Generac Power Systems 53187 - Manual (Page 49)

PARTI
THE REVOLVING FIELD AC GENERATOR
1 SECTION 1.8
INSULATION RESISTANCE TESTS
Section 1.8
INSULATION RESISTANCE TESTS
Effects of Dirt and Moisture on Genera­
tors
If m oisture is perm itted to rem ain in contact w ith the
generator w indings, som e of it w ill be retained in voids
and cracks of the w inding insulation. This w ill cause a
reduced insulation resistance and, eventually, genera­
tor AC output w ill be affected. W inding Insulation used
in 'NP' and "IM" series generators is m oisture resistant.
How ever, prolonged exposure to w ater, high hum idity,
salt air, etc., w ill gradually degrade the resistance of
w inding insulation.
Dirt can m ake the problem even w orse, since dirt
tends to hold m oisture into contact w ith the w indings.
Salt (as from sea air) w ill also w orsen the problem ,
since salt tends to absorb m oisture from the air. W hen
salt and m oisture com bine, they m ake a good electrical
conductor.
Because of the detrim ental effects of m oisture, dirt
and salt, the generator should be kept as dry and clean
as possible. Stator and rotor w inding insulation resis­
tance should be tested periodically. If insulation resis­
tance is low , drying m ay be necessary. If resistance is
still low after drying, replacem ent of defective w indings
m ay be required.
Insulation Resistance
Insulation resistance is a m easurem ent of the in­
tegrity of the insulating m aterials that separate the
electrical w indings from the generator's steel core.
Most w inding failures are caused by a breakdow n in the
insulation system . In m any cases, low insulation resis-.
tance is caused by m oisture that has collected w hen
the generator is shut dow n. The problem can often be
corrected by drying out the w indings.
Norm ally, the resistance of the insulation system is
on the order of m illions of ohm s. It can be m easured
w ith a device called a "m egger" or "m egohm m eter"
(m eg is for m illion) and a pow er supply. The pow er
supply voltage varies, but the m ost com m on is 500
volts. A m egger voltage greater than 500 is not recom ­
m ended.
Another device used to check for insulation break­
dow n is called a "Hi-Pot Tester". See Figure 1. A typical
Hi-Pot Tester has a "Pilot" lam p w hich turns on to
indicate that pow er is availabie; and a "Breakdow n"
lam p w hich turns on if the insulation is breaking dow n
under an appiied voltage. The Hi-Pot Tester show n also
has a "Voltage Selector" sw itch w hich allow s the user
to select the voltage that w ill be applied to the w inding
being tested.
DANGER: MEGGERS AND HI-POT TESTERS
DELIVER A HIGH VOLTAGE TO WINDINGS
BEING TESTED. DO NOT TOUCH THE GENER­
ATOR, THE WINDING BEING TESTED, OR THE
TEST
LEADS WHEN PERFORMING TESTS.
CARELESS USE OF SUCH TESTERS CAN RE­
SULT IN DANGEROUS ELECTRICAL SHOCK.
To m easure insulation resistance, connect one test
lead to the leads of the w inding to be tested. Connect
the other test iead to the generator fram e. Make sure
the ieads of the part being tested are not touching m etal
parts of the generator. If the "Neutral" line of a stator is
grounded, it m ust be disconnected.
CAUTION: Before attempting to test Insulation
resistance, first disconnect all electronic com­
ponents, regulators, diodes, surge protectors,
protective relays, etc. The high tester voltages
will destroy such components._______________
Figure 1. One Kind of H l-Pot T ester
Using a Hi-Pot Tester
The Hi-Pot Tester show n in Figure 1 is one of m any
brands that are com m erciaily available. Insulation
breakdow n tests outlined In this section use the tester
show n in the illustration. The tester show n has a
"Breakdow n" lam p w hich turns on to indicate an insu­
lation breakdow n. If you are using another tester or
m egger, follow the m anufacturer’s instructions care­
fully. Do not exceed the voltages reconvnended in this
m anual.
All stator leads m ust be isolated from ground and
connected together. On system s w ith a grounded neu­
tral, the neutral can be isolated from ground and used
as a test point.
CAUTION: Do not apply tester voltages to the
stator or rotor windings longer than 1 second.
Turn the tester switch ^0 N"; check that the Pilot
lamp is "ON"; wait 1 second while observing the
"Breakdown" lamp; turn the tester switch
"OFF".
PAGE 1.8-1