Section 3
INSULATION RESISTANCE TESTS
TEST BETWEEN PARALLEL WINDINGS:
Connect the tester leads across Stator leads No. 11P and 33. Apply a voltage of 1500 volts. If an insulation breakdown is indicated, clean and dry the Stator. Then, repeat the test between parallel windings. If the Stator fails the second test, replace it.
Figure 3-2. – Stator Leads
TESTING ROTOR INSULATION
To test the Rotor for insulation breakdown, proceed as follows:
1.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
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).
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 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 megger voltages must be disconnected before testing. Isolate all Stator leads (Figure
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 meter.
MINIMUM INSULATION |
| GENERATOR RATED VOLTS |
| |
RESISTANCE | = | __________________________ | +1 | |
1000 | ||||
(in “Megohms”) |
|
| ||
|
|
|
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 the Stator.
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 posi- tive (+) 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 VOLT- AGE LONGER THAN 1 SECOND. FOLLOW THE MEGGER MANUFACTURER'S INSTRUCTIONS CAREFULLY.
ROTOR MINIMUM INSULATION RESISTANCE:
1.5 megohms
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