Regulation, Efficiency, and Power Factor

Transformer regulation is the difference in the secondary terminal voltage at full load and at no load, expressed as a percentage of the full-load voltage. For the resistive load of Fig. 3(a), (b), and (c),

[12]

Since with low values of leakage reactance E_s - E_L = IR,

[13]

provided that R includes the primary winding resistance multiplied by the factor (N₂/N₁)² as well as the secondary winding resistance. If leakage reactance is not negligibly small, approximately

[14]

Efficiency is the ratio

[15]

where losses include both core and winding losses.

A convenient way of expressing power factor is

[16]

Equation 16 gives the power factor of a transformer plus its load.

One of the problems of transformer design is the proper choice of induction to obtain low values of exciting current and high power factor. Low power factor may cause excessive primary winding copper loss, low efficiency, and overheating.