Important Short Questions and Answers: Electrical Engineering and Instrumentation - Transformer

TRANSFORMERS

1. Distinguish between core and shell type transformer.

In core type, the windings surround the core considerably and in shell type the core surround the winding.

2.   What is an ideal transformer and how does it differ from a practical transformer.

Ø An ideal transformer is one which does not involve any power loss and does not have any change in frequency. It decreases or increases the input voltage.

Ø Practical transformer is one which involves power loss and does not have any change in frequency. It decreases or increases the input voltage.

3. Define voltage regulation in a transformer.

When a transformer is loaded with a constant primary voltage, the secondary voltage decreases for lagging Power factor load, and increases for leading power factor load because of its internal resistance and leakage reactance. The change in secondary terminal voltage from no load to full load expressed as a percentage of no loads or full load voltage is termed as regulation.

4. Write down the emf equation of a transformer.

Emf induced in primary coil E1 = 4.44 fΦ mN1 volt Emf induced in secondary coil E2 = 4.44fΦ mN2 volt

Where f is the frequency of AC input Φ m is the maximum value of flux in the core N1, N2 are the number of primary and secon dary turns.

5. Define a transformer.

The transformer is a static piece of apparatus by means of which electrical power is transformed from one alternating current circuit to another with desired change in voltage and current without any change in the frequency. It works on the principle of mutual induction.

6. Why transformers are rated in kVA?

Copper loss of a transformer depends on current and iron loss on voltage. Hence total losses depend on Volt- Ampere and not on the power factor. That is why the rating of transformers is in kVA and not in kW.

7. Why the core of a transformer is is laminated?

The purpose of laminating the co re in a transformer is to reduce eddy current loss.

8. Draw the equivalent circuit of a transformer.

9. An 1100/400 V, 50 Hz single phase transformer has 100 turns on the secondary winding. Calculate the number of turns on its primary.

We know V1 / V2 = k = N2 / N1

Substituting 400/1100 = 100/N1

N1 = 100/400 x 1100 = 275 turns.

10.            What are the advantages of a transformer?

Ø Less I2R loss in the transmission line

Ø Less voltage drop in the line

Ø Efficiency of the transmission line is increased

Ø Volume of the conductor required is less.

11.            Define voltage transformation ratio of transformer.

The ratio of secondary induced emf to primary induced emf is called as voltage regulation ratio devoted by K.

12. Draw the no load phasor diagram of a transformer.

13. Define all day efficiency.

All day efficiency is the ratio energy (in kwh) delivered in a 24 hours period to the energy (in kwh) input for the same length of time.

All day efficiency = Output in kwh/Input in kwh (for 24 hrs)

14.            Give the factors that determine the thickness of the lamination or stampings.

i.                   Iron loss

ii.                 Frequency

15.            What are the properties of ideal transformer?

It has no loss

ii)  Its winding has zero resistance.

iii)              Leakage flux is zero i.e 100% flux produced by primary links with the secondary

iv) Permeability of core is so high that negligible current is required to establish the flues is it.

16.            What are the losses occurring in a transformer?

i) Core losses

ii)  Copper losses

17.            What is meant by core or iron losses?

Core or iron losses are caused as the core gets subjected to an alternating flux.

18. What is meant by copper loss?

The copper losses are due to the power wasted in the form of I2R due to the resistances of the primary and secondary windings.

19. What is meant by eddy current loss?

The induced emf in the core tries to set up eddy currents in the core and hence responsible for the eddy current losses.

20. What is meant by hysteresis losses?

Due to alternating flux set up in the magnetic core of the transformer, it undergoes a cycle of magnetization and demagnetization.

Due to hysteresis effect there is loss of energy in this process which is called hysteresis loss.

21. What do you mean by step down transformer?

If the secondary voltage is greater than primary value, the transformer is called step down transformer.

22. What are the functions of no-load current in a transformer?

No-load current produces flux and supplies iron loss and copper loss on no-load.

23.            What are the typical uses of auto transformer?

Ø To give small boost to a distribution cable to correct for the voltage drop

Ø As induction motor starters

Ø As furnace transformers

Ø As interconnecting transformers

Ø In control equipment for single phase and 3 phase elective locomotives