Generator Test Procedures

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.
Page 1

2. Test Technique
The test techniques most commonly used (and visualized by means of figure 1) are:
• Measuring the voltage drops ∆U across each pole
• Measuring the actual impedance Z’ of each pole
In the first-mentioned case the series connection of all field windings is energized by an appropriate
AC source and the voltage drop ∆U across each pole is measured. Poles with turn-to-turn shorts in
their field windings will show a remarkably reduced voltage ∆U in comparison with the average value
of ∆U. The impedance Z’ of a pole can be calculated by ∆U/I~.
In the second-mentioned case each pole is energized separately by an AC source and the
impedance Z’ = ∆U/I’ is recorded; field windings with turn-to-turn failures will show remarkably
reduced values of Z’.
It must be kept in mind that Z’ is influenced to a remarkable extent whether the magnet wheel is in
its stator or not!
In cases where the pole connectors are not accessible, only the total impedance Z’ can be
determined by measuring U~ and I~. The higher the number of poles, the more difficulty to detect a
single turn-to-turn short!
Special test techniques are necessary in order to detect turn-to-turn short which depend on
centrifugal forces. i.e. the rotor must have a certain speed above which turn-to-turn failures occur. In
the simplest way in such cases the rotor is energized by an AC source via the slip-rings and both I~
and U~ are recorded in relation to the rotor speed (limitations see above!) and Z(n) is calculated.
Another special test technique is using surge voltages in order to detect voltage-depending turn-to-
turn failures.
General Requirements
Under normal circumstances some disassembly work is necessary to ensure accessibility of the pole
connectors.
In cases where the pole connectors are not accessible and in cases where the expected turn-to-turn
failures depend on rotor speed there is the request of a well experienced technician in order to
detect such failures.
© This QM-Instruction is properly of PNG Power and must neither be copied nor used in any other
way without the written consent of PNG Power. Neither is it to be handed over nor in other way
communicated to a third party. Infringement will lead to prosecution.
Page 2