SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL...

26
Dr. NNCE III/06 Sem QB-DEM 1 SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF ELECTRICAL MACHINES UNIT I INTRODUCTION PART-A (1 MARK) 1. Which of the following gives materials in order of decreasing values of permeability? (A) Cobalt, Nickel, Aluminium, Iron (B) Permalloy, Nickel, Air, Water (C) Silver, Copper, Aluminium, Iron (D) Pure iron, Silicon iron. Aluminium, Bismuth. 2. In electrical machines the material preferred for pole shoes of electro-magnets is (A) Pure iron (B) Aluminium (C) Copper (D) Lead. 3. The percentage of silicon in transformer stampings is usually limited to (A) 0.4% (B) 1.4% (C) 4% (D) 14%. 4. Find the odd one out (A) Absorptivity (B) Emissivity (C) Refractive index (D) Dielectric strength. 5.Diamagnetic: Paramagnetic: (A) Water: Air (B) Iron: Steel (C) Permalloy: Super-alloy (D) Nickel: Cobalt. 6. If the permeability of a material is 0.999991. it can be classified as (A) Diamagnetic material (B) Paramagnetic material (C) Ferro-magnetic material (D) Ferrite. 7.Which of the following material has the highest permeability? (A) Nickel (B) Cobalt (C) Pure iron (D) Permalloy. 8.For a simplex lap winding, the commutator pitch is equal to (A) +1 (B) ± 1 (C) -l (D) ± 2. 9.If the resistivity of aluminum is 0.0283 x 10 -6 Ω-m, the resistivity of copper could be (A) 0.017 x 10 -5 Ω-m (B) 0.0382 x 10 -5 Ω-m (C) 0.0832 x 10 -6 Ω-m (D) 0.0889 x 10 -6 Ω-m 10.Which variety of copper is used for overhead conductors? (A) Hard drawn copper (B) Native copper (C) Annealed copper (D) Purest form of copper. 11.Annealed copper is used in (A) Low voltage cables (B) Flexible wires (C) Machine windings (D) all of the above. 12.A ACSR conductor has central core of steel surrounded by a bunch of aluminium wires. In such conductors (A) Current flows through aluminium conductors only (B) Current flows through steel wire only

Transcript of SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL...

Page 1: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

1

SUBJECT CODE: EE 2355

SUBJECT NAME: DESIGN OF ELECTRICAL MACHINES

UNIT –I INTRODUCTION

PART-A (1 MARK)

1. Which of the following gives materials in order of decreasing values of

permeability?

(A) Cobalt, Nickel, Aluminium, Iron

(B) Permalloy, Nickel, Air, Water

(C) Silver, Copper, Aluminium, Iron

(D) Pure iron, Silicon iron. Aluminium,

Bismuth.

2. In electrical machines the material preferred for pole shoes of electro-magnets is

(A) Pure iron

(B) Aluminium

(C) Copper

(D) Lead.

3. The percentage of silicon in transformer stampings is usually limited to

(A) 0.4%

(B) 1.4%

(C) 4%

(D) 14%.

4. Find the odd one out

(A) Absorptivity

(B) Emissivity

(C) Refractive index

(D) Dielectric strength.

5.Diamagnetic: Paramagnetic:

(A) Water: Air

(B) Iron: Steel

(C) Permalloy: Super-alloy

(D) Nickel: Cobalt.

6. If the permeability of a material is 0.999991. it can be classified as

(A) Diamagnetic material

(B) Paramagnetic material

(C) Ferro-magnetic material

(D) Ferrite.

7.Which of the following material has the highest permeability?

(A) Nickel

(B) Cobalt

(C) Pure iron

(D) Permalloy.

8.For a simplex lap winding, the commutator pitch is equal to

(A) +1

(B) ± 1

(C) -l

(D) ± 2.

9.If the resistivity of aluminum is 0.0283 x 10-6 Ω-m, the resistivity of copper could

be

(A) 0.017 x 10-5 Ω-m

(B) 0.0382 x 10-5 Ω-m

(C) 0.0832 x 10-6 Ω-m

(D) 0.0889 x 10-6 Ω-m

10.Which variety of copper is used for overhead conductors?

(A) Hard drawn copper

(B) Native copper

(C) Annealed copper

(D) Purest form of copper.

11.Annealed copper is used in

(A) Low voltage cables

(B) Flexible wires

(C) Machine windings

(D) all of the above.

12.A ACSR conductor has central core of steel surrounded by a bunch of

aluminium wires. In such conductors

(A) Current flows through aluminium

conductors only

(B) Current flows through steel wire

only

Page 2: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

2

(C) Current flows through both steel as

well as aluminium conductors

(D) Majority of current flows through

aluminium and negligible amount flows

through steel.

13.Which property of copper enables it to be drawn into thin wires and sheets?

(A) Ductility

(B) Elasticity

(C) Toughness

(D) Tenacity.

14.Which is non-magnetic material?

(A) Nickel

(B) Cobalt

(C) Aluminium

(D) Gadolinium.

15.Which of the following is the least desired property in magnetic materials for

electrical engineering applications?

(A) High magnetic permeability

(B) High electrical resistivity

(C) Large hysteresis loop

(D) All of the above.

16.Above Curie point

(A) a ferromagnetic material becomes

paramagnetic

(B) a ferrite becomes an insulator

(C) a insulating material becomes a

ferrite

(D) a diamagnetic material, becomes a

paramagnetic material.

17. For a simplex lap winding, the winding pitch is equal to

(A) + 2

(B) - 2

(C) ± 1

(D) ± 2.

18.If yb is the back pitch and u is the no. of coil sides per slot, then split coils can be

avoided if the following quantity is an integer

(A) (yb + 1)/ μ

(B) (yb + 1 )

(C) (yb - 1)/ μ

(D) (yb - 1)

19.Which of the following is a paramagnetic material?

(A) Palladium

(C) Pure iron

(B) Lead

(D) All of the above.

20.Which of the following is a ferromagnetic material?

(A) Pure iron

(B) Lead

(C) Bismuth

(D) Water.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

B A C D A A D B A A D D A C C A D C A A

PART-B (2 MARKS)

21. How the design problems of an electrical machine can be classified?

The design problems of an electrical machine are classified as follows

Electromagnetic design

Mechanical design

Thermal design

Dielectric design.

Page 3: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

3

22. What is magnetic circuit?

The magnetic circuit is the path of magnetic flux. The mmf of the circuit creates flux

in the path by overcoming the reluctance of the path. The equation which relates flux,

mmf and reluctance is given by,

Flux = MMF Reluctance

23. What are the constituents of magnetic circuit in rotating machines?

The magnetic circuit of rotating machine consists of air-gap, stator and rotor cores,

and stator and rotor teeth.

24. What is loss curve?

The loss curve is a graph showing the relation between iron loss and magnetic field

intensity, H. It is used to estimate the iron loss of the magnetic materials and it is

supplied by the manufacturers of stampings or laminations.

25. What is leakage flux?

The leakage flux is the flux passing through unwanted path. The leakage flux will

not help either for transfer or conversion of energy.

26. What is fringing flux?

The bulging of magnetic path at the air-gap is called fringing. The fluxes in the

bulged portion are called fringing flux.

27. What are the factors which modify the reluctance of air-gap?

The factor which modify the reluctance of air-gap are

Slots.

radial ventilating ducts

Non-uniform air-gaps.

28. Define total gap contraction factor, Kg. (AU-APRIL/MAY2007)

The total gap contraction factor, Kg is defined as the ratio of reluctance of slotted

armature with ducts and the reluctance of smooth armature without ducts.

29. Define field form factor. (NOV/DEC2009) (AU-APRIL/MAY2012)

The field form factor, Kf is defined as the ratio of average gap density over the

Pole pitch to maximum flux density in the air-gap.

30.What is real and apparent flux density? (AU-NOV/DEC2009)

The real flux density is due to the actual flux through a tooth. The apparent flux

density is due to total flux pass through the tooth. Since some of the flux passes

through slot the real flux density is always less than the apparent or total flux

density.

