Voltage regulation of an alternator is given by:

V_NL: Voltage at no load

V_FL: Voltage at full load

Voltage Regulation (V.R.):

• The voltage regulation is defined as “the rise in voltage at the terminals, when the load is reduced from full load rated value to zero, speed and field current remaining constant”.
• With the change in load, there is a change in the terminal voltage of an alternator or synchronous generator. The magnitude of this change not only depends on the load but also on the load power factor.
• It is also defined as “the rise in voltage when full load is removed divided by the rated terminal voltage when speed and field excitation remains the same.” It is given by the formula,
  ​

V.R. = ((VNL - VFL) / (VFL)) × 100%

Where, 

VNL: Voltage at no load

VFL: Voltage at full load

Case 1:
VNL > VFL: Then V.R will be positive and the power factor will be lagging or unity.

Note: V.R under unity power factor is less than voltage regulation under lagging power factor,

Case 2:
VNL < VFL or VNL = VFL: Then VR will be negative and zero respectively and the power factor will be leading.

Conclusion:

From the above concept,

Curve D represents V.R at leading power factor.

Curve C represents V.R at a somewhat unity power factor.

Curve A and Curve B represent V.R at lagging power factor.

Voltage regulation in Transformer:

Voltage regulation is the change in secondary terminal voltage from no load to full load at a specific power factor of load and the change is expressed in percentage.

E2 = No-load secondary voltage

V2 = Full load secondary voltage

Voltage regulation for the transformer is given by the ratio of change in secondary terminal voltage from no load to full load to no load secondary voltage.

Voltage regulation 

\(= \frac{{{E_2} - {V_2}}}{{{E_2}}}\)