Vidya Digital Library - sudhinpk · PDF fileAnswerall questions. I. (a) ... 2 the no. of conductors that should be placed in each pole face to ... Given full load efficiency as 0.83,

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C 6130 (Pages 3) Name .

Reg. No .

SIXTH SEMESTER B.TEeH. (ENGINEERING) DEGREEEXAMINATION, JUNE 2010

EE 04 605-ELECTRICAL MACHINE DESIGN

(2004 Admissions)

Time : Three Hours Maximum: 100 Marks

Answer all questions.

I. (a) What are the factors to be considered for selecting the no. of poles ofa D.C . machine?

(b) Derive the output equation of a D.C. machine.

(c) Differentiae betw een the specific electric and ~agI!etic loading of a transformer.

(d) What are the different categories CTs? How CTs are different from ordinary transform ers ?

(e) Explain the various methods employed in the reduction of harmonics in alternators.T-

(0 Enumerate the various factors to be considered while selecting the no. of armature slots in asynchronous machine.

(g) Explain the phenomenon of cogging in induction machines.

(h ) What is meant by disper sion coefficient in an induction machine ? How it affects theperformance of the machine?

(8 x 5 =40 marks)

n. (a ) (i) What is commuta tion in D.C. machines ? What are the steps to improve commutation ?

(6 marks )

(ii) A d.c. machine has the following name plate data and physical properties: 550 V, 275 kW,900 r .p.m., 6 poles wave wound armature winding in 180 slots with 8 conductors in eachslot, 70% of armature surface covered' by'poles. Find

1 the armature m.rn .f per pole at rated armature current.

2 the no. of conductors that should be placed in each pole face to provide adequat .f lo

compensating winding.

3 the number of turns required on each of the 6 commutating poles to provide a fluxdensi ty across on effective gap length of 5 mm if the machine has no compensatingwindings.

(9 marks

Or

Turn ove r

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2 C 6130

(b) Calculate the diameter and length of armature for a 7.5 kW, 4 pole, 1000 r.p.m., 220 V shuntmotor. Given full load efficiency as 0.83, maximum gap flux density as 0.9 Wb/m2 , specific

electric loading =30000 ampere conductors per metre, field form factor =0.7. Assume thatmaximum efficiency occurs at full load and the field current is 2.5% of rated current. The pole

face is square.

III. (a) (i) Explain th e design procedure for CTs.

(15 marks)

(7 marks)

(ii ) What are the different types of core designs employed in transformers? How they areselected?

(8 marks)

Or

(b ) Determine the main dimensions ofthe core, the no. ofturns and the cross-section of conductors

for a 5 kvA, 11000/400 V, 50 Hz single ph ase core type distribution transformer. The netconductor area in the vindow is 0.6 times the ne': cross-section of iron in the core. Assume a

square cro ss-section for the core, a flux density of 1 Wb/m2, a current density of 1.4 Almm2

and a window sl¥J.ce factor 0.2. The height of the window is 3 times its width.

(15 marks)

IV. (a) (i) Enumerate the various factors to be considered in the design of the magnetic circuit of an

alternator.

(5 marks)

(ii) The fie ld coils of a sa lient pole alternator are wound with a single layer winding of bare

copper strip 30 mm de ep, with separating insulation 0.15 mm thick. Determine a suitablewinding length, number ofturns and thickness of conductor to develop an m.m.f. of 12,000

A with a potentia l difference of 5 V per coil and with a loss of 1200 W /m 2 of total coilsurface. The mean length of turn is 1.2 m. The resistivity of copper is 0.021 !lm.

(10 marks)

(b)

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Determine the main dimensions of a 75000 kVA, 13.8 kV, 50 HZ, 62.5 r.p.m., 3 phase starI

connected alternator. Also find the number of stator slots, conductors per slot, conductor areaand work out the winding details. The peripheral speed shouldlbe about 40 mls. Assume

average gap density =0.65 Wb/m2, ampere conductors per meter =40 ,000 and current densityis 4 Almm2.

(15 marks)

V. (a) (i) Explain the significance oflength of airgap in the design of rotor of an induction motor .

(5 marks) -



3 C6130

(i i ) A 15 kW, 3 phase 6 pole 50 Hz squirrel-cage induction motor has the following data :

Stator bore diameter =0.32 m ; axial length of stator core =0.125 m ; no. of stator slotsis 54 ; number of conducto rs per stator slots = 24 ; current in each stator conductor =

17.5 A ; full load power factor =0.85 lagging.

Design a sui table cage rotor giving number of rotor slots. section of each bar and section ofeach ring. The full load speed is to be about 950 r.p.m. approximately. Use copper for rotorbars and end rings. Resistivity of copper is 0.02 Om .

(10 marks)

Or

(b) Determine the main dimensions, number of radial ventilating duets, number as stator slotsand the number of turns per phase of a 3.7 kW, 400 V, 3 phase, 4 pole , 50 Hz squirrel-cageinduction motor to be started by a star-delta starter. Work out the winding details.

