Back To Basics

As we briefly mentioned earlier, transformers can use a number of methods to dissipate heat.

The method used depends primarily on the amount of heat that needs to be dissipated, and the application surroundings. This includes indoor vs. outdoor and hazardous vs. non-hazardous installations.

There are two main transformer design types to deal with the problem: oil filled and dry.

Liquid-Filled Transformers

Many transformers are contained in a tightly fitted sheet-metal case or tank of oil. Oil provides good electrical insulation, and carries heat away from the core and windings by convection. This type of transformer is referred to as a Liquid-Filled Transformer.

Figure 37. Liquid-Filled Transformer with Banks of Cooling Fans

Figure 38. Liquid-Filled Transformer with Radiators

Small transformers might be cooled sufficiently by just allowing the oil to circulate inside the tank. Larger transformers might use fans or radiators to cool the oil. Fans for additional cooling and oil pumps for circulation could be required on even larger transformers.

In addition to oil, other cooling liquids are used, such as silicone. Silicone might be used in an application where oil is not suitable, such as where flammability is an issue.

Sealing the transformer case or tank is important, especially in the case of an oil-filled unit. Any penetrating moisture can reduce the insulating quality of the oil. Also, oxygen can cause oil decomposition, resulting in sludge.

Dry Type Ventilated Transformers

A transformer designed to operate in air is called a Dry Type Transformer. The design does not require the assistance of a liquid to dissipate excess heat. Natural or fan-assisted circulation through ventilation openings is all that is required to meet temperature classification requirements.

Figure 39. Dry Type Transformer

Because a liquid is not used, a tank is not required. However, dry type transformers are contained in some type of an enclosure.

You may hear the phrase Core and Coils when dealing with dry type transformers. This refers to the inside parts of a dry type transformer, primarily the core and windings, mounted on a base as one unit, without an enclosure. A core and coils unit could be considered by a customer intending to include it as part of an assembled enclosure being supplied by others.

Dry Type Specialty Transformers

A variety of specialty dry type transformers exist. Typically not ventilated, this type of transformer usually has a small rating, and is capable of moving excessive heat away from the core and coils naturally , without the need for ventilation openings or other heat dissipation means.

In most designs, this is accomplished by surrounding the core and coils with special material mixtures which absorb the heat and provide a solid seal. This type of transformer is ideal for hazardous locations, and is usually referred to as an Encapsulated Transformer.

Figure 40. Small Special Application Encapsulated Dry Type Transformer