Transformer MCQs Part 7

Contents

- 1 Mutual flux in transformer remains constant at all loads since
- 2 Certain transformer takes 0.4 A and absorbs 60 watts when the primary side is connected to a 230 V, 50 Hz, while secondary being open circuit. The iron loss current is
- 3 If a is defined as turns ratio of transformer and is equal to 5, then such a transformer will be
- 4 If a transformer core possesses air gaps then
- 5 Certain transformer has an efficiency of 85% and works at 200 V, 5 kW. If secondary voltage is 240 V, considering the power factor to be unity, what is value of secondary current

## Mutual flux in transformer remains constant at all loads since

Mutual flux in transformer remains constant at all loads since:

- Leakage flux is small
- It uses iron core
- Applied voltage and frequency are constant
- None of these

Correct answer: 3. Applied voltage and frequency are constant

## Certain transformer takes 0.4 A and absorbs 60 watts when the primary side is connected to a 230 V, 50 Hz, while secondary being open circuit. The iron loss current is

Certain transformer takes 0.4 A and absorbs 60 watts when the primary side is connected to a 230 V, 50 Hz, while secondary being open circuit. The iron loss current is:

- 0.13 A
- 0.26 A
- 0.35 A
- 0.5 A

Correct answer: 2. 0.26 A

Solution:

Formula for no load primary power is: Wo = Vp Io cos Φo

Iron loss current (Iw) = Io cos Φo = Wo/Vp = 60/230 = 0.26 A

## If a is defined as turns ratio of transformer and is equal to 5, then such a transformer will be

If a is defined as turns ratio of transformer and is equal to 5, then such a transformer will be:

- Step up transformer
- Step down transformer
- Any of above
- None of above

Correct answer: 2. Step down transformer

Explanation:

Turns ration (a) = Np/Ns = Vp/Vs

Suppose a = 5, and Vp = 2

Then Vs = 2/5 = 0.4

Since Vs < Vp, the transformer is a step down transformer

## If a transformer core possesses air gaps then

If a transformer core possesses air gaps then:

- Its hysteresis losses are decreased
- Magnetizing current increases
- Eddy current increases
- Reluctance of magnetic path is decreased

Correct answer: 2. Magnetizing current increases

## Certain transformer has an efficiency of 85% and works at 200 V, 5 kW. If secondary voltage is 240 V, considering the power factor to be unity, what is value of secondary current

Certain transformer has an efficiency of 85% and works at 200 V, 5 kW. If secondary voltage is 240 V, considering the power factor to be unity, what is value of secondary current:

- 9.73 A
- 17.70 A
- 25.8 A
- 33.83 A

Correct answer: 2. 17.70 A

Explanation:

Vs * Is * PF = %Eff * P

Is = (%Eff * P)/(Vs * PF)

Is = ((85/100)* 5*1000)/(240 * 1) Is = 17.70 A