Rockwell Automation 825 Smart Motor Manager User Manual - Manual (Page 19)

Introduction 1-5
Publication 825-UM001B-EN-P January 2001
Insulation Aging
The aging of insulation material is a chemical process that is accelerated by continuous
overtemperature. It may be assumed that a winding temperature that is constantly 10 K
higher than the temperature limit reduces the motor life by half. This “life law” shows that
particular attention must be paid to adhering to the permitted operating temperature for long
periods of time. (Note that overtemperatures of short duration and infrequent occurrence do
not seriously affect the life of the machine.)
The Bulletin 825 Smart Motor Manager's ability to accurately limit excessive current
conditions greatly aids in extending motor life. In practice, it may be expected that there will
be reduced loads and pauses, so that when the temperature limit is reached, the motor life will
not be impaired.
Figure 1.5 Reduction in Average Life (E
M
) of a Motor when Winding is
Continuously Overheated
Rotor Temperature
The rotors of squirrel-cage induction motors with simple construction (no insulation) may
continuously attain higher temperatures than rotors in motors with insulated windings.
However, in larger motors, the concentration of the rotor losses during starting is higher than
the concentrations of losses in other parts of the motor. The starting time of such motors is
therefore limited by the thermal capacity of the rotor. These types of motors are commonly
referred to as “rotor-critical” motors. Critical to the rotor are the mechanical stresses caused
by the temperature rise, unsoldering of the rotor bars, and, for EExe motors (motors for use
in the chemical industry), the high temperature as a source of ignition.
Operational Requirements for Installation
Monitoring the application parameters and process data of an installation can be very
important. Even a slight change in the starting and operating behavior of the motor can
indicate an impending fault. The Smart Motor Manager helps eliminate potential trouble
before major repairs are necessary and loss of production occurs.
0
25
50
70
100
%
+5K +10K +15K +20K
E
M
ϑ
G
E
M
Average motor life
ϑ
G
Temperature limit of the insulation