The four-letter cooling designation tells you what carries the heat away and how hard it is being pushed. ONAN is the quiet self-cooled baseline; every step to forced cooling buys rating and costs you noise. Reading the code correctly matters because noise limits, enclosure ratings and altitude correction all hang off the same decision.
Decoding the four-letter code
The letters read in pairs. For oil-immersed units the first pair describes the internal cooling medium and how it moves; the second pair describes the external medium and how it moves.
| Letter position | Meaning | Common values |
|---|---|---|
| 1st — internal medium | What carries heat out of the windings | O = mineral oil or ester · K = other insulating liquid · A = air (dry-type) |
| 2nd — internal circulation | How the internal medium moves | N = natural convection · F = forced by pump · D = forced and directed through the windings |
| 3rd — external medium | What the heat is rejected to | A = air · W = water |
| 4th — external circulation | How the external medium moves | N = natural · F = forced by fan or pump |
Dry-type transformers use the shorter two-letter form, because air is both the internal and external medium.
Oil-immersed cooling classes
| Code | Meaning | Typical use | Noise |
|---|---|---|---|
| ONAN | Oil natural, air natural | Standard distribution ratings, up to roughly the lower MVA range | Quietest — no fans or pumps |
| ONAF | Oil natural, air forced | Adds rating headroom via radiator fans; common step above ONAN | Fan noise added at the forced stage |
| OFAF | Oil forced, air forced | Larger power transformers; pumps and fans | Pump and fan noise |
| ODAF | Oil directed, air forced | High ratings where hot-spot control governs; oil positively directed through winding ducts | Pump and fan noise |
| OFWF | Oil forced, water forced | Where cooling water is available, often indoor or space-constrained sites | Pump noise; no air discharge to manage |
Each step up in cooling gives additional rating headroom — confirm the headroom figure per design and rating; do not assume a generic multiplier. What matters practically is that a unit may carry two or three ratings on the same nameplate, one per cooling stage, and that the higher rating is only available with the fans or pumps running.
Dry-type cooling: AN and AF
| Code | Meaning | Rating | Noise |
|---|---|---|---|
| AN | Air natural | Base self-cooled rating | Quietest condition |
| AF | Air forced | Raised rating with fans | Fan noise added; rises further with load |
The AF stage typically adds 40–50% additional rating headroom, which is why it is specified to cover the period a redundant unit is out of service in an N+1 scheme. But it is also the reason a data hall specified at AN can fail its noise limit the moment the fans start.
Three interactions that catch people out
1. Forced cooling is a hidden noise source
The core and windings generate noise under all conditions, but forced-air operation adds fan noise on top and usually coincides with higher load, which raises core and winding noise as well. A noise-sensitive installation must be specified at the peak cooling condition, not at the quiet natural-cooled rating.
Remember also that the declared figure from a type test per IEC 60076-10 is a sound power level LWA, not a sound pressure level. In a free field, sound pressure at 1 m is approximately LWA − 11 dB, falling a further 6 dB each time distance doubles — and a hard-walled plant room measures higher than that estimate.
2. Enclosure rating and cooling are one decision
A well-sealed enclosure such as IP54 or IP65 severely restricts air exchange, which limits heat dissipation. An outdoor or washdown-rated dry-type transformer usually needs forced cooling or derating to hold its temperature rise, and forced cooling then raises noise. Decide enclosure rating and cooling method together, or you will solve the ingress problem and create a thermal one.
Where noise is the concern, note that structure-borne sound beats airborne sound: anti-vibration mounting is usually the most cost-effective acoustic measure, before any enclosure treatment.
3. Altitude and ambient change the rating
Above 1000 m, reduced air density lowers cooling effectiveness, so IEC 60076-2 requires temperature-rise correction and the transformer may need derating. A site at 2500 m changes the usable rating, not just the paperwork. State altitude and ambient temperature at enquiry stage so the design can be corrected before manufacture.
How to specify cooling
- Start from the load profile. If the unit rarely approaches its rating, ONAN or AN is sufficient and quietest.
- Use a forced stage for contingency, not for baseline duty. AF or ONAF is at its best covering the period a redundant unit is out of service.
- Specify noise at the peak cooling condition. Ask for the guaranteed sound pressure at a stated distance, not the LWA at the natural-cooled stage.
- Fix enclosure rating and cooling together. IP54 plus AN usually means derating.
- State altitude and ambient. Correction is required above 1000 m.
- Confirm the rating at each cooling stage in writing. A nameplate carrying two ratings is normal; knowing which applies when is your responsibility.
Frequently asked questions
What does ONAN stand for?
Oil Natural, Air Natural. Oil circulates by natural convection and heat dissipates to ambient air without fans or pumps. It is the baseline self-cooled rating and the quietest operating condition.
What is the difference between ONAN and ONAF?
ONAF adds forced air over the radiators, raising heat dissipation and the permissible continuous rating above the ONAN figure. The trade-off is added fan noise and a mechanical component needing maintenance and a power supply.
Why does forced cooling make the transformer noisier?
Forced-air operation adds fan noise on top of the core and winding noise, and usually coincides with higher load, which increases that core and winding noise too. Specify noise-sensitive sites at the peak cooling condition.
What is the difference between OFAF and ODAF?
Both force oil and air. ODAF positively directs oil through the winding ducts where heat is generated, making it more effective at controlling hot-spot temperature on larger and more heavily loaded units.
What do AN and AF mean on a dry-type transformer?
AN is Air Natural, the self-cooled base rating. AF is Air Forced, raising the continuous rating with fans. AF is noisier, so specify noise at the peak cooling condition.
Does a higher IP enclosure rating affect cooling?
Yes, significantly. IP54 or IP65 restricts air exchange and limits heat dissipation, so an outdoor dry-type unit usually needs forced cooling or derating — and forced cooling raises noise. Decide both together.
Does altitude change the cooling rating?
Yes. Above 1000 m, IEC 60076-2 requires temperature-rise correction and possibly derating. State altitude and ambient at enquiry stage.