Overload faults in industrial electric motors and how to solve them

One of the most common causes of electric motor failure is overload. Whether it is mechanical or electrical overload failure; no matter the type, electric motor overload occurs when the output torque of the motor is not sufficient to move the load. This article will discuss how to detect an overload in electric motors, what causes the overload and how to replace an overloaded motor correctly.

How to tell if your engine has been overloaded

An electric motor can operate in an overload situation for a period of time, days or even weeks. In fact, many motors have a rating called service factor that allows for short, periodic bursts of overloads, but not prolonged overloads. Overloading the engine operation for a prolonged period will cause the engine to eventually fail. The main symptoms of overload in electric motors are excessive current draw, insufficient torque and overheating. Here are some ways to detect an overload in an electric motor:

• Smell: The engine may smell hot or burnt during and after operation.
• Listen: the engine may make more noise and have additional vibrations as the shaft works harder to rotate the load.
• Observe: the motor itself, stator windings, rotor or wiring may appear to be burnt.

Failures in overloaded electric motors

If you have already implemented overload measures, you will find that the motor no longer starts due to a tripped circuit breaker or a blown fuse.

If you have noticed a change in operating efficiency and wish to test for overload before an electric motor failure occurs, there are a few different tests to try. Using a multimeter with a clamp-on attachment will allow you to test for excessive, insufficient or fluctuating current situations that lead to an overload of the electric motor. Vibration analysis can also be used to test worn bearings and may even help detect shaft misalignment. You can also try testing the temperature during motor operation to look for fluctuations and increases.

Causes of electric motor overload faults

Now that we know that overloading is one of the most important main causes of failure in electric motors. We know that overloading occurs when the available output torque of the motor does not match what is needed for the application. And now that we know how to recognise overload damage in electric motors, let's take a look at the factors that cause an electric motor to run overloaded.

Mechanical overload of electric motors

Mechanical overload are the non-electrical problems that cause an electric motor to over-run and draw additional current to compensate. This additional current will damage the windings and cause the electric motor to fail.

Some of the causes of mechanical overload failures in electric motors are:

• When the load is too heavy, it will demand a torque output from the engine that exceeds the engine rating.

• Worn or damaged bearings due to contaminants, old and decomposed lubrication, vibration or overheating will put pressure on the rotating shaft, requiring a higher current to keep the motor running.

• Misalignment of the bearings on the shaft or misalignment of the motor shaft with respect to the load will also cause the motor to operate in an overload situation.

Steps for detecting overload faults in electric motors and solutions

Some of the key areas that could be responsible for engine overloads that may need further investigation are:

• It is possible that there is a mechanical overload on the motor causing repeated tripping and even damage to the electric motor winding. In this case, it is important to reduce the load as quickly as possible, as buying a new motor can be an expensive proposition and will increase operating costs.

• The main power source to the motor should be shut down immediately to conduct an investigation to identify the exact reason for the trip. It is also beneficial to ensure that the overload relay system is set correctly. Generally, this is set at 110% of the total power capacity of the motor.

• In many cases, it may be necessary to properly align the transmission components with the engine to avoid mishaps of this nature.

• The exact voltage being supplied to the motor should be checked periodically, including the possibility of loose contacts that could cause tripping or blown fuses.

• The actual source of the relay trip point must be located and replaced, if defective.

• The electrical wiring to the motor should be checked to ensure that all connections are made in accordance with the requirements. In the case of single phase or open, all the different combinations available should be checked to ensure that all connections are in order, especially red to blue, blue to yellow and red to yellow phases.

• Use a clamp on meter to check the amps when the motor is running to identify if an overload is occurring. If the amps registering on the meter reflect that there is an excess of what is recommended by the motor manufacturer, then this is a sure sign that there is a mechanical overload, which should be reduced as soon as possible.

• There may be instances of less power or amps reaching the motor and yet a trip occurs. This is a sure sign that there is a defect in the motor.

Electrical overload of motors

Electrical overload occurs when the motor overload failure is due to a fluctuating, excessive or insufficient voltage situation in the power supply or motor controller. The amount of overload the motor can withstand for short periods will be indicated by the service factor rating on the nameplate of the electric motor. For operation in low voltage situations, the National Electrical Manufacturers Association (NEMA) recommends operating the motor at not less than 90 % of the nameplate voltage rating and 110 % in overvoltage situations. Shorted conductors inside the motor can also cause an overload fault.

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

Other areas related to electric motor failures and their remedy

Once the correct diagnosis is made to identify the root cause of the problem, more than half the battle is won. However, it may be necessary to continually check the future voltage being supplied to the engine.

In a three-phase motor, which is most commonly used for commercial purposes, the amount of power consumed by a motor must be equal from all phases. Any imbalance can cause a problem. If there is an imbalance of more than 10% between any of the phases, there may be several causes, which must be located and corrected immediately to avoid further mishaps.

In some cases, when a new engine is installed, it may refuse to run. The most obvious reasons could be incorrect wiring of the motor or a problem with the motor itself. The cause of this malfunction must be established quickly and a suitable solution found. There may be cases where the engine has been running, but refuses to start another time. The reason for this could be that the circuit breaker or fuse tripped due to an overload or that the motor went to ground and shorted out.

The other reasons that could be behind a motor failure are voltage fluctuation (mainly low voltage), a failing capacitor or a damaged stator. Worn bearings can also be a cause and should be replaced when the problem is detected.

Whatever the reasons for an overloaded electric motor, it is essential that you get to the root of the problem as soon as possible to avoid further complications and avoid the resulting increased costs that can dent your bottom line.

Replacement of an overloaded electric motor

Once you have validated that the electric motor has failed due to an overload, there are a few things to do before purchasing a replacement motor. You will need to perform some tests to find the exact root cause of the overload and troubleshoot any problems that may be present. Understanding the intended application is critical to avoid another overload failure in the electric motor. Knowing your power capabilities, understanding possible motor control limitations, correctly reading the motor nameplate and understanding the motor's capabilities, and learning its intended application can help you choose the right electric motor and motor control with the correct output ratings.

Summary

Most, if not all, electric motor manufacturers do not offer a warranty if the product has been damaged due to overload. That's why you need to understand what can cause motor overload, learn to recognise and understand your intended application so you can avoid damage to your expensive assets. Our electric motor experts are constantly writing in this specialised blog about main electrical problems in industrial engines to help you understand the nameplate and motor application so you can get a replacement motor that will last.