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BRISTOLFacul ty fEngineer ing

Date: Thursday f May 2001Time: 13.30 16.10Instructionso Candidates:

MODULE NT]MBERtiEE035H2MODULENAMEELECTRICAL TECHNOLOGY

. ExaminationypeLevel2 Frnal

AttemptFOURquestionsAll questionsarryEQUAL marks

QUESTTON(a) Define the meaningof power factor irrespectiveof the nature of voltage andcurrent'

[2 marks](b ) The majorityof industrialmotorsare nductionmotors,which havea reasonablyhighpower actor i.e 0.9or so)at full load.Explainwhy thesemachines aveavery ow power actorat ight oadsor no-load i.e. n orderof 0.1).

[3 marks](c) A threephase,50 Hz, 415 volts inductionmotor hasan efliciency of 90Yoand tdevelopsa power of 135 KW at a power factor of 0.75 lagging A bank ofcapacitors s connected n delta across he supply terminalsto raise its powerfactorto 0.95 agging.Eachof the capacitancenits s built of five similar83 voltscapacitors.(i ) Draw the system'sschematic iagram 12marlcl(ii) Demonstrate he concept of power factor correction with the power

trianglephasordiagram 13marksl(iii) Determinehe capacitancef eachcapacitor. [15 marks]

?--+ * l\ I\-, IIII\ .\r,.-.\) \,-

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ModuleName:ModuleNumber: ElectricalTechnologyUEEO35H2 Page2Examination ype:Final2000t01

QUESTTON(a ) Given he apparent oweras S : P + jQ:VI* and he mpedance f the oadasZ:R+jX, assume currentof I flows hrough he oad.Show he realandreactive ower n termsof voltageV. [3 marksl(b) A three-phaseenerator upplies ower o threebalanced,hree-phaseoadsvia a

three-phaseransmissionine which hasan impedance f 0 3 + j3 0 Q/phase Theloadsare connectedn parallel.The first load s absorbing total of 120KW and90 KVAr The secondoad s deltaconnected ndhasan mpedance f165-j39 O/phase. he third loadabsorbs 00KW at 0.6 power factor eadingThe ine to neutral oltageat the oadend s 240V ,(i)

(ii)(i i i)

Draw the hreephase nd he single ine diagrams f the abovesystem.[5 marks]

Calculatehe total currentdrawnby loads. [12marks]Calculatehe magnitude f the ine voltaseat the senerator nd.

[5 marks]

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ModuleName:ModuleNumber: ElectricalTechnologyUEEO35H2 Page3Examination ype:Final2000101

QUESTTON3(a) The ratingand esistancesf a dc generator regivenas:20 KW, 220voltsand1800 pm

Armature esistance:.1 )Seriesield resistance..05 )Shunt ield resistance:10 )Thegenerators connected sa longshuntcompound.(i ) Draw a schematic iagram or the generator onnection.(ii) Determinehe erminalcurrent.(iii) Calculatehe field and he armature urrent(iu) Compute he armature ack emf(v) Calculate he effective ield currentgiven he data or thecurveat a ratedspeed f 1800 pm n the following able.

[2marks][2 marks][4marks][4 marks]

magnetisation[5marks]

E" (V) 5 25 50 r00 1 5 0 200 215 225 240Ir (A) 0 0 0 . 1 0 2 5 0 5 0 75 1 0 1 2 5 1 5 2 .0(b) Assuming hegenerator elivers linecurrentof 5A onspeedof the generator t no-load no-load, alculatehe

[8 marksl

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ModuleName:ModuleNumber: ElectricalTechnologyUEEO35}f2 Page4Examination ype:Final2000/01

QUESTTON4(a) Figure Q4 shows a composite magnetic circuit consisting of three differentmagneticmaterialsof different permeabilities nd engthswith an air gap.Deducean expressionor the total reluctance f the circuit.

[5 marks](b) If permeabilities,engthsand crosssectionalareas or variouspartsof figure Q4aregivenas:

Ll rcast1".1 800 Lcassteel 0.33 m A, : 2 x l0-3 m2Frcast i ron:160 Lcar t i ron 0.33 m Ai :4 x 10-3 m2l. t rmumetal1000 I-rnumetal :0.20m A.:4 x 10-3 111'lf ,o: 4n.70-7 Lairsup _ 0.02 cm Atu 4 x l0-3 ^'and he cast ron is woundwith a coil of 500 urns,calculate:(i) The ampereurnsrequired o producea flux densityof 8Xl0-4Wb

(ii) Calculate he current n the coil.[12marksl[3 marks]

(c) In Figure Q4 assuminghe relativepermeabilities f the cast ron, cast steelandmumetal cores tends to infinity(+ co ) and the coil has N turns with totalresistance f R. If this coil is connectedo a sinusoidal ource v(t)], wherev(t) - VCos rrlt,show hat he averagemagneticenergystored n the coil is:W: YzL 12 [poNt &irg"pV2 I [4L^br"Bt]

[5 marks]

4q

5

\{'xcg" I FigureQ4

Ls

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ModuleName:ModuleNumber: ElectricalTechnologyUEEO35Tf2 Page5Examination ype:Final2000/0

QUESTTON(a)

(b)

(c )

Discussbriefly why the transformer's ating s given n KVA ratherthanKW.[3 marts]

A factory has wo identicalsingle-phaseransformersin terms of electricalparameters tc).Presently ne ransformer upplies ower o loads.Occasionally,the ratingof the loadexceedshe ratingof the transformer.With the aid of aschematic iagramdemonstrate ow the second ransformer ould be connectedto the existingsystem onfigurationo meet he demand.[3 marks]

A 4.8 KVA, 50 Hz, 2401480 olts, single-phaseransformerhas equivalentresistanceand reactance eferred to the low voltage side as 0.5Q and l.5C)respectively The secondary ide of the transformer uppliespowerto a load,at aratingof 3/aof the full loadcondition.(i) Draw the equivalent ircuitdiagramof the system [3marks](ii) Calculate resistanceand reactanceof the transformer referred to thesecondary. [3 marks](iii) Determine he load terminal voltage of the transformer or 0.8 laggingpower actor [8 marks](i") Compute he input power of the transformer, iven the core (iron) lossas100watts. [5 marks]

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Module Name:Module Number: ElectricalTechnologyUEEO35TT2 Page6Examination ype:Final2000101

QUESTTON6A threephasestar connectedoad of 2000 KVA, 0.8 power factor is suppliedat 6.6 KV,50 Hz by meansof a 33KV transmission ine, 20 Km long and a three phasestar-startransformerwith 5: ratio The resistance ndreactance er Km per phaseof each ine are0.4 Q and0.5 Q respectively. he transformer arameters er phaseare:Primary esistance:.5C)Primary eactance:3 .2Q Secondaryesistance:.35C)Secondaryeactance..65C)

(i ) Draw a one-lineand hreephase ircuit diagramof the network.

(ii) Calculate he loadcurrent.[4 marks][3 marksl

(iii) Determinehevoltage ecessaryt he sending ndof the ransmissionline when 6.6 KV is maintained t the oadend.(i") Calculatepower ossof the network.

[15marks][3 marks]