What Causes Motor Overload?

August 15, 2022

One of the most common causes of electric motor failure is overload. Mechanical or electrical overload; it doesn't matter what the type, overload is when the motor's output torque isn't sufficient to move the load. This post will look at how to detect overload, what causes overload, and how to replace an overloaded motor properly.

How to Tell if Your Motor Has Been Overloaded

Your electric motor may run under an overload situation for a while, days or even weeks. In fact, many motors have a rating called Service Factor that allows for periodic short bursts of overloads, just not prolonged overload. A motor run overload for a prolonged period will eventually fail. The primary symptoms of motor overload are excessive current draw, insufficient torque, and overheating. Here are a few ways to spot overload in your electric motor:

  • Smell: The Motor may smell hot or burnt during and after operation.
  • Listen: Your motor may be louder and have additional vibration as the shaft works harder to turn the load.
  • Look: Your motor, stator windings, rotor, or other wiring may appear burnt (see photos).

  • Additionally, if you already have overload measures in place, you'll find that the motor will no longer start because of a tripped circuit breaker or a blown fuse.

If you have noticed a change in the operational efficiency and want to test for overload before a failure has happened, there are a few different tests to try. Using a multimeter with a clamp attachment will allow you to test for over-, under-, or fluctuating current situations that lead to overload. Vibration Analysis can be used to test for worn-out bearings and might even help detect shaft misalignment. You might also want to try testing the temperature throughout the motor's operation to look for fluctuations and increases.

Causes of Motor Overload

We now know that overload is one of the leading causes of motor failure. We know overload is when the motor's available output torque doesn't match what is needed for the application. And we know how to recognize overload damage, let's now take a look at the factors that cause a motor to run overloaded.

Mechanical Overload

Mechanical Overload is the non-electrical issues that cause an electric motor to overwork, and it will draw additional current to compensate. This additional current will cause damage to the windings and lead to motor failure.

  • When the load is too heavy, it will demand a torque output from the motor that exceeds the motor's rating.
  • Bearings worn or damaged due to contaminants, old and broken-down lubrication, vibration, or overheating will put a strain on the spinning shaft, requiring a higher current to keep the motor running.
  • Misalignment of the bearings on the shaft or misalignment of the motor shaft to the load will also cause the motor to work in an overload situation.

Electrical Overload

Electrical Overload is when the motor's overload failure is due to an over-, under-, or fluctuating voltage situation at the source of power or the motor controller. The amount of overload your motor can withstand for short periods will be stated by the service factor rating on the motor's nameplate. For running in under-voltage situations, NEMA recommends running the motor at no lower than 90% of nameplate rated voltage, and 110% in over-voltage situations. Short-circuited conductors within the motor can also cause an overload failure.

Both types of overload cause deterioration of the thermal insulation and damage all phases of the motor's windings.

(photo taken from - https://easa.com/resources/failures-in-three-phase-stator-windings)

Replacing an Overloaded Motor

Once you have validated that your electric motor has failed due to overload, you have a few things to do before purchasing a replacement motor. You'll need to do some testing to find the exact root cause of the overload and fix whatever issue might be present. Understanding your intended application is paramount in avoiding another overload failure. Knowing your power capabilities, understanding your motor control's possible limitations, correctly reading your motor nameplate and understanding the motor's capabilities, and learning your intended application can help you choose the electric motor and motor control with the correct output ratings.


Most, if not all, electric motor manufacturers do not provide a warranty if the product has been damaged due to overload. That is why you must understand what can cause motor overload, learn how to recognize it, and understand your intended application so you can avoid damage to your costly assets. Our experts here at eMotors Direct are available to help you understand your motor nameplate and application so that you can get a replacement motor that will last.

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