31. List the methods used for estimating the mmf for teeth. (AU-NOV/DEC2008)

The following are the three methods used for estimating the mmf for teeth are

Graphical method three ordinate method (simpson’s rule) Bt1/3 method.

32. Define specific permeance

Specific permeance is defined as the permeance per unit length (or) depth of field.

33.What is unbalanced magnetic pull?

The unbalanced magnetic pull is the radial force acting on the rotor due to non

uniform air-gap around armature periphery.

34. List the various techniques employed to overcome unbalanced magnetic pull.

Page 4: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

4

The various techniques employed to overcome the unbalanced magnetic

pull are

The length of the rotor is small and diameter is made higher.

Ball bearings are employed and rotor is dynamically balanced.

The combinations of rotor and stator slots which produce vibrations are

avoided.

35.What factor decides the number of turns in a winding? (AU-NOV/DEC2009,AU-

APRIL/MAY2007)

The factors deciding the number of turns in the winding are (i) the emf per turn

andflux density. (ii) The emf per turn depends on the type of insulation employed.

36. What is the effect of salient poles on the air-gap mmf? (AU-APR/MAY2007)

In salient pole machines the length of air-gap is not constant over the whole pole

pitch. Hence the effective air-gap length is given by Kglg where Kg is the gap contraction

factor. Also for calculating mmf, the maximum gap density Bg at the centre of the pole is

considered instead of average gap density. MMF for air-gap in salient pole machine =

800,000 BgKglg.

37. What are the problems encountered in estimating the mmf for teeth?

The problems encountered in estimating the mmf for teeth are

The flux density in different section of a tooth is not uniform.

The slot provides another parallel path for the flux, shunting the tooth.

38. What are the factors which modify the reluctance of air-gap?

The reluctance of air-gap is modified by slots, radial ventilating ducts and non

uniform air-gaps.

39.Define leakage coefficient. (AU-APRIL/MAY2012)

The leakage coefficient is defined as the ratio of total flux to useful flux.

Leakage coefficient Cl = Total flux Useful flux

40.Define gap contraction factor for ducts.(AU-NOV/DEC2008,AU-APR/MAY2007)

The gap contraction factor for ducts, Kgd is defined as the ratio of reluctance of

airgapwith ducts to reluctance of air – gap without ducts.

Kgd = Reluctance of air-gap with ducts Reluctance of air-gap without ducts

41. Define gap contraction factor for slots.

The gap contraction factor for slots, Kgs is defined as the ratio of reluctance of

air-gap of slotted armature to reluctance of air-gap of smooth armature.

Kgs= Reluctance of air - gap of slotted armature

Reluctance of air-gap of smooth armature

42. State any two methods of cooling of turbo alternators. (AU-

MAY/JUNE2008,NOV/DEC2009)

The two methods of cooling of turbo alternators are

i. Air cooled ii. Direct cooled.

43. What is optimum design? (AU-APRIL/MAY2009)

The design is said to be optimum when the following are met.

(i) Min loss (or ) Max

efficiency,

(ii) Min cost,

(iii)Min Volume,

(iv) Min Weight.

44. Define stacking factor. (AU-APRIL/MAY07)

Page 5: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

5

Stacking factor is defined as the ratio of actual length of iron in stacks of

assembled core plates to total axial length of stack.

45. What is rating?

Rating of a motor is the power output or the designated operating power limit

based upon certain definite conditions assigned to it by the manufacturer.

46. Mention the different types of duties of a machine.

The following are the types of duty as per IS: 4722 -1968

1. S1 = continuous duty

2. S 2 = short time duty

3. S3 = Intermittent periodic duty

4. S4 = Intermittent periodic duty with starting

5. S5 = Intermittent periodic duty with starting and barking

6. S6 = continuous duty with Intermittent periodic loading

7. S7 = continuous duty with starting and barking

8. S8 = continuous duty with periodic speed changes.

47. What is Carter’s gap co-efficient?

The Carter’s gap co-efficient (Kcs) is the ratio of slot width to gap length. The

formula which gives value of Kcs directly is Kcs = 11+ 5lg/Ws for parallel sided

-1 Y – √

48. Write down the expression for calculation of reluctance of air gap with slotted

armature.

The expression for calculation of reluctance of air gap with slotted armature is

Sg = lg/ μys L = lg / μ0 L (ys - KcsWs ).

49. Define Specific electric loading? (AU-APRIL/MAY 2013)

ac = Total armature ampere conductors/armature periphery at airgap.

50. What are the major considerations in design? (AU-APRIL/MAY 2013)

Cost

Durability

Compliance with performance criteria as laid down in specifications

PART-C (16 MARKS)

51. What are the methods of determining the temperature rise of windings and

other parts of an electrical machine?(AU-APRIL/MAY 2013)

52. A laminated steel tooth of armature of a d.c machine is 30mm long and has a

taper such that the maximum width is 1.4 times the minimum. Estimate the

mmf required for a mean flux density of 1.9wb/m2 in the tooth. B-H

characteristics of steel is given below:

Bwb/m2 1.6 1.8 1.9 2.0 2.1 2.2 2.3

HA/m 3700 10,000 17000 27000 41000 70000 1,09,000

(AU-NOV/DEC2007)

53. Determine the apparent flux density in teeth of a dc machine when the

real flux density is 2.15 web/m2. Slot pitch is 28mm, slot width is 10mm and

the gross core length 0.35 m.The number of ventilating ducts is 4. Each duct

Page 6: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

6

is 10mm wide. The magnetizing force for a flux density of 2.15 wb/m2 is

55000H/m.The iron stacking factor is 0.9 NOV/DEC2007 (AU-

NOV/DEC2009)

54. Derive formula for temperature difference between the centre and

overhang of a conductor placed in a slot. Find this temperature difference

if Electrical resistivity of conductor=0.021*10-6Ωm Current density in the

conductor=4A/mm2Thermal resistivity of the conductor=0.0025Ωm Length

of the conductor=50cm (NOV/DEC2007)

55. Explain the occrence of eddy current loss in conductors of electrical

apparatus. (AU-NOV/DEC2007)

56. State different types of magnetic material used in electrical engineering and

ention salient properities of each of theses materials. (AU-NOV/DEC2007)

57. Derive the expression for total MMF for airgap in a magnetic circuit. (AU-

NOV/DEC2009)

58. Calculate the mmf required for the air gap of a machine having core

length=0.32m including 4 ducts of 10mm each, pole arc=0.19m;slot

pitch=65.4mm;slot opening=5mm; air gap length=5mm; flux per

pole=52mwb;given carter’s coefficient is 0.18 for opening=1,and for

opening=2 (AU-NOV/DEC2009)

59. What is leakage reactance? What are the components of leakage

flux?Explain in details.( AU-NOV/DEC2009)

60. Explain the direct cooling of turbo alternator with neat schematic.(AU-

NOV/DEC2009,APRIL/MAY2010)

61. Estimate the effective gap area per pole of a 10 pole, slip ring IM with the

following data:

Stator bore=0.65m;core length=.25m no of stator slot=90, stator slot

opening=3mm, rotor slots=120; rotor slot opening=3mm; air gap length

=.95mm; cartre’s coefficient for ducts=.68; cartre’s coefficient for

slots=.46, number of ventaliting ducts=3 each on rotor and stator, width

of each ventaliting duct=10mm (APR/MAY2010)

62. Explain the effect of saliency on the mmf required for air gap of salient pole

machine. APR/MAY2010

63. The field coil of dc m/c are excited with 2.5A at 230 V. The weight of copper

in the coils is 60 kg. Estimate the rate at which the temperature will begin

when the coils are excited from cold conditions. Specific heat of copper is 390

J/kg-C° (AU-APRIL/MAY2010)

64. State and explain advantages of hydrogen cooling as applied to turbo

alternator. (AU-MAY/JUNE2009)

65. Draw the cross sectional view of rotor slot and explain the method involved

in direct hydrogen cooling of rotor of turbo alternator. (AU-

MAY/JUNE2009)

