Testing Rotor Insulation, Megohmmeter | Generac Power Systems 5414

Section 3

Insulation Resistance Tests

		2
		6
		11
		22
	Stator	33
	Stator	44
		44
		11S
		22S

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 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 (-) test lead to a clean frame ground (like the Rotor shaft).

POSITIVE (+)

TEST LEAD

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 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 meter.

MINIMUM INSULATION		GENERATOR RATED VOLTS
RESISTANCE	=	__________________________	+1
RESISTANCE	=	1000	+1
(in “Megohms”)		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 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 CAREFULLY.

ROTOR MINIMUM INSULATION RESISTANCE:

1.5 megohms

Page 14

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Generac Power Systems 5414, 5412, 5411 Testing Rotor Insulation, Megohmmeter, Positive + Test Lead

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