Transformer Ratings

When a transformer is to be used in a circuit, the voltage, current, and power-handling capabilities of the primary and secondary windings must also be considered in addition to the turns ratio.

The maximum voltage that can safely be applied to any winding is determined by the type and thickness of the insulation used. When a thicker insulation is used between the windings, a higher maximum voltage can be applied to the windings.

The power-handling capacity of a transformer is dependent upon its ability to dissipate heat. If the heat can safely be removed, the power-handling capacity of the transformer can be increased. This is sometimes accomplished by immersing the transformer in oil, or by the use of cooling fins. The power-handling capacity of a transformer is measured in either the volt-ampere unit or the watt unit.

Two common power generator frequencies (60 hertz and 400 hertz) have been mentioned, but the effect of varying frequency has not been discussed.

If the frequency applied to a transformer is increased, the inductive reactance of the windings is increased, causing a greater ac voltage drop across the windings and a lesser voltage drop across the load. However, an increase in the frequency applied to a transformer should not damage it. But, if the frequency applied to the transformer is decreased, the reactance of the windings is decreased and the current through the transformer winding is increased. If the decrease in frequency is enough, the resulting increase in current will damage the transformer. For this reason, a transformer may be used at frequencies above its normal operating frequency.