your machine back into action.  The use of an inverter or VFD with the motor should be taken into consideration during rewinding as additional insulation materials and multiple resin treatments using the VPI (vacuum pressure impregnation) process will be needed for longer motor life.  Require Partial Discharge Testing of the new winding to verify the winding insulation is suitable for VFD use.

Additionally, machined surface tolerances must be checked to insure the new bearings are installed correctly and are undamaged. Remember, any replacement bearings should be the same as those provided by the manufacturer of the motor. The grease reservoir cavity should also be carefully addressed.

Consider any problems with ventilation. Sometimes, a more efficient motor with an increased wire capacity allows manufacturers to cut down on fan size. Look out for broken blades and any issues with airflow.

Measure and Test

Finally, test the operation of your motor to against the motor nameplate FLA (full load amperage).  Check the exact operating speeds, and measure no-load and full-load ratings comparatively. Typically, motors will operate at the best efficiency levels when they are 75-80% loaded. Oversizing motors is not a significant issue on efficiency, though power factor is lower at reduced loads.

Rewinding motors may not always be the right choice for your specific needs, as extreme damage can prompt the need for a motor replacement. However, in many cases, a careful rewinding process can maximize reliability and uptime for a company. For larger motors, rewinding can be faster than obtaining a new motor, with the opportunity to reduce operating costs, improve reliability and even improve your return on investment.

Contact our expert team today for advice on rewinding your motor.

Motor Rewinding: Best Practices was last modified: August 9th, 2017 by admin
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Jerry is the owner of Sloan Electromechanical and is active in all aspects of the company. He is passionate about doing the work RIGHT and proposing the best product solution, hence the Sloan team is focused on aligning company values with client values. Please post your questions or comments and Jerry will respond. For a faster or confidential response, please contact Jerry directly 619-515-9691 or LinkedIn www.linkedin.com/pub/jerry-gray/17/332/5a1
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