Voltage Regulation of Alternator or Synchronous Generator

The voltage regulation of an alternator or synchronous generator is defined as the rise in the terminal voltage when the load is decreased from full-load rated value to zero. The speed and field current of the alternator remain constant.

In other words, the voltage regulation of the alternator can be defined as the change in terminal voltage from no-load to full load rated value divided by the full-load rated voltage, i.e.,

$$\mathrm{Per\:unit\:voltage\:regulation =\frac{|𝐸_{𝑎}| − |𝑉|}{|𝑉|}}$$

Also, the percentage voltage regulation of the alternator is given by,

$$\mathrm{Percentage\:voltage\:regulation =\frac{|𝐸_{𝑎}| − |𝑉|}{|𝑉|}× 100\%}$$

Where,

• |𝐸_𝑎| is the magnitude of generated voltage (or no-load voltage) perphase

• |𝑉| is the magnitude of full-load rated terminal voltage per phase

The voltage regulation is like the figure-of-merit of an alternator. The smaller the value of the voltage regulation of a synchronous generator or alternator, the better is the performance of the alternator. For an ideal alternator, the value of the voltage regulation is zero.

The voltage regulation of an alternator depends upon the power factor of the load, i.e.,

• An alternator operating at a unity power factor has a small positive voltage regulation.

• An alternator operating at a lagging power factor has a large positive voltage regulation.

• An alternator operating at lower leading power factors, the voltage rises with increase of the load and hence, the voltage regulation is negative.

• For a certain leading power factor, the full-load voltage regulation is zero. In this case, both the full-load and no-load terminal voltages are the same.

Voltage Regulation of Alternator using Direct Loading Method

In the direct load test, the alternator is run at synchronous speed and its terminal voltage is adjusted to its rated value (V). Now, the load is varied until the ammeter and wattmeter connected in the test circuit indicate the rated values at the given power factor. Then, the load is removed and the speed and the field excitation of the alternator are kept constant and the no-load voltage (E_a) of the alternator is recorded. The voltage regulation of the alternator can be determined using these values as follows −

$$\mathrm{Percentage\:voltage\:regulation =\frac{|𝐸_{𝑎}| − |𝑉|}{|𝑉|}× 100\%}$$

The direct load test of alternator for determining the voltage regulation is suitable only for small alternators of power rating less than 5 kVA.

For large alternators, the following three indirect methods are used to determine the voltage regulation of the alternator, which are given as follows −

• Synchronous impedance method or EMF method.

• Ampere-turn method or MMF method.

• Zero power factor method or Potier triangle method.