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Moisture can cause severe problems in electrical generators. It can lead to costly generator repairs. This article gives information on how to check for the effects of moisture on generators and how to help prevent moisture from damaging the generator.
Effects Of Moisture On Windings
If moisture is allowed to remain in contact with an electrical winding, some of the moisture will eventually be absorbed. This will lower the resistance of the winding insulation. The insulation used on the windings of Caterpillar generators is moisture resistant, but constant exposure to moisture will gradually lower the insulation's resistance.
Dirt can make the problem worse because it can hold the moisture in contact with the insulation. Salt (from sea air) can also make the problem much worse. This is because salt tends to absorb moisture from the air. When the salt and moisture combine, they make a good electrical conductor.
If moisture has caused the main stator winding to have a low resistance to ground, current can flow from the main stator winding to ground when the generator is operated. Because of the high voltage between this winding and ground, the current flow could become high enough to cause additional break down of the insulation and damage the winding. Generator down time and significant repair cost can be the result of insulation breakdown.
If moisture has caused the excitor stator winding to have a low resistance to ground, a current can flow from the excitor winding to ground. The voltage between this winding and ground is much less than between the main stator winding and ground, so an insulation breakdown is not likely to occur. But an electrolysis process can cause copper to be eroded from the winding until the winding becomes an open circuit.
Rotor windings are not as sensitive to moisture as the main stator winding or the excitor stator winding. This is because the rotor is electrically isolated from ground and much lower voltages are involved.
Generators should be stored in a clean, dry area with minimal ambient temperature variation. Ambient temperature variation can cause condensation to form moisture on the generator windings. One method to stop condensation is to use space heaters or similar devices to heat the inside of the stored generator. Before a generator that has been stored is put into service, perform a megohmeter test on the main stator and excitor stator windings.
Measuring Insulation Resistance
The insulation resistance of an electrical winding can be measured with a megohmeter. This instrument applies a high voltage (500 to 1000 volts) between the insulated winding and ground and then measures the very small current that flows in that circuit. It converts that very small current flow into a resistance reading.
Before using a megohmeter to check the insulation resistance of the main stator winding, disconnect any external grounds, loads, meters or controls. Also disconnect the voltage regulator, terminals 20, 22, 24 and 26. Connect all the sections of the stator windings together in the normal high or low voltage connections. Connect one lead of the megohmeter to one of the stator windings and the other lead to the generator frame or ground. Follow the megohmeter manufacturer's directions and measure the stator windings insulation resistance. The megohmeter reading must not be less than 1 megohm. If the reading is less than 1 megohm, the winding must be dried.
Before using a megohmeter to check the insulation resistance of the excitor field winding, disconnect the F1 and F2 excitor field leads from the regulator. Connect one megohmeter lead to either the F1 or F2 field lead; make sure the other field lead is not touching anything. Connect the other megohmeter lead to the generator frame or ground. Follow the megohmeter manufacturer's directions and measure the resistance of the excitor field insulation. The megohmeter reading must not be less than 1 megohm. If the reading is less than 1 megohm, the winding must be dried.
One megohm is an approximate valve for determining if the winding insulation resistance is sufficient. That is, it can be possible to operate the generator with less than one megohm resistance without failure. However, a generator with low winding insulation resistance is far more likely to have a failure than a generator with acceptable insulation resistance.
Methods For Drying Generator Windings
One of the best methods for drying the generator winding and preventing a moisture build-up is the use of space heaters. The same type space heaters used in marine generator applications can be installed. Operate the space heaters for at least 4 hours, then repeat the megohmeter test. Continue to operate the space heaters until the megohmeter reading is greater than 1 megohm.
Another method that can be used to dry the winding is to put the generator in an oven. Heat the generator to approximately 65°C (150°F) for four hours. Repeat the megohmeter test.
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If an oven is used for drying, use a forced air type rather than a radiant type. Radiant ovens can cause localized overheating. The oven temperature should not be more than 85°C (185°F) or insulation damage may result.
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Reconditioning
If the megohmeter test reads less than 1 megohm after drying or if a reading of 1 megohm or higher cannot be maintained with space heaters, the insulation has deteriorated and should be reconditioned. Generator reconditioning should be done by an electrical shop. During reconditioning the generator is disassembled. The windings are throughly cleaned and oven dried. If megohmeter tests of the winding insulation resistance can be brought up to an acceptable level, the winding may be resealed by dipping in a tank of insulating varnish.
Recommended Periodic Insulation Resistance Checks
Use a megohmeter to check generator winding insulation resistance periodically. The frequency of the megohmeter test is determined by the generator's environment and by previous megohmeter test indications.
Megohmeter test every 6 months if the generator is installed in an enclosed building with relatively low humidity and minimal temperature variations.
Megohmeter test every 3 months if the generator is not protected from the elements by an enclosed building.
Megohmeter test every week and use space heaters if the generator is exposed to a sea water environment or if the surrounding conditions are very humid (relative humidity above 75%) or if a recent megohmeter test reading was less than 3 megohms. In applications where salt and high humidity are present, space heaters must be operated whenever the generator is not operating under load. This is the only way to maintain megohmeter test readings above 1 megohm.