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Electric Motors: Why Do New Ones Fail Sometimes?

09/02/2020

2020-06-Engineer blowing off steam over new motor failureOne of your mission-critical motors failed and while it's out for repairs or rebuild, you installed a new one. There are no words to describe how you felt when that brand new electric motor failed, as well. You’ve got more downtime now, along with additional repair costs.

Why did that new electric motor fail?

A Historical Perspective on Motor Durability

Today's electric motors aren't made the same as they used to be: modern motors are made with less steel, many times not as much service factor, and sometimes they aren't as durable. These changes are actually reflected in the motor frame classifications. Back in 1954, U frame motor classifications were introduced with standardized dimensions. These motors were heavy, rugged, and reliable with the right maintenance. In fact, some would say that they were over engineered: more powerful and heavier than they really needed to be to get the job done.

Things to Consider When a New Motor Fails

If you’re trying to figure out why your new motor just failed, you should consider what happened with the previous motor, the environmental conditions, the operating conditions, whether the motor was installed correctly, and the common failure modes for electric motors.

Is is the right motor?

Are you sure you put the right new motor in place? Was everything checked out prior to putting it in place? There are so many sizes and options when it comes to motors, maybe a mistake was made and you should evaluate what motor is needed.

What Happened with the Previous Motor?

When a brand new electric motor fails, one of the first things you need to consider is how the previous motor in that application failed. Was a cause of failure found? Was it addressed? Could it have been diagnosed incorrectly? If it was diagnosed incorrectly, that could be the cause of issues with the new motor.

Environmental Conditions

Environmental conditions can affect motor performance, even when your motor is brand new. So it makes sense that when a new motor fails you should take a look at how it was stored. For example, motors need to be kept warm -- if temperatures drop, they could reach an incapable dew point and lead to problems with corrosion.

Your surplus motors need to be kept in a dry environment where they are protected from contaminants like dirt and dust. Moisture combined with contaminants can wreak havoc on even the best motors.

Operating Conditions

Some motor failures are not due to a flaw in the motor itself or in its installation, but in the operating conditions. Power quality, vibration, and overcurrent can also be factors that affect motor performance and lead to the failure of new motors. It’s also important to make sure that you actually provided the right motor for the right application. If a motor is subjected to operational overload it could lead to serious damage and failure, no matter how good the quality of the motor may be.

Installation

If your motor was not installed correctly or mounted properly, that can lead to problems. An imbalanced shaft or looseness in the drive shaft can lead to vibration and accelerated wear. You need to be aware of mounting issues such as soft foot, which refers to the feet of the motor not being even. Soft foot leads to misalignment, vibration, and, once again, accelerated wear. It is also critical that the motor be electrically attached in a proper manner.

Common Failure Modes

When trying to determine why a new motor failed, it is also helpful to review the common failures modes.

Shaft voltages can damage the bearings in your electric motor. When these voltages exceed the electrically insulating capabilities of the grease in the bearings, the voltages will spark. Over time, those sparks cause pits and grooves in the races, which in turn leads to vibration and eventual bearing failure.

Transient voltages can come from a variety of sources and can damage brand new motors. Possible sources include everything from weather issues affecting the local power distribution system to nearby equipment cycling on and off. These transient voltages damage the insulation on your motor windings.

Stray currents that are the result of inductance and capacitance in conductors, signal frequency, or voltage levels are known as sigma currents. They can lead to overheating in the windings and sometimes cause the motor circuit to trip.

Harmonic distortion results from unwanted sources of high frequency AC currents or voltages. These unwanted currents supply additional energy to the windings -- but that energy merely circulates in the windings. The result is energy losses that take the form of heat which can damage the windings over time.

If you have a variable frequency drive, you may encounter issues with PWM (Pulse Width Modulation) signal reflections. These reflections are generated when there is a mismatch in impedance between the load and the source. PWM reflections are usually caused by the natural degradation of the motor over time, improper component selection, or improper installation. The result of PWM signal reflections is the breakdown of winding insulation.

In a three-phase motor, all three voltage phases should have the same magnitude. A voltage imbalance leads to excessive current in one or more phases. That excessive current, in turn, leads to increased operating temperatures that can cause a breakdown in the winding insulation.

Conclusion

It’s frustrating and disheartening when a brand new motor fails, but there are specific considerations that will help you track down what caused it. It might be as simple as an installation issue!

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Author & contact information: Hunter Shields  hshields@hecoinc.com (312)415-2096

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