Guardian Technologies 4270 - Manual (Page 44)

one meter test lead to Wire 1. Connect the other test lead to a
clean ground. The meter should read “Continuity”.
RESULTS:
1. Repair, reconnect or replace any defective wire(s).
2. If wires check good, go to Test 10.
Figure 7-6. – Brush Leads
TEST 10 - CHECK BRUSHES & SLIP RINGS
DISCUSSION:
Brushes and slip rings are made of special materials
that will provide hundreds of hours of service with little
wear. However, when the generator has been idle for
some time, an oxide film can develop on the slip
rings. This film acts as an insulator and impedes the
flow of excitation current to the Rotor.
If Test 4 resulted in less than one-half rated output
voltage, it is possible that the brushes and slip rings
are at fault.
PROCEDURE:
1. Gain access to the brushes and slip rings.
2. Remove Wire 4 from the positive (+) brush terminal.
3. Remove the ground wire (1) from the negative (-) brush.
4. Remove the brush holder, with brushes.
5. Inspect the brushes for excessive wear, damage, cracks, chip-
ping, etc.
6. Inspect the brush holder, replace if damaged.
7. Inspect the slip rings.
a. If slip rings appear dull or tarnished they may be
cleaned and polished with fine sandpaper. DO
NOT USE ANY METALLIC GRIT TO CLEAN
SLIP RINGS. (A 400 grit wet sandpaper is rec-
ommended).
b. After cleaning slip rings, blow away any sandpa-
per residue.
RESULTS:
1. Replace bad brushes. Clean slip rings, if necessary.
2. If brushes and rings are good, go to Test 11.
TEST 11- CHECK ROTOR ASSEMBLY
DISCUSSION:
During the “Fixed Excitation Test” (Test 4), if AC out-
put voltage did not come up to about one-half rated
volts, one possible cause might be a defective Rotor.
The Rotor can be tested for an open or shorted condi-
tion using a volt-ohm-milliammeter (VOM).
Also see Chapter Three, “INSULATION RESIS-
TANCE TESTS”.
PROCEDURE:
Gain access to the brushes and slip rings. Disconnect
Wire 4 and Wire 1 from their respective brushes and
remove the brush holder. Then, test the Rotor as fol-
lows:
1. Set a VOM to its “Rx1” scale and zero the meter.
2. Connect the positive (+) meter test lead to the positive (+) slip
ring (nearest the Rotor bearing). Connect the common (-) test
lead to the negative (-) slip ring. Read the resistance of the
Rotor windings, in OHMS.
ROTOR RESISTANCE *
MODEL QP75D: 15.25 OHMS
* Resistance values in ohms at 20° C. (68° F.). Actual readings
may vary depending on ambient temperature. A tolerance of
plus or minus 5% is allowed.
3. Set the VOM to its “Rx1 K” or “Rx10,000” scale and zero the
meter.
4. Connect the positive (+) meter test lead to the positive (+) slip
ring, the common (-) test lead to a clean ground (such as the
Rotor shaft). The meter should read “Infinity”.
Figure 7-7. – Rotor Assembly
POSITIVE (+)
TEST LEAD
4 1
Section 7
DIAGNOSTIC TESTS
Page 42