Check Ac Output Voltage; Test Excitation Circuit Breaker - Guardian 4721 Repair Manual

PREPACKAGED LIQUID
COOLED AC GENERATORS
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 ter-
minals. 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 RUN-
NING. HIGH AND DANGEROUS VOLTAGES
WILL BE PRESENT AT THE TEST TERMI-
NALS. 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 SECURE-
LY 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 mag-
netism 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
PART 2
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 exci-
tation current cannot pass. On current units with
liquid-cooled 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 volt-
age regulator from the stator excitation (DPE) wind-
ing, 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 avail-
able to the voltage regulator or to the rotor. Stator AC
power winding output will then be reduced to a volt-
age 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. Excitartion Circuit Breaker
SECTION 2.4
DIAGNOSTIC TESTS
Page 31