Guardian 4721, Repair Manual

Page: 39 / 115

PREPACKAGED LIQUID
COOLED AC GENERATORS
SECTION 2.4
DIAGNOSTIC TESTS
PART 2
Page 35
f. With the meter still set to a. high resistance
scale, check for a shorted condition between
parallel windings as follows:
(1) Connect one meter test probe to stator
lead 11.
(2) Connect the second meter test probe to
stator lead 33. The meter should read
INFINITY.
NOTE: Read section 1.5, “Testing, Cleaning and
Drying” carefully. If the winding tests good, per-
form an insulation resistance test. If the winding
fails the insulation resistance test, clean and dry
the stator as outlined in Section 1.5. Then, repeat
the insulation resistance test. If the winding fails
the second resistance test (after cleaning and
drying), replace the stator assembly.
TEST 9 - RESISTANCE CHECK
OF ROTOR CIRCUIT
DISCUSSION:
To verify the zero current draw reading and measure
the rotor circuit.
PROCEDURE:
1. Disconnect Wire 4 from the voltage regulator. It is locat-
ed third terminal from the top of the volt regulator.
2. Set VOM to the “Rx1” scale.
3. Connect one test lead to Wire 4. Connect the other test
lead to a clean frame ground. Note the resistance reading.
15 kW . . . . . . . . . . . . . . . . . . . . . . 5.8 ohms.
20 kW . . . . . . . . . . . . . . . . . . . . . . 8.7 ohms.
25 kW . . . . . . . . . . . . . . . . . . . . . . 10 ohms.
RESULTS:
1. If the resistance reading is correct, check your VOM
meters fuse and repeat Test 4.
2. If INFINITY is measured on your VOM meter, go to
Test 10.
TEST 10 - CHECK BRUSHES AND SLIP RINGS
DISCUSSION:
The function of the brushes and slip rings is to pro-
vide for passage of excitation current from stationary
components to the rotating rotor. Brushes are made
of a special long lasting material and seldom wear out
or fail. However, slip rings can develop a tarnish or
film that can inhibit or offer a resistance to the flow of
electricity. Such a non-conducting film usually devel-
ops during non-operating periods. Broken or discon-
nected wiring can also cause loss of excitation cur-
rent to the rotor.
Figure 6. Brushes and Slip Rings
PROCEDURE:
1. Remove the brush holders from the rear bearing carrier.
Visually inspect the brushes for cracks, chipping, or other
damage. Replace brushes in pairs, if necessary.
2. If the slip rings appear dull or tarnished, they may be
cleaned with fine sandpaper. DO NOT USE ANY METAL-
LIC GRIT TO CLEAN SLIP RINGS. After polishing the
slip rings, use low pressure air to blow away any sandpa-
per residue.
3. Test unit operation.
RESULTS:
1. Repair, replace or reconnect wires as necessary.
2. Replace any damaged slip rings or brush holder.
3. Clean and polish slip rings as required.
TEST 11 - TEST ROTOR ASSEMBLY
DISCUSSION:
A rotor having completely open windings will cause
loss of excitation current flow and, as a result, gener-
ator AC output voltage will drop to “residual” voltage.
A “shorted” rotor winding can result in a low voltage
condition.
PROCEDURE:
1. Disconnect the brush wires or remove the brush holder,
to prevent interaction.
2. Set a VOM to its “R x 1” scale and zero the meter.
1
1
4
BLACK
BLACK
4
RED
RED
RUSH WITH RED LEAD
1
+ -