Drying out a motor with a welder 1

[motorgirl]

I know that a motor winding that is wet can be dried using a welding machine, but I have not been able to find any guidelines on how to do it. Can any one point me to guidelines or tell me what procedure they follow?

Motorgirl

 

[rovineye]

This is a Navy instruction. Better guidance to measure insulation resistance is out there and it is pretty general in its entirety.

300-5.3.6 DRYING WITH CIRCULATING CURRENTS. It is sometimes difficult to dry large machines satisfactorily
by the use of external heating only. After removal of as much moisture as possible by this method, drying
may be hastened by circulating current throughout the windings from an external, low voltage, current source.
This should be provided with means for adjusting the voltage to limit the current through the windings. Exciter
sets or voltage arc welding sets are suitable sources of current. When the windings which are to be dried are of
equal resistance and current-carrying capacity (such as the phase windings on the stators of ac machines or the
windings on field poles), they may be grouped in series or parallel depending upon which is best suited to the
voltage available. A decision as to whether direct or alternating current is to be used should be based upon the
following considerations:
a. Direct current should be used for the field windings of both ac and dc machines for dc armatures.
b. Either direct or alternating current may be used for the stator windings of ac generators and motors except
that alternating current should be used only when the rotor of the machine is removed. Otherwise the rotor
may be heated excessively.
300-5.3.6.1 Drying Small Machines. Circulating current from an external source may also be used for drying
small machines. When there are a number of these of the same size and rating, their windings may be connected
in series to suit the voltage available.
300-5.3.6.2 Drying with Machine’s Operating Power. A generator which is in running condition can be dried
without an external source of current by short circuiting the stator phase windings or armature leads, applying
partial field, and driving the machine at reduced speed. The current should be carefully controlled by means of
the field and speed to prevent overheating.
300-5.3.6.3 Precautions Using Circulating Current. The following precautions should be observed when drying
machines by circulating current:
a. Even though the voltage required for circulating current through windings is usually low, it is important that the winding be reasonably free from moisture before this drying method is employed. Current should not be
circulated through any winding which has an insulation resistance of less than 50,000 ohms at room temperature.
b. Current should not be conducted into a dc armature through brushes resting on a stationary commutator. This
will cause localized heating of the commutator. The armature should be rotated continuously by some external
means if it is necessary to dry by means of circulating current.
c. When drying by circulating current, the temperature must be increased slowly. Embedded temperature detectors
or thermometers on the outside of the insulation will not indicate the hottest spot or copper temperatures.
Temperatures measured by embedded detectors or by the hot resistance method should not exceed 91°C (195°F). Temperatures measured by thermometers should not exceed 77°C (170°F).
d. When using circulating current for drying ac stators, it is important that the stator end windings be heated sufficiently to drive out the moisture. The end windings on large machines have a large radiating surface, and
unless heating with circulating current is supplemented with external heating, the temperature of the end
windings will be considerably less than the temperature of the embedded section of the coils and the result
will be insufficient drying of the end windings. The use of external heaters to increase the temperature of the
end windings will avoid this difficulty. In addition, on machines of 1,000 kW or more, it is advisable to untape
the end windings to facilitate moisture escape.
300-5.3.7 INSULATION RESISTANCE AND DRYING PROGRESS. The degree to which the properties of insulation are restored by drying may be determined to some extent by measuring the insulation resistance to ground. Readings should be taken when the drying is started, checked at regular intervals thereafter as long as the drying continues, and plotted on semi-logarithmic paper with the logarithm of insulation resistance ordinate and time as abscissa. Usually the resistance will drop as the machine warms up, reach a minimum, and then start to rise rapidly at first and then more slowly as the drying progresses. The value may decrease slightly at times and then increase again at a slower rate as the moisture is driven out, indicating that the drying is nearly completed. An erratic curve may indicate leakage paths to ground or weak insulation. Duplicate machines may show entirely different response to drying.
300-5.3.7.1 Preferred Method of Measuring Insulation Resistance. The preferred method of measuring insulation
resistance is to use an insulation resistance measuring instrument of the handcrank generator type. Another
method is to use one or two 45-volt B batteries and a 150-volt high resistance (at least 15,000 ohms) voltmeter
of known internal resistance. If no B batteries are available, it is permissible to use a low voltage (not more than 120-volt) dc circuit provided a voltmeter test is first made to see if the circuit is grounded. See paragraph
300-3.2.4 for instructions on how to make this test and detailed instructions on how to measure insulation resistance by the voltmeter method.
300-5.3.7.2 When Drying is Completed. As drying continues, the general trend of the insulation resistance values
indicate the progress in eliminating moisture. Drying should be continued until either the insulation resistance
readings show no abrupt changes and do not increase more than 5 percent over a 12-hour period or the polarization
index is greater than 3.0. The final value of insulation resistance for machine windings at the completion of
drying, adjusted to 25°C, should not be less than the applicable minimum value given in the insulation resistance
tables of paragraph 300-3.4.8.

 

[jraef]

More information on drying out motors and equipment can be found here:

faq237-1157


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