Introduction to Circuit and Motor Protection – MOTOR PROTECTION (5)

Sizing Proper Heater Coils
The National Electric Code specifies the amount of protection that must be provided to a motor. For example, a three-phase, AC induction motor with a service factor of 1.15 would require three overloads selected to trip at no more than 125% of a motor’s full load current rating.

The selection of the appropriate size of heater components is important. The standard tripping curves of heater elements are specified at an ambient temperature of 40°C to correspond with the temperature at which most motors are specified for ambient conditions. At a temperature of 40°C, Underwriters Laboratory requires that an overload relay trip in 4 hours of operation at 100% the trip current, in 8 minutes at 200%, and in 30 seconds at 600% (see Figure 40).


NEMA rates heater elements by the amount of time it takes to melt the alloy when the motor is drawing 6 times the full load current. These heaters are grouped into three categories: class 10, class 20, and class 30. Class 10 heaters will melt the alloy in 10 seconds, the class 20 heaters will
melt the alloy in 20 seconds, and the class 30 heaters will melt the alloys in 30 seconds (see Figure 41). Class 10 heaters should be used in applications to protect hermetic motors like compressors in air conditioning systems, submersible pumps, and other motors with short lockedrotor times. Class 20 heaters are the most common since they can be used in many types of applications. Class 30 heaters should only be used in motor applications with high inertia loads, such as driving grinding wheels and fly wheels.

Manufacturers provide tables to use as a reference in choosing the proper heater for a motor pulling the indicated amount of current. When choosing a heater, it is important to make sure that the one you are referencing in the table has the appropriate amount of current. The current of a heater can be calculated at 125% of the full load current, or without the 125% required by the code.

to be continued…………..

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