Transformer MCQs Part 23

Contents

- 1 A transformer has full load copper losses of 400 W. The copper loss at half of full-load will be
- 2 A low voltage electrical system employs a 240V to 40 step down transformer for safety
- 3 If Zs and Zp are secondary and primary load impedances of a step-up transformer, then which of following is correct
- 4 Leakage flux in a transformer
- 5 At very low frequencies transformer operates poorly, since

## A transformer has full load copper losses of 400 W. The copper loss at half of full-load will be

A transformer has full load copper losses of 400 W. The copper loss at half of full-load will be:

- 400 W
- 200 W
- 100 W
- 50 W

Correct answer: 3. 400 W

Soution:

Copper losses vary as square of current

At half-full load = (1/2)^2 * Full load Cu loss = ΒΌ * 400 = 100 W

## A low voltage electrical system employs a 240V to 40 step down transformer for safety

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 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:

- Zp > Zs
- Zp = Zs
- Insufficient data

Correct answer: 1. Zp < Zs

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

Since N2 > N1; Zp < Zs

## Leakage flux in a transformer

Leakage flux in a transformer:

- Helps in transfer of energy
- Produces mutually induced emf
- Is negligible at full load
- Is reduced by interleaving the primary and secondary windings

Correct answer: 4. Is reduced by interleaving the primary and secondary windings

## At very low frequencies transformer operates poorly, since

At very low frequencies transformer operates poorly, since:

- Primary reactance is very high
- Permeability of core is increased

- None of these

Correct answer: 3. Magnetizing current is abnormally high