transfarmers and generators

SRI SAIRAM COLLEGE OF ENGINEERING

￨ら 6 : 7 C e 0 ￨
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USN 17EE33
Third Semester B.E. Degree Examination, Dec.20l8 lJan.20l9
Transformers and Generators
Time: 3 hrs. Max.Ⅳlarks:100
Note: Answer any FIVE full questions, choosing ONE full question from each module.
‐
Module… 1
Draw and explain the fuH load phasOr dhgrams of lφ transformers for hggingたading and
upfload. (o7卜 larks)
Ho、v an day efflciency is different fronl standard efliciency?Bricfly cxplaill. (o6卜 larks)
A5KVA 200/1000V,50Hz,lφ tranSfOrmcr gave theゎ 110Wing test results:
OC Test(LV side):2oOVj‐ 1.2A,90W
SC Tcst(HV sidC):50V,5A,HOW
り Caにuhto the paramctcrs ofthe cqllivJent clcuk rettrrcd to the Iフ V sdc.
il)CalCulate the output secondary vo“ agc
When delivcring1 3kヽV at O.8pf lagging, the input prirnary voltage being 200V. Find thc
percentage rcgulatiOn also. (o7卜 Iarks)
OR
Enumerate the advantages of 3$ transformers compare to I { transformers. (06 Nlarks)
Show that open Delta connection of 3Q transformers has KVA rating of 57.1% of that of
A-A connection. Show the connection diagram. (07 1larks)
Two l$ furnaces working at l00V are connected to 3300V,3$ nrains through Scott
connected transformers. Calculate the current in each line of the 3Q mains when the powcr
taken by each furnace is 400kw at a pf of 0.8 lagging Neglect lossess in the transtbfl].Jrr..ur,
Ⅳ
lodule-2
2 a.
b.
C.
3 a.
b.
C.
What ai0 the conditions to be satisfied for parallel operation of two transformers? trxplain
briefly. '' (06 ltarks)
Derive expression for load shared betrveen trvo transformers connected in parallel when
voltage ratios are different. (07 Nlarks)
Explain how stabilization is achieved due to the tertiary winding. (07 rrarks)
OR
a. With the help of sketches explain the working of on load tap changer. (07 1rarks)
b. Derive expression for saving of copper in auto transformer compared to tw'o u,inding
transformer. (07 Marks)
c. Two lQ transformers with equal turns have impedances of (0.5 + j3)f) and (0.6 + j l0)Q with
respect to the secondary. If they operate in parallel. determine how they will share a total
load of100kW at pfO.8 1agging.
l of2
(06 NIarks)

ａ
ｂ

ｃ
５
Module‐ 3 ‐
‐￨￨￨
Write a short note on Noise in transformers. ￨・ ‐
￨￨‐
Etthh the mcthods uscd to rcduce harmonた sh3●llternators.
briefly. ‐
・
・
￨・
OR■1
a.Der市c EMF cquation ofan 3φ ALernato■ ‐
‐
b. VVhat is an armature reaction? W‐ ith a‐ ncat diagranl, explain
nlachine under nornlal working c9■ dLiOns. ‐ ‐
What is commutation? What are the methods available for improving commutation? Explain
17EE33
(06 Ⅳ
larks)
(06 Plarks)
(08 Marks)
(06 Marks)
affnature reaction in DC
(08 Ⅳ
larks)
(06 PIarks)
vcctor diagranl.
(06 NIIarks)
synchronization
(08ルlarks)
(06 Plarks)
C.
7 a.
b.
C.
8 a.
b.
C.
Whh a vector dね gram,exphⅢ wlこh10nous rcactancc ofan● lernator.
I Plodule‐ 4
Deine vohage reguht● nっ Fan ahernator and cxplⅢ Ⅲ
‐
lS Sttnincance wlh a
What is synchroniζatiOn of ahernators?What ardlthc cond託 lons br propcr
ofan alternator?How 3φ altcrnatOrs arc synchronizcd?
Write a short notes on power angle characterStics ofan altcrnator.
OR
With a neat circuit diagram, explain slip test to determine direct axis reactance and
quadrature axis reactance of an salient pole synchronous Generator. (07 Marks)
Explain the behaviour of synchronous generator on no load under variable excitation
connected to infinite bus bar. (06 Marks)
With a phasor diagram, explain the concept of two reaction theory in a salient pole
alternator. (07 Nlarks)
9 a.
b.
Mod■le‐ 5
Enumerate the methods availableゃ r‐ detcrmining thc
cxplain lllinfrncthods in dctail. ■
A3.5 MVA.Y― conncctcd altcrllator ratcd at 4!60V
fbllowins data
W
Ii in amps 50 100 150 200 250 300 350 400 450
Vot(linc)in VOltS 1620 3150 4160 4750 5130 5370 5550 5650 5750
a field current of 2004- is found necessary to circulate Iplon SC of the alternator. Calculate
by i) EMF method ii) MMF method. The voltage regulation at full load 0.8 pf lagging.
Neglect resistance, comment on result obtained. (10 Marks)
vOltage regulation of an alternator,
・
(10 Plarks)
at 50Hz has the OCC given by the
(0`Ⅳlarks)
(07 PIarks)
(07ルlarks)
OR
l0 a. Write a note on capability curves of synchronous generator.
b. What is hunting in synchronous machines? Explain the role of damper winding.
c. Explain ZPF method of predetermination of regulation of alternator.
*****
2 of2

