# Transformer MCQs Part 15

Transformer MCQs Part 15

## If Zs and Zp are secondary and primary load impedances of a step-up transformer, then which of following is correct

If Zs and Zp are secondary and primary load impedances of a step-up transformer, then which of following is correct:

1. Zp < Zs
2. Zp > Zs
3. Zp = Zs
4. Insufficient data

Correct answer: 1. Zp < Zs

Explanation: Zp = (N1 / N2) ^2 * Zs

Since N2 > N1; Zp < Zs

## Iron core is provided in transformer so as to

Iron core is provided in transformer so as to:

1. Reduce core losses
2. Increase its weight
3. Proivde tight magnetic copling
4. None of these

Correct answer: 3. Provide tight magnetic coupling

Explanation: Iron core in transformer ensures that tight magnetic coupling is present between two windings. In case of practical transformer, the coefficient of coupling can be as high as 0.99

## The auto-transformer is preferred in situations where the voltage transformation ratio (K) is

The auto-transformer is preferred in situations where the voltage transformation ratio (K) is:

1. Very high
2. Very low
3. Unity
4. Near unity

Explanation:

Wa = (1 – K) Wo

Where

Wa = Wt of Cu in autotransformer

Wo  = Wt of Cu in ordinary transformer

Saving in Cu = Wo – Wa = kWo

If K = 0.1 saving of Cu is only 10%

However, If K = 0.8, saving of Cu is 80%, the higher the K is, the greater is saving of Cu.

Therefore the nearer the value of K to 1, the greater is the saving of Cu.

## The no-load ratio of a 50 Hz single phase transformer is 6000/220 V. The value of flux in core is 0.06 Wb. What is the number of primary turns

The no-load ratio of a 50 Hz single phase transformer is 6000/220 V. The value of flux in core is 0.06 Wb. What is the number of primary turns

1. 10
2. 12
3. 17
4. 26

From formula: Es = 4.44 * f *   Ns * Φm

220 = 4.44 * 50 * Ns *0.06

Ns = 17

## Usually, transformers with rating less than 5 kVA are

Usually, transformers with rating less than 5 kVA are:

1. Natural air cooled
2. Oil cooled
3. Water cooled
4. None of these

Correct answer: 1. Natural air cooled