Applying Motor Temperature Ratings

By: Stephen Horne

Motor temperature ratings are given by the type of insulation used on the wire as well as the utilization rate. These two parameters determine the expected lifetime of the motor windings.

The vast majority of motors used today are rated for 40 °C. These motors utilize Class F insulation with a Class B temperature rise (the utilization rate). Occasionally, you will encounter a Class H motor rated for 45 °C or 50 °C. So what is the difference between these two classes and what are benefits of one over the other? And what on earth do motor temperature ratings have to do with rabbits?

Motor Insulation Class Temperature Ratings

Each insulation class receives a maximum temperature rating. These temperature ratings represent the temperature at which the winding insulation begins to breakdown. This table from The Engineering Toolbox shows the electrical insulation systems rated by standard NEMA classifications to maximize allowable operating temperatures:


  • T(°F) = [T(°C)](9/5)+32

1 Allowable temperature rises are based upon a reference ambient temperature of 40°C. Operation temperature is reference temperature + allowable temperature rise + allowance for “hot spot” winding.Example Temperature Tolerance Class F:
40°C + 105°C + 10°C = 155°C

How Motor Insulation Utilization Rates Affect Winding Life

The next discussion is how a motor is utilized. It is clear that if we utilize a motor insulation to its full capacity, it will breakdown quicker. As a result, most class F motors are utilized at a much lower temperature limit. This has several advantages. The primary advantage is extending the life of the insulation by keeping it cooler. This then reduces the skin temperature of the motor, which limits the heat rejection, which keeps the bearings and grease cooler, as well as the package in which the motors are installed.

The following graph from Marathon Electric demonstrates how insulation utilization rates affect the winding life. Here they show how using Class H insulation to a Class F rise extends winding life. I have added a line which shows how this same longevity is achieved by using a Class F (155º) motor utilized to a Class B (130°C) rise.Motor_Temperature_Ratings_Chart
As the graph shows us, the longevity of the two motors is the same—so what are the advantages of Class H insulation? The only real advantage is the maximum allowable operating temperature of the motor. Since the design and the performance of the motor remains the same, it is difficult to realize this gain.

Considerations for Using Class H instead of Class F Insulation Motors

It’s important to bear in the mind that the insulation only serves the purpose of protecting the motor winding. In addition to the winding, a motor also has bearings, which utilize grease. No matter what insulation you have on a motor winding, the thermal capacity of the insulation will exceed the thermal capacity of the grease. Continuous operation at these higher temperatures will cause rapid degradation of the grease and likely also cause a bearing failure—unless the grease is regularly replenished. Although there is high temperature grease available, the bearing itself will still run hot and degrade at a more rapid rate.

Taking a big picture look, grease is really not all that expensive and probably would not be a cost factor in terms of needing to replace it more regularly. The issue, however, is the more critical an activity is, the greater the danger if protocol is not followed. If someone forgets to grease the bearing, a bearing failure would occur. While this could also be said for normal greasing, the risk is all the more heightened with temperature.

Motor manufacturers have honestly already weighed in on the cost, benefits, and risks associated with Class F and Class H insulation motors. Nearly all motor manufacturers use Class F insulation with Class B rise in general industrial use TEFC motors, in essence, making this the industry standard. Class H is an option and while it may not be cost-prohibitive, it bears pointing out that it may not be convenient for a replacement or when repair is needed since it may not be a standard offering. This can mean extended periods of downtime.

So why do some specify Class H motors? This brings us to rabbits. Class H is quite often specified because there is the assumption that “more is better”. If the motor insulation has a higher ambient rating, then there’s a feeling of added security. And, in a sense, this is true—especially if the room is hot or if the package the motor is installed in is hot. But, really, when comes down to it, this is like building a five foot fence around a garden to keep out rabbits instead of a three foot fence. Yes, it is higher and the rabbits will not get in, but the three foot fence would do same job and the benefit of the higher fence is difficult to justify and does not guarantee a successful garden.

Truthfully, a well-designed package in an adequately ventilated room will not require the additional motor insulation and will withstand high heat events. However, if Class H insulation is still desired, or if the installation environment requires additional thermal capacity, don’t neglect to consider these other thermal considerations:

• Motor grease lubricant life
• Oil lubricant life
• V-belt life/capacity (if applicable)
• Equipment operating temperature range
• Conductor sizes
• Variable frequency drive (VFD) sizing (if applicable)

The most conservative approach would be to consider these requirements and de-rate the power rating of the equipment accordingly. Simply increasing the insulation class on the motor will not reduce the operating temperature of the motor. As stated before, the motor operating temperature is a function of the winding design, frame size, and cooling capacity. Reducing operating temperatures can only be accomplished by changing the motor design or increasing the heat sink capacity of the motor. Increasing the insulation class of the motor only increases the thermal limit of the windings, not the motor. This also does not address the thermal capacity of the power carrying conductors, VFD’s, breakers, and other mechanical devices. You can also acquire a motor rated for 50 °C which utilizes Class F insulation for a desired hp, but it will likely be in a larger frame size in an effort to increase the heat transfer of the motor. To truly account for high thermal conditions, a more comprehensive approach is required…and that’s all folks.


One thought on “Applying Motor Temperature Ratings

  1. I had never took into account that motors are utilized at a lower temperature limit, which extends the life of insulation. I can definitely see the benefits of this, as a longer lasting motor can improve efficiency and reduce costs. Really insightful read; thank you.

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