Heat kills, and the old standard of using your hand to judge the temperature of a motor and if it was over heating no longer applies.  NEMA Insulation Classes do away with guessing, and give the motor manufacturer a defined framework to operate in.

The Surface temperature of the motor is typically 30°C lower than it is at the windings. So if we look at a Class F insulated motor which will be happy running at 155°C and then subtract 30°C, we get a surface temperature of 125°C. This doesn't necessarily mean it is running too hot or operating improperly (by the way, we strongly advise against touching anything that is 125°C).  To put it simply, today's motors can simply be too hot to handle, even when all is working as it should be.

Motor winding insulation max temperature ratings carry NEMA designations.   These ratings are defined as:

Class: A 105 Degrees C
Class: B 130 Degrees C
Class: F 155 Degrees C
Class: H 180 Degrees C

NEMA specifies allowable temperature rises for motors at operating under full load (and at service factor, if applicable). The allowable temperature rises are based upon a reference ambient temperature (40°C) and are determined by a "resistance method", once the motor has achieved thermal equilibrium under load, the resistance of the windings is measured. The resistance of the winding is a function of temperature of the winding. 

The allowable temperature rises (at full load) for a 1.0 S.F. motor are:

A= 60°C
B= 80°C
F= 105°C
H= 125°C

For a 1.15 S.F. motor, the NEMA allowable temperature rises (at service factor)  are

A= 70°C
B= 90°C,
F= 115°C.

For a Class F insulated, 1.0 S.F. motor, if we add the NEMA allowable rise of 105°C  to the reference ambient temperature (40°C), results in the motor having an operating temperature of (105+40)=145°C.  

This gives us a 10°C temperature differential between a Class F insulation maximum temperature rating (155°C) and an allowable maximum temperature (145°C) which gives an allowance for the "hotspot" temperature in the interior of the winding. The overall winding resistance is of course the sum of the resistance of the cooler end turns, and the warmer windings in the stator slots.

Motor Insulation Temperature Ratings (NEMA) Temperature Rises
1.0 SF Motor 1.15 SF Motor
Class Temp. Ambient Hotspots Rise @ 1.0 Rise @ 1.15
A 105 +40 +5 60 70
B 130 +40 +10 80 90
F 155 +40 +10 105 115
H 180 +40 +15 120 not defined

Although  not specified by NEMA , it is now common practice within industry to refer to the allowable temperature rise for a given class of insulation, as a temperature rise letter. For example, an 80°C rise is often referred to as a 'Class B', as 80°C is the maximum allowable temperature rise for a 1.0 S.F. motor with Class B insulation and a 40°C ambient temperature. This practice means that a motor with Class F insulation and an 80°C rise is  referred to as an 'F/B' motor.

Modern insulation materials means Class F  insulation is commonly used for motor windings. With modern designs, a 'Class B' temperature rise is readily achievable. Therefore Class F insulation with a Class B temperature rise gives us a thermal margin of 25°C, potentially increasing the life of the motor by up to 5 times.

The effects of Thermal Deterioration on Insulation Life

Once you exceed a certain temperature threshold, the insulation deteriorates at an increasing rate which approximately doubles for every 10°C increase in temperature.

For example, class F insulation loses ½ it’s mechanical strength after experiencing 20,000 hours at its rated temperature.  Obviously the insulation will not simply fail at this point, but it will have been significantly weakened.

• 20,000 hours (2.5 years) at 155°C
• 10,000 hours (1.25 years) at 165°C, or likewise, 40,000 hours (5 years) at 145°C
• 5,000 hours (<1 year) at 175°C, or likewise, 80,000 hours (10 years) at 135°C

In the real world,  motors don’t continuously run at one temperature since both the load and the ambient temperatures vary. However, once the deterioration has occurred, it is non-reversible. Lowering the operating temperature can prevent further deterioration though.

Finally, don't forget that there is a 10°C difference between temperature measurements by Resistance versus by Embedded Detectors (resistance elements or thermocouples). A Class F temperature
rise of 105°C by Resistance, is 115°C by embedded thermal sensor. So remember to setup the correct thermal protection level within the drive.

Drives and Automation Ltd are a ‘one-stop’ independent shop for a full range of industrial automation products and system integration services. We provide drive modules, motors, control systems and PLC / SCADA solutions. Independent advice is provided on the most suited product by application. In addition we offer a variety of drive accessories.  We are the UK agent for the Sicme Motori range of AC and DC motors.