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Summary:
"How To (and Not To) Write a Motor Repair Specification"
With
trilingual summary
Major users
of electric motors often produce detailed specifications for new
purchases, especially for machines of large size. Specifications
governing motor repair and reconditioning can be equally important.
Writers of such a document should avoid several troublesome
practices, such as:
1. Mismatch
between the document title or scope and its content. If a
specification is written only for a particular motor type or size
range, its content should be confined to those topics. If meant to be
general, it should cover the entire product scope. One common mistake
is to include provisions meaningful only for horizontal motors, but
apply the specification to the servicing of vertical machines.
2. Lack of
distinction between repair in general, rewinding only, and
reconditioning. Specifications written solely for rewinding should be
so titled, and not invoked for general repairs.
3. Failure
to cite or make use of pertinent industry standards prescribing test
and measurement procedures.
4.
Inaccurate or imprecise language. Measurable quantities should be
quantified, not described in subjective terms such as "large,"
"small," "excessive," "normal," or "adequate." Such descriptions will
not mean the same thing to every service shop.
5. Technical
inaccuracy or incompleteness. For example, some have specified
replacement of aluminum rotor cage bars with copper in the same
slots, with the bars sized to produce the same cage resistance. That
inherently changes cage reactance also, thereby altering locked-rotor
current and breakdown torque. Another common error is in requiring
high potential testing of a reconditioned winding at the standard
test voltage for a new winding.
6. Undefined
test procedures, such as "checking for unbalanced phase impedance"
without specifying a procedure or providing a basis for judging the
results, or calling for use of a "motor circuit analysis tester."
Specification writers should also consider instructing the service
center to stop work at certain "hold points" where the user can
inspect the work to gather information or choose an alternative
procedure.
No matter
how the specification is written, it should never be a substitute for
careful evaluation of service center capability before having the
work done.
From "How
To (and Not To) Write a Motor Repair Specification"
...by
Richard L. Nailen, EA Engineering Editor - published in
Electrical Apparatus
March 2005
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