MRO Magazine

Focus on Motors: Hot Stuff!

By Cyndi Nyberg   

You've just installed a newly rewound motor, and it's running hot. In fact, the frame is getting so hot that you can't put your hand on it. You didn't change the design and it was tested before it was sent it out. You're probably wondering if the...

You’ve just installed a newly rewound motor, and it’s running hot. In fact, the frame is getting so hot that you can’t put your hand on it. You didn’t change the design and it was tested before it was sent it out. You’re probably wondering if the motor been rewound incorrectly. What should you do?

Your first thought likely is to call the rewind shop and have it figure out what is wrong or rewind the motor again. Before you consider this, there are a few things to check to see if the motor is, in fact, running properly. It is quite possible that the motor ran hot before it failed. Did someone put their hand on the frame to check the temperature before it had to be rewound?

The fact is that there is no way to determine the winding temperature of a motor just by feeling the frame. The maximum temperature rating, based on the class of insulation, applies to the winding temperature at the hottest spot inside the motor. The temperature of the frame can be between 20C and 40C (or less, or more) cooler, depending on the design of the frame and the enclosure.

The insulation class of a winding is determined by the maximum temperature it can withstand before complete breakdown. The majority of motors manufactured today use a Class F (155C) or Class H (180C) system. Many service centres will use a Class H system on all rewinds. Assuming that the ambient temperature is 40C, the NEMA standard states that at 1.0 service factor, the maximum winding temperature rise for a Class F system is 105C.


Even though the insulation system may be able to withstand very high temperatures, for every 10C rise in total winding temperature, the insulation life will be cut in half.

But a hot frame doesn’t mean the motor is running too hot.

The following example will illustrate that the motor frame can be quite hot even if it is running perfectly fine. Let’s assume that a particular motor is wound with Class F insulation and has a 40C ambient temperature. Let’s also assume that the motor is a high-efficiency design that operates with a Class B temperature rise (80C winding temperature rise).

At full load, we can assume that the total winding temperature is somewhere around 120C, well below the design limit for the Class F insulation. If we estimate that the frame temperature is 40C lower than the winding temperature (which is fairly liberal), the frame is going to be about 80C on the surface. That’s hot, as 80C equals 176F. I don’t know about you, but I certainly couldn’t hold my hand on a frame that hot for any length of time!

Check the amps

The best way to determine if the motor is running properly is to check the current draw with an ammeter. If the current is less than or equal to the nameplate current, then you are probably not overheating the motor. If the current is higher than the nameplate reading, then it is possible that the motor is running too hot. Overloading, high or low voltage, or high ambient are the most common causes of motor overheating.

In general, old NEMA U-frame motors run cooler than today’s T-frames, mainly because the insulation systems were not as good, so the motors were required to run at much lower temperatures. If a U-frame motor is replaced with a T-frame, it may be alarming to feel that it runs much hotter, when actually it may be running even more efficiently than the old one.

Explosion-proof motors

There is one case where it is very important that the frame surface temperature be kept somewhat cool. An explosion-proof motor requires that the surface temperature be below the ignition point of the hazardous material in the environment.

These motors come equipped with temperature sensors that will trip the motor offline when the winding temperature exceeds a certain level. Underwriters Laboratory sets standard temperatures based on the gas, vapour or dust present in the environment. However, there is no standard for other general-purpose motors.

The best way to determine whether a motor is running within a safe operating temperature is to use temperature sensors to monitor the winding temperature. You can also monitor the current to get a basic idea of how a motor is running. However, do not rely solely on the frame surface temperature.

Cyndi Nyberg is a technical support specialist with the Electrical Apparatus Service Association (EASA). For details, visit


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