USN
3 a.
b.
4 a.
b.
5 a.
b.
C.
15EE33
(08 Marks)
tap changer for
(08 Marks)
a.
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Third Semester B.E. Degree Examination, Dec.2018 lJan.2D19
Transformers and Generators
Time: 3 hrs. Max. Marks: 80
Note: 1. Answer any FIVE full questions, choosing one full question from each module.
2. Assume Missing data any
Module-1
With the help of phasor diagram, explain the operation of practiealtransformer on load.
(08 Marks)
A 3-phase I000KVA, 660A11100V transformer is delta connected on primary and star
connected on secondary. The primary resistance/ph is l.8Q and secondary resistance/ph is
0.025C). Find the efficiency w-hen secondary is supplying full load at 0.8p.f and the iron loss
is l5kN. Also determine efficiency on full load unity p.f.
(08 Marks)
OR
Explain star zig - zag- star and open deha connection with the help of connection diagram
and phasor diagram. Mention its advantages applications. (10 Nlarks)
The parameters of l0KVA, 500/250V, 50Hz . single - phase transformer are as lollows :
Primary resistance : 0.2A Primary'reactance = 0.4O
Secondary resistance : 0.5O Secondary reactance 0.lO
Exciting circuit resistance and reactance are 1500f, and 750Q respectively. Find out results
(06 1Iarks)
of o'c and S'c test'
Module-2
What is the need for parallel operation oftransformer? Mention the conditions to be satisfled
for parallel operation and explain. (08 Marks)
An autotransformer is used to supply a resistive load of 5kW at 400V. Supply voltage is
440Y. Neglecting the losses calculate the currents in various parts of the winding. Find the
percentage of copper saving effected due to use of the autotransformer instead of equivalent
2 a.
b.
two winding transformer.
OR
With a neat diagram, explain the construction and operation of on load
transformer.
Two transformers gave the followings test results: with the LV side shorted, transformer A
takes current of 10A at 200V. power input is 1000W. Similarly transfbrmer B takes 30A at
200V; the power input is 1500W. On open circuit both transfbrmers gave a secondary
voltage of iZOOV when I IKV is applied to the primary terminals. These terminals of the
two tiansformers are connected in parallel. Calculate the load shared by each transformer.
(08 Marks)
Module-3
Explain the current in rush phenomenon in transformer. (04 Marks)
With neat diagram, explain the commutation process in DC machines' (06 Nlarks)
A 4- pole. 3 iphase. 50Hz star connected alternator has 60 slots. with 4 conductors per slot'
The coits are short pitched by 3 slots. If the phase spread is 60'. find the line voltage
induced for a flux pei pole orb.q+: wb, sinusoidally distributed in space' All the turns per
(06 )larks)
phase are ln serles.
I of 2

6 a.
b.
9 a.
b.
OR
With a neat diagranl, explain how sumpnesis test iS used to flnd efncicncy and voltage
regulation ofa transbrmer? .・￨.￨■‐
￨ (0`PIIarks)
A4-pole,lap wound armature running at 1400rpm del市 ers a current of 100A and has 64
conductor segments.The brush width is equalto l.4 collnFnutator segments and inductance
of cach armature coil is O.05mH.Calcilatё the value of the reaCtance voltage assuming
15EE33
(06Ⅳ〔
arks)
(04 Plarks)
(08 Marks)
(08 Marks)
(08ルIarks)
・ I Module… 4 1
7 a. Explain the effect ofvariatiOn19fcxcitation ofan altcrnator supplying constant load.
(08 Plarks)
b. A synchronous gcneratoF has a direct axis synchrOnOus rcactance of O.8pu and a quadrature
axis synchronous reactance of 05pu.It is supplyinglil1 1。 ad at ratcd vokage at O.8p.g lag.
linear commutation.
c. What are the methods used to reduce the harmonics in alternator?
Find the open circuit voltage.
OR
a. Explain two reaction theory as applied to synchronous machines. (08 Marks)
b. Two identical, three phase star connected alternators, operating in parallel share equally a
totalload of l000kW at 6600V and 0.8 power factor lagging. The freld of the first generator
is excited so that the armature current is 50A lagging. Find
i)Armature currcnt ofsecond machine
H)ThC powcr factor o,(譲ch machinc. ・・
・￨
Modile‐5・
Exphh MMF mcthod ofddermmhg voLI扉 regulatiDn ofanlaltcrnaton
A 3 - phase. t0KVA" 400V. 50Hz star connected alternator supplies the rated load at 0.8
power tactor lagging. If the armature resistance is 0.5Q, and synchronous reactance is l0Q,
find the voltage regulation. (08 Marks)
l0 a. With suitable graphs, explain tt',. capaci$lurr"u for an alternator. (08 Marks)
b. A 2300V. 50H2. 3 - phase star connected alternator has an effective armature resistance of
0.2O. A field current of 35A produces a current of l50A on short circuit and an open circuit
emf 780V (line). Calculate the voltage regulation at 0.8 p.g. lagging and 0.8 leading for the
lull load current of 25A. (08 Marks)
*****
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