What an air cooled transformer actually is

An air cooled transformer is a transformer whose heat is dissipated directly to the surrounding air instead of being carried away by insulating oil. In practice, most air cooled units are dry-type transformers: the core and windings are encapsulated in cast resin or impregnated with insulating varnish, and cooling is achieved by natural convection, radiation, and — on larger ratings — forced ventilation from fans. Because there is no oil tank, no conservator, and no oil fire load, the air cooled transformer is the default choice for indoor installations where fire safety, environmental protection, and minimal maintenance matter more than the last few percent of capital cost.

This overview explains how these units work, what the ratings on the nameplate mean, which specifications actually change the price, and how to buy an air cooled transformer without paying for features you do not need.

How air cooling works: AN, AF and AN/AF ratings

Cooling class is the single most misunderstood item on a dry-type nameplate. Three designations cover almost every air cooled transformer sold today:

• AN (Air Natural): heat is removed by natural air circulation only. Silent, fan-free, and virtually maintenance-free, but the continuous rating is limited by how fast the enclosure can shed heat.
• AF (Air Forced): built-in fans push air across the winding ducts, raising the permissible output — typically 25% to 40% above the AN rating on the same core and coil.
• AN/AF (dual rating): the same transformer carries a lower continuous rating with fans off and a higher rating with fans running. This is the most cost-effective arrangement for loads with predictable peaks, because you pay for one unit instead of oversizing.

Fans are switched by winding temperature sensors, so the AN/AF air cooled transformer runs quietly at normal load and only brings the fans in during peak periods or high ambient temperatures.

Air cooled versus oil immersed: which one belongs in your project

Both technologies are mature and reliable; the choice is usually dictated by location rather than by performance.

• Fire and environmental risk: an air cooled transformer contains no flammable liquid, so it suits buildings, basements, tunnels, hospitals, and densely occupied sites where oil-filled equipment would require fire walls, oil containment pits, and extra insurance.
• Installation: dry-type units are lighter, need no oil handling or draining, and can be placed close to the load, which shortens low-voltage cable runs.
• Efficiency and cost: oil-immersed transformers such as the S13 and S20 series still win on price per kVA at larger ratings and tolerate harsher outdoor environments. For ratings roughly from 30 kVA to a few MVA indoors, the air cooled transformer is normally the better lifetime choice.
• Maintenance: no oil sampling or dissolved gas analysis, but periodic cleaning of dust from winding ducts becomes essential — a dirty air cooled transformer loses cooling capacity quickly.

Specifications that genuinely affect performance and price

When comparing quotations, look beyond the kVA figure. These parameters explain most of the price gap between two seemingly identical offers:

• Winding material: copper windings cost more but offer lower losses, better short-circuit strength, and more stable long-term performance than aluminium.
• Insulation class and temperature rise: class F (155 °C) is standard; class H (180 °C) allows a higher temperature rise and therefore a smaller, cheaper core and coil for the same output, at the cost of a shorter thermal margin. 100 K or 125 K temperature rise designs are commonly quoted.
• Loss levels: no-load and load losses determine the energy bill for the next 20 years. A low-loss design such as the SCB13, SCB14 or SCB18 series costs more up front and less every month afterwards.
• Enclosure protection: IP20 for clean indoor rooms, IP23 or IP31 where dripping water or dust is possible, IP54 for dusty or semi-outdoor locations. Each step up adds cost and slightly reduces heat dissipation.
• Voltage class: 10 kV and 35 kV dry-type units are standard; higher voltage classes change the insulation structure and the price accordingly.
• Accessories: forced-air fans, PT100 temperature sensors, digital temperature controllers, anti-condensation heaters, and surge arresters are frequently listed as options — confirm whether they are included before comparing totals.

Where an air cooled transformer is normally installed

Typical applications include commercial and office buildings, hospitals and laboratories, subway and railway stations, data centres, water treatment plants, shipyards, and process industries. In renewable energy projects, dry-type units are often used on the low-voltage side of wind and solar converters where fire risk must be minimised. Where the environment is wet, salty, or heavily polluted, an oil-immersed or specially protected design is usually preferred.

Installation and maintenance points that protect your investment

• Respect the altitude limit — above 1,000 m the cooling capability drops and the unit must be derated.
• Keep the specified clearances around the enclosure; an air cooled transformer that cannot breathe will run hot and lose life expectancy.
• Check ambient temperature assumptions, typically 40 °C maximum. A plant room running at 50 °C needs a larger unit or forced cooling.
• Clean winding ducts and fan blades on a fixed schedule, and record winding temperatures after each service.
• Use infrared thermography on terminals and inspect fan operation at every maintenance interval.

How to buy without overpaying

Price is driven by four variables: rating, winding material, loss level, and enclosure or accessory package. The fastest way to cut cost without cutting quality is to buy directly from the factory that winds the coils, rather than through a trading layer. A manufacturer with in-house casting, winding, and testing can match the temperature rise and loss class to your actual load profile instead of quoting a generic catalogue item.

Runsheng Transformer Co., Ltd. produces SCB13, SCB14 and SCB18 dry-type transformers alongside S11, S13, S20 and S22 oil-immersed units, SH15 amorphous alloy transformers, and 35 kV preassembled substations. Every air cooled transformer is supplied with a manual, inspection report, test certificate, and lifelong quality tracking, and units are shipped at factory-direct prices from maintained stock.

Quick checklist before you place an order

• Rated power in kVA, primary and secondary voltage, and vector group
• Cooling class required: AN, AF, or AN/AF
• Insulation class and temperature rise
• Loss level or efficiency standard to be met
• Enclosure IP rating and installation environment
• Accessories: fans, temperature controller, sensors, heaters
• Test reports, drawings, and delivery schedule confirmed in writing

Buying an air cooled transformer is mostly a matter of matching cooling class, loss level, and enclosure to the real site conditions — then buying that specification from the source. Get those three right and the unit will run quietly for decades. Contact us at phone +86 13406394988, e-mail runsheng@7813transformer.com, or WhatsApp 008613508930968 for a specification review and a factory-direct quotation.