Copper losses in both primary and secondary windings, and core losses are converted into heat in a transformer. These losses are all measured in watts. This heat must be conducted out of the windings and core and dissipated to the surrounding, or ambient air. There's a time delay involved in getting this heat out; as a result, the transformer temperature increases. This temperature will continue to increase until a condition of equilibrium is reached, one where the amount of heat generated in the transformer equals the amount of heat being dissipated.

The difference in temperature between a nonoperating transformer ("cold" condition) and one at full load equilibrium point ("hot" condition) is called temperature rise. It usually is measured in degrees Centigrade.

Measuring temperature rise

The core temperature is measured with a thermometer, with readings taken with the transformer "cold" and "hot." With these two readings, the temperature rise is calculated. For example, if we have a reading of 25 [degrees] C "cold" and 75 [degrees] C "hot," then the temperature rise is 50 [degrees] C.

The average winding temperature rise is determined by measuring the resistance of a winding when it's "cold" and again when the winding temperature has stabilized under full load. From the difference in the resistance readings, the average temperature is calculated for each winding.

Hot spot temperature

Although, the resultant temperature rise is averaged over the whole winding, the inside of a winding is hotter than its outside, in reality. The hottest spot is at some point inside the coil having the longest thermal paths to the outside air. This hot spot temperature differential is determined by the manufacturer on prototype units; it's usually expressed as a temperature increase over the average temperature.

The hot spot temperature differential is defined by industry standards (NEMA and ANSI) for each insulation class (type and temperature rating of insulation used on windings). Obviously, the hot spot temperature is the limiting temperature for a transformer's insulation system. In other [TABULAR DATA OMITTED] words, the temperature rise must be limited by design and application so that the total temperature does not exceed the temperature rating of the insulation used.

For example, if an insulation is rated at 105 [degrees] C maximum, the manufacturer must allow a 10 [degrees] C differential between average and hot spot temperatures in a winding. If the room ambient is 40 [degrees] C, then the allowable temperature rise is as follows.

Average rise = 105 [degrees] C (hot spot) minus 10 [degrees] C minus 40 [degrees] C (ambient) = 55 [degrees] C

Thus the transformer must be capable of withstanding a 55 [degrees] C average temperature rise.

Standard insulation classes

Insulation class is usually specified by degree Centigrade rise, which, as previously discussed, is based upon the maximum allowable rise by resistance in a maximum ambient of 40 [degrees] C, such that the maximum hot spot temperature is not exceeded. The table above clarifies this.

The total maximum temperature allows for a maximum ambient of 40 [degrees] C. If the ambient temperature is below this, the transformer will run cooler; if above, it will run too hot, unless a special lower temperature rise design is used.


Many types of insulation must be impregnated with a treating varnish to improve their temperature rating, resistance to moisture, or dielectric and mechanical strength. The coils also must be impregnated.

A good impregnating material will fill many of the small holes in a winding: For example, the space between and around two round wires. Impregnation also seals the edges of the coil and, in general, prevents the entrance of moisture and air.

Filling these small holes eliminates small pockets and, in general, improves the conduction of heat out of the coil.

The completed core-and-coil assembly then is treated; this molds the complete assembly into one solid mass. Thus, the coils can't move the core, so mechanical wear on the insulation is minimized.

The core laminations are cemented together by the varnish in this process, thus improving their mechanical strength while preventing movement of individual laminations. This helps reduce the sound level.

Air cooling

Air is the cooling medium for dry-type transformers. These transformers are divided into three groups: potted or encapsulated, exposed core, and cabinet type.

The potted or encapsulated type uses a filling material, either thermoplastic or thermosetting, surrounding the core and coils. This filling material is usually loaded with a good heat conducting material such as silica sand; this material comes in close contact with the coils and core, and carries their heat to the transformer case, where it's dissipated into the air.

The exposed core type uses a shell type core that surrounds the coils. A protective case is usually placed on one end and another case with the wiring compartment on the other, leaving the core exposed. Heat conduction is primarily from the coils to the core to the outside air.

Cabinet types have the core and coil completely surrounded by a cabinet that is appreciably larger than the core and coils. This cabinet has ventilating openings at the bottom and top so that a chimney effect is created. Thus, the air flowing up through the cabinet carries the heat away.

It's extremely important that air cooled unit are installed in freely circulating air.