Procedure for trending temperatures on Electric Motors
Make sure the motor is at operating temperature (steady-state).
The first time the survey is performed, take a photograph of the motor. Take a thermograph (infrared picture) also, if you have an infrared (IR) camera. But you do not need an IR camera for this. If you have one – use it, but a $100 spot radiometer will do just fine.
Take a can of barbeque grill paint (E=~.95), SAFELY clean and/or paint the points where you plan on taking the temperatures.
Enter the date.
Enter the high temperature (HT). Get the high temperature from the inboard bearing.
Enter the ambient temperature (AT). Get the ambient temperature from the motor foot or a painted spot on the floor near the motor.
The Temperature Rise (Delta-T) will automatically show in the spreadsheet and on the graph.
If the trend is higher temperatures every time you check the motor:
Increase the frequency (say from monthly to weekly) and see if the Delta-T continues to rise and watch it closer.
Check other connected components, i.e., outboard bearing, winding, coupling.
Add other testing, i.e. airborne ultrasonic analysis, vibration analysis, oil and wear particle analysis, eddy current analysis.
General Notes:
Pick a temperature scale at the beginning of the trending. (Either °C – Celsius or °F – Fahrenheit and always use the same scale. Keep the surface clean with an air hose, so dust does not build up on the inboard bearing and insulate it.
After blowing it off, give the bearing a few minutes to come back to steady state temperature.
This trending is predicated on the fact that the Delta-T (HT – AT = Delta-T) will not change if the motor stays in the same condition.
If the motor’s inboard bearing gets hotter over time, something is wrong. Increase frequency and check other components. The motor’s inboard bearing is the pivot point of all other connected components, if something goes wrong anywhere, the bearing heats.
Don’t worry about being off a few degrees here or there. The idea is that you will be able to see a trend. Even if some errors are introduced, so as long as you use the same repeatable method, you will see the trend.
