PART 1
GENERAL INFORMATION
Page 22
Section 1.5
Testing, Cleaning and Drying
VISUAL INSPECTION
When it becomes necessary to test or troubleshoot an
alternator, it is a good practice to complete a thorough visual
inspection. Remove the access covers and look closely for any
obvious problems. Look for the following:
•Burned or broken wires, broken wire connectors, damaged
mounting brackets, etc.
•Look for loose or frayed wiring insulation, loose or dirty
connections.
•Check that all wiring is well clear of rotating parts.
•Verify that the Generators voltage output matches Utility
voltage.
•Look for foreign objects, loose nuts, bolts and other
fasteners.
•Clean the area around the Generator. Clear away paper,
leaves, snow, and other objects that might blow against the
Generator and obstruct its air openings.
•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. Set the “megger”
to a voltage setting of 500 volts when testing stators and
rotors.
•DO NOT EXCEED 500 VOLTS
•DO NOT APPLY VOLTAGE LONGER THAN 1 SECOND
•FOLLOW THE “MEGGER” MANUFACTURERS INSTRUCTIONS
CAREFULLY
Testing Stator Insulation
Warning: Megger HIGH voltages could cause damage
to other components on the Generator. Take the proper
precautions before testing.
BEARING
BA
0
4
44
22
33
11
STATOR
0
4
44
11
11
44
2 6
2
6
0
4
11
44
2
6
0
4
11
44
2
6
CLOSEST TO
6
5
2
1
4
3
PIN #
C1
TO C1 SEE TABLE FOR PINOUT
CONFIGURATION A
CONFIGURATION B
0
4
11
44
2
6
0
4
11
44
2
6
6
5
2
1
4
3
PIN #
C1
Figure 20. Stator Output Leads
Isolate all stator leads (Figure 20) and connect all the stator
leads together.
Use a megger power setting of 500 volts. Connect one megger
test lead to the junction of all the stator leads, the other test
lead to frame ground on the stator can. Read the number of
megohms on the meter.
To calculate the MINIMUM acceptable megger readings use the
following formula:
MINIMUM INSULATION
GENERATOR RATED VOLTS
RESISTANCE =
__________________________
+1
(in “Megohms”)
1000
Example: Generator is rated at 120VAC. Divide “120” by
“1000” to obtain “0.12”. Then add “1” to obtain “1.12”
megohms. Minimum insulation resistance for a 120VAC
stator is 1.12 megohms.
120
+ 1 = 1.12 megohms
1000
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 Megger to test for shorts between isolated windings as
outlined in “Stator Insulation Tests.”
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
•DO NOT APPLY VOLTAGE LONGER THAN 1 SECOND
•FOLLOW THE “MEGGER” MANUFACTURERS INSTRUCTIONS
CAREFULLY
ROTOR MINIMUM INSULATION RESISTANCE:
1.5 megohms
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