Please excuse what might me a basic question. I spend most of my time doing vibration analysis and thermography is one of the extra tools in the bag.

At my plant we tend to use 70°C as a rule of thumb temperature for investigating motor problems. Anything less tends to be ignored until the next survey. Obviously this depends on the thermal profile and whether it is a localised hot spot or not. The first question is; does this seem too high/low?

The second question is; At what temperature would you write a motor off and change it, because it's reached the point of no return. Is there a rule of thumb temperature where I can consider that the insulation, bearings, grease, etc has been cooked and the motor is past saving?

Thanks.

Limiting temperatures for insulation are quite high. Continuous temperature ratings for insulation are 130C for class B, 155C for class F, 180C for class H.

Limiting temperature for grease are lower. Depending on the type of grease, you could start seeing accelerated aging as low as 90C.

Above are internal temperatures which you can't see. We don't really know the exact difference between the temperature you see outside and the true internal temperature. Most people apply perhaps a 10C - 20C allowance for the temperature difference and subtract it from those limits above. So at 70C if temperature is on the housing and appears to be coming from the bearing, you might start worrying about the grease. Also keep in mind that housing temperature usually spikes up after greasing.

I think Pete gave excellent advise and guidelines. However, I like to see additional info such as PI, vibration data, etc..., a little more than temps alone. I like to use them as a flag and perform additional testing to make a final call.

Great point Sam. I certainly didn't mean to imply the motor should be yanked based on temperatures I mentioned. Only that those should trigger investigation. For example, if the whole side of the motor is hot (not bearings), check the current load against nameplate, and check the cooling.

Thanks for the input guys. I'm starting to formulate some sort of idea now.

The grease that we use on site for motors is Renolit LXEP2 which I'm told can go up to 160°C for short periods of time without deteriorating.

I've also found the limiting temperature for class A insulation at 110°C

If I remember correctly a bearing begins to change metallurgically (temper) at approx 115-125°C. That's why these temperatures should be avoided when hot fitting a bearing.

So depending on the insulation class, it's looking like anything from 110°C and above may instigate the start of a failure.

I've got an old article here by Ron Newport (Condition Monitoring Training Institute, British Columbia) which says "When measuring the outside surface of a motor casing, it is safe to predict that the insulation temperatures on the inside will be at least 20°C above the measured casing temperature."

Allowing for the difference in external and internal temp means we could be looking at anything from 90°C upwards.

I would just like to ask further on the effect on greasing. The spike on the temperature after greasing, is it not because you have applied too much grease on the bearing. Meaning over greasing.

I think greasing is carried out in different ways. Motor running and not! A vented plug is recommended and if running, inject very, very slowly to a calculated quantity. If motor is not running you can overgrease and no immediate affect on temperature. Running and an immediate effect can be noticed with only rapid injection; overgreasing and you'll see long term elevated temperatures.

Assume you don't know nor have a program with exact quantities per a given frequency - monitor via vibration, add 5 grams, monitor, wait 8-10 minutes, add 5 grams more, monitor and repeat process. I think it will be obvious. Do record everything.

Got some time on your hands? Machine down! Know bearing and cavity size? Remove grease fitting and plug and pull or remove grease via vacuum and start over measuring quantity.

I tend to look at a lot of drive systems running conveyor systems. As others have mentioned its very difficult to get meaningful analysis out of Thermography without combining it with another technology, such as MCA or Vibration.

Its probably worth mentioning the importance of historical data however, as it can have far greater practical value when what you dig out of textbooks.

You might want to go back over your old thermographs and table up the recorded temperatures for different motor ratings. With these you can figure out the tolerances for normal operating conditions. When a motor jumps up above these limits you then know somethings up.

But back to your original post, once a 2.2kW motor goes above 70 degrees C I'd flag it to the engineers. Larger motors however would normally run that hot.

On greasing, can you comment on using ultrasound instrucment that has a grease adapter. The grease instrument is connected to the ultrasonic equipment and to your grease gun.