A dry type transformer works on the same principle as every other transformer. What differs is how the windings are insulated and how the heat leaves the unit, and those two differences explain most of the constraints you run into when specifying one.
Basic Working Principle
Alternating current in the primary winding produces an alternating magnetic flux in the core. The flux links the secondary winding and induces a voltage in it. The secondary voltage is set by the turns ratio, and the power available on the secondary is limited by how much heat the unit can shed, not by the ratio itself.
Core Construction
The core is built from laminations of grain-oriented silicon steel, stacked and insulated from each other so that eddy currents cannot circulate freely through the section. The laminations give the flux a low-reluctance path and keep the core loss down. Core loss is present whenever the unit is energised, loaded or not, which is why no-load loss matters even for a transformer that spends much of its life lightly loaded.
Winding Construction
Conductors are usually copper, and aluminium appears in cost-sensitive designs. Insulation is applied as enamel, paper or aramid tape, varnish, or resin, depending on the class required. In a cast resin transformer the completed winding is cast in epoxy, so the insulation is also the structural containment and the winding becomes a rigid block that resists moisture and fault forces.
Load loss appears as I²R heating in the conductors. At higher currents it is not evenly distributed: skin and proximity effects push current toward the conductor surface, and a rectifier or VFD load adds harmonic content that increases the effective loss again.
Cooling Methods
Air is the only cooling medium, and it can be moved or left to move itself.
- AN: natural air cooling, by convection and radiation, with no fans.
- AF: forced air cooling, with fans moving air across the windings. Fans raise the rating a unit can carry in the same footprint, but they are moving parts and the rating depends on them running.
A unit is often rated both ways, for example as AN/AF, so the base rating is available with the fans out of service and the higher rating only with them running.
Insulation Systems
Solid insulation classes are defined by the temperature the material is expected to survive over its life. Class B is rated at 130 °C, Class F at 155 °C, and Class H at 180 °C. The class does not set the operating temperature of the unit; it sets the limit the design is working against, and the gap between that limit and the actual hot spot is what determines how long the insulation lasts.
Thermal ageing roughly doubles for every 6 to 8 °C of sustained extra hot spot temperature in the mid range, which is why a unit running at the top of its class at continuous full load will not reach the same service life as one with margin.
The practical question when comparing units is which winding temperature rise the manufacturer guarantees at rated load in the specified ambient, not which class is printed on the nameplate. See the dry type transformer range or contact us with the site conditions.
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