66. Compute apperant magnetic flux density in the teeth of a dc machine when

the real flux density is 2.15 w/m2. Slot pitch is 28mm. Slot width is 10mm and

the gross core length is 0.35 m.the number of ventilating ducts is 4,each

10mm wide. The magnetizing force for a flux density of 2.15web/m2 is 55000

Page 7: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

7

A/m.. The iron stacking factor is 0.9. (AU-

MAY/JUNE2009,NOV/DEC2009,NOV/DEC2009,APRIL/MAY2012)

67. Determine the air gap length of a dc m/c from the following particulars;

Gross length of the core:0.12m; no of ducts:1; width of each duct:10mm;

slot pitch:25mm; slot width:10mm; carter,s coefficient for slots and

ducts:0.32; gap density at pole

centre:0.7wb/sqm;fieldMMf/pole:3900AT; MMF required for iron parts

of the magnetic circuit:800AT(AU-NOV/DEC2009,APRIL/MAY2012)

68. Discuss the properties and specification of magnetic materials used in the

manufacture of electrical machines. (AU-NOV/DEC2009)(AU-APRIL/MAY

2013)

69. Mention the major factors that impose technical limitation on the design of

electrical m/c.Explain.(AU- NOV/DEC2009)

70. Describe the step by step procedure for the design of a dc rotating m/c, by

listing out the assumptions made.( AU- NOV/DEC2009)

UNIT-II DC MACHINES

PART-A (1 MARK)

71.For simplex wave and lap windings, the back pitch yb and front pitch yf are as

follows:

(A) yb is odd, yf is even

(B) yb is even, yfis odd

(C) bothyb and yf are odd

(D) bothyb and yf are even.

72.All of the following are ferromagnetic materials EXCEPT

(A) Nickel

(S) Bismuth

(C) Silicon

(D) Mild steel.

73. Diamagnetic: Copper:: Ferromagnetic:

(A) Iron

(B) Lead

(C) Air

(D) Water.

74. Diamagnetic: Ferromagnetic:

(A) Copper: Silver

(B) Air: Water

(C) Iron: Steel

(D) Lead: Cobalt.

75. Which of the following gives materials in order of increasing values of

permeability?

(A) Water, Iron. Copper, Silver

(B) Water, Air, Pure iron. Permalloy

(C) Cobalt, Aluminium, Copper, Silver

(D) Silicon iron. Palladium, Lead,

Silver.

76.By adding silicon to ferromagnetic, materials

(A) Electrical resistivity increases and

also magnetic permeability increases

(B) Electrical resistivity decreases and

also magnetic permeability increases

(C) Electrical resistivity decreases and

magnetic permeability increases

(D) Electrical resistivity increases and

magnetic permeability decreases.

77.Which of the following can be worked with higher flux densities?

(A) Mild steel

(B) Silicon iron

(C) Cold rolled silicon steel

(D) Hot rolled silicon steel.

78.Hysteresis loss varies with frequency (f) as

(A) f (B) f1.6 (C) f 2

Page 8: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

8

(D) f2.6

79.Hysteresis loss varies with maximum flux density (B) as

(A) B

(B) B1.6

(C) B2

(D) B2.6.

80.In large bulky materials, the uni-axial anisotropy can be induced by

(A) Cold rolling

(B) Magnetic annealing

(C) Magnetic quenching

(D) Any of the above.

81.Magnetostriction

(A) Grain oriented magnetic properties

(B) Represents the upper limit to which

magnetic properties can be induced

(C) Change in dimensions resulting from

magnetisation of ferromagnetic materials

(D) None of the above.

82.An ideal insulating material should have low

(A) Insulation resistance

(B) Dielectric strength

(C) Dielectric loss angle

(D) Mechanic strength.

83.The class of insulation used for large rating HV alternators

(A) Y

(B) A

(C) B

(D) C.

84.Asbestos has

(A) Fibrous structure

(B) Crystalline structure

(C) Grain oriented crystalline structure

(D) None of the above.

85.Asbestos can bear a temperature up to __________ without loosing its insulating

properties

(A) 100°C

(B) 200°C

(C) 300°C

(D) 400°C.

86.Fiber glass insulation can be used up to temperature of

(A) 60°C

(B) 80°C

(C) 130°C

(D) 180°C

87.Raw form of natural rubber is known as

(A) malamine

(B) Milk of lime

(C) Epoxy

(d) Latex.

88.Which of the following is the characteristic of ceramic insulators?

(A) These are hard, strong and dense

(B) These are stable at high temperatures

(C) These are unaffected by commercial

acids and alkalies

(D) All of the above.

89.The number of commutator segments in a dc machine is equal to the no. of

(A) coil-sides

(B) Turns

(C) Coils

(D) Slots.

90.Dielectric strength of PVC is around

(A) 5 kV/mm

(B) 10 kV/mm

(C) 15 kV/mm

(D) 30 kV/mm.

71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90

C B A D B C C A B D C C D A D C D D C D

PART-B (2 MARKS)

Page 9: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

9

91. What is the range of specific magnetic loading in D.C. machine? (AU-

APR/MAY07)

The usual range of specific magnetic loading in D.C. machine is 0.4 to 0.8

Wb/m2.

92. What is the range of specific electric loading in D.C. machine?

The usual range of specific electric loading in D.C. machine is 15000 to 50,000

amp.cond./m.

93. What are the factors to be considered for the selection of number of poles in

D.C.machine?

The factors to be considered for the selection of number of poles in D.C.

Machines are frequency of flux reversals, current / brush arm and armature mmf per

pole.

94.What are the parameters that are affected by the number of poles?

The parameters that are affected by the number of poles are weight of iron and

copper, length of commutator and dimension of brushes are the parameters affected by

number of poles.

95.Why square pole is preferred?

Square pole is preferred because the length of the mean turn of field winding is

minimum. Hence to reduce the copper requirement a square cross-section is preferred

for the poles of D.C. machine.

96.What is square pole and square pole face?

In square pole, the width of the pole body is made equal to the length of the

armature. In square pole face, the pole arc is made equal to the length of the armature.

97.Define winding pitch. (AU-APRIL/MAY2012)

The winding pitch is defined as the distance between the starts of two

consecutive coils measured in terms of coil sides.

98. What is back pitch? (AU- APRIL/MAY2007)

The distance between top and bottom coil sides of a coil measured around the

back of the armature is called the back pitch. The back pitch is measured in terms of coil

sides.

99.What is front pitch?

The front pitch is the distance between two coil side connected to the same

commutator segment. It is measured in terms of coil sides.

100.Definecommutator pitch. (AU- NOV/DEC2009)

The commutator pitch is defined as the distance between the two commutator

segments to which the two ends (start and finish) of a coil are connected. It is measured

in terms of commutator segment.

101.What is equalizer connection?

The equalizer connections are low resistance copper conductors employed in lap

winding to equalize the induced emf in parallel paths. The difference in induced emf in

parallel paths is due to slight unsymmetrical in flux per pole in dc machines.

102.What is the need for brushes in D.C. machine?

The brushes are used in D.C. machines to collect or draw current from the

rotating armature.

103.What are the effects of armature reaction? AU- (MAY/JUNE 2008)

Page 10: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

10

The various effects of armature reaction are reduction in induced emf, increase in

iron losses, delayed communication, sparking and ring firing.

104.How the polarities of interpole are decided?

The polarities of the interpole must be that of the main pole just ahead (in the

direction of rotation) for a generator and just behind (in the direction of rotation) for a

motor.

105.What are standard periods for short time rating of a machine?

The standard periods for short time ratings are 10, 30, 60 and 90mts.

106.State different losses in a D.C. generator. (AU- APRIL/MAY2007)

The different losses in a D.C. generator are

Copper losses

Armature copper loss

Field copper loss

Magnetic losses (iron or core losses)

Hystersis loss (ii) Eddy current loss (iii) Mechanical loss

107.State the relationship between number of armature coils and number of

commutator segments in a D.C. machine.

