B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2010
Third Semester
Electronics and Communication Engineering
EC2201 — ELECTRICAL ENGINEERING
(Regulation 2008)

Time: Three hours
Answer ALL Questions

PART A — (10 × 2 = 20 Marks)

  1. What are the conditions to be fulfilled for the self-excitation of a dc shuntgenerator?
  2. What are the functionS of interpoles and how are the interpoles windingsconnected?
  3. The emf per turn of a single phase, 6.6 kV/440 V, 50 Hz Transformer isapproximately 12V Calculatethe number of turns in the HV and LV windings andthe net cross-sectional area of the core for a maximum flux density of 1.5 T.
  4. Define voltage regulation of a transformer.
  5. Why cannot an induction motor run at synchronous speed?
  6. Why are single phase induction motors not self-starting?
  7. What are the causes of faulty starting of a synchronous motor?
  8. What are the applications of stepper motors?
  9. Why are insulators used with overhead lines?
  10. Define skin effect.

PART B — (5 × 16 = 80 Marks)

(i)Describe with a neat sketch, the construction of a d.c. machine.
(ii)A separately excited dc generator running at 1000 r.p.m. supplied
110 A at 220 V to a resistive load. If the load resistance remains
constant, what will the load current if the speed is reduced to
800 r.p.m? Armature resistance is 0.02 Ω . Field current is
unaltered. Assume a voltage drop of 1 V per brush. Ignore the effect
of armature reaction.
Or
(i)Derive from the first principle, an expression for the torque
developed in d.c. motor.
(ii)In a brake test on a dc shunt motor, the load on one side of the
brake was 35 kg and on the other side 5 kg. The motor was running
at 1500 r.p.m. its input being 34 A at 400 V. The diameter of the
pulley is 50 c.m. Determine the torque and efficiency of the motor.
(i)
From first principles, derive the emf equation of a transformer. Also
show that the voltage induced per turn is the same, whether it is
primary or secondary.
A single phase transformer with a ratio of 6.6. kV/415 V takes a
no-load current of 0.75 A at 0.22 p.f. If the secondary supplies a
current of 120 A at 0.8 p.f. calculate the total current taken by the
primary.
Or
(ii)
(i)
Develop an equivalent circuit for a single phase two winding
transformer.
Calculate the full-load efficiency at 0.8 p.f. and the voltage at the
secondary terminals when supplying full load secondary current at
unity power factor, for a 4 kVA, 200/400 V, 50 Hz, single phase
transformer of which the following are the test results :
(ii)
OC test (on primary) :
SC test (on secondary) : V = 17.5 V ; I = 9 A; W = 50 W
(i)Explain with neat sketches, the principle of operation of a three-
phase induction motor.
A 6 pole, 3 phase, 50 Hz induction motor develops a maximum
torque of 30 Nm at 960 r.p.m. Determine the torque exerted by the
motor at 5% slip. The rotor resistance per phase is 0.6 Ω .
Or
(ii) V = 200 V ; I = 0.8 A ; W = 50 WDiscuss briefly the various methods of speed control of three-phase
induction motors.
2
(i)Derive the emf equation of an alternator. Discuss the effect of
winding factor on the induced emf.
(ii)Explain the speed-torque characteristics of a reluctance motor.
Or
(i)A 500 kVA, 3.3. kV, 3-phase, star-connected alternator is found to
give a short circuit current of 290 A at normal field current. Its
effective winding resistance per phase is 0.7 Ω . Estimate the full
load of voltage regulation by EMF method for 0.8 pf lagging.
(ii)Explain the speed-torque characteristics of a hysteresis motor.
(i)Draw the single diagram of a typical a.c. power supply scheme.
(ii)With a neat sketch, explain the intersheath grading of cables.
Or
(i)Explain briefly the advantages of EHVAC transmission over
EHVDC transmission.
(ii)Draw the layout of a substation with the main equipments.

