In all drying out of electrical windings, the regulation of temperature should be controlled carefully. Maximum drying temperatures on windings should not exceed 194oF (90oC) as measured by thermometer. This will prevent not only the rapid thermal deterioration of the insulation but damage from the high vapor pressures that would be obtained if steam were produced.
Several methods are available for drying equipment. Probably the most satisfactory solution to the problem is when the windings can be placed in an oven with suitable temperature control and proper air circulation. Banks of infrared lamps may be used when this is not possible, or a suitable housing may be built around the machine, using steam coils or electric resistance type units for a source of heat. Openings should be provided for the free circulation of air. Blowers may be used to increase the air movement. Vacuum drying has also been effectively used to expedite the return of equipment to service. Certain precautions are necessary if this method is used, and it should be carried out only by experienced personnel.
Another method often used is to circulate low-voltage current through the windings. This should not be done, however, until the insulation resistance has reached a value of at least 100,000 ohms. Look for Megger® Insulation Testers that have kilohm ranges in order to perform this check prior to application of current. These models include the MIT400/2 Series, the MIT320 and MIT330. For immediate post-catastrophe environments, where line power may be unavailable and batteries in short supply, Major Megger models 212159, 212359, and 210170 offer always-ready hand-crank operation. Measurement is performed at only 3 to 6 V, so it will not damage even the worst of insulation. Welding sets may be used to provide the current. The flow should be limited to only a fraction of nameplate amperes, and a careful check must be maintained on maximum temperatures on the insulated parts.
On ac generators, drying current may be provided by driving the units at less than rated speed with the terminals short-circuited and excited with very low values of field current. Here again, this should be done only by those with experience in such methods.
During drying operations, when insulation resistance values are used as an indicator of the suitability of windings for service or for application of test potential, the drying must be continued for a sufficient time to make sure that the values are reliable. Often the resistance curve will take one or more sharp dips before leveling off or continuing to increase in a positive direction. This is due to moisture working out of the windings. When the machine is completely dried out, further work is required to remove any remaining dust. This may be done through the use of dry compressed air at pressure not exceeding 40 psi.
With respect to particular classes of electrical equipment damaged by flooding, the procedures listed below should be followed.
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