The relationship between number of armature coils and number of

commutator segments in a D.C. machine is βc = Π D c

C Where βc = Commutator segment pitch

C = Number of coils

Dc = Diameter of commutator

108.Write the expressions for output coefficients of D.C machine. (AU-APRIL/MAY

2013)

The expressions for output coefficients of D.C machine is

Co = Π2Bav ac x10-3 kW/m3 rps

109.State any two guiding factors for the choice of number of poles.(AU-

APRIL/MAY 2013)

(I) The frequency of flux reversal in the armature core generally lies between

25 to 50 Hz. Lower values of frequency are used for large machines.

(II) The value of current per parallel path is limited to about 200A. Thus the

current per brush arm should not be morethan 400A.

PART-C (16 MARKS)

110. Calculate suitable dimension of armature core to give a square pole face for a

50kW,4pole,600rpm DC shunt generator with full load terminal voltage of 220v.The

maximum gap density is .83wb/sqm and armature ampere conductor/metre are

30000.assume full load armature voltage drop as 3% of the rated terminal voltage

and that the field current as 1% of rated full load current .The ratio of pole arc to

pole pitch is 0.67 (AU- NOV/DEC2009, MAY/JUNE2009)

111.Determine the main dimension of a 200kw,250v,6pole,1000rpm generator.the

maximum value of flux density in the airgap is 0.87wb/m2 and the ac=31000.the ratio

of pole arc to pole pitch is o.67 and the η=91%.assume the ratio of length of core to

pole pitch=0.75 (AU- NOV/DEC2009)

Page 11: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

11

112.Discuss various methods to reduce the effects of armature reaction (AU-

NOV/DEC2009)

113.A 4pole 50HP DC shunt motor operates with rated voltage of 480 volt at the

ratrd speed of 600 rpm it has wave wound armature with 770conductors .the leakage

factor of pole is 1.2.the pole are of circular cross section.the flux density in the pole is

1.5wb/m2 compute diameter of each pole (AU- MAY/JUNE2009)

114.Derive the output equation of dc machine (AU- APRIL/MAY2010,

NOV/DEC2009,2007,APRIL/MAY2012)

115.Calculate the diameter and the length of armature for a

7.5kw,p=4,N=1000rpm,v=220v shunt motor,given:full load η=0.83 maximum gap

flux density=0.9wb/m2 ,ac=30000amp.cond/m,field form factor= 0.7.assume that the

maximum efficiency occurs at full load and the field current is 2.5% of rated current

the pole face is square. (AU- APRIL/ MAY2010)(AU-APRIL/MAY 2013)

116.Explain the choice of specific electric loading, specific magnetic loading on DC

machine( APR/ MAY2010, NOV/DEC2009,2007)

117.Write short notes on design of field poles and field coils in dc machine. (AU-

APR/ MAY2010)

118.Determine the main dimension of a 5kw,250v,4pole,1500rpm generator.the

maximum value of flux density in the airgap is 0.42wb/m2 and the ac=15000.the ratio

of pole arc to pole pitch is o.66and the η=87%. (AU-

NOV/DEC2007,APRIL/MAY2012)

119.Explain the various steps involved in the design of commutator and brushes of a

d.c. machine.(AU-APRIL/MAY 2013)

UNIT-III TRANSFORMERS

PART-A (1MARK)

120.Gaseous insulating material

(A) Teflon

(B) Sulphur hexafluoride

(C) Stealite

(D) Phologopite mica.

121.Silver finds application generally as

(A) Contact material

(B) Fusing material

(C) Low temperature insulating material

(D) None of the above.

122.When I is the current in amperes, the temperature rise at which fuse wire will

melt is proportional to

(A) I (B) I 2 (C) 1/ I (D) 1/ I2

123.The fusing current depends on

(A) diameter of fuse wire

(B) Number of strands of wire

(C) Length of the fuse wire

(D) all of the above.

124.Class H insulation is normally used in

(A) Traction motors

(B) Submersible pump motors

(C) Single phase motors

(D) Dc motors.

125.An ideal liquid insulating material should have low

(A) Heat conductivity

(B) Dielectric strength

(C) Mechanical strength

(D) Volatility.

Page 12: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

12

126.The electric breakdown strength of an instating material depends on

(A) Composition of the material

(B) Thickness of the material

(C) Time of application of voltage

(D) all of the above.

127. When V is the voltage applied across the insulation, the dielectric loss for an

insulating material will be proportional to

(A) V

(B) V2

(C) 1/ V

(D) 1/ V2

128.When δ is the dielectric loss angle, the value of tan δ increases with

(A) Decrease in temperature

(B) Presence of moisture

(C) Voltage

(D) all of the above.

129.Which has lowest dielectric strength?

(A) Cotton

(B) Empire cloth

(C) Porcelain

(D) Mica.

130.Silicon steel E330 indicates that

(A) It has 33% silicon

(B) It has normal specific loss at 50 Hz

(C) It is cold rolled grain oriented

(D) all of the above.

131.Which of the following silicon steel will have least specific lose?

(A) E11

(B) E42

(C) E31

(D) E41

132. The diameter of a 10 SWG wire will be closer to

(A) 0.1 mm

(B) 1mm

(C) 3 mm

(D) 13 mm.

133. In silicon steel designation like E320 the first figure 3 indicates

(A) The percentage of silicon

(B) The specific loss at 50 Hz

(C) The specific lose at 100 Hz

(D) The grain oriented cold rolled

variety.

134.In silicon steel designation like E320 , the second figure 2 represent

(A) Specific loss

(B) Direction of grain orientation

(C) Magnetic variety

(D) Non-magnetic variety.

135.In high nickel alloys the percentage of nickel is

(A) 5 to 10 percent

(B) 10 to 20 percent

(C) 30 to 50 percent

(D) 70 to 80 percent.

136.Pumps, blowers and fans are usually called for

(A) Continuous duty

(B) Short time duty

(C) Intermittent periodic duty

(D) Intermittent periodic duty with

starting and braking.

137.Which of the following is normally called for continuous duty with starting and

braking?

(A) Machine tools

(B) Compressors

(C) Sirens

(D) Escalators.

138.In lap windings, the equalizer rings arc used to save

(A) armature winding from carrying

circulating currents

(B) commutator from carrying

circulating currents

(C) Brushes from carrying circulating

currents

(D) Armature core from eddy currents.

139.Which of the following calls for intermittent duty with starting and braking?

Page 13: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

13

(A) Pumps

(B) Conveyors

(C) Wind lasses

(D) Cranes.

120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139

B A B D A D D B B A C A C A A D A A C D

PART-B (2MARKS)

140.What are the different losses in a transformer? (AU- APRIL/MAY07)

The different losses in a transformer are

1. Core or Iron loss

2. Copper loss.

141.State merits of 3 Ø transformers over 1 Ø transformers.

The merits of 3 Ø transformers over 1 Ø transformers.

(i) Less weight (ii) Less cost.

142. What is the range of efficiency of a transformer?

The range efficiency of a transformer will be in the range of 94% to 99%. Notes

on design of transformer core

143.What is transformer bank?

A transformer bank consists of three independent single phase transformers with

their primary and secondary windings connected either in star or in delta.

144.Define window space factor (AU-APRIL/MAY07,APRIL/MAY2012,

APRIL/MAY 2013)

The window space factor is defined as the ratio of copper area in window to total

area of window.

145.What are the factors to be considered for selecting the cooling method of a

transformer? (AU-APRIL/MAY2012)

The factors to be considered for selecting the cooling method of a transformer

(i)KVA rating of transformer,(ii) size, (iii)application and (iv) the site condition where it

has to be installed.

146.How the heat dissipates in a transformer?

The heat dissipation in a transformer occurs by

(i) Conduction,

(ii) Convection

(iii) Radiation.

147.Why cooling tubes are provided?

Cooling tubes are provided to increase heat dissipating area of the tank.

148.What are the various types of transformers?( AU- APRIL/MAY07)

The various types of transformers based on construction are

Core type Shell type.

