'fED (10) - 4036

(REVTSTON - 2010)

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FOURTH SEMESTER DIPI,OMA HXAMINAI'ION IN EI,ECTRIC,A.I,AND ELECTRONICS BNGINEERING - OCTOBER. 2016

POLYPHASE CIRCUITS AND I'RANSFORMERS

fTime: 3 hows

(Maximum mark : 100)

PAK| - A

Maximum marks: l0)

Marks

I Answer the following questions in one or two sentences. Each question carries

2 marks.

l. Write the relation between KW, KVA and KVAR.

Z. Write the equation for power factor from wattmeter readings in two wattmeter

method of measuring three phase power-

3. Give the components of no load culrent in an ideal transformer.

4. List the pararneters obtained from the short circuit test of a transformer.

5. Give any two leasons of providing tertiary winding in transformers' (5x2=10)

PAKI''BMaximum marks: 30)

II Answer any five of the following questions' Each question carries 6 marks'

1. Describe the method of 3 phase voltage generation and draw the wave form

of three phase suPPlY.

Show that a balanced delta connected load having equal impedance z < o

may be replaced by an equivalent star connected load of 3z < a per phase'

Obtain the condition for maximum efficiency of a transformer'

Derive the EMF equation of a transformer'

with necessary connection diagam explain the procedure of conducting SC

and OC test of a single phase transformer'

A5KVAtransformerhascorelossof35Wandcopperlossof40Watfullload.ItoperateonratedKVAat0.8powert.actorlaggingfbr6hours,halfratedKVAat0.5powerfactorlaggingforl2hoursandonnoloadfbrtheremaining of the day. Calculate the all day efficiency'

7. I-ist the special purpose transformers and their use'

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(5x6=30)

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Marks

PART - C

(Maximum marks: 60)

(Answeronefu||questionfromeachunit.Eachfullquestioncarries15marks.)

' Uxtr-l

III (a) Describe the methods of interconnection of phases in three phase system and

obtain tire iine and phase values. In Y connection with the help of vector8

diagpm.

(b) A delta connected balanced 3 phase load is supplied from a three phase 400V

supply. The line current is 20A and the power taken by the load is 10000w'

Find (i) the impedance in each branch. ()) fne line cunent, power consumed

arrdpowerfactorifthesameloadisconnectedinstar.T

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(a) Show that the sum of watffneter readings in trvo wattnetter method of meastring

power in three phase system (balanced load) gives three phase power with the

help of vector diagran.

(b) State the advantages of tluee phase system over single phase system'

Uxrr - II

(a) A 40KVA 3300/240V 50Hz single phase transformer has 600 tums on primary.

Determine (i) thc number of tums in secondary (ii) ma,rimum value of flrx in core.

(iii) the approximate value of primary and secondary full load current'

Intemal <irops in windings are to be ignored'

(b) Explain why the net flux in the core of a transformer remains constant

irrespective of load condition.

On

(a) A 30KVA 24001120Y,50H2 transformer has H.V winding resistance of- 0.lf)and leakage reactance of 0.22Q. The ll/ winding resistance is 0.035Q and

leakage reactance of 0.0120. Find the equivalent winding resistance, reactance

and impedance referred to (i) HV side (ii) LI/ side.

(b) Describe the different losses in a transformer and the lactors affecting these

losses.

Usrr - III

(a) Obtain the equivalent circuit of a 2001400V 50H2, single phase transformer

from the following test data.

OC test : 200V 0.7A, 70W on LV side.

SC test : i5V l0A. 85W on I-{V side.

Also calculate the secondary terminal voltage when delivering 5KW at 0.8

lagging power fbctor.

(b) Give a brief description about the 3 phase transformer connection.

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(a) 'fhe following test results were obtained flom a 20KVA. 2200/220y, 50LIz,single phase transfonner.

OC test :220y,1.1A, 125W, on iV si,Je.

SC test : 52.7y,8.4A,287'N, on HV side. If the transformer is loadedat UPF with 220v on secondary calculate the marimum efficiency and theload at which this occurs.

(b) Describe with circuit diagram the procedure of direct load test on singlephase transformers to obtain the efficiency and regulation.

Urrr -- IV(a) Explain about any one type of on load tap changing arrangement in a

transformer with a neat diagram.

(b) Explain oil natural cooling method employed in transformer with necessarydiagram.

On

(a) Compute the saving of copper in autotransformer.

(b) Explain about the insnument transformers in common use.

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