The 50 Failure Modes of Electric Motors
Electric motors are essential to numerous plants operations, no matter the industry, which is why understanding their 50 failure modes can help you develop a better maintenance program in your plant.
Electric motors are essential for making sure that plants are running smoothly and effectively. If one fails, it can mean costly downtime for the plant and create a variety of safety hazards. There are a number of different failure modes out there, so by understanding them, the lifespan of a motor can extend from two to 15 years.
The key is moving from the reactive category of the PF curve to the predictive phase. By using ultrasound technology, such as the Ultraprobe 15,000, you can detect problems before they start to create serious damage in the motor. Because there are so many different components within a motor, a failure mode can emerge in a variety of places. There are between 8 and 10 components within a motor, each with its own failure modes, bringing the total to around 50, so by properly addressing them, you can greatly extend the life of your motor.
Failures in motor housing can crop up from improper installation, physical damage, corrosion and material buildup. While motor housing may not seem like a true performance component, these shortcomings can ultimately affect the way others perform.
For instance, a soft foot could lead to bearing failures, shaft bending and broken or cracked feet. This emerges if a motor, when placed in a flat surface, does not have all its feet flat on the surface. Material buildup can heat up the operating temperature of the motor, ultimately leading to damage on other parts of the motor, such as bearings.
Motor stator failure modes emerge from physical damage, contamination, corrosion, high temperature, voltage imbalance, broken supports and rewind burnout procedures. A lot of times, these can emerge from motor repair shops.
Stator failures occur due to the rewind burnout of the windings. This often happens before the motor can be rewound requiring emergency repairs. But because the plant will need the motor returned as soon as possible, hasty repairs can end up damaging the stators by improperly heating the housing and the stator. This can also lead to motor inefficiencies.
Rotors are composed of numerous layers of laminated steel and the rotor windings are composed of bars of copper or aluminum alloy that is shorted on both sides with shorting rings. These components can then fail through thermal stress, physical damage, imbalance, broken rotor bar, contamination and improper installation.
Physical damage on rotors can develop after certain emergency maintenance tasks including bearing replacement, motor rebuilds and during a disassembly and reassembly process. Generally speaking, motor bearings should not be changed at plant locations and especially on critical equipment.
Imbalanced motor rotors are common, but this can put a lot of strain on bearings. This will ultimately lead to a rotor making contact with a stator and creating another point of failure. Again, improper rebuilding tactics, such as overheating, can damage rotor components as well.
By establishing precision balance standards, you can be sure you are preventing these kinds of imbalance failures.
Motor bearings within an electric motor can emerge from improper handling and storage, improper installation, misalignment, improper lubrication, start/stop processes, contamination, overhung loads and motor fan imbalance.
Contamination is one of the biggest reasons for bearing failure modes. This occurs when foreign contaminants or moisture enter the bearings, usually during the lubrication process. You can take steps to prevent contamination during the regreasing process to ensure that they are kept out.
It is also important that your motor is properly outfitted for the task for which it was selected. This means using the right bearings for its application. Motors that are using sheaves or sprockets that are mounted on the shaft will need roller bearings in the motor, which are common among most standard motors.
Lubrication can always be a major cause of failure because there are so many different places where one can improperly apply lubrication. Too much or too little lubrication, along with the improper form of lubrication, can lead to premature wear and tear. All motor greases should be polyurea-based, and not all purpose greases. One should always take the plug out of the bottom so that old grease can be drained properly. Also, release valves can help prevent over greasing.
The UE Grease Caddy can be a great tool for listening to when lubricating a motor.
Motor bearing seal failures tend to emerge from improper lubrication or installation.
Motor fans tend to fail from physical damage, ice buildup, foreign materials and corrosion. Fans help keep the temperature down on a motor, which is essential to making sure that the rest of the components are performing well.
The motor fan guard failures can also lead to a larger motor failure. This tends to happen through physical damage and plugging. By taking the time to keep them clean, you can go a long way in preventing fan guard failures.
Motor insulation and windings
When it comes to motor insulation and windings, there are a number of potential issues. Contamination and moisture can lead to winding failures. Often times, this is because they are not stored in ambient areas. Overheating is another issue that can cause a motor failure. Insulation breakdown, cycling and flexing, along with AC drive stress, round out the possible failure modes for this category.
The life of the insulation in a standard electric motor is based on the temperature at which the motor operates. This means for an electric motor that is operating at a particularly high temperature, you could be cutting back on its lifespan. In fact, for every 18 to 20 degrees Fahrenheit, the insulation life is cut in half. While better insulation can extend the lifespan, temperature is easily one of the biggest factors in this instance. This means bringing in cooler outside air.
Insulation breakdown can be a big problem, as it will cause windings to short out. These problems can be detected through MCE testing and thermography. Winding shorts from turn to turn can crop up from contaminants abrasion, vibration or voltage surges.
Cycling and flexing is another problem that typically occurs from frequent start and stop operations from the motor. This kind of an operation cycle can lead to the frequent heating and cooling of windings and insulation, which can lead to wear and tear, such as holes, ultimately leading the motor to short and fail.
Motor shaft failure modes occur due to physical damage, improper manufacturing, improper installation and corrosion. For instance, installing a motor improperly can cause certain components, such as the motor casing, to corrode and create imbalance.
How to make your motor last
Now that we are aware of the various types of motor failure modes, we can take better steps toward creating a preventive maintenance plan.
Many maintenance tasks can be addressed through a weekly hands on inspection. Make sure to grease the motors as needed with the proper motor rated grease. Add grease or oil only when needed. Incorporating an ultrasound assisted lubrication program can go a long way in preventing bearing failure.
There are a number of ongoing tasks you can do to ensure that motors are in their best performance conditions. Keep your motors clean and at the proper temperature with consistent airflow, and store motors properly to keep moisture from contaminating them. Also, keep moisture and chemicals away from the motor so as to prevent contamination.
There are also a number of precision maintenance steps you can take in order to enhance the performance of your motors and reduce wear and tear. Always align your motors to under .003 in all three planes, while also taking care to eliminate soft feet. Specify the precision balance of the rotor to .05 in per second. Finally, only use certified motor rebuild shops, because as we discussed earlier, improper repairs can lead to greater damage down the line.
In terms of predictive maintenance measures, use motor circuit evaluation to detect all motor failures. Vibration analysis can be used for a number of other motor failures while mechanical ultrasound can be used for bearings, rotor bars and electrical failures – also use oil analysis on sleeve bearings with oil reservoirs.
There are a number of other ultrasound applications as well. Failures tend to first appear in bearings, meaning that the Ultraprobe 15,000 can be a great way to detect Stage 1 failures. The device is also great at detecting over or under lubrication. As ultrasound becomes an increasingly integral part of maintenance operations, so too are its applications. It can be used to detect electrical failures like arcing, rotor bar problems and rotor imbalance, along with alignment and soft foot issues.
Generally speaking, when a motor fails, you need to decide if it is worth rebuilding or buying a new motor. Using a motor decision flowchart can help guide this decision. Talk with a CMRP to find a decision flow chart for your operations.
Finally, you can get a lot more out of your motors by taking proactive maintenance steps. Purchase precision motors for all of your critical applications and always use precision maintenance for installation, alignment, balance and lubrication.
By adhering to these steps, you can extend the lifespan of your motors and limit downtime in your plant, effectively speeding up operations, limiting cost and improving performance.