Categories: Condition Monitoring,Machinery Diagnostics,Machinery Maintenance,Other Topics

TROUBLESHOOTING MOTOR UNBALANCE

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By on June 20, 2017

Electric motors are almost always well-balanced when they leave the motor manufacturer.  But occasionally, motor unbalance problems are picked up with vibration monitoring tools, such as the OneProd Hawk.

Here are some easy troubleshooting methods to determine the cause of unbalance in the motor.

An electric motor’s rotor, like any rotating mass, cannot become out of balance, unless something is either added to it, or taken away from it.  But the components attached to the rotor can become unbalanced, such as:

  1. The cooling fan. If motor unbalance is diagnosed, inspect the cooling fan for dirt build up, or broken fan blades.  If the cooling fan is dirty, clean it, and recheck the vibration.  If the fan is broken, it should be replaced.
  2. The coupling hub. Many, but not all, coupling hubs are balanced.  More likely, the hub could either be experiencing:
  • Eccentric bore.  Use a dial indicator to check for runout on the coupling hub.  If the hub is bored out of center by more than 2-3 mils, it should be replaced.
  • Improper key stock length. There are a handful of methods to determine proper key stock length, but the easiest is:

Hub length + key slot length ÷ 2 = length of key stock

In addition, the coupling should be assembled with the keys opposed 180 degrees, to help counterbalance any residual unbalance forces.

An easy field method to narrow down the unbalance problem, if vibration levels indicate motor unbalance:

  • Uncouple the motor, and re-check vibration. If the vibration drops substantially, the problem could be misalignment, not unbalance.  Or, the coupling element is unbalanced.
  • If this does not reduce the unbalance, remove the coupling hub from the motor shaft, and re-measure. If vibration is reduced, check to make sure the hub is concentric, and the key length is sized correctly.

If motor vibration is still excessive, inspect the cooling fan.  If no problem is found, the motor should be sent to a motor shop for balancing.

About the Author

Stan Riddle joined VibrAlign in 2008. He has over 35 years experience in aligning industrial machinery. Stan received his AAS Degree in Machinist Technology from Surry Community College in Dobson, NC, and also holds a diploma in Industrial Systems Technology from Forsyth Technical Community College in Winston-Salem, NC, where he was also an instructor in the program.

Stan began his maintenance career working as a machinist and millwright for companies such as Weyerhaeuser, R.J. Reynolds, and Tyco Electronics. He also has over 25 years experience in Predictive Technologies, such as vibration analysis, thermography, oil analysis, and ultrasonic inspection. He is a certified Level III Vibration Analyst with the Vibration Institute, and is a Past Chairman and Board Member of the Piedmont Chapter.

Stan and his wife live in Yadkinville, NC.

2 responses to “TROUBLESHOOTING MOTOR UNBALANCE”

  1. Rick Diese says:

    In addition, the coupling should be assembled with the keys opposed 180 degrees, to help counterbalance any residual unbalance forces.

    I’m not sure I agree with this statement. One could easily make it worse no better. One would be assuming both keys are too long, not sure on the odds of this. Could you have just as easily said “try different coupling positions,, relative to each other, and see if one is better than the others. Or am I missing something?

  2. Stan Riddle says:

    Thanks for your comment Rick! It is an interesting discussion.

    If a shaft is well-machined, and not bent, it should be well-balanced. But if it is machined for a key slot, removing metal would slightly unbalance it. Broaching the coupling hub for a key would do the same thing as machining the shaft. If the hub is factory-balanced, no problem. If the key was cut to the proper length, it should counteract residual unbalance forces from the key slots. It would not matter if the keys were opposed or not.

    But keys usually are generally not cut to length with the same precision as machining the slots. It could be too long, too short, or just right. It’s random, so you are correct. But at least in the industries I’ve worked in, it is standard procedure to oppose the keys. You certainly could try positioning them differently, and find a good counter-balance point – if you have the time.

    In my experience, I have reduced vibration a few times by opposing they keys. But I’ve never increased vibration by doing it.

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