Electrical Machines–I All Units Question Bank–2012 Edition (Version 1)

DEPARTMENT OF ELECTRICAL AND ELECTRONICS

ENGINEERING

QUESTION BANK

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SUB.NAME: ELECTRICAL MACHINES -I

BRANCH : ELECTRICAL AND ELECTRONICS ENGINEERING

YEAR : II

 SEMESTER : IV

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UNIT I

INTRODUCTION

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PART – A

1. Mention the types of electrical machines.

2. State Ohm’s law for magnetic circuit.

3. Define leakage flux

4. Define magnetic reluctance

5. State stacking factor.

6. Mention some magnetic materials

7. What is magnetostriction?

8. Define statically induced emf.

9. Define dynamically induced emf.

10. State Fleming’s right hand rule.

11. State Fleming’s Left hand rule.

12. What are the losses called as core loss?

13. Define coercivity.

14. What are factors on which hysteresis loss?

15. What is core loss? What is its significance in electric machines?

16. What is eddy current loss?

17. How hysteresis and eddy current losses are minimized?

PART – B

1. (a) Explain about the magnetization curve of Ferro –magnetic material. (8) (b) Derive the relation between mutual inductance and self inductances of two magnetically coupled coils. (8)

2. (a) Explain AC operation of magnetic circuits. (8) (b) Explain in detail about hysteresis and eddy current losses. (8)

3. (a)Write in detail about magnetically induced emf & force and derive the relevant expression. (8) (b) An electromagnetic relay has an exciting coil of 800 turns. The coil has a cross section of 5 cm x 5cm. Find

1. coil inductance if the air gap length is 0.5 cm.

2. field energy stored for a coil current of 1.25 A.

3. Permeance at air gap. (8)

4. Explain in detail about three basic rotating electric machine types. (16)

5. (a) Compare magnetic and electric circuit. (8) (b) An iron rod 1.8 cm diameter is bent to form a ring of mean diameter 25cm and wound with 250 turn of wire. A gap of 1mm exists in between the end faces. Calculate the current required to produce a flux of 0.6mWb. Take relative permeability of iron as 1200. (8)

6. (a) Explain the two different types of magnetic circuits with neat diagram. (8) (b) When two coils are connected in series, their effective inductance is found to be 10H .When the connections of one coil are reversed , the effective inductance is 6H.If the coefficient of coupling is 0.6, calculate the self inductance of each coil and the mutual inductance. (8)

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UNIT II

TRANSFORMERS

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PART-A

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

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

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

4. Define voltage regulation of a transformer?

5. Define all day efficiency of a transformer?

6. Why transformers are rated in KVA?

7. What are the typical uses of auto transformer?

8. What are the applications of step-up & step-down transformer?

9. How transformers are classified according to their construction?

10. Explain on the material used for core construction?

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

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

13. List the arrangement of stepped core arrangement in a transformer?

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

15. Can the voltage regulation go negative if so under what condition?

16. Distinguish power transformers & distribution transformers?

17. What is the purpose of providing Taps in transformer and where these are provided?

18. What are the typical uses of auto transformer?

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

20. Why are breathers used in transformers?

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

PART-B

1. (a) Explain the principle and operation of auto transformer. (8) (b)Draw and explain the no load phasor diagram of a single phase transformer. (8)

2. (a) Derive the emf equation of single phase transformer. (8) (b) A 120kVA, 6000/400V, Y/Y, 3-phase, 50Hz transformer has a iron loss of 1800W.

The maximum efficiency occurs at ¾ full loads. Find the efficiency of the transformer at

(i) Full load and 0.8 pf

(ii) The maximum efficiency at unity pf. (8)

3. A100 kVA, 6.6kV/415V, single phase transformer has an effective impedance of (3+8j) _ referred to HV side. Estimate the full load voltage regulation at 0.8 pf lagging and 0.8 leading pf. (16)

4. (a) Explain the working of auto transformer and prove that when transformation ratio approaches unity, the amount copper used approaches smaller value. (8)

(b)The emf per turn of a single phase, 6.6kV/440V, 50 Hz transformer is approximately 12V. Calculate the number of turns in the HV and LV windings and the net cross sectional area of the core for a maximum flux density of 1.5T. (8)

5. (a) Obtain the equivalent circuit of a 200/400V,50Hz,single phase transformer from the following test data: OC test: 200V,0.7A,70W on LV side SC test: 15V, 10A, 85W on HV side. (10) (b)With the help of circuit diagrams, explain any two types of three phase transformer connections. (6)

7. Draw the circuit diagrams for conducting OC and SC tests on a single phase transformer. Also explain how the efficiency and voltage regulation can be estimated by these tests. (16)

