efficiency of transformer is maximum at which power factor?

The efficiency of a transformer is maximum at a power factor of unity (1.0). At this power factor, the load is purely resistive, meaning that all the power supplied is effectively converted into useful work without reactive power losses. When the power factor is less than unity, additional reactive power is present, which can reduce the overall efficiency of the transformer. Therefore, for optimal performance, transformers operate most efficiently at a power factor close to 1.