Which statements about voltage drop in an alternator are correct?

I. The voltage drop due to armature resistance (IaRa) is in phase with the armature current (Ia).

II. At unity power factor, the voltage drop IaRa directly subtracts from the generated EMF (E).

III. For a leading power factor load, IaRa contributes to an increase in terminal voltage (V).

IV. Armature resistance has no effect on voltage regulation.

Armature reaction in an alternator

In an alternator, armature reaction refers to the effect of the armature current on the magnetic field of the machine. This interaction results in a distortion of the magnetic field, leading to changes in the terminal voltage.

The voltage drop due to armature reaction is minimized when the power factor is unity, meaning the load is purely resistive. In such a case, the armature reaction has the least impact on the voltage, resulting in the minimum voltage drop.

There are three causes of voltage drop in the alternator.

• Armature circuit voltage drop due to resistance
• Armature reactance
• Armature reaction

The first two factors always tend to reduce the generated voltage, and the third factor may tend to increase or decrease the generated voltage. The nature of the load affects the voltage regulation of the alternator.

Explanation

• Statement I is correct. The voltage drop due to armature resistance (IaRa) is in phase with the armature current (Ia) because resistance and current are always in phase in an AC circuit.
• Statement II is correct. At unity power factor, the voltage drop IaRa directly subtracts from the generated EMF (E) because the current and voltage are in phase, so the resistive drop subtracts directly from EMF.
• Statement III is incorrect. The terminal voltage is given by: V = E - I_aR_a. IaRa is a resistive drop and always reduces terminal voltage, regardless of the power factor.
• Statement IV is incorrect. Armature resistance does affect voltage regulation, as it causes a voltage drop under load.