Guardian 4721, Repair Manual

Page: 15 / 115

GENERAL INFORMATION
SECTION 1.4
TESTING, CLEANING AND DRYING
PART 1
Page 11
MEGGERS:
The normal resistance of generator winding insulation
is on the order of millions of ohms. This high resis-
tance can be measured with a device called a "meg-
ger". The megger is a megohm meter ("meg" stands
for million) and a power supply. The power supply
voltage varies between megger models and is selec-
table on some models. The most common power sup-
ply voltage is 500 volts. Use of power supplies
greater than 500 volts are not recommended on
prepackaged generators.
CAUTION: Before attempting to measure
Insulation resistance, first disconnect and
Isolate all leads of the winding to be tested.
Electronic components, diodes, surge pro-
tectors, relays, voltage regulators, etc., can
be destroyed if subjected to high megger
voltages.
HI-POT TESTER:
A "Hi-Pot" tester is shown in Figure 1. The model
shown is only one of many that are commercially
available. The tester shown is equipped with a volt-
age selector switch that permits the power supply
voltage to be selected. It also mounts a breakdown
lamp that will illuminate to indicate an insulation
breakdown during the test.
Figure 1. One Type of Hi-Pot Tester
STATOR INSULATION TESTS
GENERAL:
Units with liquid cooled engine and 1 -phase stator
windings are equipped with (a) dual stator AC power
windings, and (b) an excitation or DPE winding.
These units are not equipped with a battery charge
winding. Stator winding insulation tests consist of (a)
testing all windings to ground, (b) testing between iso-
lated windings, and (c) testing between parallel wind-
ings. Figure 3 represents the various stator AC output
leads on 1 -phase units with liquid-cooled engine.
TEST ALL WINDINGS TO GROUND:
1.Disconnect and isolate Stator Leads 11, 22, 33, 44,
2 and 6.
2.Connect terminal ends of all stator leads together.
Make sure all wire terminal ends are completely
isolated from frame ground.
3.Connect the red test probe of the Hi-Pot tester to
the terminal ends of all stator leads. Connect the
black tester probe to a clean frame ground on the
stator can. Then, proceed as follows:
a. Turn the Hi- Pot tester switch OFF
b. Plug the tester cord into a 120 volts AC wall
socket and set its voltage selector switch to
"500 volts".
c. Turn the tester switch ON and observe the
breakdown lamp. After one (1) second, turn the
tester switch OFF.
If the breakdown lamp turned on during the one (1)
second test, clean and dry the stator. Then, repeat
the test. If breakdown lamp comes on during the sec-
ond test, replace the stator assembly.
Figure 3. Stator Winding Leads (Liquid Cooled Units)
TEST BETWEEN ISOLATED WINDINGS:
1.Connect the red test probe to stator lead 2, the
black probe to stator lead 11.
2.Set the tester switch to "500 volts".
3.Turn the tester switch ON and check that the pilot
lamp is lighted.
4.Wait one (1) second while observing the tester
breakdown lamp. DO NOT EXCEED ONE SEC-
OND. After one (1) second, turn the tester switch
OFF.
5.Connect the red test probe to stator lead 2, the
black probe to stator lead 33. Then, repeat Steps 2,
3 and 4.
2
6
11
22
33
44
!