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TEST 6 – CHECK FIELD BOOST (12-20 KW)
DISCUSSION:
See “Field Boost Circuit” in Section 2.2. Field boost
current (from the circuit board) is available to the
rotor only while the engine is cranking. Loss of field
boost output to the rotor may or may not affect power
winding AC output voltage. The following facts apply:
• Asmallamountofvoltagemustbeinducedintothe
DPE winding to turn the voltage regulator on.
• If rotor residual magnetism is sufficient to
induce a voltage into the DPE winding that is
high enough to turn the voltage regulator on,
regulator excitation current will be supplied
even if field boost has failed. Normal AC output
voltage will then be supplied.
• Ifrotor residualmagnetismhas beenlostor isnot
sufficient to turn the regulator on, and field boost
has also been lost, excitation current will not be
supplied to the rotor. Generator AC output voltage
will then drop to zero or nearly zero.
5.54 VDC
Figure 3. Field Boost Test Points
PROCEDURE:
1. Disconnect Wire 4 from the voltage regulator, third
terminal from the top (see Figure 3).
2. Set a VOM to read DC volts.
3. Connect the positive (+) VOM test probe to the terminal
end of disconnected Wire 4.
4. Connect the common (-) VOM test probe to the
grounding lug.
5. Crank the engine while observing the VOM reading.
While the engine is cranking, the VOM should read
approximately 4-6 Volts DC. When engine is not crank-
ing, VOM should indicate “zero” volts (see Figure 3).
6. Reconnect Wire 4.
RESULTS:
1. If normal field boost voltage is indicated in Step 6,
replace the voltage regulator.
2. If normal field boost voltage is NOT indicated in Step 6,
check Wire 4 (between regulator and circuit board) for
open or shorted condition. If wire is good, replace the
circuit board.
TEST 7 – TESTING THE STATOR WITH A VOM
(12-20 KW)
DISCUSSION:
A Volt-OHM-Milliammmeter (VOM) can be used to test
the stator windings for the following faults:
• Anopencircuitcondition
• A“short-to-ground”condition
• Ashortcircuitbetweenwindings
Note: The resistance of stator windings is very
low. Some meters will not read such a low resis-
tance, and will simply indicate CONTINUITY.
Recommended is a high quality, digital type meter
capable of reading very low resistances.
LINE
WIRE 11 TEST POINT
WIRE 44 TEST POINT
LOAD
Figure 4. Test 7 Test Points
PROCEDURE:
1. Isolate the generator from the transfer switch by dis-
connecting the load wires from the main breaker inside
the generator.
2. Disconnect Stator Leads 22 and 33 from the neutral
connection and separate the leads.
3. Disconnect and isolate Wires 2 and 6 and Wires 11 and 22
from the voltage regulator.
4. Make sure all of the disconnected leads are isolated
from each other and are not touching the frame during
the test.
Page 44
PART 2
AC GENERATORS
SECTION 2.4
DIAGNOSTIC TESTS