149.Mention the main function of cooling medium used in transformers.( AU- MAY/JUNE

2008)

The main functions of cooling medium used in transformers are

Transfer the heat by convection from the heated surface to the tank surface. Create good level

of insulation between various conducting parts.

150.How much heat is dissipated by radiation and convection?

The heat dissipated by radiation and convection are i) 6 watt/m2/C by radiation.

(ii) 6.5 watt/m2/C by convection .

151.Why stepped core are generally used for transformer?

Page 14: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

14

The stepped core are generally used for transformer because of the following reason

LV and HV coils are circular, for better utilization of space, for reducing the mean

length of LV

And HV turns and resulting in saving of copper material.

152.What are the important properties of transformer steel?

The important properties of transformer steel are (i) High permeability, (ii) high

resistivity, and (iii) low coercive force.

153.Why the area of yoke of a transformer is usually kept 15 – 20% more than that of

core? (AU- NOV/DEC2009)

The area of yoke of a transformer usually kept 15 – 20% more than that of core so

that the flux density in the yoke is reduced, resulting into reduction in iron losses for

yoke.

154.What is the cause of noise in transformer? (AU- APRIL/MAY2007)

The cause of noise in the transformer is mainly because of magnetostriction effect and

also loosening of stamping and mechanical forces produced during working.

155.What are the advantages of using higher flux density in the core? (AU-

NOV/DEC2009)

The advantages of using higher flux density in the core are (i) reduction in the core and

yoke section for same output,(ii) reduction mean length of LV and HV turns, resulting in

saving of copper material,(iii) reduced overall size and weight of transformer.

156.List various disadvantages of using higher flux density in design of core.

The disadvantages of using higher flux density in design of core (i) Increased

magnetizing current and iron losses, saturation of magnetic material, lower efficiency, because

of higher no-load losses,(ii) higher temperature rise of transformer.

157.Why the cross-section of yoke is taken greater than core section?

The cross-section of yoke is taken greater than core section in order to reduce flux

density in the yoke, thereby reducing iron losses and no-load current in the yoke section.

158.What are the types of winding, commonly used for LV winding?

The types of winding, commonly used for LV winding are (i) Cylindrical winding with

rectangular conductors and (ii) helical winding.

159.What are the drawbacks of sandwich winding?( AU- APRIL/MAY2008)

The drawbacks of sandwich winding are (i) Requires more labour in its maintenance,(ii)

more difficult to insulate different coils from each other and from yoke.

160.Define Stacking factor

Stacking factor is defined as the ratio of net iron area to the gross iron area. It is denoted

by Ki.

161.Define iron space factor (AU- NOV/DEC2009)

Iron space factor is defined as the ratio of gross core area to the area of circumscribing

circle. It is denoted by Ki.

162.What is leg Spacing?

The leg spacing is the distance between the centers of two adjacent limbs. It is denoted

by D.

163.What are the cooling methods used for dry type transmformers?(AU-APRIL/MAY

2013)

Air natural cooling

Air blast cooling

Page 15: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

15

PART-C (16MARKS)

164.Derive the output equation of a 1 phase transformer in terms of core and

window area.(AU- NOV/DEC 2009)

165.Calculate the core and window areas required for a 1000 kVA, 6600/400V, 50Hz, 1

phase core type T/F. Assume density=1.25Wb/m2 and a current density of 2.5 A/mm2

voltage per turn is 30. Window space factor is 0.32.( AU- NOV/DEC 2009)

166.The tank of a 1250 kVA natural oil cooled T/F has the dimensions, length, width and

height 1.55m*0.65m*1.85m respectively. The full load loss is 13.1 kW. Find the number of

tubes for this transformer. Assume W/m2_C due to raditation=6 and that due to

convection is 6.5.Improvement in convection due to provision of tubes is 40% maximum

temperature rise is 40C. Length of each tube is 1m.Dia of the tube is 50mm. Neglect the

top and bottom surfaces of the tank as regard to cooling. (AU- NOV/DEC

2009,APRIL/MAY2012)

167.Calculate the main dimensions of a core of 100KVA,2000/400 V,50Hz, 1 phase shell

type transformer. Voltage/turn=10 V.Peak flux density in the core is 1.1

wb/m2.Windowspace factor is 0.33. Ratio of core depth to width of central limb=2.5. Ratio

of window height to width=3.0 current density in the winding is 2A/mm2.Stacking

factor=0.9. (AU- MAY/JUNE2009)

168.A 250 KVA, 6600/400V,3phse core type T/F has a total loss of 4800 watts at full load.

The transformer tank is 1.25m in height and 1m*0.5m in plan. Design a suitable scheme

for tubes if the average temperature rise is to be limited to 35®C.The diameter of tubes is

50mm and is spaced 75mm from each other. The average height of tubes is

1.05m.Specified heat dissipation due to radiation and convection is respectively 6 and 605

W/m2/®C. Assume that convection is improved by 35 percent due to provision of tubes.

(AU- NOV/DEC 2009, NOV/DEC 2007, APR/MAY 2010)

169.Derive the output equation of transformer and hence discuss the rating of both

single phase and 3 phase transformer. ‘’(AU- APR/MAY 2010, NOV/DEC 2009)

170.Determine the dimensions of core and yoke for a 200 KVA, 50 Hz, single phase

core type transformer. A cruciform core is used with distance between adjacent

limbs to 1.6 times the width of core laminations. Assume voltage /turn 14V, Max.

Flux density=1.1wb/m2, window space factor=0.32, current density=3 A/mm2, and

stacking factor=0.9. The net iron area is 0.56 d2 in a cruciform core .where‘d’ is the

diameter of circumscribing cirle. Also the width of largest stamping is 0.85 d(AU-

APR/MAY 2010, NOV/DEC 2007,APRIL/MAY2012)

171.Write short notes on design of cooling system for a transformer.(AU-

APRIL/MAY 2010)

172.Discuss about the design of width of window for optimum output in a transformer.

(AU- APRIL/MAY 2010)

173.Explain the design of transformer tank with cooling tubes in details.(AU-

NOV/DEC 2009,APRIL/MAY2012)

174.Describe the methods of cooling of transformers. (AU-APRIL/MAY 2013)

175.A single phase, 400V,50Hz transformers is built from stampings having a relative

permeability of 1000. The length of the flux path is 2.5m,the area of the cross section of the

core is 2.5 x 10-3m2 and the primary winding has 800 turns. Estimate the maximum flux

Page 16: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

16

and no load current of the transformer. The iron loss at the working flux density is

2.6W/Kg. Iron weighs 7.8 x 103 Kg/m3.Stacking factor is 0.9 (AU-APRIL/MAY 2013)

UNIT-IV INDUCTION MOTORS

PART-A (1 MARK)

176.The percentage of overload on a continuous rating machine is

(A) zero

(B) 5%

(C) 10%

(D) 50%.

177.Open circuit test on a transformer is conducted to obtain

(A) The leakage impedances

(B) The ohmic loss

(C) Hysteresis loss only

(D) Core loss only

(E) Eddy current loss only.

178.Heat transfer capability of hydrogen is nearly __________ times that of air

(A) 2

(B) 3

(C) 7

(D) 17.

179.The density of hydrogen is __________ that of air

(A) 14

(B) 1/4

(C) 1/7

(D) 1/14

180.Turbo-alternators of rating __________ and above are normally hydrogen

cooled.

(A) 10 MW

(B) 20 MW

(C) 25 MW

(D) 50 MW.

181. Iron losses of a machine are

(A) Directly proportional to flux density

(B) Directly proportional to the square of

flux density

(C) Inversely proportional to flux

density

(D) Inversely proportional to the square

of flux density.

182.While considering hysteresis loss in a transformer, under which of the following

the loss will not increase?

(A) When flux density is increased by

10%

(B) When thickness of lamination is

caused by 10%

(C) When frequency is increased by 10%

(D) When both thickness and frequency

are increased by 10%.

183.The shaft of electric motors is generally made of

(A) Mild steel

(B) Cast iron

(C) Copper

(D) Aluminum alloy.

