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Chapter: Electrical machines - Transformer

Important Short Questions and Answers: Electrical machines - Transformer

Electrical machines - Transformer -

TRANSFORMERS

 

1. Mention the difference between core and shell type transformers.

 

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

 

2.              What is the purpose of laminating the core in a transformers ?

 To reduce eddy current loss.

 

3.              Give the emf equation of a transformer and define each term

 Emf induced in primary coil E1 = 4.44 fΦmN1 volt

 

Emf induced in secondary coil E2 = 4.44fΦmN2 volt

 

Where f is the frequency of AC input

Φm is the maximum value of flux in the core

N1, N2 are the number of primary and secondary turns.

 

4. Does the transformer draw any current when secondary is open ? Why ?

 

Yes,it (primary) will draw the current from the main supply in order to magnetise the core and to supply iron and copper losses on no load . There will not be any current in the secondary since secondary is open.

 

5. Define voltage regulation of 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 pf 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 load or full load voltage is termed as regulation .

 

 %regulation down = (0V2-V2) x 100/0V2

 %regulation up = (0V2-V2) x 100/V2

 

5.              Full load copper loss in a transformer is 1600 watts. What will be the loss at half load ?

 

 

If x is the ratio of actual load to full load then copper loss = x2(full load copper loss).Here Wc = (0.5)2 x 1600 = 400 watts

 

7. Define all day efficiency of a transformer .

 

It is the computed on the basis of energy consumed during a certain period , usually a day of 24 hrs.

 

ηall day = output in kWh /input in kWh for 24 hrs.

 

8. 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 are in kVA and not in kW.

 

9. What are the typical uses of auto transformer ?

 

(i)To give small boost to a distribution cable to correct for the voltage drop. (ii)As induction motor starters.

 

(iii)As furnace transformers

 

(iv)As interconnecting transformers

 

(v)In control equipment for single phase and 3 phase elective locomotives.

 

10. What are the applications of step-up and step-down transformers?

 

Step-up transformers are used in generating stations. Normally the generated voltage will be either 11 kV or 22 kV. This voltage is stepped up to 110 kV or 220 kV or 400 kV and transmitted through transmission lines. (In short it may be called as sending end). Step-down transformers are used in receiving stations. The voltage are again stepped down to 11 kV or 22 kV and transmitted through feeders. (In short it may be called as receiving end). Further these 11 kV or 22kV are stepped down to 3 phase 400 V by means of a distribution transformer and made available at consumer premises. The transformers used at generating stations and receiving stations are called power transformers.

 

11. How transformers are classified according to their construction? Or

 

Mention the difference between “CORE” and “SHELL” type transformers.

 

Or

 

What are the two types of cores used ? Compare them. Transformers are classified according to their construction as,

 

(i)Core type (ii)Shell type (iii)Spirakore type.

 

Spirakore type is a latest transformer and is used in big transformers. In “core” type, the windings(primary and secondary)surround the core and in “shell” type, the core surround the windings.

 

12. Explain on the material used for core construction.

 

The core is constructed of transformer sheet steel laminations assembled to provide a continuous magnetic path with a minimum of air gap included. The steel used is of high silicon content sometimes heat-treated to produce a high permeability and a low hysteresis loss at the usual operating flux densities. The eddy current loss is minimized by laminating the core, the laminations being insulated from each other by light coat of core-plate vanish or by an oxide layer on the surface .the thickness of laminations varies from 0.35 mm for a frequency of 59 Hz and 0.5 mm for a frequency of 25 Hz.

 

13. When will a Bucholz relay operate in a transformer?


Bucholz rely is a protective device in a transformer. If the temperature of the coil exceeds its limit, Bucholz relay operates and gives an alarm.

 

14.         How does change in frequency affect the operation of a given transformer?

 

With a change in frequency, iron loss, copper loss, regulation, efficiency and heating varies and thereby the Operation of the transformer is affected

 

15.         What is the angle by which no-load current will lag the ideal applied voltage?

 

In an ideal transformer, there are no copper loss and no core loss,(i.e. loss free core).The no load current is only magnetizing current. Therefore the no-load current lags behind by an angle of 90°. However the windings possess resistance and leakage reactance and therefore the no-load current lags the applied voltage slightly less than 90°.

 

16.         List the advantages of stepped core arrangement in a transformer.

 

(i)                       To reduce the space effectively.

 

(ii)                      To obtain reduced length of mean turn of the windings. (iii)To reduce I2R loss.

 

17.         Why are breathers used in transformers?

 

Breathers are used to entrap the atmospheric moisture and thereby not allowing it to pass on to the transformer oil. Also to permit the oil inside the tank to expand and contract as its temperature increases and decreases. Also to avoid sledging of oil i.e. decomposition of oil. Addition of 8 parts of water in 1000000 reduces the insulations quantity of oil. Normally silica gel is filled in the breather having pink colour. This colour will be changed to white due to continuous use, which is an indication of bad silica gel, it is normally heated and reused.

 

 

18. What is the function of transformer oil in a transformer?

 

Nowadays instead of natural mineral oil, synthetic oils known as ASKRELS (trade name) are used. They are noninflammable; under an electric arc do not decompose to produce inflammable gases. PYROCOLOR oil possesses high dielectric strength. Hence it can be said that transformer oil provides, (i) good insulation and (ii) cooling.

 

19.         A 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 that V1 / V2 = k = N2 / N1

 

Substituting in above equation 400/1100 = 100/N1 N1 = 100/400 x 1100

 

= 275 turns.

 

20.         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.

 

21. How will you transfer the quantities from one circuit to another circuit in a transformer?

 

1.Secondary to primary

2.Primary to secondary

Symbol Value Symbol Value

 

V2 V2/k VL kV1

I2 kI2 IL I1 /k

 

R2  R2/k2 RL  k2R1

X2  X2/k2 XL’ k2X1

ZL  ZL/k2

 

 

22.         Can the voltage regulation of a transformer go to negative? If so under what condition?

Yes. If the load has leading power factor.

 

 

23.         Distinguish between power transformer and distribution transformer.

 

Power transformers have very high power ratings in the order of MVA. They are used in generating and receiving stations. Sophisticated controls are required. Voltage ranges will be very high. Distribution transformers are used in consumer side. Voltage levels will be medium. Power ranging will be small in order of kVA. Complicated controls are not needed.

 

 

24. What is the purpose of providing ‘taps’ in transformer and where these are provided?

In order to attain the required voltage, ‘taps’ are provided. Normally it will be

provided at low voltage sides

 

25. Give the method of reducing iron loss in a Transformer

 

The iron losses are minimized by using high-grade core material like silicon steel having very low hysteresis loop and by manufacturing the core in the form of laminations.

 

26. State the condition for maximum efficiency

Copper losses = Iron lossess

 

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