Guardian Technologies 5308, 4582 & 4987, 4583 Megohmmeter, Stator Insulation Resistance Test

Section 3

DESCRIPTION & COMPONENTS

The resulting direct current from the BCR is delivered to the 12 VDC receptacle, via Wire 13A, CB1, and Wire 15A. This receptacle allows the capability to recharge a 12 volt DC storage battery with provided battery charge cables.

Insulation Resistance

The insulation resistance of stator and rotor wind- ings is a measurement of the integrity of the insulat- ing materials that separate the electrical windings from the generator steel core. This resistance can degrade over time or due to such contaminants as dust, dirt, oil, grease and especially moisture. In most cases, failures of stator and rotor windings is due to a breakdown 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, they can usually be corrected by drying the windings. Cleaning and drying the windings can usu- ally eliminate dirt and moisture built up in the genera- tor windings.

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 500 volts when testing stators or rotors. 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 3-9) and connect all of the stator leads together. FOLLOW THE MEGGER MANUFACTURER’S INSTRUCTIONS CAREFULLY.

Use a megger power setting of 500 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.

The MINIMUM acceptable megger reading for stators may be calculated using the following formula:

MINIMUM INSULATION		GENERATOR RATED VOLTS
RESISTANCE	=	__________________________	+1
		1000
(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 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 Tests”.

Also test between parallel windings. See “Test Between Windings” on next page.

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 AND DO NOT APPLY VOLTAGE LONGER THAN 1 SECOND. FOLLOW THE MEGGER MANUFACTURER’S INSTRUCTIONS CAREFULLY.

ROTOR MINIMUM INSULATION RESISTANCE:

1.5megohms

* CAUTION: Before attempting to measure

Insulation resistance, first disconnect and Isolate all leads of the winding to be tested. Electronic components, diodes, surge protec- tors, relays, Voltage Regulators, etc., can be destroyed if subjected to high megger volt- ages.

HI-POT TESTER:

A“Hi-Pot” tester is shown in Figure 3-8. The model shown is only one of many that are commercially available. The tester shown is equipped with a voltage selector switch that permits the power supply voltage to be selected. It also mounts a breakdown lamp that will illuminate to indicate an insulation breakdown dur- ing the test.

Figure 3-8. – One Type of Hi-Pot Tester

Stator Insulation Resistance Test

GENERAL:

Units with air-cooled engines are equipped with (a) center tapped AC power windings, (b) an excitation

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