184.The critical speed of the shaft for an electric motor should be

(A) Away from the operating speed

(B) Exactly the same as operating speed

(C) Half the operating speed

(D) Double of the operating speed.

185.The shaft of electric motors is generally supported in

(A) Magnetic bearings

(B) Bush bearings

(C) Ball or roller bearings

(D) cast iron bearings.

186.Ball and roller bearings are generally lubricated by

(A) Graphite

(B) Grease

(C) water

(D) Air.

187.In case of ball bearings, the ball are made of

Page 17: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

17

(A) Cast iron

(B) Mild steel

(C) Carbon chrome steel

(D) None of the above.

188.A ball bearing is secured to the shaft by means of

(A) Key

(B) Slot

(C) Pin

(D) Sleeve.

189.In case of 110 MW generators, the shaft is supported on

(A) Ball bearings

(B) Roller bearing

(C) Needle bearings

(D) Bush bearings.

190.Which of the following is classed as anti friction bearing?

(A) Ball bearing

(B) Roller bearing

(C) Needle bearing

(D) All of the above.

191.The force exerted by a lifting magnet varies with distance (D) as

(A) D

(B) D2

(C) 1/D

(D) 1/D2

192.The force-stroke curve for a lifting magnet will be of the shape shown in

(A) figure A

(B) Figure B

(C) Figure C

(D) Figure D.

193.When B is the flux density; A is the area of the working gap and μ is the

permeability, the force exerted by the attractive type magnets is given by

(A) μA / 2B

(B μ B / A2

(C) A2B1/ 2μ

(D) B A2/ 2μ

194.Transformer-core laminations are made of

(A) Cast iron

(B) Wrought iron

(C) Silicon steel

(D) Cast steel.

176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194

B A B D A D D B B A C A D A A D A A C

PART-B (2 MARKS)

195.What are the main dimensions of induction motor? (AU- APRIL/MAY2007)

The main dimensions of induction motor are stator core internal

diameter and stator core length.

196.Why wound rotor construction is adopted? (AU- NOV/DEC2009)

The wound rotor has the facility of increasing the rotor resistance

through slip rings. Higher values of rotor resistance are needed during starting to

get a high value of starting torque.

197.What is rotating transformer? (AU- APRIL/MAY2007)

The principle of operation of induction motor is similar to that a

transformer. The stator winding is equivalent to primary of a transformer and the

Page 18: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

18

rotor winding is equivalent to short circuited secondary of a transformer. In transformer

the secondary is fixed but in induction motor it is allowed to rotate.

198.What type of starter cannot be used for squirrel-cage motors?

The starter which cannot be used for squirrel cage motor is rotor

resistance starter.

199.What are the materials used for slip-rings and brushes in induction motor?

The slip-rings are made of brass on phosphor bronze. The brushes are

made of metal graphite which is an alloy of copper and carbon.

200.What are the factors to be considered for the choice of specific magnetic

loading? (AU- APR/MAY2007)

The choice of specific magnetic loading depends on power factor,

iron loss and over load capacity.

201.What are the factors to be considered for the choice of specific electric loading?(

AU- NOV/DEC2009)

The choice of specific electric loading depends on copper loss, temperature

rise, voltage rating and over load capacity.

202.How the induction motor can be designed for best power factor?

The induction motor can be designed for best power factor by choosing Pole pitch,

203.What are the different types of stator windings in induction motor?( AU-

APRIL/MAY07)

The different types of stator windings are mush winding, (ii) lap winding

and (iii) wave winding.

204.What is full pitch and short pitch or chorded?

is called full

pitched winding. When the coil span is less than

called short pitched or chorded.

205.Why short chorded windings are employed in induction motor? (AU-

NOV/DEC2009)

For short chorded windings the length of mean turn will be lesser than the full

pitch coils. Hence it results in reduction of copper. Also the short chorded winding

eliminates certain harmonic magnetic fields.

206.What type of slots is preferred in induction motor?

Semi enclosed slots are preferred for induction motor. It results in less air gap

contraction factor giving a small value of magnetizing currents, low tooth pulsation

loss and much quieter operation (less noise).

207.What is slot space factor? (AU- APRIL/MAY2007)

The slot space factor is the ratio of conductor (or copper) area per slot and

slot area. It gives an indication of the space occupied by the conductors and the space

available for insulation. The slot space factor for induction motor varies for 0.25

to0.4.

208.Write the expression for length of mean turn of stator winding.

The expression for length of mean turn of stator, Lmts = 2L +

209.Which part of induction motor has maximum flux density? What is the

maximum value of flux density in that part?

The teeth of the stator and rotor core will have maximum flux density.

The maximum value of flux density in the teeth is 1.7 Sb/m2.

Page 19: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

19

210.What are the factors to be considered for estimating the length of air-gap in

induction motor? (AU- NOV/DEC2009,APRIL/MAY2012)

The following factors are to be considered for estimating the length of air-gap.

1. Power factor 2. Overload capacity 3. Pulsation loss

4. Unbalanced magnetic pull 5. Cooling 6. Noise

211.List the undesirable effects produced by certain combination of rotor and stator

slots.

The following problems may develop in induction motor with certain combination

of rotor and stator slots.

The motor may refuse to start, (cogging)

The motor may run at sub synchronous speed (crawling)

Severe vibration may develop and the noise will be excessive.

212.What is crawling and cogging?

Crawling is a phenomena in which the induction motor runs at a speed lesser

than sub synchronous speed. Cogging is a phenomena in which the induction motor

refuses to start.

213.Explain the phenomena of cogging.(AU- NOV/DEC2009,APRIL/MAY2012

When the number of stator and rotor slots are equal, the speeds of all the

harmonics produced by stator slotting coincide with the speed of corresponding,

rotor harmonics. This harmonics of every order would try to exert synchronous

torques at their corresponding synchronous speeds and the machine would refuse

to start this is known as cogging.

214.What are the methods adopted to reduce harmonic torques?

The methods used for reduction or elimination of harmonic torques are chording,

integral slot winding, skewing and increasing the length of air-gap.

215.What are the different types of windings used for rotor of induction motor?

(AU- APRIL/MAY07)

The different types of windings employed in induction motor rotor are mush

winding the double layer bar type winding.

216.Write down the equation for output co-efficient in an induction motor.(AU-

APRIL/MAY 2013)

Output co-efficient Co=1.1 KwBavac x 10-3

217.What is meant by an Ideal short circuit current?(AU-APRIL/MAY 2013)

Ideal short circuit current is defined as the current drawn by the motor at standstill

if its resistance is neglected.

PART-C (16 MARKS)

218.Derive output equation of a three phase induction motor. (AU- NOV/DEC

2007,2009)

219.State and explain factors governing the choice of ampere conductors per

meter in the design of a 3 phase IM. (AU- NOV/DEC 2007)

220.Compute the main dimension of a 15 KW, 3 phase, 400 V, 50Hz, 2810 rpm

squirrel cage IM having efficiency of 88% and full load power factor of 0.9. Assume

specific magnetic loading equal to 0.5 wb/m2 and specific electric loading equal

to 25000 A/mThe rotor peripheral speed maybeapproximately 20m/sec

atsynchronous speed.(AU- NOV/DEC 2007,APR/MAY 2010,MAY/JUNE2009)

Page 20: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

20

221.Explain the following of 3 phase IM (AU- NOV/DEC 2009)

(a) Output equation

(b) Choice of average flux density in air gap

(c) Choice of ampere conductor/meter.

222.Explain the rotor design of squirrel cage IM. (AU- NOV/DEC 2009)

223.A 90 kw, 500v, 50Hz, 3 phase ,P=8 IM has a star connected stator

winding accommodated in 63 slots with 6 conductor per slot. If the slip ring

voltage on open circuit is to be about 400 v, find a suitable rotor winding. Given

efficiency=0.9, power factor=0.86(AU-NOV/DEC 2009,APRIL/MAY2012)

224.Derive the O/P equation of 3 phase IM and discuss the choice of specific

electric loading. (AU- APRIL/MAY 2010,APRIL/MAY2012)

225.Write short notes on design of wound rotor.( AU- APRIL/MAY 2010)

226.Explain various losses in a 3 phase IM and its efficiency. (AU- APRIL/MAY

2010)

227.A 11kw ,3 phase ,P=6,50Hz, 220V, star connected induction motor has 54 ststor

slots, each containing 9 conductor. Calculate the value of bar and end ring currents.

