Fahad's Electrical Encyclopedia — Transformers

ONAN and ONAF Transformer Cooling Systems

Decoding transformer cooling codes: the meaning of the four letters in ONAN and ONAF, the difference between them, and what happens when the cooling fans fail.

Trainer Fahad Refai Oil, Insulation & Cooling Quick read

On the nameplate of every oil-filled transformer you'll find a four-letter code such as ONAN or ONAF. These are not riddles; they are a precise description of the cooling system, and understanding them is essential to reading the transformer's true capacity.

The key to the code: four letters

PositionDescribesCommon symbols
First letterInternal cooling mediumO = mineral oil
Second letterMethod of internal medium circulationN = natural, F = forced (pump)
Third letterExternal cooling mediumA = air, W = water
Fourth letterMethod of external medium circulationN = natural, F = forced (fans)

Reading common codes

  • ONAN: oil circulates naturally by convection + natural air around the radiators. The simplest and most reliable system.
  • ONAF: natural oil + forced air, with fans blowing air over the radiators, raising the transformer's thermal capacity.
  • Many transformers carry two ratings together, such as ONAN/ONAF: a base capacity without fans and a higher capacity when they are running.
An important field scenario

An ONAF transformer whose fans have failed: its thermal capacity effectively drops to the ONAN rating. Continuing at full load means exceeding the allowable temperature, and the load must be reduced or urgent repairs made — thermal protection may also intervene.

Interview question: What does the symbol ONAF mean on a transformer's nameplate? And what happens if the fans fail?

Sample answer: ONAF means: oil (O) circulates naturally (N) internally, and air (A) is forced (F) externally by fans. If the fans fail, the transformer loses part of its thermal capacity and effectively operates at the lower ONAN rating, so the load must be reduced or the fans restored to service quickly — otherwise the temperature rises and protection intervenes.

Common Mistake

Operating an ONAN/ONAF transformer at its maximum capacity without verifying that the fans are working. The higher rating on the nameplate is conditional on forced cooling, and ignoring this condition silently shortens the life of the insulation.

Related Topics

The Function of Transformer Radiators Causes of Transformer Overheating Understanding the Transformer Nameplate The Function of Transformer Oil

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The Function of Transformer Radiators Transformers Guide Understanding the Transformer Nameplate