Test 10 - Test Rotor Assembly; Test 11 - Check Ac Output Voltage; Test 12 - Check Ac Output Frequency - Generac Power Systems CorePower Repair Manual

Section 2.4
Diagnostic Tests
Results
1. Resistance Test: If the VOM indicated a very high
resistance or INFINITY, the windings are open or partially
open.
2. Grounded Condition: Any resistance value other than
INFINITY indicates a grounded winding.
3. Shor ted Condition: Any resistance value other than
INFINITY indicates a shorted winding.
Note: If the winding tests good, perform an insulation
resistance test. If the winding fails the insulation
resistance test (using a meg-ohm-meter), clean and dry the
stator. Then, repeat the insulation resistance test. If the
winding fails the second resistance test (after cleaning and
drying), replace the stator assembly.
TEST 10 – TEST ROTOR ASSEMBLY
Discussion
A rotor having open windings will cause loss of excitation
current flow and as a result generator AC output voltage will
drop to "residual" voltage. A "shorted" rotor winding can result
in a low voltage condition.
Procedure
1. Remove the brush assembly from the slip rings to prevent
interaction.
2. Set a Volt-Ohm-Milliammeter (VOM) to measure
resistance.
3. Connect one meter test lead to the positive slip ring
(nearest the rotor bearing) and the other test lead to the
negative slip ring, measure and record the resistance.
4. Connect one meter test lead to the positive slip ring and
connect the other meter test lead to a clean frame ground,
measure and record the resistance.
Results
1. If the VOM indicated a resistance of 40 ohms +/- 20
ohms, (GOOD) refer back to flow chart.
2. If the VOM did not indicate a resistance within the allowed
tolerances, replace rotor.
3. If the VOM indicated CONTINUITY in Step 4, replace the
rotor assembly.
TEST 11 – CHECK AC OUTPUT VOLTAGE
Discussion
A Volt-Ohm-Milliammeter (VOM) may be used to check the
generator output voltage. Output voltage may be checked at the
unit's main circuit breaker terminals. Refer to the unit's DATA
PLATE for rated line-to-line and line-to-neutral voltages.
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PART 2
 DANGER! Use extreme caution during this test. The
generator will be running. High and dangerous voltages
will be present at the test terminals. Connect meter
test clamps to the high voltage terminals while the
generator is shut down. Stay clear of power terminals
during the test. Make sure meter clamps are securely
attached and will not shake loose.
Procedure
1. With the engine shut down, connect the AC voltmeter test
leads across the Blue and Black Wire terminals of the
generator main circuit breaker. These connections will
permit line-to-line voltages to be read.
2. Set the generator main circuit breaker to its OFF or "Open"
position. This test will be conducted with the generator
running at no-load.
3. Start the generator, let it stabilize and warm up for a
minute or two.
4. Take the meter reading. On unit's having a rated line-to-
line voltage of 240 volts, the no-load voltage should be
about 242-252 volts AC.
5. Shut the engine down and remove the meter test leads.
Results
1. If zero volts or residual voltage is indicated, proceed to
Test 4.
2. If the voltage reading is higher than residual, but is lower
than the stated limits, go to Test 12.
3. If a high voltage is indicated, go on to Test 12.
NOTE: "Residual" voltage may be defined as the voltage
that is produced by rotor residual magnetism alone.
The amount of voltage induced into the stator AC power
windings by residual voltage alone will be approximately 2
to 16 volts AC. If a unit is supplying residual voltage only,
either DC field excitation voltage is not reaching the rotor
or the rotor windings are open and the excitation voltage
cannot pass.
TEST 12 – CHECK AC OUTPUT FREQUENCY
Discussion
The generator AC frequency is proportional to the operating
speed of the rotor. The 2-pole rotor will supply a 60 Hertz
AC frequency at 3600 rpm. The unit's AC output voltage is
proportional to the AC frequency. For example, a unit rated
240 volts (line-to-line) will supply that rated voltage (plus or
minus 2 percent) at a frequency of 60 Hertz. If, for any reason,
the frequency should drop to 30 Hertz, the line-to-line voltage
will drop to a matching voltage of 120 volts AC. Thus, if the
AC voltage output is high or low and the AC frequency is
correspondingly high or low, the engine speed governor may
require adjustment.
AC GENERATORS