The numbers of rotor bars is 64. The machine has an efficiency of 86% and a

power factorof 0.85. the rotor MMF may be assumed to be 85 % of stator

MMf. Also find the bar and the end ring sections if the current density is

5A/mm2(AU- MAY/JUNE2009,AU-APRIL/MAY2012)

228.Determine the main dimensions of a 3.7kw,400v, 3 phase, P=4,50Hz, squirrel

cage IM. Assume :Average flux density inthe air gap=0.45wb/m2,ac=23000,ŋ=0.85

and power factor=0.84. takeL/τ=1.5(AU- NOV/DEC 2009)

229.Estimate the stator core dimensions, number of stator slots and number of

stator conductor per slots for a 100kw, 3300v, P=12, 50Hz, star connected slip

ring IM. Assume :Average flux density in the air gap=0.4wb/m2,ac=25000,ŋ=0.9

and power factor=0.9. Winding factor=0.96. chose main dimension to give best

power factor. The slot loading should not exceed 400 ,Amp.conductors.( AU-

NOV/DEC 2010,AU-APRIL/MAY 2013)

230.Describe the effect of dispersion co-efficient due to the following factors in an

induction motor: (AU-APRIL/MAY 2013)

Over load capacity

Airgap length

Number of poles

Frequency

UNIT V SYNCHRONOUS MACHINES

PART-A (1 MARK)

231.Skewing of rotor slots helps in

(A) Improving beat transfer

(B) Reducing noise

(C) Suppressing undesirable harmonics

(D) all of the above.

232.The harmonics in rotating machines are generated due to

(A) Non-sinusoidal field form

(B) Slotting of the stator core

(C) Both (A) and (B) above

(D) None of the above.

233.The effect of harmonics in rotating machines can be minimized by

Page 21: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

21

(A) Use of longer air gap

(B) Skewing the poles

(C) Use of distributed winding

(D) all of the above.

234.Which of the following measure will not help in reducing the effect of

harmonics?

(A) Connecting 3 phase winding in star

or delta without neutral

(B) Use of short pitch coils or fractional

slot windings

(C) Chamfering the pole shoes of salient

pole machines

(D) Reducing the number of slots per

pole per phase.

235.In dc machines the number of poles is generally decided by

(A) Frequency of flux reversals

(B) Weight of iron parts

(C) Weight of copper

(D) all of the above.

236.In dc machines by increasing the number of poles, all of the following reduce

except:

(A) Weight of copper

(B) Weight of iron parts

(C) Frequency of flux reversals

(D) Overall size of the machine.

237.Transformer action requires a

(A) Constant magnetic flux

(B) Increasing magnetic flux

(C) Alternating magnetic flux

(D) Alternating electric flux.

238.Which component of the no load current of the transformer is opposite in phase

to the induced emf? s

(A) Magnetizing component

(B) Core loss component

(C) Both (A) and (B) above

(D) None of the above.

239.In transformers, the cylindrical winding with rectangular conductors is

generally used for

(A) Low voltage winding

(B) High voltage winding

(C) Tertiary voltage winding

(D) Any of the above.

240.Cylindrical winding on transformers is generally not used beyond

(A) 6 kV

(B) 30 kV

(C) 66 kV

(D) 132 kV.

241.The disadvantage of cylindrical winding is

(A) Increased eddy current loss

(B) High copper loss

(C) Low mechanical strength

(D) all of the above.

242.For transformer laminations

(A) Hot rolled silicon steel is preferred

(B) Cold rolled silicon steel is preferred

(C) grain oriented silicon steel is

preferred

(D) Any of the above steel can be used.

243.The percentage of silicon in the core steel is

(A) 1 to 2 percent

(B) 2 to 3 percent

(C) 4 to 6 percent

(D) 8 to 10 percent.

244.The stacking factor will be least for

(A) Square core

(B) Cruciform core

(C) Three stepped core

(D) Four stepped core.

245.In static transformers mechanical forces are produced due to

(A) Vibrations

(B) Gap between

laminations

(C) Interaction of

current flowing in the

conductor and leakage

flux around it.

(D) None of the above.

Page 22: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr. NNCE III/06 Sem QB-DEM

22

246.The thickness of laminations of the core of a power transformer usually

(A) 0.003 to 0.05 mm

(B) 0.03 to 0.05 mm

(C) 0.03 to 0.5 mm

(D) 3 to 5 mm.

247.Transformer action requires a

(A) Short circuiting

(B) Increasing resistivity

(C) Reducing resistivity

(D) all of the above.

248.In an oil filled transformer, oil is provided for

(A) Cooling

(B) Insulation

(C) Lubricating

(D) Both cooling and insulation

(E) Preventing accumulation of dust.

249.The dimensions of a dc machine primarily depend on

(A) KW output

(B) Work done per revolution

(C) Exposed surface

(D) None of the above.

The various losses of a 60 HD, 220 V 850 rpm dc motor are as follows: - Copper losses

3.2 kW, core losses 0.8 kW, mechanical losses 0.1 kW. Now let us consider another dc

machine otherwise similar to this machine but having its linear dimensions 1.5 times.

250.The copper losses of new machine will be

(A) 1.5 x 3.2 kW

(B) (1.5)2 x 3.2 kW

(C)(1.5)3x3.2Kw

(D) 1/(1.5)2 x3.2 Kw

251.The iron losses of new machine

will be

(A) (1.5) x 0.8 kW

(B) (1.5)2 x 0.8 kW

(C) (1.5)4 x 0.8 kW

(D)(1.5)5x0.8kw

231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251

C C D D D C B B A A C D C A C C C D B C c

Page 23: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr.NNCE III/06 Sem QB-DEM

23

252.What are the two types of poles used in salient pole machines?

The two types of poles used in salient pole machines are Round poles and

Rectangular poles.

253.List the factors to be considered for the choice of number of slots.

The factors to be considered for the choice of number of slots are

1. Balanced winding 2. Cost 3. Host spot temperature in winging 4. Leakage

reactance 5. Tooth losses 6. Tooth flux density.

254.What is the limiting factor for the diameter of synchronous

machine?

The limiting factor for the diameter of synchronous machine is the peripherals peed. The

limiting value of peripheral speed is 175 m/sec for cylindrical rotor machines and

80m/sec for salient pole machines.

255.Write the expression for air-gap length in cylindrical rotor machine.

The expression for air-gap length in cylindrical rotor machine is

Length of air- -6KgBav

256.What is skewing?( AU- APRIL/MAY2007)

Skewing is twisting either the stator or rotor core. The motor noise, vibrations, cogging

and synchronous cusps can be reduced or even entirely eliminated byskewing. In order

to eliminate the effect of any harmonic, the rotor bars should beskewed through an

angle so that the bars lie under alternate harmonic poles of the same polarity or in other

words the bars much be skewed through two pitches.

257.What are the advantages and disadvantages of large air-gap length in

inductionmotor?( AU- NOV/DEC2009)

The advantages and disadvantages of large air-gap length in induction Motor are

Advantage:

A large air-gap length results in higher overload capacity, better cooling

reduction in noise and reduction in unbalanced magnetic pull.

Disadvantage:

The disadvantage of large air-gap length is that it result in high value of

magnetizing current.

258.What are the different types of induction motor? How they differ from each other?

The two different types of induction motor are squirrel cage and slip ring type

The stator is identical for both types but they differ in the construction of rotor.The

squirrel cage rotor has copper or aluminum bars mounted on rotor slots and short

circuited at both ends by end rings.The slip – ring rotor carries a three phase winding.

