June 20, 2017
TROUBLESHOOTING MOTOR UNBALANCE
By Stan Riddle
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:
- 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.
- 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.