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,
check for loose parts or mounting bolts.
When cleaning the control equipment, use a vacuum
cleaner or hand bellows. Accumulations of dust or dirt
around components can impair the natural flow and
If oil enters the windings, insulation breakdown
may be imminent, and the winding will probably have
to be rewound. However, patches of oil should be
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
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 212°F.
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.