Check Ac Output Frequency; Check And Adjust Governor Board And Stepper Motor - Guardian 4721 Repair Manual

SECTION 2.4
DIAGNOSTIC TESTS
3. Connect the positive (+) VOM test lead to the positive
(+) rotor slip ring (nearest the rotor bearing); and the
common (-) test lead to the negative (-) slip ring. The
meter should read (approximately) as follows:
15 kW . . . . . . . . . . . . . . . . . . . . . . 5.8 ohms.
20 kW . . . . . . . . . . . . . . . . . . . . . . 8.7 ohms.
25 kW . . . . . . . . . . . . . . . . . . . . . . . 10 ohms.
4. Now, set the VOM to a high resistance scale (such as "R
x 10,000" or "R x 1K"). Again, zero the meter.
5. Connect the positive (+) VOM test lead to the positive
(+) slip ring and the common (-) test lead to a clean
frame ground. The meter should indicate INFINITY.
RESULTS:
1. Replace rotor assembly if it is open or shorted.
2. If rotor tests good, perform "Insulation Resistance Test"
in Section 1.4.
NOTE: Be sure to read Section 1.5, "Testing,
Cleaning and Drying", carefully. If the rotor tests
good, try performing an insulation resistance test.
Clean and dry the rotor if it fails that test. Then,
repeat the test. If the rotor fails the second insula-
tion resistance test, it should be replaced.
Figure 8. The Rotor Assembly
TEST 12 - CHECK AC OUTPUT FREQUENCY
DISCUSSION:
The generator AC frequency is proportional to the
operating speed of the rotor. A 4-pole rotor (having
two north and two south magnetic poles) will supply a
60 Hertz AC frequency at 1800 rpm. A 2-pole rotor
will supply a 60 Hertz AC frequency at 3600 rpm. The
unit's AC output voltage is proportional to the AC fre-
quency. 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.
Page 36
PART 2
PROCEDURE:
1. Connect an accurate AC frequency meter across the
Wires 11 and 44 terminals of the generator main line
circuit breaker (see Figure 1, Section 2.4).
2. Start the engine, let it stabilize and warm up at no-load.
3. When engine has stabilized, read the frequency meter.
The no-load frequency should be about 61-63 Hertz.
RESULTS:
1. If the AC frequency is high or low, go on to Test 13.
2. If frequency is good, but voltage is high or low, go to Test
14.
3. If frequency and voltage are both good, tests may be
discontinued.
TEST 13 - CHECK AND ADJUST GOVERNOR
BOARD AND STEPPER MOTOR
DISCUSSION:
The generator is equipped with a "voltage over fre-
quency" type AC voltage regulator. The units AC out-
put voltage is generally proportional to AC frequency.
A low or high governor speed will result in a corre-
spondingly low or high AC frequency and voltage out-
put. The governed speed must be adjusted before
any attempt to adjust the voltage regulator is made.
Troubleshooting the stepper motor governor system
is generally limited to checking inputs to the control
module, adjustments, settings, electrical connections,
and testing the stepper motor windings.
NOTE: A fuel supply problem can sometimes be
mistaken for a speed control problem. Verify
whether a problem is fuel related before attempt-
ing to readjust the governor pot / switch settings.
The following is a list of possible problems and tests
to perform, followed by descriptions of the tests.
PROBLEM:
• Overspeeding
TEST PROCEDURES:
Check the frequency switch setting, TEST 13A, TEST
13B, TEST 13C, TEST 13D.
PROBLEM:
• Instability, erratic operation
TEST PROCEDURES:
Adjust linkage and check for binding, adjust control
module pots, TEST 13C, TEST 13D.
PROBLEM:
• Low speed, no fuel
PREPACKAGED LIQUID
COOLED AC GENERATORS