Any time current flows through a wire — such as the windings in a motor — resistance in the wire, as well as other losses, cause heat to be generated. This heat will eventually transfer to other parts of the motor, causing them to degrade over time. One of the key components affected by this heat is the insulation system that protects the motor windings. To address the potential for failure of the insulation due to heat, the NEMA MG-1 standard establishes four classes of motor insulation, which define the insulation system’s ability to endure a specified temperature while providing a specific operating life.
Insulation life refers to not to a sudden, catastrophic failure of winding insulation, but rather to a gradual aging and degradation of the system’s insulating properties. If the insulation reaches a point where it cannot withstand the applied voltage, a short-circuit of the windings can occur.
How motor insulation classes are determined
The NEMA insulation classes specify the maximum allowable temperature of the motor winding insulation — under continuous operation — that will provide a life of 20,000 hours.
The maximum allowable temperature assumes an ambient temperature of 40° C and adds to that an allowable temperature rise plus an additional amount (often referred to as thermal margin) to allow for hotspots within the windings.
In addition to a maximum ambient temperature of 40° C, the insulation class ratings also assume a service factor of 1.0 and an altitude of 3300 feet above sea level (beyond which, the thinner air has a reduced cooling capacity). However, adjustment tables and calculations are available to determine the reduced maximum operating temperature for conditions outside of those specified by the insulation class. For example, if the ambient temperature is higher than 40° C, the allowable temperature rise should be reduced by the amount that the ambient temperature exceeds 40° C.
NEMA motor insulation classes
The NEMA insulation classes currently in use are A, B, F, and H, although newer motors are rarely built with class A insulation, which has a maximum winding temperature of 105° C.
The maximum winding temperature increases by 25° C with each step up in insulation class, as shown below.
Recently, motor manufacturers have begun specifying both the insulation class and the allowable temperature rise, with a rating such as “F/B.” The first letter indicates the insulation class, as specified above, and the second letter indicates the allowable temperature rise.
In this case, the maximum winding temperature is 155° C (per insulation class F), and the allowable temperature rise is 80° C (per insulation class B). Adding 80° C to the ambient temperature of 40° C, plus the 10° C hotspot margin of insulation class F, gives a maximum temperature of 130° C, rather than the 155° C of a typical insulation class F motor. This means that the “F/B” motor has an additional 25° C safety margin, which can provide significantly longer insulation life (and, therefore, motor life).
Adjustments to insulation life
The maximum temperature specified by the insulation class will provide an insulation life of 20,000 hours with the motor operating at full load. According to the Arrhenius equation, for every 10° C that the maximum temperature is exceeded, the insulation life will be reduced by 50 percent. Conversely, for every 10° C that the motor is operated below the maximum temperature, the insulation life will be doubled.
Methods of winding temperature measurement
Temperature rise is calculated based on the change in resistance of the windings, with a correction for any change in ambient temperature between the start and end of the test.
Δt = temperature rise (°C)
R2 = hot winding resistance (Ohms)
R1 = cold winding resistance (Ohms)
t1 = ambient temperature at start of test (°C)
t2 = ambient temperature at end of test (°C)
If temperature is measured by devices embedded in the motor, the temperature rise can be 10° C higher than that specified with the resistance-based calculation.
Note: motor insulation can also be checked by directly measuring the insulation resistance, as described here.
NEMA vs IEC insulation classes
Although the NEMA insulation classes are widely recognized in North America, the IEC 60034-1 standard is often used for motors made or sold outside of North America. The IEC ratings align with the NEMA ratings for classes A, B, F, and H, but add an additional rating of class “E.”