Anna University - Question Bank

UNIT – I

PART – A

  1. Define electric motor.
  2. Define electric generator.
  3. Define excitation.
  4. What are the methods of excitation?
  5. What do you mean by self excited machine?
  6. What do you mean by separately excited machine?
  7. State the function of commutator.
  8. What do you mean by commutation?
  9. What is the purpose of brushes?
  10. By which material yoke can be manufactured?
  11. Why the brushes are made upon carbon?
  12. What are the types of self excited generator?
  13. Define shunt generator.
  14. Define series generator.
  15. Define compound generator.
  16. Draw the open circuit characteristics of dc separately excited generator.
  17. What do you mean by magnetization curve?
  18. Draw the load characteristics of separately excited dc generator.
  19. Draw the characteristics for DC shunt generator.
  20. Draw the characteristics of DC series generator.
  21. Draw the characteristics of DC compound generator.
  22. What is the principle of operation of dc generator?
  23. Write down faradey's law of electromagnetic induction.
  24. What is the principle of operation of DC motor?
  25. State lenz's law.
  26. How Lenz's law is helpful for the operation of DC motor?
  27. Write the significance of back e.m.f.
  28. Define back e.m.f.
  29. Write the e.m.f. equation of DC generator.
  30. Write down the voltage equation of DC motor.
  31. Write the torque equation of DC motor.
  32. Draw the characteristics of DC shunt motor.
  33. Draw the characteristics of DC series motor.
  34. Draw the characteristics of DC compound motor.
  35. Differentiate differentially compound and cumulatively compound motor.
  36. What are the applications of DC shunt motor?
  37. What are the applications of DC series motor?
  38. What are the applications of DC compound motor?
  39. Why shunt motor is called as constant speed machine?
  40. Why starter is needed in DC motor?
  41. What is the advantage of 4 point starter over 3 point starter?
  42. State the function of no volt coil.
  43. State the function of over load release.
  44. What is the advantage of conducting Swin-burne's test?
  45. Draw the diagram of ward-leanord system of speed control.
  46. By which method, we can control the speed of the motor below the base speed?
  47. By which method, the speed control of the DC motor be above the base speed?
  48. What are the various methods of speed control of DC motor?
  49. Differentiate armature voltage control method and field flux control method.
  50. What do you mean by brake test?

PART – B

  1. Explain in detail the constructional details of DC machine.
  2. Derive the e.m.f. equation (8)
  3. Draw and explain the characteristics of DC generator.
  4. Derive the torque equation/
  5. Write down the significance of back e.m.f.
  6. Write down the principle of operation of DC motor.
  7. Draw and explain the characteristics of DC motor.
  8. Draw and explain 3 point starter.
  9. What is the draw back in 3 point starter and how it is overcome in 4 point starter?
  10. Explains the procedure of swin burne's test.
  11. Draw and explain the speed control methods of DC shunt motor.
  12. Explain in detail about the ward-leonard system of speed control of DC motor.
  13. Problem.

UNIT – II

PART – A

  1. State the principle of operation of transformer.
  2. What are the types of transformer according to its construction?
  3. Differentiate core type transformer and shell type transformer.
  4. Write down the e.m.f equation of transformer.
  5. Write down the transformation ratio.
  6. Define magneting current component.
  7. Define working current component.
  8. Draw the vector diagram of transformer an no load.
  9. How the resistance of the secondary be referred in the primary?
  10. Draw the approximate equivalent circuit of the transformer.
  11. Draw the vector diagram for the transformer under resistive load.
  12. Draw the vector diagram for the transformer under inductive load.
  13. Draw the vector diagram of the transformer under comparative load.
  14. When the transformer is said to be ideal?
  15. Define voltage regulation of transformer.
  16. Define voltage regulation up.
  17. Define voltage regulation down.
  18. By conducting no load test, which losses can be found out?
  19. Which losses can be found out by conducting short circuit test?
  20. What are the tests to be conducted on the transformer?

PART - B

  1. Derive the e.m.f. equation of transformer.
  2. Explain in detail about the transformer on no load.
  3. Draw and explain ideal transformer. Draw its vector diagram.
  4. Obtain the equivalent circuit of the transformer by conducting suitable tests.
  5. Draw and explain in detail about the transformer an load.
  6. Derive the regulation formula for the transformer.
  7. Explain in detail about the testing procedures for the transformer.
  8. Problem.

UNIT – III

PART – A

  1. Write the working principle of induction motor.
  2. How RMF is produced with the help of 3 phase supply?
  3. Define slip.
  4. Write the working principle of 3f induction motor.
  5. Why 3f induction motor is called as rotating transformer?
  6. Can the rotor speed of induction motor be synchronous speed?
  7. What are the types of induction motor?
  8. Write down the torque equation of 3f induction motor.
  9. Draw the slip-torque characteristics.
  10. Draw the speed-torque characteristics.
  11. State the relation between P2, Pc and Pm.
  12. Draw the Power flow diagram of 3 phase induction motor.
  13. Draw the equivalent circuit of 3 f induction motor.
  14. Why starter is necessary in AC induction motor?
  15. What are the types of starters available to start 3 f induction motor?
  16. Why single phase induction motor is not self starting?
  17. How single phase induction motor is made to start?
  18. How the rotor is classified in 3f induction motor?
  19. How Rmf is produced in the stator of the induction motor
  20. What is Rmf?

