Application Guide

63-7062 10
Factors Affecting Motor Temperature and Insulation Life
As indicated earlier, three-phase electric motors are extremely rugged machines. With few moving parts sealed from the local
environmental effects, except temperature, the motor can be expected to provide reliable service for many years. When
discussing this subject with end users, often the comments made are significantly different. Where the motor is considered
relatively unreliable and regularly
burning out
, random motor inspections are made on various sites. The majority are found to
work at only 80% of full load. Fig. 8 indicates the life expectancy of Grades A, B, F and H types of insulation.
NOTE: Most modern motors are manufactured using Class F insulation.
2
4
100
100 120 140 160 180 200 250 300
6
8
1,000
2
4
6
8
10,000
2
4
6
8
100,000
1,000,000
10,000,000
Temperature (C°)
A
B
F
H
Fig. 8. Insulation life expectancies.
NOTE: The
Ten Degree Rule
indicates that a ten degree temperature increase cuts insulation life span approximately in half
and vice-versa.
Clearly, with a life expectancy of in excess of 1,000,000 operational hours (over 100 years) at 284°F, motor burn-out should not
be the problem it is often claimed to be. When investigations are made into a motor
burn out
, the failure can often be attributed
to one of the variables indicated in Fig. 9.