UNIT - I PRINCIPLES OF DYNAMICS (9)

MODEL QUESTION PAPER-OCT/NOV 2014

Regulations: Reg.No:

BANNARI AMMAN INSTITUTE OF TECHNOLOGY

(An Autonomous Institution Affiliated to Anna University)

SATHYAMANGALAM 638 401

END SEMESTER THEORY EXAMINATIONS - OCT/NOV 2014

III Semester

ELECTRICAL MACHINE DESIGN

11E501

DEGREE & BRANCH : B.E.(EEE)

Time : 30 Min. Maximum : 20 MarksPART - A (20 x 1 = 20 Marks)Answer All Questions (Instructions to the Candidate : Write the appropriate answer in Page No.3 of your Answer book)

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

A)10 MW

B)20 MW

C)25 MW

D)50 MW.

2.The length cf air gap to be provided in ______ is primarily determined by power factor

consideration.

A)d.c. series motor

B)d.c. shunt motor

C)induction motor

D)synchronous motor

3.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)Represents the maximum value of B

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

A)coil-sides

B)turns

C)coils

D)slots.

5.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.

6.In d.c. machines the usual limit of slot pitch is

A)Between 5 to 10 mm

B)Between 10 to 15 mm

C)Between 15 to 20 mm

D)Between 25 to 35 mm

7.In a d.c. machine, the value of pheripheral speed should not, normally, exceed

A)10 m/s

B)20 m/s

C)30 m/s

D)40 m/s

8.If a DC motor stops after running for some time, the probable cause could be any of the following EXCEPT:

A)Weak or no field

B)Brushes ahead of neutral

C)Insufficient power supply

D)Insufficient torque developed.

9.The maximum permissible water content in transformer oil is

A)zero

B)10 ppm

C)25 ppm

D)50 ppm.

10.An air core transformer as compared to iron-core transformer has

A)Less magnetic core loss

B)More magnetic core loss

C)No magnetic core loss

D)Less ohmic loss.

11.Oil for transformer cooling should have high

A)flash point

B)viscosity

C)sludging tendencies

D)moisture content.

12.Tap changing facility is generally provided on

A)high voltage transformers

B)distribution transformers

C)current transformers

D)step up transformers.

13.For avoiding cogging in induction motor the difference between the number of stator and rotor slots should not be

A)P

B)3P

C)5 P

D)8P

14.The magnetizing current of an induction motor will be __________ for __________ air gap

A)Larger, smaller

B)larger, larger

C)Smaller, larger

D)Smaller, smaller.

15.When D is the diameter and L is the length of rotor of an induction motor, the best power factor can be obtained when (P = no. of poles)

A)D=L

B)D = PF

C)D = (PL)

D)D = 1.35 P L.

16.In induction motor if the number of rotor slots is equal to the stator slots then

A)the motor will run smoothly

B)the motor will mil at zero slip

C)the noise level will be low

D)the motor may refuse to start.

17.Turbo alternators are characterized by

A)short diameters and great axial lengths

B)short diameters and narrow axial lengths

C)large diameters and narrow axial lengths

D)large diameters and great axial lengths.

18.A synchronous machine having large length of the air gap will have

A)higher noise level

B)smaller unbalanced magnetic pull

C)lower limit of stability

D)higher value of inherent regulation.

19.In a synchronous generator in order to eliminate the fifth harmonic the chording angle should be

A)9

B)18

C)27

D)36.

20.The use of salient poles on high speed alternators will cause

A)excessive windage loss and excessive noise

B)reduced windage loss and low noise

C)reduced windage loss but excessive noise

D)excessive windage loss but reduced noise.

Time : 2 Hrs. 30 Min. Maximum : 80 MarksPART - B (10 x 2 = 20 Marks)Answer All Questions

21.Distinguish between specific electric loading and specific magnetic loading.

22.In what way the air gap length influence the design of machines?

23.List the factors that influence the separation of D and L of a dc machine.

24.State the relationship between number of armature coils and number of commutator segments in D.C. machine.

25.Define window space factor.

26.Mention clearly the condition for maximum efficiency for a transformer

27.Give any three advantages and disadvantages of large air gap length in induction motor.

28.Why does induction motor designed with high specific electric loadings have smaller over load capacity?

29.Write down the main consideration in the selection of specific loadings for the design of synchronous motor.

30.Differentiate critical speed from run-away speed.

PART - C (5 x 12 = 60 Marks)31.a)(i) Discuss the three methods of calculating the mmf of tapered teeth.

