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 Summary: " The fine
art of load testing d-c motors" With
trilingual summary
Most of the many tests for brush-type
d-c motors are intended to diagnose motor condition. In addition to
the methods of evaluating insulation integrity, common to all
electrical machinery, procedures exist to properly position brushes,
ensure correct field polarity, and evaluate commutation.
However, to check temperature rise
(particularly in the armature), establish torque capability, or
determine efficiency, a load test is required. As with a-c motors,
three basic methods exist. The most straightforward is a dynamometer
test. Besides the need for a loading device, this method has two
disadvantages. First, it requires a d-c power source capable of
supplying full motor load. Second, efficiency calculated only as the
ratio of input to output is subject to considerable error.
A second method is the "pump-back" or
"opposition" test. This requires two identical machines with their
fields adjusted so that one operates as a motor, the other as a
generator. External power (either electrical or mechanical) supplies
only the machine losses. The simplest of three commonly-used test
circuits requires the least equipment, but also yields the least
precise value of efficiency.
Loss segregation (the remaining method
of evaluating full-load performance) gives the best efficiency
result. However, it does not permit direct measurement of armature
temperature or evaluation of commutation under full load. D-C motor
losses differ in several ways from those in an induction machine.
Since the armature magnetic field is alternating, d-c motor core
loss occurs in the rotor rather than in the stator. The brush-commutator
interface involves a constant friction loss plus a "contact loss" that varies with armature
current. Losses occur in the field supply circuit. Copper loss is
present in both the shunt field winding (constant with field
excitation) and in series field or interpole windings (variable with
load).
Although ways of evaluating those
losses have been widely published, no current IEEE test standard
exists for d-c machines. The applicable document, IEEE 113-1985, was
withdrawn some years ago. Loss segregation is dealt with in detail
by IEC 60034-2, which says little about pump-back testing.
From
" The fine art of load testing d-c motors" ...by
Richard L. Nailen, EA Engineering Editor -
published in Electrical Apparatus July 2004
© 2004
Barks Publications, Inc. All Rights Reserved.
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