Two dry type transformers can carry the same nameplate kVA and still behave differently, because the cooling class decides how much of that rating is available all day and how much needs help from a fan. It is worth understanding before the drawing is fixed.
Reading the letters
The cooling designation uses letters for the cooling medium and how it moves. For a dry type unit the medium is air, written A. The letter after it describes the circulation: N for natural, meaning air moves on its own by convection, and F for forced, meaning fans push air across the windings.
Oil immersed units use O instead of A, which is why you see ONAN and ONAF on oil transformers. The logic is the same.
AN: air natural
AN is the simplest and quietest arrangement. No fans, no auxiliary power, nothing to maintain. It suits the majority of indoor dry type installations where the load is steady and the room is ventilated.
The trade-off is capacity. With air moving only by convection, the winding can shed a limited amount of heat, so for a given frame the AN rating is the lower of the two.
AF: air forced
AF adds fans that push air through the cooling ducts. The same transformer carries more load because heat leaves the winding faster. This is useful when a site needs extra capacity on top of the AN rating without buying a larger unit.
The cost is noise, a small auxiliary power draw and one more thing to maintain. Fans also need control, so that they run when they are needed and not before.
AN/AF: a dual rating
A common arrangement is AN/AF. The transformer carries the AN rating continuously with the fans off and a higher AF rating with the fans running. Fan control is triggered by winding temperature, usually from the temperature sensors already fitted for alarm and trip.
This gives a quiet base rating for normal load and headroom for peaks. It also means the design must be checked against the ambient in both modes.
What to weigh up
Fans are the only moving part on a dry type transformer, so they are the only part likely to fail. Deciding whether you want that dependency — and whether the site can power and control it — matters more than the small capacity gain on paper.
Hebei Youge builds dry type transformers in AN and AN/AF arrangements. Send the load profile and the ambient and we will advise which one fits.