Assume :

Average flux density in the gap = 0.45 Wblm2, ampere conductors per metre = 23000,efficiency =0.85 and power factor =0.84.

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(15 marks)

[4 x 15 = 60 marks)

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58373 Name:Reg.No.

SIXTH SEMESTER B.TECH DEGREE EXAMINATION, JUNE 2009

EE.04.605 - Electrical Machine Design

Time: Three hours

PART-A(Answer all questions)

Maximum: 100 marks

1. a.

b.c.d.e.f.g.h.

Briefly explain the factors to be considered in the choice of armature winding in aDC machine.What are the different losses occurring in a DC machine.Explain the factors to be considered in the design of insulation in transformers.Write down the design procedure for a CT.Differentiate the specific magnetic and electric loading of an alternator.What are the various losses occurring in an alternator?Explain the magnetic circuit of an induction machine.Explain how the harmonic torques can be reduced in an induction machine.

(8x5=40 marks)

PART-B

II. a. 1. What is armature reaction in DC machines? Explain the effects of armaturereaction in the performance of the machine and show how it can beminimized? (8 marks)

11. The armature ofa 12 pole, 500 KW, 550 V generator has a complex lapwinding consisting of 2484 conductors. There are 621 commutator segments.The ratio of pole arc to pole pitch is 0.7.i. Calculate the demagnetizing mmf per pole at rated full load current if the

brushes are shifted through three segments from the geometric neutralaxis. What is the cross magnetizing mmf per pole. (5 marks)

11. Calculate the number ofconductors that must be provided in each poleface if a compensating is used for the machine. (2 marks)

ORb. A 150 KW, 230 V, 500 rpm de shunt motor has a square field coil. Find its main

dimensions, number ofpoles and air gap length. Assume the average gap densityover the pole are as 0.85 W6/rr?" , and the ampere conductors per meter as 29000.The ratio of width of pole body to pole pitch is 0.7. The efficiency is 91%.Assume that the mmfrequired for air gap is 55% of armature mmf and the gapcontraction factor is 1.15. (15 marks)

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III. a. i. Derive the output equation for a 3-phase transformer. (5 marks)ii. Elaborate on the different cooling methods employed for various types of

transformers. (10 marks)OR

b. Calculate the approximate overall dimensions for a 200 KVA, 6600/440 V,50 Hz, 3 phase core type transformer. The following data may be assumed:emf per tum = 10 V; maximum flux density = 1.3 W6/m'2.; current density = 2.5Almm'2.; window space factor = 0.3; overall height = overall width; stackingfactor = 0.9. Use a 3 stepped core. For a 3 stepped core, width oflargeststamping = 0.9d and net iron area = 0.6 d2.where d is the diameter of thecircumscribing circle. (15 marks)

IV. a. i. Explain the various methods employed for the elimination ofhannonies in asynchronous machine. (5 marks)

ii. Determine the main dimensions ofa 75000 KVA, 13.8 KV. 50 Hz, 62.5 rpm,3 phase star connected alternator. Also find the number of stator slots,conductors per slot, conductor area and work out the winding details. Theperipheral speed should be 40mls . Assume average gap density = 0.65 Wb/m2.ampere conductors per metre = 40000 and current density = 4A1mm .Assume any missing data. (10 marks)

ORb. 1. Explain the use of damper windings in synchronous machines. (5 marks)

ii. A 1250 KVA. 3 phase 6500 V, salient pole alternator has the following data:air gap diameter = 1.6m; length of core = 0.45 m; number ofpoles = 20;armature ampere conductors per metre = 28000; ratio ofpole arc to pole pitchis 0.68; stator slot pitch =28 mm; current density in damper bars =3A1mm2 .

Design a suitable damper winding for the machine. (10 marks)

V. a. Determine the main dimensions, turns per phase , number of slots, conductorcross section and slot area ofa 250 hp, 3 phase , 50 Hz, 400 V, 1410 rpm clipring induction motor. Assume Bav. = 0.5W6/m ,ac = 30000 Aim,efficiency = 0.9 and power factor = 0.9, winding factor = 0.955 , currentdensity = 3.5A1mm'-. The slot space factor is 0.4 and the ratio of core length topole pitch is 1.2. The machine is delta connected. (15 marks)

ORb. i. Write down the procedure for the design ofa wound rotor. (5 marks)

ii. A 90 KW, 500 V, 50 Hz, 3 phase 8 pole induction motor has a starconnected stator winding accommodated in 63 slots and 6 conductors perslot. If the slip ring voltage on open circuit is to be about 400 V, findsuitable rotor winding, stating:a). number of slots , b). number of conductors per slot, c). coil span,d). slip ring voltage on open circuit, e). approximate full load current perphase in rotor. Assume efficiency = 0.9, power factor = 0.86. (10 marks)

(4xI5=60 marks)

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Vidya Digital Library - sudhinpk · PDF fileAnswerall questions. I. (a) ... 2 the no. of conductors that should be placed in each pole face to ... Given full load efficiency as 0.83,

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