As we know, alternator is an electrical device that converts mechanic energy into potential energy. It is also called synchronous generator because of it rotate on its synchronous speed on different load condition. We have already discussed and alternator in details so you can visit this link. Here we are going to discuss about voltage regulation of alternator. Generally, voltage regulation means difference between induced voltage and terminal voltage. An alternator need to produce a fixed amount of voltage for any condition i.e. either loaded or unloaded. Field excitation will responsible for maintaining constant voltage by changing excitation. 

Voltage regulation of alternator

On unloaded condition, induced emf voltage and terminal voltage of alternator are same but this case will be changed for loaded condition under constant speed and excitation i.e. induced emf voltage is greater than terminal voltage i.e. Eph > Vph. Under no load, there is no current flow to load and armature current will be zero i.e. Ia = 0 so there is no armature flus and no armature reaction. Which will cause no voltage drop that mean Eph and Vph are same. So we calculate voltage regulation for loaded condition. The voltage regulation of alternator is the change in terminal voltage when the full load is removed at constant filed excitation and speed i.e. 

Voltage regulation = |Eph| - |Vph|

If voltage regulation is divided by terminal voltage, this is known as percentage voltage regulation which is multiplied with 100 and expressed in percentage i.e.

Percentage voltage regulation = [ (Eph - Vph) / Vph ] x 100%

Also, voltage regulation of alternator is only not depends on load current ( or armature current). It is also depends on power factor of the system. This can be easily understand by using the figure below.

From figure we can see that for unity and lagging power factor, terminal voltage is dropped from Eph as load current is increasing and voltage regulation become positive. This process is changed for leading power factor, terminal voltage is increasing as load current is increasing. In such case, voltage regulation becomes negative as Vph more than Eph. The given figure is known as load characteristics of the alternator which is the relationship between terminal voltage and load current.

Determination of voltage regulation

There two type of methods to measure voltage regulation i.e.

1. Direct load test 

This test is done as we have formula of voltage regulation. For that, we need to have terminal voltage at full load and at no load. Firstly, we run the synchronous generator at synchronous speed and adjust the excitation to get its rated terminal voltage. Now, load is given and increase it to full rated given power factor. Then, the load is removed but its other condition remain same i.e. constant speed and excitation. At that time, voltage is recorded which is known as open circuit or no load voltage. After that it is easy to calculate voltage regulation by using formula,

percentage voltage regulation = (E - V) x 100% / V

where E = no load voltage

V = rated terminal voltage

This method is only suitable for small power alternator.

2. Indirect methods

For small alternators, direct load test can be applied but what for large alternator? For large alternator, indirect methods is used. Different types of methods are available in indirect methods. They are:- a. synchronous impedance method (EMF methods), b. Ampere-turn method (MMF method), c. zero power factor method (Potier method).