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A Case Study: Heat Transfer In Action

Over the past several weeks we’ve talked about how heat “moves,” or what is called heat transfer (steady state and transient) and the modes of transfer (conduction, convection and radiation). While it is important to have a solid understanding of the theoretical basics, how does all this really apply to what we do every day as thermographers? Let’s look at a simple case study that illustrates something similar to what many of us see every day in our work places.

Over 90% of electricity used in industry passes through a motor at some point. Not all of that energy is converted into rotational force and as a result, motors get warm! If they get too warm, the very thin layers of insulation on the windings are damaged and the motor fails. Motors are designed and rated to operate at a given temperature. Most motors depend heavily on forced convection to keep them cool. When ambient air temperatures increase or when airflow is reduced, cooling is less effective and internal temperatures rise.

A Class F motor, for instance, can safely approach internal temperatures of 155°C (311°F). Heat transfer to the surface of these motors however, is very complex, and as a result, the surface can typically be approximately 20°C (36°F) cooler than the internal temperature. That means the surface of the Class F motor we are looking at could still be close to 135°C (275°F)—not something you’d want to get too close to, much less touch! But the important thing to know is that it is even hotter on the inside.

In many plants and factories smaller motors are basically “throw-away” assets, even though the cost of their failure and their subsequent replacement is not insignificant. Often there are just too many of them to monitor individually or they are in places that are too challenging to access with ease. Thermography can be used to quickly assess the temperature of such motors, even from a distance, to determine those that are warmer than they should be. Once these are flagged, they can be further maintained as schedules and resources allow. This motor (left), operating at a temperature exceeding it’s rating, was simply blown free of dust using compressed air. An hour later, it (right) was operating at normal temperatures! One of our students recently left a Level I course and inspected similar motors in her chemical plant. Her work over the next month caused her management to credit a $100K savings!

The motor (left) was plugged with production dust. An hour after it was blown clear with compressed air (right), it was operating at normal temperatures—heat transfer at work! Unfortunately, by the time this particular motor was found and cleaned, further testing revealed that permanent damage had been done. Even so, preventing a catastrophic failure resulted in total savings in the thousands of dollars.

We know heat transfers happen all around us. We also know that really understanding how heat transfer works means we are able to use thermography to help prevent and reduce costly damage to machine assets. Think about how you can put this theoretical knowledge into practice today.

Next week we’ll head back outside to talk a bit more about the power of that great big heater in the sky, the Sun.

Thinking Thermally,

John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner

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