Biju Patnaik University of Technology 2008-5th Sem B.Tech (B Tech)digital signal processing . - Question Paper

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BPUT(B Tech) fifth semester digital signal processing ques. paper.

Total number of printed pages B.Tech

CPEE 5301

Fifth Semester Examination - 20C^q

L

ELECTRICAL MACHINE - I Full Marks-70

Time -3 Hours

Answer Question No. 1 which is compulsory and any five from the rest

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The figures in the right-hand margin indicate marks.

Answe r the following questions : 2x10

(a) A dc. shunt motor draws an armature current of 12 A from a 220 V d₍c, supply while developing a gross torque ot 20 Nm in the armature running at 1200 RPM. What will be the induced back emf in the

armature ?

(to) Whfli wiffl be the frequency ol mducsd emi in the conductors of the armaiure vr-ndmg of a 6-pote d c general running m 750 RPVi *>

(c J Monnon with mason Ihe typo o* d c molof used tOf traction loads

(d) Whal ⁱs the disadvantage of a 3-poffU sianer as compared 10 a 4.-point starter used to siart dc shunt motors ¹⁷

(e) Explami in brief why iho compensating windings in d c machines am connoctjcW/l In series with armature

(J) What is Voltage regulaflion* m case o a transformer ? For which type o< load power factor will this regulation be zero ?

(g) The iron losses in a TiOO-kVA transformer are 1000 W, The full load copper Josses are 1.25 limes the iron losses. Determine the load in kVA to obtain maximum efficiency of the transformer.

fh) Explain why the cross section of the forte rt a core type transformer are stepped instead of being circular.

(1) Draw the per phase phasor diagram of a cylmdnca* fotor synchronous generator supplying unity power lactof load. Show Ihe mmf space phasors in the same dtagram

(j) Calculate the V/inding factor of a 3-phase balanced winding having a coil span of

9 slots with 54 stator slots and 6 poles.

(a) A 220 V d.c, shunt motor has an armature resistance of 0.4 ohm and field resistance of 220 ohms. The no-toad input line current of the motor from the 220 V d.c mains is 4,5 A while the no-load speed of the motor is 1500 RPM, The motor takes 22 A from the same 220 V d.c mains while driving the full load torque at 1470 RPM:. Find the induced back emf at no-

load and ft! MWosd What wtB bo the ratio of tfm fuiHoad ttxpuo to ncMoad I0*tjue developed by the machine⁹ 2*2

(b) Explain bnofty the commuiafon process in a d c nsaefiin* Describe to* Kho 'rterpofes' and the oomponsatrtg v.-wvung help in achieving bettor commuUiinx'

3 + 3

3 (a) Draw the field mm! and armature mmf waveforms Over one pate pt#> *n cgs o$ a d.c generator Show itiefein the FisuJtant waveform of \hoso I wo mmfs Explan 'cross'maousaton' and '00 -magnetisation effects of armature reaction. 2+1+2

(b> A 230 V d c shun! generator has armature circuit resistance (including brushes) of 0*4 ohm and field circuit resistance of 120 ohms If this machine 15 run as a motor by connecting it to a 230 V d.c, mains, find theiratio o,t speed as a generator to Ihe speed as a motor The line currenl in each case is 45 A, 5

⁴ tel Brake Tesr was performed m a d e shunt

mot Of and the following readings were obtained Ttie Hoad on one side of the brake band was 40 kg and on the other Side, irt was 5 5 kg The motor was taking a current of 60 A from 400 V d.c mains white running al 1470 RPM. The pulley diameter is 110¹ cm. 'Neglect the belt thickness and determine the torque, output and the efficiency of the motor. 5

(b) Derive I he expression for the torque developed m a d.c. motor. Draw and explain the speed-iorque characteristic separately in case ot shunt and series motors. 5

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5 (a) The maximum efficiency ol a 150 kVA_f

2200/220 V, 50 Hz, single phase transformer is 38,5% and occlirs at half-full load at unity power factor Determine its fulMoad efficiency at 0.8 leading power factor. ⁴

The prvTWiry of a tsryjv phase tv*) wncfcng iransformer is rated at 12 A and tQO V On opcn-circtiit, lh** readings Iraforred fo HV Sidej are V=1?00 V. V_; - 600 V, f₀ ?- 0 52 A. P₀ a ISO W Or hortctrcoi', the readings (on HV side) arc iL " 1? A

* 14 v and P. =- 450 W Draw an equivalent circuit for iho transformer and determine the parameters WHa: vifi\ bo the output voltage across a foad mped* ancei of 20 *j I S ohms ** +2

Dovotop Step by stop Ihe exact eqyrvakml circui! ot a single pha:><? iwo winding transformer **

(a)

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A single phase autotransformot is to deliver a power of 350 kW at unity power tactor at 350 volts with an input voltage Of 230 volts. Determine the kVA rating of each section of the autolransforrner winding,

lb)

3

What is Polarity Test in case of a single phase two winding transformer ? Explain clearly witlh the help oi ihe connection diagram. 3

(c)

fa) Deduce the expresses for output real and reactive powers from the equivalent orcuft (per phase) m case of a cylindrical rotor synchronous generator. 5

(bj A 150 IkVA* 3300 V, 50 Hz, star connected three phase alternator has an effective stator resistance 0.125 ohms per phase. During Open Circuit Test' ot the machine, for a *ie)d current of 8.5 A, the terminal voltage is measured to be 600 V line*to-line. While performing the Short Circuit Test' on the same machine,, the short circuit current is 120 A for the same field current ol 8.5 A. Find Ihe terminal voltage per phase when rated full load at 0.8 leading power factor is switched off from the generator without changing its excitation and speed. What is the load angle 'delta' in this case ? ⁵

8. (a) Explain the method of constructing the potier Triangle* from the Open Circuit "Characteristic', 'Zero Power Factor

CftarsGtofisJjc' and Short Grcurt Charac* teristc" of a cyfandncal totor synchronous geooraior What ts tho drflereoce between ihe potter reactance* and the actual leakage reactance' of ttxs machine ?

4+1

(b) Calculate the no iod term/nai voltage ot a 3-phase, 6-pota, star connected altar-nator running, at 1000 RPM having the following daia

Sinusoidal dtslnfouted flu v per poto * 60 mWb.