Electrical Engineering

Frequently Asked Electrical Lab Viva Questions

FREQUENTLY ASKED QUESTIONS

 

 

1. Conduct a suitable test on the given dc series motor and draw the Electrical characteristics and determine the torque developed at half rated output.

 

 

ANS: Conduct actual loading test(Break test) and draw the characteristics and from it determine the torque corresponding to half rated output

 

 

 

2. Conduct a suitable test on the given dc shunt generator and determine the critical resistance.

 

Ans: Conduct the OCC test on the dc generator. Draw a tangent to the initial part of the OCC… The slope of that line gives the critical resistance

 

 

 

 

3. Conduct a suitable test on the given dc shunt generator, draw the internal characteristics and find out E.M.F induced when the load current is 10A.

 

 

4.Plot the mechanical characteristics of the given d.c series motor by conducting a suitable test and determine the efficiency at 3/4^th full load.

 

 

ANS: Conduct actual loading test(Break test) and draw the characteristics and from it determine the torque corresponding to half rated output

 

 

 

5 Draw the equivalent circuit of the given single phase transformer by conducting a suitable test.

 

ANS: Perform OCC and SCC and obtain the circuit parameters and draw the circuit

 

 

 

6.Pre determine the efficiency of the given single phase transformer at 3/4^th full load.

 

find out no load losses n full load copper losses frm OC n SC tests

 

eff = output/(output + iron loss + (3/4)^2*Cu loss)

 

 

 

 

7.Conduct a suitable test on the given dc shunt generator and draw the load curve. Also find out the voltage drop across the load at ¾ ^th full load

 

 

8.Determine the regulation of the given d.c compound Generator at ¾^th full load current.

 

 

9.Conduct a suitable test on the given dc machine and predetermine the O.C.C at ¾ ^th rated speed and find voltage induced at 110% of its excitations under open circuit condition.

 

Perform OCC at 1500*0.75 rpm

 

 

 

10. Find the critical speed of the given d.c machine by conducting a suitable test.

 

See quest 2

 

 

11.Predetermine the efficiency of the given d.c machine by conducting a suitable test at half rated load.

 

 

12.Draw the equivalent circuit of the given three phase transformer by conducting a suitable test

 

 

13. Pre determine the regulation of the given single phase transformer at 3/4^th full load and 0.8pf leading.

 

Use formula where cos Φ = 0.8 and current + .75 full load current.

 

 

14. Conduct a suitable test and determine the fraction of load at which maximum efficiency occurs in the given single phase transformer.

 

 

Use formula for load at max efficiency

 

Load corresponding to max efficiency

 

=( full load* √(iron loss/ full load copper loss))

 

 

15. Pre determine the maximum efficiency of the given single phase transformer.

 

Ans: By conducting SC and OC tests, find out the iron loss and full load Cu loss. Then, find the load at which CU loss = iron loss by:

 

(x^2)*Full load cu loss = iron loss..

 

Where ‘x’ is the fraction of full load current.

 

Find out output = x* rated output. Find efficiency = output/(output +iron loss+ cu loss)

 

 

 

16. Conduct a suitable test and measure the reactive power using single wattmeter for the given balanced 3 phase load.

 

 

17. Conduct a suitable test and determine the power factor of the given 3 phase balanced load.

 

 

18. Conduct a suitable test and determine the torque at 2% slip on the given 3 phase induction motor.

 

 

19. Conduct load test on the given 3 phase induction motor and determine the efficiency at half the rated input current.

 

(Use star-delta starter for starting)

 

 

20. Conduct a suitable test and determine the efficiency at half rated load on the given 3 phase induction motor.

 

 

21. Conduct a suitable test and separate the different losses on the given dc machine.

 

 

22. Conduct a suitable test and separate the different losses on the given single phase transformer.

 

23. Find regulation of single phase Transformer at zero pf

 

ANS : sc test

 

 

 

 

 

DC Motor viva qns.

 

 

 

1) How can be the direction of rotation of motor be reversed?

 

 

2) If the direction of terminals of a dc series is reversed, what will happen?

 

 

3) What will happen if ac supply is given to: a) dc shunt motor b) dc series motor. Explain your answer

 

 

4) What is the need for starter for a dc motor?

 

 

5) What will happen if a shunt motor is connected to the supply line directly?

 

 

6) A dc motor is found to stop running after a short period of time.What do think could be the possible causes? How can you remedy each?

 

 

7) Comment on the running of a a)dc series b) shunt motor when supplied with only half the rated voltage

 

 

What will happen to the losses in a dc motor when its supply voltage is: a) doubled b) halved

 

 

9) Which is the most efficient method of increasing the speed of a 3.75kW dc shunt motor?

 

 

10) What is the main advantage of Ward-Leonard system used for speed control?

 

 

11) What are the various speed control methods used to bring down speed of a motor from rated?

 

 

12) Give three main applications of dc shunt and series motors

 

 

13) Where are dc compound motors used?

 

 

14) Which motor is preferred for traction purposes? Why?

 

 

15) What is meant by regenerative braking? What are its advantages? Give one of its

 

main applications

 

 

16) What is the nature of the emf wave produced in the armature conductors of a dc motor?

 

17) Give the type of dc motor used in the following cases: a) lathes b) cranes and hoists c) shears and punches d) machine tools

 

 

18) Why is the shaft torque of a dc motor less than its armature torque?

 

 

19) Sketch the N-T characteristics of dc series and shunt motors

 

 

20) Explain the significance of the back emf in a dc motor

 

 

21)What is meant by the shaft torque of a motor?