PART - B

1. Explain the constructional details of 3 f induction motor.

2. How Rmf is produced with the help of 3 f AC supply?

3. Explain in detail the principle of operations of 3 phase induction motor.

4. Explain the procedure to obtain the equivalent circuit of 3 phase induction motor.

5. Derive the torque equation of 3 phase induction motor.

6. Draw and explain the characteristics of 3 phase induction motor.

7. Explain in detail about (i) Star-delta starter (ii) Auto transformer starter (iii) DOL starter.

8. Write short notes on single phase transformer.

9. Why single phase induction motor is not self starting? Explain the methods available to start the motor.

10. Write short notes on speed control of 3 f induction motor.

UNIT IV

PART – A

  1. What are the types of synchronous machines?
  2. What is the basic principle of operation 3f alternator?
  3. Why stationary armature is there in 3f alternator?
  4. Write down the e.m.f. equation of alternator.
  5. What are the types of 3 f alternator?
  6. Define voltage regulation of alternator?
  7. What are the methods to find out voltage regulation?
  8. Why synchronous impedance methods is called as pessimistic method?
  9. What are the methods to find out voltage regulation
  10. Why synchronous impedance method is called as pessimistic method?
  11. Define synchronous impedance.
  12. Why ampere-turns method is called as optimistic method?
  13. Draw the phasor diagram for voltage regulation in synchronous impedance method.
  14. How emf is induced in brushless alternator?
  15. What is the function of rotating diode assembly in brush can alternator?
  16. What do you mean by brushless excitation?
  17. How reluctance motor is running?
  18. What is the operating principle of hysteresis motor?
  19. What are the types of stepper motor?
  20. How do you find step angle in stepper motor?
  21. Give the application of stepper motor.

PART - B

  1. Derive the emf equation of alternator.
  2. Write down the procedure of finding voltage regulation of alternator using synchronous impedance method.
  3. Write down the procedure for finding voltage regulation using ampere-turns method.
  4. Explain in detail the construction of synchronous machine.
  5. Write short notes on (i) Brushless alternator (ii) Reluctance motor (iii) Hysteresis motor (iv) Stepper motor.

UNIT – V

PART – A

  1. What are the various sources of electric power generating system?
  2. What is the purpose of step up transformer in electrical power system?
  3. What do you mean by carona loss?
  4. What is meant by Ferranti effect?
  5. What are the advantages of AC transmission?
  6. What are the advantages of DC transmission?
  7. What are the types of distribution system?
  8. What is EHUAC?
  9. Define EHUDC.
  10. What is the purpose of circuit breakers.
  11. Define insulators.
  12. Why the transmission is done through under ground cable?
  13. What are the advantages of under ground cables?
  14. What are the types of under ground cables?
  15. What are types of insulators?
  16. What is pin insulator? Where it is used?
  17. What is stay insulator? Where it is used?
  18. What are the purposes of insulator?
  19. Define service mains.
  20. Define grid.
  21. Differentiate feeders and distributors.

PART - B

  1. Explain in detail, the single line diagram of electrical power system.
  2. Explain in detail the various types of transmission and distribution systems.
  3. Distinguish EHUAC and EHUDC systems.
  4. Explain in detail the substation lay out.
  5. Write short notes as (i) Insulators (ii) cables.

EC2201-ELECTRICAL ENGINEERING

QUESTION BANK

UNIT I - D.C. MACHINES

PART A

1.Define electric motor.

The electric motor is a machine that converts electrical energy into mechanical energy or motion.

2.Define electric generator.

The electric generator is machine that converts mechanical energy into electrical energy.

3.What is a prime mover?

The basic source of mechanical power which drives the armature of the generator is called prime mover.

4.What are the different types of Motors?

1. Self Excited Motors

2. Seperately Excited Motors

a. Series Motor

b. Shunt Motor

c. Compound Motor.

i.Cumulative Compound

ii. Differential Compound

5.State the principle of working of D.C. motor.

An electric motor is a machine which converts electrical energy into mechanical energy. Its action is based on principle that when a current carrying conductor is placed in a magnetic field it experiences a mechanical force whose direction is given by Fleming’s left hand rule.