(ii) Compare and contrast direct and indirect cooling of turbo alternator.( 6 )( 6 )

(OR)

31.b)(i) Determine the air-gap length of a dc machine from the following particulars: gross-length of core = 0.12m, number of ducts = one and is 10 mm wide, slot pitch = 25 mm, slot width = 10 mm, carter's coefficient for slots and ducts = 0.32, gap density at pole center = 0.7 Wb/m2; field mmf/pole = 3900 AT, mmf required for iron parts of magnetic circuit = 800AT,

(ii) Calculate the apparent flux density at a section of the teeth of an armature of a dc machine from the following data at that section : slot pitch= 24 mm, slot width = tooth width = 12 mm, length of armature core including 5 ducts of 10 mm each = 0.38 m, iron stacking factor = 0.92. True flux density in teeth at that section is 2.2 Wb/m2 for which the mmf is 70000 AT/m.

( 8 )(4)

32.a)Explain the various steps involved in the design of the brushes and commutator for dc machine.(12 )

(OR)

32.b)Determine the diameter and length of armature core for a 55 KW, 110V, 1000 rpm, 4 pole shunt generator, assuming specific electric and magnetic loadings of 26000 amp.cond./m and 05 Wb/m2 respectively. The pole arc should be about 70% of pole pitch and length of core about 1.1 times the pole arc. Allow 10 ampere for the field current and assume a voltage drop of 4 volts for the armature circuit. Specify the winding used and also determine suitable values of the number of armature conductors and number of slots.(12 )

33.a)(i) Derive the output equation for 3 phase transformers.

(ii) Estimate the main dimensions including winding conductor area of a 3=phase, delta-star core type transformer rated at 300 KVA, 6600/440 V, 50 Hz. A suitable core with 3-steps having a circumscribing circle of 0.25 m diameter and a leg spacing of 0.4 m is available. Emf/turn = 8.5V, ( = 2.5 A/mm2 Kw = 0.28, Sf = 0.9 (stacking factor). (6)(6)

(OR)

33.b)The tank of 1250 KVA, natural oil cooled transformer has the dimensions length, width and height as 1.55 x 0.65 x 1.85 m respectively. The full load loss = 13.1 KW, loss dissipation due to radiations = 6 W/m2-0C, loss dissipation due to convection = 6.5 W/m2-0C, Improvement in convection due to provision to tubes = 40 %, Temperature rise = 400C, Length of each tube = 1 m, Diameter of tube = 50 mm. Find the number of tubes for this transformer. Neglect the top and bottom surface of the tank as regards the cooling.

(12 )

34.a)(i) Determine the D and L of a 70 Hp, 415V, 3-phase, 5-Hz, star connected, 6 pole induction motor for which ac = 30000 amp.cond/m and Bav = 0.51 wb/m2. Take ( = 90 % and pf = 0.91. Assume ( = L. Estimate the number of stator conductors required for a winding in which the conductors are connected in 2-parallel paths. Choose a suitable number of conductors/ slots, so that the slot loading does not exceed 750 amp. cond.

(ii) Give a detailed procedure for the design of rotor bars and end rings of a squirrel cage induction motor.

(6 )(6)

(OR)

34.b)A 90 kW, 500 V, 50 Hz, three phase, 8 pole induction motor has a star connected stator winding accommodated in 63 slots with 6 conductors per slot. The slip ring voltage on open circuit should be 400 V approximately. Design a suitable rotor winding and state (i) Number of slots in rotor (ii) Number of conductors per slot (iii) Coil span (iv) Slip ring voltage on open circuit (v) Full load current per phase in rotor. Assume efficiency of 90% and power factor of 0.86.

(12 )

35.a)(i) Explain the choice of specific magnetic and electric loadings of synchronous machines

(ii) Determine the output coefficient for a 1500 KVA, 2200 volt, 3-phase, 10-pole, 50 Hz, star connected alternator with sinusoidal flux density distribution. The winding has 600 phase spread and full pitch coils. ac = 30000 amp.cond/m, Bav = 0.6 Wb/m2. If the peripheral speed of the rotor must not exceed 100 m/sec and the ratio pole pitch to core length is to be between 0.6 and 1, find D and L. Assume an air gap length of 6 mm. Find also the approximate number of stator conductors.

(6)(6)

(OR)

35.b)Determine the main dimensions for a 1000 KVA, 50 Hz, 3-phase, 375 rpm, alternator. The average air gap flux density is 0.55 Wb/m2 and the ampere conductors per metre are 28000. Use rectangular poles and assume a suitable value for ration of core length to pole pitch in order that bolted on pole construction is used for which the maximum permissible peripheral speed is 50 m/s. the run-away speed is 1.8 times the synchronous speed.

(12)

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