AC motors

Is there a general guideline other than the rating on the motor that anyone is using to determine how hot is too hot for a bearing area on a motor? I take a look with IR monthly on NH3 compressors along with an ultrasound inspection. Is there a guideline based on the HP of the motor? I have read that a 250 HP motor should be no hotter than 140 F. A motor vendor says the they don't get worried until it hits the 165 to 170 F range. I have attached an IR that shows the temp is increasing. I welcome any and all thoughts.


Original Post


Here is a link to article discussing motor allowable temps, below is quote from article

Many improvements have been made in motor insulation in the past couple of decades, such that a Class F (or better) insulation system is common for motor winding insulation. With conservative motor design, a 'Class B' temperature rise is quite achievable. For a 1.15 S.F. motor wound with Class F insulation, a Class B temperature rise at full load translates to a thermal margin of 155-(40+90) = 25°C. This means insulation life will be approximately 5 to 6 times that which would be expected with a motor operating at its rated insulation temperature (i.e. for every 10°C lower temperature, the insulation life is

Too hot is when the lubricant melts out to a puddle under the motor end bell.

I see an IR images taken 9 days apart. I do not know if the machine was loaded or just started, the past history of the temps on this bearing, if bearing has been greased recently, any alignment or maint activities, thrusting, all which can affect the bearing temps. If you are also using ultrasound, is it too increasing? What about the front bearing, is it just as hot? 200F is too hot for a bearing, if the oil from the grease leaks out, the bearing is too hot. Whether a 5hp or 250hp 200F is hot

I would highly suggest you get vibration data on the bearing to confirm any bearing defect

Hope this helps


The ultrasound and vibe data have agreed that there is an issue with this bearing, and now that I am seeing an increase in infrared temperatures, I have attempted to expedite the need for a scheduled repair. Again I thank you for your response.

The rating on the motor is the winding classification, it doesn't really apply to the bearing. These are effectively treated differently than the motor winding. I would not use the winding classification to determine the expected bearing temperature. If you are getting via data, plus ultrasound suggesting that you have a problem, then you definitely have a problem. The temperature increasing is really telling you that failure has begun, it is not really possible to predict how long it will take to totally fail. While the temperatures are not that high (around 68C), this is the surface temperature, it is probably 20 C more inside the bearing, so approx 88C. I would think this is on the high side for a bearing.

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