6.State the various types of D.C. machines based on excitation.

(1)Separately excited machine.

(2)Self exited machine.

(a)Shunt wound

(b)Series wound.

(c)Compound wound.

7.What is the purpose of Yoke in a d.c. machine?

(1)It provides mechanical support for the poles and acts as a protecting cover for whole machine.

(2)It carries the magnetic flux produced by the poles.

8.What is the need for laminated poles and armature?

The purpose of using lamination is to reduce the loss due to eddy current.

9.Explain the function of commutator in D.C. generator and D.C. Motor.

The function of the commutator is to facilitate the collection of current from the armatureconductors. It converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit.

10.What are the effects of armature reaction?

The effect of magnetic field set up by the armature current on the distribution of flux under main poles of a generator is known as armature reaction. It has two effects

(1) it demagnetizes the main flux

(2) it cross magnetizes or distorts it.

11.What are the methods of neutralizing the effects of armature reaction?

(1)By connecting compensating winding in series with the armature.

(2)By having a small poles fixed to the yoke and spaced in between the main poles(Inter poles).

12.What is commutation and what is the effect of poor commutation?

The process by which current in the short circuited coil is reversed while it crosses the m.n.a. (magnetic neutral axis) is called as Commutation. The effect of poor commutation is sparking at the brushes.

13.What are the methods to improve commutation?

(1)Resistance commutation.

(2)E.m.f. commutation.

14.What are Interpoles and state their use.

Interpoles small poles fixed to the yoke and spaced in between the main poles. Interpoles are used to neutralize the cross magnetizing effect of armature reaction.

15.How can the voltage in a D.C. generator be increased?

The voltage in a D.C generator can be increased by increasing the main field flux and thespeed of armature.

16.What is critical resistance of a D.C. shunt generator?

The value of resistance of shunt field winding beyond which the shunt generator fails to build up its voltage is known as “critical resistance”.

17.What do you mean by residual flux in D.C. Generator?

The magnetic flux retained in the poles of the machine even without field supply is called the residual flux.

18.What do you mean by external characteristic of D.C. generator?

It is the relationship between the load current and the terminal voltage.

19.What is the effect of armature reaction in D.C. generator?

Due to the demagnetizing effect of armature reaction field pole flux is weakened and so the induced EMF in the armature is decreased.

20.What is the use of compensating winding in D.C. generator?

The function of compensating winding is to neutralize the cross magnetizing effect of armature reaction.

21.Why D.C. series generator is called as rising characteristic machine?

In D.C. series generator, with increase in load, it’ terminal voltage is also increased. So it is called as rising characteristic machine.

22.What do you mean by saturation of field pole?

If we are going on increasing the field current, after certain limit, any more rise in current will not have a rise in flux. This condition is called saturation of field poles.

23.Name the parts of a D.C. machine in which iron losses occur.

Iron losses occur in yoke, field pole and armature core.

24.What are the losses of a shunt machine assumed as constant?

Core losses, mechanical losses and shunt field copper loss assumed as constant in shunt machine.

25.What is the condition for maximum efficiency of a D.C. machine?

Efficiency of a D.C. machine will be maximum when variable losses are equal to constant losses.

26.What are the applications of D.C Series generator?

These are used for series arc lighting, series incandescent lighting and as a series boosterfor increasing the voltage in D.C. transmission lines.

27.What is the use of shunt generator?

Shunt wound generator with field regulations are used for light and power supply purposes. These are also used for charging of batteries on account of its constant terminal voltage.

28.What causes sparking at the brushes?

It is either due to self-induction of the coil undergoing commutation or due to improper pressing of brush over the commutator surface.

29.Explain how you would reverse the direction of rotation of a D.C shunt motor.

The direction of rotation of a D.C. shunt motor can be reversed either by changing the direction of field current or armature current.

30.What are the factors that govern controlling of motor speed?

  • flux per pole
  • Voltage.

31.Suggest, with reason, a method of speed control of D.C motor above its rated speed.

The speed of a D.C motor can be obtained more than the rated value by field control method, because the flux per pole can be decreased to any value by decreasing the field current. (Field current can be decreased by inserting a field regulator)

32.How can the speed of a D.C shunt motor be controlled?

By varying the field current as well as armature voltage speed can controlled.

33.What will be the effect of adding resistance in the field circuit of a D.C shunt motor?

When the motor is running on no-load, the speed will increase, if additional resistance is connected in the field circuit. But the speed will decrease if it runs with load as torque produced decreases.