Goff 2460, Operation And Maintenance Manual

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Longer motor bearing life is assured by proper alignment, proper belt and chain
tension and proper lubrication. Please follow all instructions given in this manual when
attempting any replacements or repairs.
High external thrust from the driven unit is usually carried by the top bearing or
thrust bearing. If replacement of the thrust bearing becomes necessary, the new bearing
must be the same type and mounted in an identical manner. When angular contact type
bearings are replaced, the new bearing must have the same thrust capacity.
Aside from a thorough maintenance program, one of the best ways to guarantee
economical performance and long motor life is to make sure your motors operate at
nameplate voltage. In the case of a T-frame motor, applying too high a voltage will
reduce the motor ef ficiency. Too high a voltage will also lower the power factor of any
motor increasing power consumption, utility bills and core loss. Core loss shortens motor
life by overheating the insulation system.
Low voltage can also shorten motor life. When a voltage is used that is below
nameplate recommendations, the effective horsepower of the motor is reduced. If, for
example, a 5 HP motor is operated at 10% below rated voltage it becomes for all
purposes a 4 HP motor. The motor, however, will try to drive the load it was intended to
drive and become overloaded resulting in premature failure.
Operating voltages that are too high or too low can cause problems for motors of
all kinds, but operating a three-phase motor on an unbalanced or open-delta distribution
system can also cause serious overheating that will shorten motor life dramatically. A
motor operating with a 3.5% voltage unbalance, for example, will experience a
temperature rise of approximately 25%. Under normal conditions a totally enclosed
fan-cooled T-frame motor has a temperature rise of about 750C (167
□
F). An increase
of 25% in this case would add about 19□ C (660F) to the motor operating temperature.
The rule of thumb for motor life is that for each IODC (50CF) rise in temperature above
the rated temperature, the life of the motor is reduced one half. An increase of 19
□
C
would reduce the motor life to about one fourth of normal wear.
In deciding whether to repair or replace an electric motor several factors should
be considered. Motors with unusual electrical or mechanical features are usually more
expensive and may not always be readily available. Therefore, it is frequently faster and
more economical to repair these motors.
Because of differences in frame designs, modifications may have to be made
when replacing some standard motors. In some cases an adapter may be necessary.
Depending on the motor and cause of failure, the spiraling repair costs on parts and
service could far outweigh the expense of replacement. A discussion about the motor in
6-2