 

 

22) In case of dc shunt motors, can flux control be used to reduce the speed below rated speed?

 

 

23) Where are differentially compounded shunt motors used?

 

 

24) What may be the causes of sparking in a motor?

 

 

25) Can a dc motor be made to work as a generator? Discuss the relative ratings in both cases(ie which will be greater etc)

 

 

26) Explain the characteristics of all types of dc motors and explain their applications.

 

 

27)What will happen if field of a dc shunt motor, running at no load, gets opened accidentally?

 

 

28) What will happen if field of a dc shunt motor, running at loaded condition, gets opened accidentally?

 

 

29) Why is the speed of dc shunt motor constant?

 

 

30)Why is field rheostat in dc shunt motor circuit kept minimum during starting?

 

 

31)What are the speed control methods for obtaining speeds greater than rated speed?

 

and for speeds less than rated speed ?

 

 

 

DC Generator

 

 

1. What is meant by armature reaction? What are the components of armature reaction?

 

 

2. Which law does d c generator work on?

 

 

3. What is meant by commutator?

 

 

4. What are brushes made of ?

 

 

5. What are the different types of DC windings?

 

 

6. When are lap and wave windings used?

 

 

7. What are advantages of Lap windings over wave?

 

 

8. What are the different types of dc generators?

 

 

9. What are the applications of DC shunt generators?

 

 

10. What are the applications of DC series generators?

 

 

11. What are the applications of DC compound generators?(differential and commutatively)

 

 

12. What are the various losses in a DC machine?

 

 

13. Give the condition for maximum efficiency.

 

 

14. What are compensating windings?

 

 

15. What are compoles?

 

 

16. What is meant by commutation ?

 

 

17. what is the reason for sparking in dc machines ?

 

 

18. How can this be over come ?( methods to obtain proper commutation)

 

 

19. Why do we require equalizing connections?

 

 

20. Why parallel operation of generators required? What are the conditions for parallel operation?

 

 

21. Which type of generators is used for parallel operation?

 

 

22. Define critical resistance of a DC generator.

 

 

23. what is meant by critical speed?

 

 

24. What are the conditions for build up of voltage in a shunt generator?

 

 

25. Explain the load characteristics(external characteristics) of a shunt generator.

 

 

26. When are series generators used?

 

 

27. Why is the field rheostat of dc shunt generator kept at maximum value during start?

 

 

 

Transformer

 

 

 

1. Why rating of a transformer is in KVA?

 

2. What all factors decides the iron losses in a transformer?( Ans. Frequency, Flux density, volume of iron used)

3. What factors decides the copper losses in a transformer? (load current. Power factor of load)

4. Could a 50 Hz transformer be used on a 60 Hz supply? If so what would happen?

(yes but iron losses becomes 11 % higher)

 

5. What is the condition for maximum efficiency of a transformer? What is the load current at this condition?(copper loss=iron losses)

6. Why do you need to do Sumpners test ?(Copper and iron loss occurring simultaneously so even temperature rise of a transformer under actual conditions can be estimated, while in SCC and OCC either copper or iron loss occurs while under real operating conditions both occurs)

7. What would be the magnitude of no load current of a transformer?(10% full load current)

8. Why does a no load current flow ? (or does current flow when secondary is open)

9. Are copper losses affected by the load power factor?( yes , because load current changes)

10. How do you determine polarity of a transformer ?

11. What is regulation of a transformer ?

12. Which is a better transformer, one with a) regulation=5% or b) regulation =10% ?

( ans. Transformer (a)) )

 

13. Draw a phasor diagram of a transformer(under different conditions, no load, loaded etc.)

14. Difference between Core type and Shell type of transformers, advantages of each and when each are used .

15. What is Buccholz relay ? (To detect moisture and formation of gases inside tank due to insulation break down or due to short circuit between phase windings.)

16. Why are cores of transformers laminated ?

17. What are Askarels ?(transformer oils)

18. What are advantages of stepped core ?

19. What are the different methods of cooling a transformer ?

20. why does a transformer produce a humming sound ?

21. Give the EMF equation of a transformer.

22. Why is the OCC test conducted in the low voltage side and high voltage left open? ( as in text + ratings of the instruments are reduced, if we hav to conduct in HV side we would have required larger rating of voltmeter)

23. Why is SCC performed in the HV side and keep HV side shorted ?

 

24. How can you separate the Hysterisis and Eddy current losses from OCC test ?

25. Why are OCC and SCC conducted to determine equivalent circuit while direct

26. loading is not performed? (Ans. Energy consumption is large and difficulty to arrange very large loads)

27. What is the approximate efficiency of a transformer? (Ans. 98, because no moving parts)

28. Is efficiency affected by power factor of the load ?( see quest 9)

29. What is all day efficiency? For which type of transformer is it specified ?

30. What are auto transformers ? How are they advantageous ?

31. What are the conditions for parallel operation of transformers ?

32. Why is parallel operation necessary?

33. What happens if the conditions are not satisfied ?

34. What are advantages and when are the different configurations of three phase transformers preferred? (delta-delta, star-delta, star-star, delta-star)

35. What is open delta configuration?

36. Why is transmission done in Star configuration ?

37. What are instrument transformers? Why are they used?

38. Can a transformer be used with d.c. supply ?

39. Why is Scott connection used?

 

 

 

 

 

40. What is meant by tertiary winding of transformer? Why are they used?

(Ans. To supply station auxiliaries with low voltages, to connect static

 

capacitors for power factor correction, to reduce impedance offered to zero

 

sequence currents during earth fault, for interconnection between

 

transmission lines, for measurement purposes, used for stabilization.)

 

 

 

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