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                                                                    B.E/B.TECH DEGREE EXAMINATION,APRIL/MAY 2011
                                                                                                            sixth semester

                                                                                        (electrical and electronics engineering)

                                                                                EE2355 DESIGN OF ELECTRICAL MACHINES

(Regulation 2008)                 
                                                                                                                                Time:3hours Max.marks:100

Answer All Questions

PART A (10*2=20)
1.What is specific electric loading?

2.How materials are classified according to their degree of magnetism?

3.Name any two methods to reduce armature reaction.

4.What is slot loading?

5.Give the relationship between emf per turn and kva rating in a transformer .

6.What are the factors affecting the choice of flux density of core in a transformer?

7.How crawling can be prevented by design in an induction motor?

8.Define dispersion coefficient of an induction motor?

9.What is runaway speed of synchronous machine?

10.Give the need for damper winding in a synchronous machine?

PART B (16*5=80)
11.(a).Discuss about various duties and ratings of rotating machines and give their respective temperature -time curves? (16)
(b).A field coil has a heat dissipation surface of 0.15m^2 and length of mean turn 1 dissipates loss of 150W ,the emissivity being 34W/m^2_*c.Estimate the final steady temperature rise of the coil and its time constant if the cross section of the coil is 100*50mm^2,specific heat of copper is 390J/kg*c.The space factor is 0.56.Copper weighs 8900kg/m^3. (16)

12.(a) (i)Explain the effects of choice of no.of poles in a DC machine on (1)Frequency of flux reversals (2)Weight of iron(3) Weight of copper and(4) length of commutator.
(ii) A 5kw,250 v,4 pole,1500 rpm DC shunt generator is designed to have a square pole face. The specific electric loadings are 0.42Wb/m^2 and 15000AC/m respectively.Find the main dimensions of the machine.Assume full load efficiency =0.87 and pole arc to pole pitch ratio is 0.66. (8)

(b)(i) Discuss various methods to determine the mmf required for teeth of an Electric machine.
(ii)Determine the apparent flux density in teeth of a DC machine if the real flux density in teeth is 2.15Wb/m^2,slot pitch is 28mm,slot width is 10mm,gross core length is 0.35m,no.of ventilating ducts is 4 each 10mm wide.Magnetizing force corresponding to flux density of 2.15Wb/m^2 is 55000AT/m and iron stacking factor is 0.9 (8)

13.(a)(i) Derive the output equation of a three phase transformer.(8)
(ii)The ratio of full load mmf in a 400kva,50Hz single phase core type transformer is 2.4*10^-6.Calculate the net iron area of the transformer if the maximum flux density in the core is 1.3Wb/m^2,current density in the core is 1.3 Wb/mm^2 and window space factor is 0.26.also calculate the lad mmf. (8)

(b) A 2500 kva,6600/400v three phase core type transformer has a total loss of 4800W at full load, The transformer tank is 1.25 m 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 each tube is 50mm and are spaced 75 mm from each other. the average height of tubes is 1.05 mm.Specific heat dissipation due to radiation due to radiation and convection is respectively 6 and 6.5Wb/m^2-*c.assume that convection is improved due to the provision of tubes. (16)

14. (a) Determine the main dimensions,number of radial ventilating ducts,number of stator slots and turns per phase of a 3.7 kw,three phase,400v4 pole,50Hz squirrel cage induction motor t be started by a star delta starter.Given that the average flux density in the air gap =0.45Wb/m^2;ampere conductors per metre of armature periphery=23000 full load efficiency =0.85,full load power factor=0.84 and Kw=0.9555.tale L/r=1.5. (16)

(b).(i)Discuss the factors to be considered in estimating the length of air gap of Induction motor. (8)
(ii)Discuss the step by step procedure to design the rotor of a squirrel cage Induction motor. (8)

15.(a) Define short circuit ratio.Explain how it is determined for an alternator.Also discuss its effects on the performance of alternator. (16)

(b)(i) Derive the output equation of an AC machine.
(ii)Determine the main dimensions of a 100kva,50Hz,3 phase 375rpm alternator. The average gap flux density is 0.55Wb/m^2 and ampere conductors per metre is 28000.Given that L/r must be between 1 to 5. The maximum permissible peripheral speed is 50m/sec.the run away speed is 1.8 times the synchronous speed. (16)

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