The purpose of these tests is to check for field-coil
short-circuited turns, for number of turns/coil, or for short-circuit conductor
size. Besides tests at standstill, a test at rated speed is required, as short
circuited turns may occur at various speeds.
There are DC and alternating current (AC) voltage tests for
the scope. The DC or AC voltage drop across each field coil is measured.
A more than +2% difference between the coil voltage drop
indicates possible short-circuits in the respective coils. The method is
adequate for salient-pole rotors.
For cylindrical rotors, the DC field-winding resistance is
measured and compared with values from previous tests. A smaller resistance
indicates that short-circuited turns may be present.
Also, a short-circuited coil with a U-shaped core may be
placed to bridge one coil slot. The U-shaped core coil is placed successively
on all rotor slots.
The field-winding voltage or the impedance of the winding
voltage or the impedance of the exciting coil decreases in case there are some
short-circuited turns in the respective field coil.
Alternatively, a Hall flux probe may be moved in the airgap
from pole to pole and measures the flux density value and polarity at
standstill, with the field coil DC fed at 5 to 10% of rated current value.
If the flux density amplitude is higher or smaller than that
for the neighboring poles, some field coil turns are short-circuited (or the
airgap is larger) for the corresponding rotor pole. If the flux density does
not switch polarity regularly (after each pole), the field coil connections are
not correct.
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