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7.6.8 Drying
The windings must be dried:
•
After washing (especially a water wash and rinse)
•
If they have become humid in use or during a standstill.
Drying should always be started with an external blower or warm air. Other
means should be attempted, only if blower and warm air do not suffice.
During drying, the rate of temperature rise of the winding should not exceed 5 K
(9 °F) per hour, and the final temperature should not exceed 105 °C or 220 °F). A
sudden temperature rise or a too high final temperature can cause steam to be
formed in the cavities of the windings, which in turn can destroy the windings.
During the drying process, the temperature should be monitored periodically, and
the insulation resistance should be measured at regular intervals.
A very wet machine should be dismantled and the windings dried in an oven.
Every part should be checked. If the machine is not very wet, the winding can be
dried by passing a current through it.
If the winding is dried by passing a current through it, the source of electricity can
be e.g. a welding machine or a similar device.
NOTE: Direct current or alternate current can be used. The current must not
When drying in an oven, the temperature rise and the maximum temperature
should be monitored carefully. The oven temperature should be around 90 °C (or
194 °F) for 12 to 16 hours and then 105 °C (220 °F), for six to eight hours. These
times can vary, and the correct time should be monitored with and insulation
resistance test.
Effective drying is achieved with the right balance of heat and ventilation. The air
circulation inside the machine should be as effective as possible.
Drying in an oven with good ventilation is the most effective technique.
Unfortunately, this is not usually possible at the machine's operating site.
Therefore, either hot-air-blow or heating the windings with current should be used.
Adequate fresh-air circulation is essential, whatever heating method is used.
An insulation resistance test should be performed after drying the windings. When
drying is started, the insulation resistance decreases due to the temperature rise.
As the drying continues, however, the insulation resistance increases until it
reaches a stable value.
7.6.9 Other maintenance operations
Usually, ABB made winding are trouble free and in addition to periodical
monitoring they require only occasional cleaning and drying as described above.
If extraordinary circumstances occur and other maintenance is required, it is best
to acquire professional help. The ABB After Sales organization is happy to assist
in question regarding maintenance of electrical machine windings, for contact
information see Chapter 9. After sales and spare parts.
exceed 25 % of the nominal current, which is indicated on the rating
plate on the machine. The winding temperature should also be
continuously monitored.
Page 59
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