Test 56 - Choke Test; Insulation Resistance; The Megohmmeter; Introduction - Generac Power Systems GP Series Diagnostic And Repair Manual

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TB1
J2-1
J2 HARNESS CONNECTOR
Figure 4-62. Check TR1 Wiring
Test 56 – Choke Test
Procedure
If the generator is surging it may have a carburetion
problem. A lean condition can cause erratic RPM. Slowly
pull the choke out to see if surging stops. If it does stop,
carburetion should be checked.

Insulation Resistance

The insulation resistances of stator and rotor windings are
a measurement of the integrity of the insulating material
that separates the electrical windings from the generator
steel core. This resistance can degrade over time or due
to such contaminates as dust, dirt, oil, grease and
especially moisture. In most cases, failures of stator and
rotor windings are due to a break down 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, this can usually be corrected by drying the
windings. Cleaning and drying the windings can usually
eliminate dirt and moisture that has built up in the
generator windings.

The Megohmmeter

Introduction

A megohmmeter (often called a megger), consists of a
meter calibrated in megohms and a power supply.
Diagnostic Repair Manual
BLK
BLK
TR2
TR1
J2-12
004639
IMPORTANT NOTE: When testing stators and rotors,
set Megohmmeter to 500 volts and apply voltage for a
maximum of one second. Follow the megohmmeter
manufacturers instructions carefully. Do not exceed 500
volts or apply voltage
megohmmeter high voltages could cause damage to
other components on the generator. Take the proper
precautions before testing.

Testing Stator Insulation

See Figure 4-64. All parts that might be damaged by the
high megger voltages must be disconnected before
testing. Isolate all stator leads and connect all of the
stator leads together.
IMPORTANT NOTE: Follow the megger manufacturer's
instructions carefully.
Use a megohmmeter power setting of 500 volts. Connect
one megohmmeter test lead to the junction of all the
stator leads. Connect the other test lead to a frame
ground on the stator can. Read the number of megohms
on the meter.
To calculate the minimum acceptable megohmmeter
readings use the following formula:
Minimum Insulation
Resistance (In "Megohms")
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 calculated
minimum resistance, clean and dry the stator. Then, repeat
the test. If resistance is still low, replace the stator.
Use the megohmmeter to test for shorts between isolated
windings as outlined in
Test.
Also test between parallel windings. See
Windings.

Testing Rotor Insulation

Apply a voltage of 500 volts across the rotor positive slip
ring (nearest the rotor bearing), and a ground (i.e. the
rotor shaft).
IMPORTANT NOTE: When testing stators and rotors,
set megohmmeter to 500 volts and apply voltage for a
maximum of one second. Follow the megohmmeter
manufacturers instructions carefully. Do not exceed 500
volts or apply voltage
megohmmeter high voltages could cause damage to
other components on the generator. Take the proper
precautions before testing.
Rotor Minimum Insulation Resistance:
1.5 Megohms
Section 4 AC Diagnostic Tests
longer than 1 second.
Generator Rated Volts
=
1000
Stator Insulation Resistance
Test Between
longer than 1 second.
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