One end of each phase is connected to a slip – ring and other ends are star connected.

259.How the slip – ring motor is started?

The slip – ring motor is started by using rotor resistance starter. The starter consists of

star connected variable resistances and protection circuits. The resistances are connected to

slip-rings. While starting the full resistance is included in the rotor circuit to get high

starting torque. Once the rotor starts rotating, the resistances are gradually reduced in

steps. At running condition the slip-rings are shorted and so it is equivalent to squirrel

cage rotor.

260.What are the ranges of specific magnetic loading and electric loading induction

motor? (AU- APRIL/MAY2007)

Page 24: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr.NNCE III/06 Sem QB-DEM

24

The ranges of specific magnetic loading and electric loading in

Induction motor are Specific magnetic loading = 0.3 to 0.6 Wb/m2

Specific electric loading = 5000 to 4500 amp. Cond/m.

261.What is integral slot winding and fractional slot winding? (NOV/DEC2009)

In integral slot winding the total number of slots is chosen such that the slots

perpole is an integer. The integer should also be a multiple of number of phases. In

fractional slot winding the total number of slots is chosen such that the slots perpole is

not an integer.

262.What type of connection is preferred for stator of induction motor?

Under running condition the stator of induction motor is normally connected in

delta. (In delta connection the torque developed will be higher than the star connection).

But for reducing the starting current, the stator can be connected in star while starting

and then changed to delta.

263.Whatis Short Circuit Ratio (SCR)?( AU- MAY/JUNE 2008,

APRIL/MAY2012,APRIL/MAY 2013)

The Short Circuit Ratio (SCR) is defined as the ratio of field current

required to produce rated voltage on open circuit to field current required to circulate

rated current at short circuit. It is also given by the reciprocal of synchronous reactance,

Xd in p.u (per unit).For turbo – alternators SCR is normally between 0.5 to 0.7.

for salient pole alternator SCR varies from 1.0 to 1.5.

264.What is magnetization curve? (AU- APRIL/MAY2007)

The magnetization curve is a graph showing the relation between the magnetic

field intensity, H and the flux density, B of a magnetic material. It is used to estimate

the mmf required for flux path in the magnetic material and it is supplied by the

manufacturer of stampings or laminations.

265.What is leakage flux? (AU- NOV/DEC2009)

The leakage flux is the flux passing through unwanted path. The leakage

flux will not help either for transfer or conversion of energy.

266.What is leakage coefficient?

The leakage coefficient is defined as the ratio of total flux to useful flux.

Leakage coefficient, Cl= Total flux Useful flux

267.What are the differences between leakage flux and fringing flux?

The leakage flux is not useful for energy transfer or conversion. But the

fringing flux is useful flux. The leakage flux flows in the unwanted path. But the

fringing flux flows in the magnetic path. The effect of leakage flux on machine

performance is accounted by leakage reactance. The fringing flux increases the slot

reactance.

268.How the tertiary winding is connected? Why? (AU- NOV/DEC2009)

The tertiary winding is normally connected in delta. When the tertiary is

connected in delta, the unbalance in phase voltage during unsymmetrical faults in

Page 25: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr.NNCE III/06 Sem QB-DEM

25

primary or secondary is compensated by the circulating currents flowing in the closed

delta.

269.How the heat dissipation is improved by the provision of cooling tubes.

The cooling tubes will improve the circulation of oil. The circulation of oil is M, due to

more effective pressure heads produced by columns of oil in tubes. The .improvement in

cooling is accounted by taking the specific heat dissipation due to convection as 30% more

than that without tubes.

270.Why wound rotor construction is adopted?

The wound rotor has the facility of increasing the rotor resistance through slip rings.

Higher values of rotor resistance are needed during starting to get a high value of

starting torque.

271.What type of connection is preferred for stator of induction motor? (AU-

NOV/DEC2009)

Under running condition the stator of induction motor is normally connected in delta.

(In delta connection the torque developed will be higher than the star connection). But for

reducing the starting current, the stator can be connected in star while starting and

then changed to delta.

272.What are the factors that influence the choice of specific magnetic loading in a

synchronous machines? (AU-APRIL/MAY 2013)

Iron loss

Voltage

Transient short circuit current

stabillity

PART-C (16 MARKS)

273.Discuss the factors affecting the choice of specific magnetic loading in an alternator.

(AU- NOV/DEC 2009,APRIL/MAY2012)

274.Calculate the peripheral speed of a 1000kva,16 pole, 3 φ,375 rpm alternator. The

average air gap flux density is 0.55 wb/mm2 and ampere conductor/ meter are 28000.

takeL/τ=2 (AU-NOV/DEC 2009)

275.Compute the main dimension of a 100 MVA, 11kV, 50Hz, 150 rpm ,3φ water wheel

generator. The average gap flux density 0.65 T and ac=40000. The peripheral speed

should not exceed 65m/s at normal running speed.(AU-NOV/DEC 2009,APRIL/MAY2012)

276.Give the advantages of computer aided design of electrical apparatus.(AU- NOV/DEC

2009,MAY/JUNE2009)

277.Derive the output equation of synchronous machine and discuss the choice of specific

magnetic loading. (AU-APR/MAY 2010,MAY/JUNE2009,NOV/DEC2009)

278.Explain the constructional features of cylindrical pole and salient pole synchronous

machine. (AU- APR/MAY 2010,NOV/DEC2009)

279.Explain the effect of SCR on synchronous machine performance. (AU- APR/MAY

2010,NOV/DEC2009)

280.Determinethe main dimension for a 1000KVA, 50Hz,3 phase, 375 rpm alternator.

The average air gap flux density is 0.55 wb/mm2 and ampere conductor/ meter are

28000. Use rectangular poles and assume a suitable value for ratio of core length to pole

pitch in order that bolted on pole construction is used for which the maximum

Page 26: SUBJECT CODE: EE 2355 SUBJECT NAME: DESIGN OF · PDF fileSUBJECT NAME: DESIGN OF ELECTRICAL MACHINES ... In electrical machines the material preferred for pole shoes of electro-magnets

Dr.NNCE III/06 Sem QB-DEM

26

permissible peripheral speed is 50m/s. The run away speed is 1.8 times the

synchronous speed. (AUAPR/MAY2010,NOV/DEC2007,APRIL/MAY2012)

281.Determine the main dimension for a 75000KVA, 50Hz,13.8KV, 62.5 rpm ,

3phase star connected alternator. The peripherial speed of rotor should be about

40m/sec. Assume average gap density equal to 0.65 web/m2, ac=40,000 and

current density=4A/mm2.Assume Kw=0.955(AU- MAY/JUNE2009)

282. Find the main dimension of a 2500KVA,187.5 rpm,50Hz, 3phase,3kv, salient pole

synchronous generator. The generator is to be a vertical, water wheel type. The

specific magnetic loading is 0.6 wb/m2 and the specific electric loading is

34000 A/m.Use circular poles with ratio of core length to pole pitch=0.65.

Specify the type of pole construction a used if the runaway speed is about 2 times

the normal speed.( AU- NOV/DEC2009)

283.The field coils of a salient pole alternator are wound with a single layer winding of bare

copper strip 30mm deep, with separating insulation 0.15mm thick. Compute thickness

of the conductor, number of turns and height of the winding to develop an mmf of 12000

ampere turns with a potential difference of 5 volts per coil and loss of 1200 watts/m2 of

coil surface area. Mean length of turn is 1.2 m. Resistivity of copper is 0.021Ω/m/mm2(AU-

NOV/DEC2007)

284. Explain the step by step procedure for the design of field winding of synchronous

machine.(AU-APRIL/MAY 2013)

285. Determine a suitable number of slots and conductor per slot, for the stator winding of

a three phase 3300V, 50Hz, 300 rpm alternator. The diameter is 2.3m and the axial length

of core is 0.35m. The maximum flux density in the airgap should be approx.. 0.9 Wb/m2.

Assume sinusoidal flux distribution. Use single layer winding and star connection for

stator.(AU-APRIL/MAY 2013)