Transformer MCQs Part 5

Contents

- 1 The no-load primary current Io is about what percentage of full-load primary current
- 2 Certain transformer has an efficiency of 80% and works at 100 V, 4 kW. If secondary voltage is 240 V, considering the power factor to be unit, what is value of primary current
- 3 The transformer works on
- 4 A low voltage electrical system employs a 240V to 40 step down transformer for safety. The equivalent resistance of electrical system is 10 ohms. Considering the transformer to be ideal and overall system as lossless, what is the current in secondary coil
- 5 If a is defined as turns ratio of transformer and is equal to 5, then such a transformer will be

## The no-load primary current Io is about what percentage of full-load primary current

The no-load primary current Io is about what percentage of full-load primary current:

- 3-5%
- 10-15%
- 15-30%
- 30-50%

Correct answer: 1. 3-5%

## Certain transformer has an efficiency of 80% and works at 100 V, 4 kW. If secondary voltage is 240 V, considering the power factor to be unit, what is value of primary current

Certain transformer has an efficiency of 80% and works at 100 V, 4 kW. If secondary voltage is 240 V, considering the power factor to be unit, what is value of primary current

- 10 A
- 20 A
- 30 A
- 40 A

Correct answer: 4. 40 A

Explanation:

P = Vp * Ip * PF or Ip = P/(Vp * PF) = 4*1000/(100*1) = 40 A

## The transformer works on

The transformer works on:

- AC only
- DC only
- Both of these
- None of these

Correct answer: 1. AC only

## A low voltage electrical system employs a 240V to 40 step down transformer for safety. The equivalent resistance of electrical system is 10 ohms. Considering the transformer to be ideal and overall system as lossless, what is the current in secondary coil

A low voltage electrical system employs a 240V to 40 step down transformer for safety. The equivalent resistance of electrical system is 10 ohms. Considering the transformer to be ideal and overall system as lossless, what is the current in secondary coil:

- 2 A
- 4 A
- 5 A
- 10 A

Correct answer: 2. 4 A

Explanation: Secondary current (Is) = Vs/R = 40/10 = 4 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.