Guardian Technologies 5209 - Manual (Page 15)

section 3
DescRiption & coMponents
The resulting direct current from the BCR is delivered
to the 12 VDC receptacle, via Wire 13A, CB1, and
Wire 15A. This receptacle allows the capability to
recharge a 12 volt DC storage battery with provided
battery charge cables.
INSULATION RESISTANCE
The insulation resistance of stator and rotor wind-
ings is a measurement of the integrity of the insulat-
ing materials that separate the electrical windings
from the generator steel core. This resistance can
degrade over time or due to such contaminants as
dust, dirt, oil, grease and especially moisture. In
most cases, failures of stator and rotor windings is
due to a breakdown in the insulation. In many cases,
a low insulation resistance is caused by moisture
that collects while the generator is shut down. When
problems are caused by moisture buildup on the
windings, they can usually be corrected by drying the
windings. Cleaning and drying the windings can usu-
ally eliminate dirt and moisture built up in the genera-
tor windings.
THE MEgOHMMETER
GENERAL:
A megohmmeter, often called a “megger”, consists of
a meter calibrated in megohms and a power supply.
Use a power supply of 500 volts when testing stators
or rotors. DO NOT APPLY VOLTAGE LONGER THAN
ONE (1) SECOND.
TESTING STATOR INSULATION:
All parts that might be damaged by the high meg-
ger voltages must be disconnected before testing.
Isolate all stator leads (Figure 3-9) and connect all of
the stator leads together. FOLLOW THE MEGGER
MANUFACTURER’S INSTRUCTIONS CAREFULLY.
Use a megger power setting of 500 volts. Connect one
megger test lead to the junction of all stator leads, the
other test lead to frame ground on the stator can.
Read the number of megohms on the meter.
The MINIMUM acceptable megger reading for stators
may be calculated using the following formula:
MINIMUM INSULATION
GENERATOR RATED VOLTS
RESISTANCE =
__________________________ +1
(in “Megohms”)
1000
EXAMPLE: generator is rated at 120 volts AC.
divide “120” by “1000” to obtain “0.12”. Then add
“1” to obtain “1.12” megohms. Minimum insulation
resistance for a 120 VAC stator is 1.12 megohms.
If the stator insulation resistance is less than the cal-
culated minimum resistance, clean and dry the stator.
Then, repeat the test. If resistance is still low, replace
the stator.
Use the Megger to test for shorts between isolated
windings as outlined “Stator Insulation Tests”.
Also test between parallel windings. See “Test
Between Windings” on next page.
TESTING ROTOR INSULATION:
Apply a voltage of 500 volts across the rotor positive
(+) slip ring (nearest the rotor bearing), and a clean
frame ground (i.e. the rotor shaft). DO NOT EXCEED
500 VOLTS AND DO NOT APPLY VOLTAGE
LONGER THAN 1 SECOND. FOLLOW THE
MEGGER MANUFACTURER’S INSTRUCTIONS
CAREFULLY.
RotoR MiniMuM insulation Resistance:
1.5 megohms
* caution: before attempting to measure
insulation resistance, first disconnect and
isolate all leads of the winding to be tested.
electronic components, diodes, surge protec-
tors, relays, voltage Regulators, etc., can be
destroyed if subjected to high megger volt-
ages.
HI-POT TESTER:
A “Hi-Pot” tester is shown in Figure 3-8. The model
shown is only one of many that are commercially
available. The tester shown is equipped with a voltage
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 dur-
ing the test.
Figure 3-8. – One Type of Hi-Pot Tester
STATOR INSULATION RESISTANCE TEST
GENERAL:
Units with air-cooled engines are equipped with (a)
center tapped AC power windings, (b) an excitation
Page 13