What Causes Motor Overload?

March 7, 2023

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.
Temperature: the motor temperature will increase above the usual baseline temperature.
Motor won't start: 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.
Capacitor failure: a blown capacitor is probably due to an overload.

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.

What is 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.

What causes mechanical overload:

When the load is too heavy, it will demand a torque output from the motor that exceeds the motor's rating. This could happen in a crushing machine, where there's a lot of variation in what the machine is crushing and the load on the motor.
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.

What is 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.

What causes electrical overload:

There are issues with power being supplied by the power company.
Power imbalances in your neighbourhood or building.
Certain situations where the motor is being powered by a generator.

Electrical overload can be out of your control, so consider using an overload device.

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)

Nuisance Tripping

You may have also heard of “nuisance tripping”. One of our customers has an industrial laundry business with many washing machines, which can be overloaded by extra-large loads or by constant use without a cool-down period. Recently, one machine, in particular, wouldn’t start. As a temporary solution, they’d override the overload device by holding the “reset” button down and bypassing the overload warning to continue working. While it was a nuisance that the overload device kept tripping and causing downtime, a small issue became a large issue when they had to replace multiple overloaded motors. Moral of the story – if you have nuisance tripping, find out the root cause before the issue becomes more expensive and potentially causes more downtime.

Prevent Chronic Motor Overloading

Your motor overloaded for a reason, and we need to figure out why. To prevent future overload:

Check voltage and current (electrical overload)
Consider recent changes to process (mechanical overload)
Use an overload protection device

To rule out electrical overload, test the power source input at the motor, testing as close to the motor as possible helps you to rule out any issues with the power cable or the motor controller. You’ll check the voltage and the current to ensure that they match the motor nameplate ratings.

The voltage should be within 10% of the nameplate rating. If the voltage is too low, too high, or fluctuating or the motor is being supplied with too much current, the issue is with the motor controller or source power. If the motor is attempting to draw more current, the load may be too heavy, or the power source isn’t strong enough.

To rule out mechanical overload, consider if your process has changed recently. For example, we work with a food manufacturing company that changed the recipe of their dough. The new recipe resulted in a much stickier dough and their motors had to work harder to turn the mixer. I’m sure you can guess that they had issues with mechanical overload. Even though they had been making dough for 10 years, a simple recipe change meant they had to re-evaluate their entire system. We found some motors that could handle the new torque requirements.

If you don’t already have one, you should install an overload protection device to help protect your motor from those unexpected overload situations. You could go with a variable frequency drive, a starter, or an overload relay. It’s a Canadian Electrical Code requirement to use an overload device and it’s important to size it correctly.

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.

Is an Overloaded Motor Covered Under Warranty?

At least half of the warranty requests that we see are motor overloads and motor overloads are not covered under warranty. This is why it’s helpful to understand what caused your motor to overload, learn how to recognize it, and protect against it. If you need some help resizing a motor or finding the right overload device, contact us at the link below.

Summary

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.