The Hot Issue of Motor Temperature Ratings

Sept. 1, 2000
When you specify a motor, how do you know it can take the heat?

Understanding motor insulation classes and temperature ratings is simpler than you might think. Though we're focusing on standard AC induction motors, you can apply most of this information to DC motors. Let's begin by looking at motor temperature terms.

Ambient temperature is the temperature of the air surrounding the motor. This is the threshold point or temperature the motor assumes when shut off and completely cool.

Temperature rise is the change within a motor when operating at full load. For example; if a motor in a 78°F room operates continuously at full load, the winding temperature will rise. The difference between its starting temperature and its final elevated temperature is the motor's temperature rise.

Hot spot allowance. The standard method of measuring "temperature rise" involves taking the difference between the cold and hot ohmic resistance of the winding. This averages the temperature change of the whole winding, including the motor leads, end turns, and wire deep inside the stator slots. Since some of these spots are hotter than others, an allowance factor uses the average temperature to indicate what the temperature probably is at the hottest spot. We call this allowance factor the hot spot allowance.

Insulation class. Insulation classes group insulations by their resistance to thermal aging and failure. We designate the four common insulation classes as A, B, F, or H. The temperature capability of each class is the maximum temperature at which the insulation can operate to yield an average life of 20,000 hr. The Table, on page 78, shows the rating for 20,000 hr of average insulation life.

Insulation system. Manufacturers use several insulating components when building motors (see sidebar, on top of page 78). A chain is only as strong as its weakest link, so manufacturers base insulation system classifications on the component with the lowest temperature rating. For example, if a manufacturer uses one Class B component with F and H components, the entire system is Class B.

Putting it all together. The basic ambient temperature rating point of most motors is 40°C. A motor rated for 40°C is suitable for installation where the normal surrounding air temperature does not exceed 40°C (104°F). This is the starting point.

When the motor operates at full load, it has a certain temperature rise, which adds to the ambient temperature. For example, U frame motors originally had Class A insulation and a maximum temperature rise of 55°C. In a 40°C ambient temperature, this gives an average winding temperature of 95°C. That's 40°C (ambient) ` 55°C (rise). Manufacturers use the 10° difference between 95°C and 105°C rating of Class A insulation to handle the hot spot allowance. If you take a motor designed for a 55°C rise and Class A insulation, and change the insulation to Class B, you have an extra 25°C of thermal capability. You can use this extra capability to handle higher than normal ambient temperatures. In so doing, you extend the motor's life.

You can also use this capacity to handle higher than normal temperature rise brought on by overloads. You can get overloads from high or low voltages, voltage imbalance, blocked ventilation, high inertia loads, frequent starts, and other factors. For example: If a motor with Class A design (55°C) temperature rise has Class B insulation, then you could expect it to have a normal insulation life - even when ambient temperature is 65°C. These design criteria show that even if a motor feels hot, it may be fine (see sidebar, below).

In a T frame motor with Class B insulation, the extra 25°C of thermal capacity (Class B compared to Class A) accommodates the higher temperature rise associated with the smaller T frame motors. For example: A standard T frame motor might have a rating of 40°C ambient, 80°C temperature rise, and a 10°C hot spot allowance. When you add these three components together, you use up the total temperature capability of Class B insulation (130°C).

Changing insulation classes. By building a Class B, totally enclosed, fan-cooled, T frame motor with Class F insulation, you may increase the service factor from 1.0 to 1.15. You can use this same change of one insulation class to handle a higher ambient temperature or increase the motor's life. This could also make the motor more suitable in high elevations where thinner air has less cooling effect.

Actual insulating practice. Improvements in insulating materials have reduced manufacturing costs. As a result, most motor manufacturers use a mixture of materials, many of which have higher than required temperature ratings. Some manufacturers stopped using Class A materials altogether. This means even though many fractional horsepower motors should have a Class A temperature rise, the real insulation is Class B or better. Similarly, many motors designed for Class B temperature rise actually use Class F and H materials. This extra margin gives the motor a life bonus.

As a general rule of thumb, insulation life doubles for each 10° of unused insulation temperature capability. For example, if you design a motor to have a total temperature of 110°C (including ambient, rise, and hot spot allowance), but build it with a Class B (130°C) system, an unused capacity of 20°C exists. This extra margin raises the expected motor insulation life from 20,000 hr to 80,000 hr.

Similarly, if a motor's load is less than full capacity, its temperature rise will be lower. This automatically lowers the hot spot temperature and extends motor life. If the motor operates in a lower than 40°C ambient temperature, its life will be longer. The degree rule applies to motors operating at above rated temperature; insulation life drops by half for each 10°C of overtemperature.

Depending on design and cooling arrangements, motor surface temperature can be hot to the touch. Surface temperatures of 75°C to 95°C can exist on T frame motors. However, these temperatures do not necessarily indicate overload or impending motor failure.

Motors use these components for insulation.

- Enamel coating on the magnet wire.

- Insulation that comes to the conduit box.

- Sleeving where leads connect to magnet wire.

- Lacing string that binds the end turns of the motor.

- Slot liners (in the stator laminations) that protect the wire from chafing.

- Top sticks that hold the wire down in place inside the stator slots.

- Varnish that manufacturers dip the completed assembly in, prior to baking it. The dipping varnish seals nicks or scratches that may occur during the winding process. The varnish also binds the entire winding together into a solid mass so it doesn't vibrate and chafe when subjected to the high magnetic forces.

About the Author

Ed Cowern | P.E.

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