1. IMPEDANCE MEASUREMENT ON FIELD WINDINGS
General:
Measuring the impedance of field windings is done in order to detect turn-to turn failures in these
field windings.
Introduction
During long-time service of rotating machines it cannot be excluded that the turn-to-turn insulation of
the field windings becomes poor which can cause this insulation to fail. Reasons for these defects
often are long-time thermal ageing effects in addition with the high mechanical stresses due to
centrifugal forces, but can also be influenced by humidity or improper cleaning agents.
Single turn-to-turn failures in field winding usually are of no danger, but single turn-to-turn short can
cause local over-temperature which will consequently lead to further turn-to-turn failures. Such
shorted turns reduce the actually effective ampere-turns of a pole and thus unbalanced magnetic
forces are caused.
During machine service a single turn-to-turn failure can hardly be detected: The reduction of the
ohmic resistance of the field windings is small and thus the deviations in the exciter current are
small, too, and these deviations usually are covered by other effects (e.g. by the variation of the
ohmic resistance due to temperature). In a few cases only deviation in the dynamic behavior
(deviations in the vibration pattern) can be recognized due to the un-balanced magnetic forces.
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