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Avoiding Insulation Failure: 5 Root Causes Of Overheating Motors

Avoiding Insulation Failure: 5 Root Causes Of Overheating Motors

Electric motors, like any complex machine comprising many moving parts, are vulnerable to a host of issues such as harmonic distortion, bearing wear, and misalignment, warranting repairs or an electric motor rewind sooner than expected. But out of all these problems that can bring your equipment to a screeching halt, many maintenance experts agree that excessive heat is the one that demands immediate action.

After all, operating an overheating motor causes rapid deterioration of its winding insulation, inevitably leading to short-circuiting, leakages, and motor failure. The common rule states that a motor insulation’s life is cut in half for every 10°C of extra heat the windings receive. Leading standards organisations also conclude that as much as 30% of motor failures stem from insulation failure, with 60% of such cases pointing to excessive heat as the root cause.

Below, we go over the common causes behind an overheating motor.

1. Power condition

When experiencing poor power conditions in the facility or similar adverse operating conditions, it is recommended to derate all electric motors in service to maximise their useful life. Derating a motor involves reducing its load to ensure its operating temperature stays within the limits of its insulation class, preserving reliability. Carrying out the process correctly largely depends on several factors: the motor’s insulation class, ambient temperature, and temperature spike at nominal and under load.

Many motors now list their expected temperature rise on their nameplate, either through a number or insulation class designation. Otherwise, you will need to determine the temperature rise manually by leveraging some sort of resistance method or taking the maximum temperature of the motor’s insulation class and subtracting it from the motor’s rated maximum ambient temperature. For instance, you can assume that a motor with a 40°C maximum ambient temperature and an Insulation Class F designation will have a temperature rise of 115°C.

2. High effective service factor

A motor’s service factor (SF) is the percentage of overloading it can handle for short periods when operating within the OEM’s recommended voltage rating. For instance, a motor with an SF rating of 1.15 on its nameplate means it can put out 15% more power when overloaded for a brief duration. As such, performing longer, steady-state operations at the service factor level will lead to rapid motor health degradation due to increasingly inefficient energy use, insufficient operating or starting torque, and, most importantly, overheating.

3. Excessive voltage

Stator current is a common way of measuring load levels in electric motors, which can be easily masked by an excessive load or overvoltage condition. Many facilities often make the mistake of running their motors at overvoltage levels just to reduce the stator current. While this is done with good intentions in mind, primarily to reduce heat on the motor, the result is often not what is desired as excessive voltage can cause the motor’s current to vary but not lead to a reduction in heat or losses of energy.

For example, when you take a motor with a range of 10-200 hp and have it run at a 10% overvoltage, it will only see a 1-3% reduction in losses, which is essentially negligible. Moreover, it is never recommended to rely solely on the stator current to find out a motor’s load conditions since a stator current analysis will often detect just mild overloading despite the true values being much higher.

4. Frequent starts and stops

Frequently stopping and starting a motor is a bad habit that is highly discouraged as it places unnecessary stress on the motor. The stress of a startup is the worst thing a motor goes through in its lifetime, but these startups are often inevitable at times and can even occur hourly, if not daily, in certain environments.

To avoid failure, it is imperative to closely monitor the duration and quantity of your motor startups using online monitoring equipment to ensure that you meet the professional motor standards and guidelines relevant to your industry.

5. Environmental conditions

It should go without saying that operating in high ambient temperatures will naturally cause your motor to heat up. High humidity, clogged ducts, chemically abrasive substances in the air, and high-altitude operations are also other common reasons for the unusual temperature increase in your motor. Infrared imaging can help to quickly uncover the causes of such non-electrically induced temperature stress.


Avoiding the problems associated with excessive heat requires careful planning, the right equipment, and consistent preventative maintenance. That said, if you have considered all the potential issues yet your machinery continues to run hot, that could indicate a problem with the internals of your motor. By working with a mechanical engineering and servicing firm, you can quickly get to the bottom of your motor troubles as well as source essential products necessary to keep your assets running smoothly, such as motor grease and transformer insulating oil.