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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