Hot Tip #152 Motor Failures

Reader’s question: 
I’m having a lot of motor failures, would you point out some causes and solutions?

Motors failures break down into two categories:

  • Mechanical
  • Electrical

Mechanical

These failures are the easiest to diagnose and correct.

Bearing seize:

Solutions: 

  • Check the lubrication; there could be too little or too much lubrication.
  • Check the belts, for both tightness (could be too tight) and alignment.
  •  Motor may have been in an environment that it was not rated for; an example - installing any open motor around water is never a good idea.

Electrical:

Electrical failures are much more difficult to diagnose and correct.

Cause:

Stator (windings) short circuit:

Electrical motor failures are caused by the short-circuiting of the stator (windings). The short circuit was causes by insulation failure. Failure of the insulation can be caused by many variables, mostly related to exceeding the temperature rating. The insulation is a very thin coating applied directly onto the windings.

Failure of this insulation could be caused by:

  • Electrical spikes
  • Voltage drops
  •  Poor power factor (PF)
  •  Lack of motor ventilation

In this Hot Tip we are going to review only insulation failures due to lack of ventilation on TEFC motors, (Totally Enclosed Fan Cooled). It is crucial to keep air circulating over the motor. Lint or other contaminates in the motor cooling fan will cause both lack of air flow and mechanical stress.

When a motor is driven by an inverter at low speeds two problems can occur:

  1. Fan speed is reduced, reducing airflow.
  2. Motor heat increases due to lower operating efficiency, including lower PF.

When installing inverters on TEFC motors, remember to keep the slowest speed high enough to keep the motor cooled. Motors with cooling fans independently driven are available. On these motors the fan runs at a constant speed irrespective of main motor operating speed.

Remember, you can run most four pole motors up to 4,000 RPM, (130 Hz). You may need to run the motor at a higher speed and then reduce the speed mechanically. This keeps the cooling fan running at a high speed and improves motor efficiency and PF,

When I am replacing the drive system on old ironers, I like to design the speed reduction system such that at maximum iron roll speed, the drive motor is running at 90 to 100 Hz.

 

 


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