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When an alternator is subjected to a varying load, the voltage at the armature terminals varies to a certain extent, and the amount of this variation determines the regulation of the machine. When the alternator is loaded the terminal voltage decreases as the drops in the machine starts increasing and hence it will always be different than the induced emf. So, voltage regulation of an alternator is defined as the change in terminal voltage from no load to full load expressed as a percentage of rated voltage when the load at a given power factor is removed without change in speed and excitation. Or The numerical value of the regulation is defined as the percentage rise in voltage when the full load at the specified power factor is switched off with speed and field current remaining unchanged expressed as a percentage of rated voltage.

Hence, regulation can be expressed as,

% Regulation = (Eph – Vph / Vph ) x 100

Where,

Eph = induced emf /phase,

Vph = rated terminal voltage/phase

synchronous impedance Zs = (open circuit voltage per phase)/(short circuit current per phase)

Hence Zs = (Voc) / (Isc)

Using synchronous impedance and armature resistance synchronous reactance and hence, regulation can be calculated as follows using emf method.

Zs =√(Ra)² +(XS)²and Synchronous reactance Xs =√ ( Zs)² - (Ra)²

Hence induced emf per phase can be found as

Eph = √ [ (V cos Ø+ IRa)²+ (V sin Ø± IXS)²]

where

V = phase voltage per phase = Vph

[Note: voltage regulation for lagging power factor is always positive & for leading power factor is always negative.]

When an alternator is subjected to a varying load, the voltage at the armature terminals also varies.

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