ELECTRICAL ENGINBBRING

Candiilotes slwuJd attempt questions 1 and 5 which arecompulsory andanythree oftheremaining questions selection oJ least one question selecting at leastone question/rom e(f£h Section.
SECTION A

1. Answer any THREE ofthe following
Prove that an 1deal transfonner shown below for which n can be identical with a perfectly coupled transformer
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". L.
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1f n ,JL,I where and are the self inductances of the pnmaty and secondaty
wmdings of the lransfonner and M is the mutual mduclance
Discuss about the current ratio Oblam the expressiOn for 11 and conditions that the perfect transfonner will be an 1deal transfonner
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A farmer has two sheds in his farm to keep his farm produce and belongings He keeps vegetables M as farm produce, a goal which can eat the vegetables and a dog
that can bile the goal m the ab senee of the farmer
The farmer along has to manage both sheds withoutloosmg either vegetables or the goat Suggest a lOgic CircUlt that can alarm him under any catastrophic condition by means of a buzzer
A 3-phase mductionmotor has an effiaency of 0.9 when the load lS 37 kW. At this load, the stator copper and rotor copper loss each equals the iron loss The mechanical losses are one­third 0f the no load 10 ss. Calculate the slip
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A two-tone modulating Signal 5 cos 211;.103 4 cos 4".103t modulates a camer voltage 10 cos 2".10'1. Find vanous frequency components present and corresponding

modulation mdices Also obtam the amplitude 0f the Signals presentm each Sideband and the bandWidths


Show that the multiplication of a smus01dal Slgnal by a Signal translates the whole frequency spectrum. Can we retranslate the sp ectrum to its onglnal p 0Sl tion? If yes how?


A separately excited d.c motor supplied Wlth rated voltage and rated field currentlS dnvmg a fan dehvenng rated torque. The load torque of the fan lS proportional to square of speed. The motor exhibited linear load characlenstics such that the speed dropped by 10% from no load


to full load torque at rated voltage and field current If the supply voltage lS halved at the
same field current develops an expreSSiOn for the new speed m tenns of no Load speed (at
rated voltage), full 10 ad torque and other constants
While the Thynstors lS used m Power Electroruc convertors ItlS bemg replaced by other devices recently. Mention any two of these devices and explam how they differ from the Thynstors. NametypiCal convertorsmwhichtheThynstors arebemgreplaced
Draw the truth table of an octal-to-bmal)' encoder with the condition that the output F I when at least one of the mputs lS a I The octalmput with highest number has the highest pnority What lS the condition of outputs and F output, If the Inputs

DeSign a combmational Clfcuit usmg AND gates and half adders that can multiply two 2-bit numb ers and b,boto pro duce 4-bit numb er S3, S" and So
Figure below shows a smgly excited el ectro mecharucal system With a stationary memb er and a rotating member The stational)' member lS an electromagnet excited by a coil of N turns canymg a current I wound on a ferromagnetic core of mfinite permeability. The rotating member lS an unwound ferromagnetic core of ,nfinite penneability causmg vanable
permeance at changmg rotor position 8 from a maxtmum permeance of at 8 0 to a mlrumum penneance ofP, at 90° Assunung this vanation ofpenneance to be smus01dal develop an expreSSiOn for the developed torque on the rotating member as a function of8
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In an electric traction d.c senes motor has been used with Single-phase ac overhead lines Gradually itlS bemg replaced by another motor along with suitable speed control mechanlSm Explam this new system compnsmg the motor and Its control from the same overhead supply system and compare Wlth the one usmg d.c. senes motor


Define a uniform plane wave, Show that a unlfonn plane wave progressmg m x-direction does not have the x-component of E. Explam what does this mean phYSically


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Obtam the voltage gam and output reSiSlance 0f the compo site ampl1fier shoWllln Fig ab ave
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A rectangular wave gU1de has cross-sectional area a 7 em and b 4 em Obtam all TE
modes that will propagate frequency less than 5 GHz How these modes are deSignated and whal are the corresp onding cul-o ff frequena es
SEcnON B
Answer any THREE ofthe followmg
How do you deleffillne the Laplace transform of a function fell? Are there any restrictions on fell for this? Slale them
Delermme the Laplace transfonns of
(il Signal m the fann 0 f a tri angular pulse and

impulse tram as shown below
The strength of each impulse 1S unity

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ltianBi. P!J••

Figure below shows a coil of N turns placed m a time vatymg flux The cod direction is such that for the direction of currenl! (as shown) m the flux direction 1S an mdicaled in the figure. Assume mfinile conductivity of the conductor of the coil Usmg relevant Maxwell's equation obtams an expressiOn for e. Indicate how this expreSSiOn will be m confonnity WIth Lenz' slaw


A 3-phase mduction motorlS drivmg a constant torque load. Ifone of the supply lines to the motor lS discOlUlected discuss the changes you would observe m supply current, speed and effiCIency


Differentiate between FDM and TDM plctonally Enumerate the advantages of one over the other

24-telephones, each band limited to 3.4 kHz, are to be time divlSlon multiplexed usmg PCM. Calculate the bandWidth of the PCM system for 128 quantization levels With 8 kHz sampling frequency


What,S the order of magnitude of veloCIty of an electromagnetic wave m a good conductor as compared to sound wave m air? Discuss


Calculate the depth of penetration of a MHz wave mto copper which has 5.8 x IO,u"m and penneability equal to that 0f free space



The Slmplifi ed circuit of a radio 0scillator lS shown m Fig. ab ove. Draw Its eqU1 valent CIrcuit
and 0btam the condition 0f 0sClllation. Obtain the frequency of oscillation and the value of h"
reqU1red for the 0sClllation m the case 0f Hartley 0sClllator

A transmitting antenna at a height ofh, above a smooth perfectly flat-earth surface sends a Slgnalto arecelvmg antenna of height with ground range between them equal to a Obtam the path difference between the direct and reflected Slgnals reaching the reCe1vmg antenna

Assume


WhatlS cut-setm lmear graph Explam with an example


In the graph





checklfthe sets of branches constitute cut-sets

Show that the output of a balanced modulatorusmg selmconductor diode is DSB-SC


Obtam relationship between the skip distance and maximum usable frequency over the flat earth surface


For the pole-zero configuration 0f an lmpedance function shown below draw the schemati c diagrams of the two Foster and two Cauer networks without detemumng element values


What more mformation do you need to detenmne the element values?

Measurements on a small pennanent magnet d.c. motor lndicate that it has an armature reSistance of7.5 ohm. With an applied armature voltage of 6 V. the motor is observed to achieve a no 10 ad sp eed to 10570 r1nun whil e drawmg an armature current of 13 mA


Calculate the motor torque constant


Find the no load rotational losses of the motor Find the stall current and torque of the motor at an appli ed armature voltage of 6 V




How are the Power Senuconductor deVices sp eClfied? What do these speclfications Slgru fy?