Voltage Regulation in Transformers

The formula for voltage regulation in transformers is:

$$VR=\frac{V_{NL}-V_{FL}}{V_{FL}}\times 100\%$$

where NL and FL stand for no load and full load, respectively. When calculating voltage regulation of transformers, magnetizing branch is usually neglected.

Primary Side

Fig. 9-29 shows the equivalent circuit for the case where the resistances and inductances have been referred to the primary side, and the magnetizing branch has been neglected.

In this case, voltage regulation becomes:

$$VR=\frac{V_p-a{V}_s}{a{V}_s}\times 100\%$$

Note that at no-load, $a{V}_s=V_p$.

Secondary Side

Fig. 9-30 shows the equivalent circuit for the case where the resistances and inductances have been referred to the secondary side and the magnetizing branch has been neglected.

In this case, voltage regulation becomes:

$$VR=\frac{V_p/a-V_s}{V_s}\times 100\%$$

Note that at no-load, $V_s=V_p/a$.

Examples

• Example 9-3 is also relevant to Determining Transformer Equivalent Circuit Parameters