Guardian 4389 Diagnostic Repair Manual page 37

SECTION 2.4
DIAGNOSTIC TESTS
TEST 2- 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.
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 Wires 11 and 44 terminals of the
generator main circuit breaker (see Figure 1). 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, go on to
Test 3.
2. If the voltage reading is higher than residual, but is lower
than the stated limits, go to Test 11.
3. If a high voltage is indicated, go on to Test 11.
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, depending on the
characteristics of the specific generator. If a unit is
supplying residual voltage only, either excitation
current is not reaching the rotor or the rotor windings
are open and the excitation current cannot pass. On
Page 2.4-2
PART 2
current units with air-c c ooled engine, "field boost"
current flow is available to the rotor only during
engine cranking.
TEST 3- TEST EXCITATION CIRCUIT
BREAKER
DISCUSSION:
Unregulated excitation current is delivered to the
voltage regulator from the stator excitation (DPE)
winding, via Wire 2, an excitation circuit breaker
(CB2), Wire 162, and Wire 6. If the excitation circuit
breaker has failed open, excitation current will not be
available to the voltage regulator or to the rotor.
Stator AC power winding output will then be reduced
to a voltage that is the product of residual magnetism
alone.
PROCEDURE:
1. With the generator shut down for at least two minutes,
locate the excitation circuit breaker in the generator
panel. Disconnect wires from the breaker, to prevent
interaction.
2. Set a volt-ohm-milliammeter (VOM) to its "R x 1'scale
and zero the meter.
3. Connect the VOM test probes across the circuit breaker
terminals. The meter should read CONTINUITY.
RESULTS:
1. Replace circuit breaker if defective (meter reads
"OPEN"). Then proceed to Test 4.
2. If circuit breaker is good, go on to Test 4.
Figure 2. Excitation Circuit Breaker
AC GENERATORS