What are the most common causes of excessive heat in electric motors?

Excessive heat in electric motors can lead to performance issues and reduced lifespan.

Understanding the causes of excessive heat and implementing preventive measures are crucial for maintaining the optimal functioning of electric motors. In this post, we will explore the most common causes of excessive heat in electric motors, discuss preventative maintenance practices, and highlight the importance of monitoring factors such as the Effective Service Factor and NEMA derating curve. We will also delve into the effects of frequent starts and stops on electric motors and explain how thermography can be used to determine the heat at which motors are operating. By addressing these topics, we aim to provide valuable insights into maintaining and optimising the performance of electric motors.

What is preventative maintenance for electric motors?

Preventive maintenance for electric motors refers to the regular inspection, cleaning, and servicing of motors to prevent potential issues and ensure their optimal performance. It involves scheduled maintenance tasks such as lubrication, alignment, and electrical testing to detect and address any potential problems before they lead to motor failure.

Implementing preventive maintenance helps extend the lifespan of electric motors, minimise downtime, and reduce the risk of costly repairs or replacements. This is done with modern motor monitoring technologies that allow for the easy assessment of the entire motor system, including the power source, motor, and load. By considering the entire system rather than individual components, effective troubleshooting solutions are possible.

How does excessive heat will cause rapid deterioration of the winding insulation within motors?

A motor that would normally last 20 years in regular service, running 40°C above its rated temperature. Under these conditions, the motor would only have a lifespan of about 1/16 of its expected duration, which is a little more than one year until it fails.

💡 The common rule states that, for every 10°C of additional heat to the windings, motor insulation life is cut in half.

Excessive heat can cause rapid deterioration of the winding insulation within motors due to the following reasons:

  1. Increased thermal stress: When the motor operates at a temperature higher than its rated temperature, the insulation material can degrade more quickly. The insulation may become brittle, leading to cracks and breakdowns.
  2. Reduction in insulation resistance: Excessive heat can reduce the resistance of the insulation material, causing leakage currents to flow through the windings. This can lead to insulation breakdown and short circuits.
  3. Accelerated aging: Heat accelerates the aging process of insulation materials, causing them to lose their mechanical and electrical properties at a faster rate. This can result in insulation failure and compromised motor performance.

💡 30% of motor failures result from insulation failure, and 60% of those are caused by excessive heat in the motor.

To ensure the longevity and reliability of electric motors, it is crucial to address and control excessive heat generation through proper motor design, cooling mechanisms, and preventive maintenance practices.

What are the reasons that an electric motor overheats?

In real-world applications, ideal voltage conditions are uncommon. Excessive heat and damage to windings and bearings can occur due to losses, not just current level. The following are reasons why an electric motor may overheat:

  1. Excessive load: When a motor is subjected to a load beyond its capacity, it can lead to overheating.
  2. Poor power conditions: Industrial facilities often have poor power conditions, causing motors to operate under adverse conditions and generate excessive heat.
  3. High effective service factor: Operating a motor above its rated service factor can result in increased stress and heat generation.
  4. Excessive starts and stops: Frequent start-ups and stops can put significant stress on a motor, leading to overheating.
  5. Environmental influences: Factors such as high ambient temperature, clogged ducts, chemical substances in the air, wet operation, and high-altitude operation can contribute to motor overheating.

Addressing these reasons is important to prevent overheating and ensure the proper functioning of electric motors.

What is the Effective Service Factor for electric motors?

The Effective Service Factor for electric motors is a measurement that determines the stress on a motor-load application. It is calculated using the formula:

Effective Service Factor = (% Load / % NEMA derating)

An Effective Service Factor greater than 1.0 indicates that a motor's capacity for overload is only for short periods of time, not for longer steady-state operation. It is important to monitor the Effective Service Factor to ensure that a motor operates within healthy limits and to prevent excessive stress on the motor.

💡 NEMA specifies the allowable load level for poor voltage conditions. Preventive maintenance tools are necessary to accurately calculate operating loads and ensure safe plant operation.

The effective service factor is also important for assessing stress on a motor-load application. In a test with a 300hp motor running at nearly full load (99.7%) using a dynamometer, poor voltage distortion was observed due to an unidentified SCR defect in the power supply. The resulting NEMA derating factor of 0.85 led to an effective service factor (s.f.) of 1.17, indicating an alarm condition.

What is the NEMA derating curve?

The NEMA derating curve illustrates how the allowable load of an electric motor decreases as the unbalance percentage increases. The formula for calculating the unbalance percentage is:

% Unbalance = (100 x maximum voltage deviation from average voltage / Average Voltage)

As the level of unbalance increases, the motor's ability to handle loads decreases. For instance, a 100hp motor with a 3% unbalance factor should be derated to 90% capacity, which is equivalent to 90hp.

