Page 351 - Improving Machinery Reliability

Extending Motor Life in the Process Plant Environment 317

LOCKED ROTOR CURRENT BY HORSEPOWER

8
c
U
I ‘ ”B” Actual
500%
400%

1 1.5 2 3 5 7.5 10 15 20 25 30 40 50 60 75 1WI 125 150 200
HORSEPOWER
Figure 6-2. Locked rotor current by horsepower.

If a load cycle requires motor operation beyond its rated temperature rise for
whatever reason, the operator must be willing to accept reduced motor life or use a
larger motor.

A Bank of Motor Life
Through motor designs and selection of electrical and insulating materials, the
manufacturer builds each motor with a “bank of motor life.” The owner-operator
determines how fast motor life will be expended by the way the motor is installed,
operated and maintained.
The manufacturer cannot know how a standard motor will be applied. Thus the
motor design engineer can only specify construction and materials that will meet the
NEMA performance requirements for that motor design class and satisfy the manu-
facturer’s target for maximum warranty claims under the terms of the warranty.
Most standard motors are warranted for one year. However, some manufacturers
warrant their standard products as long as five years.
Many motors are conservatively designed to carry their rated load with a low-tem-
perature rise. In addition, these motors may use higher-grade insulation materials (6,
H, H+), coil wire, and electrical steels as well as superior ventilation systems. This
gives them a greater spread between their maximum operating temperature and the
thermal capacity or limiting temperature of the insulation system materials. This
adds significantly to the “bank of motor life” from which the motor owner can draw.