Ques. Describe a forced draught or air blast transformer.
Ans. The case is closed at the bottom and open at the top. A current of air is forced through from bottom to top as shown in fig. 1,964 by a fan.
Ques. How are the coils best adapted to air cooling?
Ans. They are built up high and thin, and assembled with spaces between them, for the circulation of the air.
Ques. What are the requirements with respect to the air supply in forced draught transformers?
Ans. Air blast transformers require a large volume of air at a comparatively low pressure. This varies from one-half to one ounce per square inch. The larger transformers require greater pressure to overcome the resistance of longer air ducts.
Fig. 1,964.—Forced draught or "air blast" transformer. As is indicated by the classification, this type of transformer is cooled by forcing a current of air through ducts, provided between the coils and between sectionalized portions of the core. The cold air is forced through the interior of the core containing the coils by a blower, the air passing vertically through the coils and out through the top. Part of the air is sometimes diverted horizontally through the ventilating ducts provided in the core, passing off at one side of the transformer. The amount of air going through the coils, or through the core, may be controlled independently by providing dampers in the passages.
Ques. How much air is used ordinarily for cooling per kw. of load?