Insulation life refers 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.
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.
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.
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.
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 the 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.
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 an 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 the start of the test (°C)
t2 = ambient temperature at the end of the test (°C)
If the 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.
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