Failure modes for electric motors

Electric motors are found everywhere in modern industry. They can be used to drive pumps, fans, conveyors, or crushers. Motors are often critical parts of operations, and in the event of a malfunction or failure, an entire production line can come to a halt, leading to enormous costs related to both lost production and damaged equipment.

To prevent such incidents, proper maintenance is crucial. There are several ways to perform maintenance: one can run equipment until it fails and then repair it, perform maintenance at specific time intervals, or conduct maintenance based on the actual condition of the equipment. The latter is known as condition-based maintenance and is becoming increasingly common. This approach allows for maintenance to be performed where and when it is needed, resulting in significant savings in maintenance costs and reduced unplanned downtime.

To perform effective condition-based maintenance, it is essential to understand the potential failure modes of the equipment. In this article, we review typical failure modes of various components in electric motors.

Overheating

Approximately half of the failures in electric motors are due to elevated temperatures. The causes of this can include overload, dirty cooling fins, cooling fan malfunction, high ambient temperature, worn bearings, electrical imbalance, and poor power quality, among others.

A rule of thumb is that the lifespan of a motor is halved for every 10 degrees Celsius increase above the motor’s maximum temperature.

The best way to avoid motor failures is to ensure that the motor operates within its allowable temperature range. Proper cleaning of the cooling fins and fan is extremely important.

Heat can be measured with Neuron Vibration, Neuron PT100, or one of the other temperature sensors in the product range. Larger electric motors often have a built-in temperature sensor in the stator windings, which can be connected to a Neuron PT100 to measure the internal temperature of the motor. This will provide a very early warning of thermal faults.

Vibration

Ball Bearings / Plain Bearings

Bearing failures are one of the main drivers for the maintenance of electric motors. Bearings are in general incredible robust when operating in optimal conditions, but they become vulnerable when the conditions are suboptimal, such as in cases of lubrication failure, misalignment of the shaft, dirt in the bearing, excess vibration, or extreme temperature. Lubrication failure is the primary cause of problems, whether it’s insufficient lubrication, excessive lubrication, incorrect type of lubrication, or old lubrication. In any case, lubrication failure will lead to either heat or vibrations.

Loose Mounts

If the motor, frame, or connected equipment comes loose from its mounts, it will result in strong vibrations. In the worst case, the motor can completely detach from the frame, causing significant damage and creating dangerous situations.

Motor Imbalance

Imbalance in the motors can be caused by loose parts, a misaligned shaft, shaft breakage, defective fan, or deposits. When imbalanced, the motor components experience greater mechanical stress, significantly shortening the motor’s lifespan.

Misaligned Shaft and Imbalance in Connected Equipment

Vibrations in connected equipment will transfer vibrations to the motor and shorten the lifespan of bearings and rotor, similar to how internal imbalance in the motor would.

Vibration can be measured with the Neuron Vibration sensor. All changes in vibration are of utmost interest, especially the sum of acceleration in all three axes, which provides very valuable and easily interpretable information about the motor experiencing an undesirable condition.

Reduced Motor Effect

If the motor performs below the expected output, it is a sign that the motor is struggling, whether it manifests as reduced speed, output, or overall performance. The causes are usually related to worn or unlubricated bearings, imbalance, electrical imbalance, or insulation failures.

Changed Power Consumption

Increased Power Consumption

If the motor shows a general increase in power consumption without a corresponding increase in output, it is a typical sign of imminent motor failure. Increased power consumption can result from insulation failures in the windings or increased mechanical resistance within the motor. However, increased power consumption can also stem from the load, so it must be investigated before drawing conclusions. Power consumption is measured with Neuron Ampere.

Electrical Imbalance

Electrical imbalance will cause increased current draw in the motor; an imbalance of just 5% can reduce the motor’s lifespan by 25%. If the imbalance also raises the motor’s temperature, degradation will accelerate even faster. Electrical imbalance can be detected by monitoring current draw (Neuron Ampere) and temperature (Neuron Vibration or temperature probes), as well as by tracking if motor protection trips more frequently (dry contact).

Abnormal Behaviour

Sudden stops, fluctuating performance, or problems with startup are typical signs that the motor is experiencing issues. Repeated startups significantly increase motor wear (known as short cycling) and are a well-known cause of motor problems.

Operating Hours

Most motors will operate according to specifications and perform well for many years. However, everything has a calculated lifespan, even under favourable conditions. To stay ahead, you can install hour meters (Neuron Hour meters) that accurately count the number of hours the motor has been running. This allows you to better estimate the remaining lifespan and plan replacements well in advance.

Links:

https://ibtinc.com/6-signs-electric-motor-failure/

https://www.fluke.com/en-us/learn/blog/motors-drives-pumps-compressors/13-causes-of-motor-failure

https://www.ecmweb.com/content/article/20893083/explaining-motor-failure

https://www.progress-energy.com/assets/www/docs/business/motor-protection-voltage-unbalance.pdf

https://www.dukeelectric.com/motors/failure/guide/