Question

In a synchronous generator, "Voltage Regulation" is negative when the load is:

• A. Purely resistive
• B. Inductive (Lagging)
• C. Capacitive (Leading)
• D. Unity power factor

Hint:

Negative voltage regulation means that the terminal voltage on load is higher than the terminal voltage at no-load.
This phenomenon is unique to leading power factor (capacitive) loads due to magnetizing armature reaction.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This question relates to the voltage regulation of an alternator (synchronous generator) and how it depends on the nature of the load power factor.

Step 2: Key Formula or Approach: Voltage regulation is defined as the change in terminal voltage when full load is thrown off, expressed as a percentage of the rated terminal voltage ($V_t$):
\[ \text{\% Voltage Regulation} = \frac{E_0 - V_t}{V_t} \times 100 \]
where:
$E_0$ = No-load generated EMF per phase
$V_t$ = Rated terminal voltage per phase on load

Step 3: Detailed Explanation:

• When the alternator is loaded, the terminal voltage ($V_t$) differs from the induced EMF ($E_0$) due to armature resistance drop ($I_a R_a$), leakage reactance drop ($I_a X_L$), and the armature reaction.

• The effect of armature reaction depends heavily on the load power factor.

• For resistive (unity power factor) and inductive (lagging power factor) loads, the armature reaction is primarily cross-magnetizing and demagnetizing respectively.

• This causes the terminal voltage $V_t$ to drop below the open-circuit voltage $E_0$ ($V_t < E_0$), leading to a positive voltage regulation.

• For capacitive (leading power factor) loads, the armature reaction has a magnetizing component.

• This magnetizing effect assists the main field flux, raising the terminal voltage above the no-load value ($V_t > E_0$) as load increases.

• When $V_t > E_0$, the quantity $(E_0 - V_t)$ becomes negative, resulting in a negative voltage regulation.

Step 4: Final Answer
Thus, negative voltage regulation occurs under capacitive (leading) load conditions, matching option (C).