8. What is the sumpner’s test? Draw the circuit diagram to conduct this test and explain its

principle. (16)

9. (a) Derive the condition for maximum efficiency in a transformer. (8)

(b) A11000/230 V, 150 KVA, 1-phase, 50 Hz transformer has core loss of 1.4kW and F.L cu loss of 1.6 Kw .Determine (i) The kVA load for maximum efficiency and the value of maximum efficiency at unity p.f. (ii) The efficiency at half F.L 0.8 pf leading. (8)

10. Explain in detail about parallel operation of single phase transformers. (16)

11. Data of a 500KVA, 3300/400 V, 50 Hz, single phase transformer is given below.

S.C test: 1250 W, 100 V –secondary short circuited with full load current in it O.C test: 1000 W –with normal primary voltage.

Calculate the full load regulation and efficiency at a power factor of 0.8(lag). (16)

12. (a) Derive the equivalent circuit of a single phase two winding transformer. (8) (b) The maximum efficiency of a single phase 250kVA, 2000/250 V transformer occurs at 80% of

full load and is equal to 97.5% at 0.8 pf .determine the efficiency and regulation on full load at 0.8pf lagging if the impedance of the transformer is 9 percent. (8)

13. Explain in detail about tap changing of transformers. (16)

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UNIT III

ELECTROMECHANICAL ENERGY CONVERSION

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PART-A

1. What is an electromechanical system?

.2. Describe multiply excited magnetic field system.

3. Define co energy.

4. How energy is stored?

5. Define field energy.

6. Write the expression for the principle of energy conversion.

7. What is the significance of co energy?

8. How the energy stored in magnetic field?

9. Give any four examples if single excited magnetic system.

10. Write the applications of singly excited and doubly excited magnetic system .

11. State the necessary conditions for the production of steady torque the interaction of stator and rotor fields in an electric machine.

PART-B

1. Derive the expression for field energy produced in a doubly excited magnetic field system? (16)

2. The magnetic flux density on the surface of an iron face is 1.6 T which is a typical saturation level

value for ferromagnetic material. Find the force density on the iron face. (16)

3. What are the special applications where the electric field is used as a coupling medium for electromechanical energy conversion? Also explain why electric field coupling is preferred in such applications? (16)

4. Find an expression for the force per unit area between the plates of a parallel plate condenser in terms of the electric field intensity. Use both the energy and co energy methods. Find the value of the force per unit area when E = 3 x 106 V/m, the breakdown strength of air. (16)

5. Explain with neat diagram and sufficient expressions, the multiply excited magnetic field systems. (16)

6. Explain i - characteristics of a magnetic system. Also derive the expression for co energy density. Assume i - relationship of the magnetic circuit is linear. (16)

7. Explain the concept of singly – excited machines and derive the expression for the electromagnetic torque. (16)

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UNIT IV

BASIC CONCEPTS IN ROTATING MACHINES

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PART-A

1. Define the term pole pitch

2. Define pitch factor

3. Define the term breadth factor

4. Write down the advantages of short pitched coil.

5. What is distributed winding?

6. Explain the following terms with respect to rotating electrical machines.

7. Write the expressions for the synchronous speed.

8. Write the mmf equation of dc machine.

9. What is meant by electromagnetic torque?

10. State the torque equation for round rotor machine.

11. Define rotating magnetic field.

PART-B

1. Derive the expression for the r.m.s value of emf induced in a.c. machines. (16)

2. Prove that mmf wave of a single phase ac winding is pulsating or standing. (16)

3. Prove that the resultant mmf wave of three phase ac winding is rotating in space with

speed but its magnitude is constant. (16)

