Page 352 - Improving Machinery Reliability

318 Improving Machinery Reliabilitj

Temperature vs Life Curves
For Insulation Systems
By IEEE 117 Method

1""
60 80 100 120 140 160 180 200 220
TOTAL WINDING TEMPERATURE - "C
INDUSTRY RULE OF THUMB - EACH 10DC REOUCTION IN MOTOR TEMPERATURE
DOUBLES THE EXPECTED LIFE OF MOTOR WlNOlNGS
Figure 6-3. Temperature vs life curves for insulation systems. (Source: Lincoln Bulletin
€7, p. 7.)

"Running Cooler"-A Relative Term

To determine whether one motor is "running cooler'' than another requires knowl-
edge of both the thermal capacity of the insulation and the final operating tempera-
ture of the windings (see Figure 6-3). As a general rule, every I0"C increase in oper-
ating temperature will decrease insulation life by one-half:
However, higher thermal capacity of the insulation system does not necessarily
equate to longer insulation (and motor) life, nor does a lower operating temperature.
A motor with Class H insulation does not automatically last longer than a motor
with Class B insulation. If the motor with the Class B insulation system (limiting
temperature of 130°C) operates at a winding temperature of llO"C, it will have
twice the insulation life (80,000 hours) of a motor with a Class H insulation system
(limiting temperature of 1 80°C) operating at a winding temperature of 170°C
(40,000 hours).