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Know the 4 Winding Failures in AC Electric Motors

06/15/2020

Medium voltage motor rewinds-1Did you know that the vast majority of electric motor failures are preventable?

There are some failures you can't prevent because everything has a limited lifespan. But many of the motors we have come into our shop for repair have suffered a failure that was not necessary. And when we talk about parts that wear out, all too often they wear out way too soon.

Winding Failures

Motor windings account for about 32% of all AC electric motor repairs. These winding failures fall into four major categories related to what causes them: thermal stress, mechanical stress, electrical stress, and environmental stress.

Thermal Stress Failures

Temperature plays a major role in motor failure. In fact, research shows that the lifespan of a motor is inversely related to temperature. This means that the hotter the motor runs, the quicker it is going to fail. In fact, the expected life of the insulation on the windings doubles for every 10C you can reduce the operating temperature. That makes it clear that you can prevent thermal stress failures.

Here's another example: operating a motor beyond its design limits can cause the temperature to rise. For every amount of load you go over your motor's operating limits, you can expect the temperature to change to be the square of that. Load cycling, and repeated starting/stopping can also cause temperatures to increase, as can a stalled motor. Again, these are conditions that you can prevent from happening.

Voltage variations can increase winding temperature, too. They occur when the motor is subjected to over or under voltage conditions. Another problem is voltage imbalance, where the supply voltage is not equal. This leads to unbalanced currents in the winding which can increase the winding temperature.

Other sources of heat include bearing temperatures (including the lubrication system), insufficient ventilation, poor positioning of air vents, heat generated by nearby machines, and losses related to belts and couplings.

Mechanical Stress Failures

Mechanical stress failures often take the form of coil movement. If the coil is poorly blocked or braced, it will move and risks damaging the winding. Another example of mechanical stress failure would be when the rotor and stator rub against each other.

And keep in mind that movement doesn't always refer to a visible, dramatic displacement. It can refer to vibration that can wear away at even the best quality insulation over time. These mechanical stress issues can have a variety of starting points including an eccentric shaft, shaft or bearing misalignment, in incorrect air gap, and extremely worn bearings.

Electrical Stress Failures

Electrical stress failures can take the form of turn to turn, coil to coil, phase to phase, coil to ground, and open circuits (or any combination of these). These types of failures can be the result of issues with winding insulation or cause problems with the insulation.

There are other ways that electrical stress can damage the windings. The use of minimal insulation or a poor quality insulating material can cause electrical stress failures to develop. Insulation can be gradually thinned to the point that conduction is possible, often as a result of poor blocking or bracing.

Another issue is partial discharge resulting from an air pocket in the insulation or contamination on its surface. These partial discharges will damage the windings by eroding the insulation over time and often exceed 6kV. Transient voltages, such as lightning strikes or failed circuit breakers, can also result in serious damage.

Environmental Stress Failures

Moisture can be a major problem for electric motors. It can be the result of a buildup of condensation, general equipment wash down, or just an extremely humid environment. Regardless of the cause, moisture can lead to shorts and other issues with the winding.

When you expose the motor and its winding to abrasive particles (sand, cement dust, fly ash), the insulation on the winding erodes over time. The buildup of dust and debris on the motor itself can be a problem, too. It will make it hard for components meant to conduct heat away from the motor, such as cooling ribs, to do their job.

And chemicals can be an issue for motor windings, and not just corrosive ones like acids. For example, fly ash and carbon black are both electrically conductive. As they begin to build up they, can cause some electrical issues that are not only hard to track down but hard on the winding insulation.

Finally, don't forget what we talked about earlier with regard to ventilation. A buildup of contamination (think dust or paper pulp!) will block vents and lead to thermal failure as the motor starts running hotter than it should. Exterior filters can also become blocked with contaminants. A clogged air filter will block off the air supply needed to keep the motor cool.

Preventable Failures

There are a host of ways that a winding can fail. The good news is this: knowing what can cause the windings on your motors to fail also means that you know how to prevent them. Remember that you can keep the vents clean, you can change air filters, you can work to keep the windings free of contamination, you can ensure that enclosures keep dust and water out, and you can keep loads within the operating range for the motor.

Finally, keep in mind that preventative maintenance and the use of an EASA accredited repair shop combined can help your motors last!

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Author & Contact Info: Nolan Crowley  ncrowley@hecoinc.com

 

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