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reading (refer to instructions for the
megger).
The minimum acceptable value is 1.5
megohms.
If the reading is low, the winding must be
dried out or repaired.
INSULATION RESISTANCE -
EXCITER STATOR
Disconnect the exciter leads Fl and F2 from
the regulator. Never subject the regulator to
a megger. Connect Fl and F2 together with
the positive megger lead. Connect the
negative megger lead to the ground stud.
Take the megohm reading (refer to
instructions for the megger).
The minimum value is 1.5 megohms.
If the reading is low, the winding must be
dried out or repaired.
INSULATION RESISTANCE -
EXCITER ROTOR
Disconnect the exciter rotor windings (6
leads from the diodes). Connect all leads
together with the positive megger lead.
Connect he negative megger lead to a good
ground on the rotor assembly such as the
mounting bolt. Take the megohm reading
(refer to the instructions for the megger).
The minimum acceptable value is 1.5
megohms.
If the reading is low, the winding must be
dried out or repaired.
MAIN ROTOR FIELD AC IMPEDANCE
TEST
THEORY: The main rotor resistance can be
measured with a very accurate meter that is
able to measure low (1 ohm) resistance but it
is difficult to determine if there are turn to
turn shorts in the field pole windings. One
shorted turn would only change a resistance
reading on the order of one half of one
percent.
The ac impedance test measures the
impedance (inductance and resistance) of
the field pole coils. Shorted turns in the field
pole windings change the coil inductance to
a much greater degree than the resistance.
PROCEDURE:
Step 1: The rotor must be supported on a
non-magnetic surface such as a wooden
skid. Do not use a steel table that would
create a magnetic "short circuit" between the
poles.
Step 2: Apply 120 volts ac to disconnected
main rotor leads Fl and F2.
Step 3: Measure and record voltages pole.
Between points "A" and "C", "C" and "D",
"D" and "E" across each "B", "B" and
(Figure 8-1).
Step 4: The voltage readings should balance
within one volt.
RESULTS: If the ac voltages are not
balanced (30v ± lv ac with 120v ac input)
across each pole, the winding has shorted
turns and should be rewound.
Refer to Marathon Electric for further
information.
60
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