The NEMA derating curve is crucial for evaluating the motor's load capacity under unbalanced voltage conditions. It ensures safe operation and prevents excessive stress on the motor.

How do frequent starts and stops effect your electric motor?

Frequent starts and stops can have a significant impact on electric motors. They can cause increased stress and wear on the motor, leading to various issues and potential failures. Some of the effects of frequent starts and stops on electric motors include:

  1. Increased mechanical stress: Every time a motor starts or stops, it undergoes mechanical stress due to the sudden changes in rotational motion. This can lead to increased wear and tear on the motor's components, such as bearings and shafts.
  2. Thermal stress: Starting a motor requires a surge of current, which generates heat. Frequent starts and stops can result in excessive heat buildup, which can degrade the motor's insulation and shorten its lifespan.
  3. Reduced efficiency: Each start consumes more energy than normal operation, resulting in higher energy consumption over time. Additionally, the motor may not have enough time to reach optimal operating conditions, leading to reduced efficiency.
  4. Increased maintenance requirements: Frequent starts and stops can accelerate the wear of motor components, necessitating more frequent maintenance and potentially increasing downtime.

To mitigate the effects of frequent starts and stops, it is important to carefully manage motor operations and consider alternative control strategies, such as soft starters or variable frequency drives, which can help reduce the mechanical and thermal stress on the motor. Regular maintenance and monitoring are also essential to identify and address any potential issues before they escalate.

How is thermography used to determine the heat electric motors are operating under?

Thermography is commonly used to determine the heat at which electric motors are operating. By using specialised thermal imaging cameras, our technicians can capture images that display the temperature distribution across the motor. These images help identify any areas of excessive heat, which may indicate potential issues or abnormal operating conditions.

Thermographic inspections can reveal poor thermal conditions, such as high ambient temperature or clogged ducts, which can lead to temperature stress on both the motor and its insulation system. Additionally, environmental factors like the presence of chemical abrasive substances in the air, wet operation, and high-altitude operation can contribute to motor overheating.

By conducting regular thermographic inspections, maintenance personnel can detect abnormal temperature patterns, diagnose potential problems early on, and take appropriate corrective measures to ensure the optimal performance and longevity of electric motors.


In conclusion, excessive heat in electric motors can have detrimental effects on their performance and lifespan. The most common causes of excessive heat include increased thermal stress, reduction in insulation resistance, and accelerated aging of insulation materials. To mitigate excessive heat, preventive maintenance practices, proper motor design, and cooling mechanisms are crucial.

It is also important to understand the reasons that can cause an electric motor to overheat, such as excessive load, poor power conditions, high effective service factor, frequent starts and stops, and environmental influences. Addressing these reasons and implementing appropriate measures can prevent overheating and ensure the proper functioning of electric motors.

Factors like the Effective Service Factor and NEMA derating curve play a vital role in assessing the stress on a motor-load application under different conditions. Monitoring these factors helps ensure that motors operate within healthy limits and prevent excessive stress.

Thermography is an effective tool for determining the heat at which electric motors are operating. Through thermographic inspections, abnormal temperature patterns can be detected, potential issues can be diagnosed early on, and appropriate corrective measures can be taken to maintain the optimal performance and longevity of electric motors.

By understanding the common causes of excessive heat, implementing preventive maintenance, and monitoring motor conditions, it is possible to extend the lifespan of electric motors, minimise downtime, and ensure their reliable operation in various applications.

How can Powerline Electric Motors help with maintenance?

Powerline Electric Motors can help with maintenance in the following ways:

  1. Routine Inspections: Our team of experts can conduct regular inspections of electric motors to identify any potential issues or signs of wear and tear. This active approach allows us to address problems before they escalate and cause major disruptions.
  2. Scheduled Servicing: We offer scheduled servicing to ensure that electric motors receive the necessary maintenance and cleaning. This includes tasks such as lubrication, alignment, and electrical testing to keep motors in optimal condition.
  3. Repairs and Replacements: In the event of motor failures or malfunctions, Powerline Electric Motors has the expertise to perform repairs and replacements. Whether it's fixing faulty components or installing new motors, our technicians can get your equipment back up and running efficiently.
  4. Upgrades and Retrofits: If you're looking to improve the performance or energy efficiency of your electric motors, we can assist with upgrades and retrofits. Our team can recommend and implement the latest technologies to enhance motor performance and reduce energy consumption.
  5. Emergency Services: We understand that unexpected motor breakdowns can occur at any time. That's why we offer emergency services to provide immediate assistance and minimise downtime for your operations.

With our experience and knowledge in electric motor maintenance, Powerline Electric Motors is dedicated to ensuring the reliability, longevity, and optimal performance of your motors. Contact us today to discuss your maintenance needs and how we can assist you.

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