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A Breakdown of The 4 Major Failure Types in AC Motor Windings

A Breakdown of The 4 Major Failure Types in AC Motor Windings

At some point in a motor’s service life, it will encounter a failure situation that can never be prevented since everything has a limited lifespan. Nevertheless, proper maintenance and electric motor overhauling can keep most preventable motor issues from developing to the point of failure. When it comes to AC motors, these issues typically affect the windings, accounting for as much as 16% of all reported costly motor failures.

In this article, we go over the four major categories of winding failures and the most common examples of each.

Mechanical stress failures

Mechanical stress failures often manifest as coil movement, where poorly braced or blocked coils will inevitably move and potentially damage the winding. Another common example of mechanical stress failure is when the stator and rotor rub against each other.

Also, keep in mind that movement does not always involve a dramatic or, better yet, visible displacement, as it may also refer to vibrations, which are capable of wearing away even the highest quality of insulation over time. The starting points for these various stress issues can stem from a wide range of causes, including an incorrect air gap, shaft or bearing misalignment, and extremely worn bearings.

Electrical stress failures

Electrical stress failures commonly stem from issues with the winding insulation or, if not, can cause problems with said insulation. They usually come in the form of open circuits, coil to coil, phase to phase, turn to turn, coil to ground, or any combination of these. Beyond that, there are other ways electrical stress damages the windings and increases the risk of failure, such as using poor-quality insulating material or only having minimal insulation.

In such cases, the insulation may be gradually worn down to the point that conduction becomes possible, often due to poor bracing or blocking.

Another issue involves partial discharges caused by contamination on the surface of the insulation or an air pocket inside it. These discharges, which often exceed 6kV, damage the windings by gradually eroding its insulation bit by bit. Transient voltages, such as failed circuit breakers or rare lightning strikes, can also cause serious damage.

Environmental stress failures

Moisture has always posed a major issue for electric motors, and it can arise in various ways, ranging from condensation buildup to extremely humid operating environments. Regardless of the cause, moisture in a motor almost always leads to shorts and other problems with the winding.

Also, a motor that gets exposed to abrasive particles like sand, dust, dirt, and other debris will see its winding insulation erode over time. The buildup of these particles on the motor itself also presents a problem in that components such as the cooling ribs, which are meant to conduct heat away from the motor, will have more difficulty doing their job.

Of course, when the interior parts get affected by contaminants, the ventilation system is often overwhelmed to the point that it can no longer function optimally. Blocked vents mean less heat can escape the system, and not enough air supply can reach the internals to cool the motor, eventually leading to thermal failure. Besides the main issue of suboptimal thermals, the increased heat will also affect other critical aspects of a motor, such as faster deterioration of its lubricative components like mineral oil transformer.

Lastly, chemicals are another cause for concern for motor windings, not just the corrosive ones like acids. Some prime examples include carbon black and fly ash, two electrically conductive materials. Once these build up in the motor, they can cause serious electrical issues that are hard on the winding insulation and notorious for being hard to track down.


As we have covered above, there are a number of ways a motor’s windings can fail. However, by knowing what these potential causes are, you can stay on top of them before they cause more serious issues. That said, should your critical assets still experience unexpected failure, a mechanical engineering and servicing provider can help you get your motors back in action as soon as possible.