Damp Windings

Take care to ensure that the drying current does not

cleanliness, brush operation, condition of brushes and

exceed 80 percent of the full-load rating. When drying

commutator, bearing temperature, and vibration.

the field winding, remove the brushes to prevent

When necessary, remove dust from the wound

marking or pitting the collector rings (which can occur

section with a vacuum cleaner, if available. If a vacuum

if the brushes carry the drying current).

cleaner is not available, use either compressed air (30

psi maximum) or a hand bellows. Be certain that the

Check the insulation resistance at regular intervals.

Remember, when using the current method, the

compressed air does not have any grit, oil, or moisture

resistance may drop temporarily as the moisture is

content in it. Use compressed air with caution,

forced to the surface of the winding. Drying should

particularly if abrasive particles, such as carbon, are

present, since these may be driven into the insulation

continue until the resistance is at least 1 megohm. The

drying process can take days in extreme cases. When the

and puncture it or may be forced beneath insulating

windings are dried out, apply a coating of insulating

tapes or other possible trouble spots. If vibration exists,

varnish.

check for loose parts or mounting bolts.

When cleaning the control equipment, use a vacuum

OIL-SOAKED WINDINGS

cleaner or hand bellows. Accumulations of dust or dirt

around components can impair the natural flow and

If oil enters the windings, insulation breakdown

cause overheating.

may be imminent, and the winding will probably have

to be rewound. However, patches of oil should be

DAMP WINDINGS

removed with a clean cloth soaked in an approved

solvent. Use the solvent sparingly, being careful that the

Moisture in windings can soften the insulation and

insulation is not saturated as this can cause softening of

reduce the dielectric strength. However, considerable

the insulation. After cleaning with solvent, apply one of

time is usually required before the moisture will harm

the drying methods.

the windings. By checking the insulation resistance of

the winding, you can spot possible trouble and take

SUMMARY

appropriate maintenance action.

In this chapter, you were introduced to the

If the insulation resistance falls below 1 megohm,

fundamentals of the various ac and dc motor and circuit

dry out the windings. Three recommended methods of

control devices to enable you to maintain, troubleshoot,

drying are oven drying, forced warm air drying, or

and repair the equipment successfully. You were given a

low-voltage current drying.

basic understanding of frequency regulators and the

Oven drying is accomplished using a maximum

troubleshooting and repair of them as well. Most

temperature of 85. The forced warm air method is

equipment installed will have a manufacturer's

simply using a fan to blow warm air across the damp

technical manual that should be used to adjust and repair

windings, The air must be dry and should be in a vacuum

t h e equipment following the recommended

with the temperature below 212F.

specifications. The Naval Ships' Technical Manual

The low-voltage current method requires

(NSTM), chapter 302, will provide additional

circulating a limited direct current through the windings.

information of value to you.

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