A range of factors, such as ambient temperature ranges, expected core and copper losses, load conditions, and the capacity to test products in accordance with recognised standards, are all important when selecting the correct transformer for a particular application. This is according to Trafo Power Solutions MD David Claassen, who says that differences in transformers can be substantial, even if the design and construction appears to be similar.

“The minimum, maximum and average temperatures in which a transformer can safely operate will affect the design and consequently the price of a transformer,” he explains.

In dry-type transformers, two main temperature winding insulation classes exist, Class F and Class H. Class F allows the transformer to operate safely at temperatures up to 155 °C without damage, while Class H allows the transformer to accommodate temperatures up to 180 °C.

Claassen says another detail to look out for is that both oil-cooled and dry-type transformers are available in multiple cooling options.

“The most common is natural air (AN), where the surrounding still air is used to keep the transformer operating within the correct temperature range. The second most common option is the forced air method, which entails moving air over the radiators, or over the core and windings. This allows the temperatures to be kept in check,” he explains. Transformers using forced cooling are able to supply additional load over and above the nominal power rating but it is important to rate the transformer at AN (natural ventilation) and use the fans as a temporary measure.

Transformer losses can be either load or core losses and no-load or copper losses. IEC standards set the maximum allowable load and no-load losses. The higher the transformer’s losses, the cheaper it is to produce and the higher the operational costs.

An additional consideration is the load that the transformer will supply or be supplied by, as the Total Harmonic Distortion that a transformer encounters significantly influences the design of the transformer.

Classen also emphasises that potential buyers must carefully assess manufacturers’ quality control programmes, in order to ensure that all raw materials and components can be tracked at all stages of the manufacturing process.