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b) What do you mean by voltage regulation and efficiency

of 1 phase transformers? Determine the expression forvoltage regulation and efficiency in terms of equivalent

circuit parameters of 1 phase transformers

c) Explain construction and working of auto transformers

A 100 kVA transformer has tis maximum efficiency of0.98 at full load with unity power factor. During the day

its loaded as follows-

For 12 hours- 20kW at 0.5 pf lagFor 6 hours- 45kW at 0.9 pf lag

For 6 hours- 80kW at 0.8 pf lagDetermine all day efficiency

5. Attempt any twoquestions 2*10=20

a) Write short notes on any twoi.

Three phase to six phase conversion

ii. Scott connection

iii. Excitation phenomenon in transformers

a) Two transformers having equal voltage ratio areoperated in parallel. Obtain expressions for the

maximum possible kVA loading of the two

transformers in parallel. State assumptions made if any

b) A two sinlge phase transformers rated 1000kVA and500 kVA have per unit leakage impedance of

(0.02+j0.06) and (0.025+j0.08) respectively. What is

the largest kVA load that can be delivered by parallelcombination of these two transformers without

overloading any one

B. Tech(SEM III) Third Sessional Test 2014-15

EMEC-I

TIME: 3 Hr. Total Marks: 100

Note: Attempt all questions

1. Attempt any fourquestions 5*4=20

a)

What d you mean by Energy and Co-Energy?What is the importance of these terms in electrical

machine?

b) Explain singly excited systems. Also mention its

examples.

c) Explain principle of electro-mechanical energy

conversion.

d)

Write short notes on following:i. Energy stred in magnetic field

ii. Generated emf in machines

e) Explain doubly excited systems. Enlist the

practical examples of doubly excited systems.

f) A magnetic circuit has a coil with 1000 turns. It

reluctance is given as- R=[8.5 +40 g]*10^3 MKS

(Following Paper ID and Roll No. to be filled in your Answer Book)

Pa er ID : Roll No.

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units, where g = air gap length in mm, betweenfixed and moveable parts. For a coil current of 2.0

amp held constant and with slow movement,calculate change in field energy stored, if length

of air gap changes from 0.20 to 0.15 cm. calculatethe mechanical force experienced by system.

2. Attempt any fourquestions 4*5=20

a) Derive emf and torque equation for DC machine. Also

derive relation between them

b) The terminal voltage of a 8 pole dc shunt generator with780 wave connected armature conductors and running

at 500 rpm at terminal voltage 240 V.the armatureresistance is 0.24 and the field resistance is 240.Find the armature current, the induce emf and flux per

pole if load resistance is 12

c) What do you mean by poor comutation? Discuss thereason for poor commutation

d) Why terminal voltage vs load currrent characteristic of

a dc shunt generator is more drooping than that of

separately excited generator

e) What do you understand by armature reaction in dcmachine? How it can be minimised?

f) A 440 V 4 pole 25 kW dc generator has a waveconnected armature winding with 486 conductors. The

mean flux density in the air gap under the interpoles is

0.5 Wb/m2on full load and radial gap length is 0.3 cms.

Calculate the number of turns required on each

interpole. Assume armature current equals line current

3. Attempt any twoquestions 2*10=20

a) Explain any 2 of the following:

i. Hopkinsons test on dc machineii. Swinburns test on dc machine

iii. Three point starters

b) A 250 V dc shunt motor has an armature resistance of

0.5 and a field resistance of 250. When driving aconstant torque load at 600 rpm the motor draws 21 A.

what will be the new speed of motor if an additional250 ohm resistance is inserted in the field circuit.

c) A 200 V shunt motor has Ra=0.1 ohm Rf=240 ohm androtational losses 236 W. on full load the line current is

9.8 A with motor running at 1450 rpm. determine:

i. Mechanical power developedii. Power output

iii. Load torqueiv. Full load efficiency

4. Attempt any twoquestions 2*10=20

a) Draw and explain the phasor diagram of 1 phasetransformer on the following-

i. Unity power factor load

ii. Lagging power factor loadiii. Leading power factor load

Also mention limitations and significance

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.