Transformer Losses and Efficiency - GATE EE Electrical Machines Free MCQ

Let's analyze each statement carefully:

Statement I:

"Maximum voltage regulation occurs at the leading power factor."

• Voltage regulation ($V.R.$) is a measure of the voltage drop in a transformer under load conditions. It is maximum at lagging power factors (e.g., inductive loads), not leading power factors.
• At leading power factors, the voltage regulation can even become negative.

This statement is incorrect.

Statement II:

"Maximum voltage regulation occurs when the load power factor angle and impedance angle of the leakage impedance are the same."

• This is true. The maximum voltage regulation occurs when the phase angle of the load current matches the impedance angle ($\tan^{-1}(X/R)$) of the transformer.
• This is because the voltage drop due to leakage reactance ($IX$) and resistance ($IR$) aligns with the load current, maximizing the overall voltage drop.

This statement is correct.

Statement III:

"Voltage regulation at zero power factor is always zero."

• At zero power factor (purely reactive load), the voltage regulation is not zero. Instead, it is dominated by the reactive component of the impedance ($IX$).
• The voltage regulation at zero power factor is typically the highest because the reactive drop is maximum.

This statement is incorrect.

Statement IV:

"Voltage regulation of a transformer may be negative at leading power factors."

• At leading power factors (e.g., capacitive loads), the reactive voltage drop ($IX$) becomes negative, leading to a rise in terminal voltage rather than a drop.
• Hence, voltage regulation can indeed be negative.

This statement is correct.

Summary of Correctness:

• Statement I: Incorrect
• Statement II: Correct
• Statement III: Incorrect
• Statement IV: Correct

Final Answer:

The incorrect statements are: