INSULATION RESISTANCE TESTS
Leads 2 & 6 = Stator Excitation Winding
Leads 11S & 22S = Voltage Sensing Leads
Leads 11 & 22, 33 & 44 = AC Power Windings
Figure 3-2. – Stator Leads
TESTINg ROTOR INSULATION
To test the Rotor for insulation breakdown, proceed as
Disconnect wires from the Rotor brushes or
remove the brush holders with brushes.
2. Connect the tester positive (+) test lead to the
positive (+) slip ring (nearest the Rotor bearing).
Connect the tester negative (-) test lead to a clean
frame ground (like the Rotor shaft).
Figure 3-3. – Rotor Test Points
3. Apply 1000 volts. DO NOT APPLY VOLTAGE
LONGER THAN 1 SECOND.
If an insulation breakdown is indicated, clean and dry
the Rotor then repeat the test. Replace the Rotor if it
fails the second test (after cleaning and drying).
A megohmmeter, often called a "megger", consists
of a meter calibrated in megohms and a power sup-
ply. Use a power supply of 1500 volts when testing
Stators; or 1000 volts when testing the Rotor. 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 3-2) and connect all of
the Stator leads together. FOLLOW THE MEGGER
MANUFACTURER'S INSTRUCTIONS CAREFULLY.
Use a megger power setting of 1500 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
The MINIMUM acceptable megger reading for Stators
may be calculated using the following formula:
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
Use the Megger to test for shorts between isolated
windings as outlined "Stator Insulation Resistance".
Also test between parallel windings. See "Test
Between Parallel Windings" on this page.
TESTING ROTOR INSULATION:
Apply a voltage of 1000 volts across the Rotor pos-
itive (+) slip ring (nearest the rotor bearing), and
a clean frame ground (i.e. the Rotor Shaft). DO
NOT ExCEED 1000 VOLTS AND DO NOT APPLY
VOLTAGE LONGER THAN 1 SECOND. FOLLOW
THE MEGGER MANUFACTURER'S INSTRUCTIONS
ROTOR MINIMUM INSULATION RESISTANCE:
GENERATOR RATED VOLTS