4. Derive the torque equation for round rotor machine. (16)

5. Explain the various concepts of magnetic fields in rotating machines. (16)

6. Explain with neat diagram the concept of mmf space wave of a single coil. (16)

7. Write in detail about mmf space wave of three phase distributed winding. (16)

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UNIT V

DC MACHINES

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PART – A

1. What is prime mover?

2. Give the materials used in machine manufacturing

3. How will you change the direction of rotation of a d.c motor?

4. What is back emf in d.c motors?

5. Under what condition the mechanical power developed in a dc motor will be maximum?.

6. What is the function of a no-voltage release coil provided in a dc motor starter?

7. Name the two types of automatic starters used for dc motors.

8. Enumerate the factors on which the speed of a dc motor depends.

9. List the different methods of speed control employed for dc series motor

10. Name the different methods of electrical breaking of dc motors.

11. Under what circumstances does a dc shunt generator fail to build up?

12. To what polarity the interpoles excited in dc motors?

13. What is back emf in d.c motors?

14. Name any four applications of DC series motor.

15. Why DC motors are not operated to develop maximum power in practice?

16. Name the starters used for series motors.

17. Name Different types of starters.

18. Name the Protective devices in a starter.

20. What are the modification in ward Leonard linger system?

21. What type of DC motors are suitable for various torque operations?

22. Define speed regulation.

23. What are the performance curves?

24. To what polarity are the interpoles excited in dc generators?

25. Why are carbon brushes preferred for dc machines?

26. What are the various types of commutation?

27. Name the two methods of improving commutation.

28. What is reactance emf in dc machine?

29. Define the term commutation in dc machines.

30. How and why the compensating winding in dc machine excited?

PART-B

1. (a) Describe with sketches the construction of a DC machine. (8) (b) Derive the EMF equation of DC generator. (8)

2. Draw and explain the no-load and load characteristics of DC shunt, series and compound generators. (16)

3. Explain the effect of armature reaction in a DC shunt generator. How is its demagnetizing and cross-magnetizing ampere turns calculated? (16)

4. Explain the process of commutation in a DC machine. (16)

5. With a aid of a circuit diagram, describe the procedure for paralleling two DC shunt generators and for transferring the load from one machine to the other. (16)

6. A 4-pole, 50 kW, 250 V, wave wound shunt generator has 400 armature conductors. Brushes are given a lead of 4 commutator segments. Calculate the demagnetization ampere-turns per pole if shunt field resistance is 50 ohm. Also calculate extra shunt field turns per pole to neutralize the demagnetization. (16)

7. A 4-pole, lap connected DC machine has 540 armature conductors. If the flux per pole is .03 Wb and runs at 1500 RPM, determine the emf generated. If this machine is driven as a shunt generator with same field flux and speed, calculate the line current if the terminal voltage is 400V.Given the RSH=450ohm and RA=2phm. (16)

8. Two separately excited DC generators are connected in parallel and supply a load of 200A. The machines have armature circuit resistances of 0.05 ohm and 0.1 ohm and induced emfs of 425V and 440V respectively. Determine the terminal voltage, current and power output of each machine. The effect of armature reaction is to be neglected. (16)

9. (a) Explain the principle of operation of a DC motor. (8) (b) A shunt machine, connected to 200V mains has an armature resistance of 0.15 _ and field resistance is 100 _. Find the ratio of its speed as a generator to its speed as a motor, line current in each case being 75 A. (8)

10. (a) Draw and explain the mechanical characteristics of DC series and shunt motor. (8)

(b) A 230V, DC shunt motor, takes an armature current at 3.33A at rated voltage and at a no load speed of 1000RPM. The resistances of the armature circuit and field circuit are 0.3 _ and 160 _ respectively. The line current at full load and rated voltage is 40A. Calculate, at full load, the speed and the developed torque in case the armature reaction weakens the no load flux by 4%. (8)

11. (a) Describe the working of 3 point starter for DC shunt motor with neat diagram. (8)

(b)Explain Ward-Leonard method of speed control in DC motors. (8)

12. (a) Derive an expression for the torque developed in a DC machine. (8)

(b) A 220V, Dc shunt motor with an armature resistance of 0.4 ohm and a field resistance of 110ohm drives a load, the torque of which remains constant. The motor draws from the supply, a line current of 32A when the speed is 450 RPM. If the speed is to be raised to 700RPM, what change must be effected in the value of the shunt field circuit resistance? Assume that the magnetization characteristic of the motor is a straight line. (8)

13. Explain the different methods used for the speed control of D.C. shunt motor. (16)

14. With the help of neat circuit diagram, explain swinburne’s test and derive the relations for efficiency (both for generator and motor) also state the merits and demerits of this method. (16)

15. (a)Explain in detail about circuit model of D.C. machine. (8)

(b) A 440 V D.C shunt motor takes 4A at no load . its armature and field resistances are 0.4 ohms

and 220 ohms respectively .estimate the kW output and efficiency when the motor takes 60A on full load. (8)

16. (a) Derive an expression for the torque developed in the armature of a D.C. motor. (8) (b) Determine developed torque and shaft torque of 220V, 4 pole series motor with 800 conductors wave-connected supplying a load of 8.2 kW by taking 45A from the mains. The flux per pole is 25m/Wb and its armature circuit resistance is 0.6 ohm. (8)

17. With the help of neat circuit diagram, explain Hopkinson’s test and derive the relations for efficiency (both for generator and motor) also state the merits and demerits of this method. (16)

18. (a) Explain in detail about different methods of excitation. (8) (b)Derive the expression for efficiency of D.C. machines. (8)

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