eksamenvraestel examination question paper module … · the following information for a 120v ... =...

Departement Elektriese, Elektroniese en Departement Elektriese, Elektroniese en Departement Elektriese, Elektroniese en Departement Elektriese, Elektroniese en RekenaarRekenaarRekenaarRekenaar----IngenieursweseIngenieursweseIngenieursweseIngenieurswese

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Module ELX 311Module ELX 311Module ELX 311Module ELX 311 27 Mei 2009

Department of Electrical, Electronic and Department of Electrical, Electronic and Department of Electrical, Electronic and Department of Electrical, Electronic and Computer EngineeringComputer EngineeringComputer EngineeringComputer Engineering

Examination Question PaperExamination Question PaperExamination Question PaperExamination Question Paper

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Module ELX 311Module ELX 311Module ELX 311Module ELX 311 27 May 2009

Student se besonderhede:Student se besonderhede:Student se besonderhede:Student se besonderhede: Student's details:Student's details:Student's details:Student's details:

Naam Name):

Studentenommer: Student number:

Tel. nr. gedurende eksamen: Tel no. during examination:

Tel. nr. na eksamen: Tel. no. after examination:

Eksameninligting:Eksameninligting:Eksameninligting:Eksameninligting: Examination iExamination iExamination iExamination information:nformation:nformation:nformation:

Maksimum punte: Maximum marks: 90

Volpunte: Full marks: 85

Duur van vraestel: Duration of paper:

3 ure 3 hours

Oopboek / toeboek: Open / closed book:

Toe Closed

Totale aantal bladsye (hierdie blad ingesluit):

Total numer of pages (including this page):

BELANGRIKBELANGRIKBELANGRIKBELANGRIK- IMPORTANTIMPORTANTIMPORTANTIMPORTANT 1. Die eksamenregulasies van die Universiteit van Pretoria geld.

The examination regulations of the University of Pretoria apply. 2. Hierdie vraestel mag slegs in Afrikaans of Engels beantwoord word.

This question paper may only be answered in Afrikaans or English. 3. ALLE berekening moet getoon word.

ALL calculations must be shown.

1. Mr. W. Badenhorst Interne Eksaminator(e): Internal examiner(s): 2

Eksterne Eksaminator: External Examiner:

Prof. MN Gitau

Q1Q1Q1Q1 Q2Q2Q2Q2 Q3Q3Q3Q3 Q4Q4Q4Q4 Q5Q5Q5Q5 QQQQ6666 Q7Q7Q7Q7 Q8Q8Q8Q8 Q9Q9Q9Q9 Q10 Q10 Q10 Q10 BBBB TotalTotalTotalTotal 12 6 8 4 8 6 4 10 5 11 16 90

ELX 311 Exam 2009 2

SECTION A / AFDELING ASECTION A / AFDELING ASECTION A / AFDELING ASECTION A / AFDELING A

QUESTION 1QUESTION 1QUESTION 1QUESTION 1 / VRAAG 1 [12] / VRAAG 1 [12] / VRAAG 1 [12] / VRAAG 1 [12] The relative permeability of the ferromagnetic material of Fig. Q1 is 1200. Neglect magnetic leakage and assume 5% increase in the effective air gap area due to fringing. All dimensions are in centimeters and the magnetic material has a square cross-sectional area.

Die relatiewe permeabiliteit van die ferromag-netiese material van Fig. Q1 is 1200. Ignoreer magnetiese lekkasie en neem aan dat die effektiewe luggapingoppervlak met 5% toeneem weens ranteffekte. Alle dimensies is in centimeter en die magnetiesemateriaal het ‘n vierkantige deursnit oppervlakte.

Fig. Q1: Magnetic circuit taken from Principles of Electric Machines and PFig. Q1: Magnetic circuit taken from Principles of Electric Machines and PFig. Q1: Magnetic circuit taken from Principles of Electric Machines and PFig. Q1: Magnetic circuit taken from Principles of Electric Machines and Power Electronics, 2ower Electronics, 2ower Electronics, 2ower Electronics, 2ndndndnd edition, p12, PC edition, p12, PC edition, p12, PC edition, p12, PC Sen.Sen.Sen.Sen.

Q1.1: First draw the equivalent magnetic circuit diagram and then calculate the different reluctances for the circuit. (7)

V1.1: Teken eers die ekwivalente magnetiese stroombaandiagram en bereken dan die verskeie reluktansies vir die stroombaan. (7)

Diagram / Stroombaan:

ELX 311 Exam 2009 3

Q1.1 cont… V1.1 verv…

Q1.2: Calculate the flux density inside the air gap. Hint: Use source transformation or superposition to solve the circuit. (5)

V1.2: Bereken die vloeddigtheid in die luggaping. Wenk: Gebruik brontransformasie of superposisie om die stroombaan op te los. (5)

ELX 311 Exam 2009 4

Question 2Question 2Question 2Question 2 / Vraag 2 [6] / Vraag 2 [6] / Vraag 2 [6] / Vraag 2 [6]

The electromagnet lift system shown in Fig. Q2 has 2500 turns on an ideal core material. The air gap is at g = 10mm and the flux density is 1.25 T. Ignore fringing effects.

Die elektromagnetiese hysstelsel in Fig. Q2 het 2500 draaie op ‘n ideale kern material. Die luggaping is by g = 10mm en die vloeddigtheid is 1.25T. Ignoreer rant-effekte.

Fig. Q2: Magnetic circuit taken from Principles of Electric Machines and Power Electronics, 2Fig. Q2: Magnetic circuit taken from Principles of Electric Machines and Power Electronics, 2Fig. Q2: Magnetic circuit taken from Principles of Electric Machines and Power Electronics, 2Fig. Q2: Magnetic circuit taken from Principles of Electric Machines and Power Electronics, 2ndndndnd edition, edition, edition, edition,

p117, PC Senp117, PC Senp117, PC Senp117, PC Sen

Q2.1: Determine the coil current. (4) V2.1: Bepaal die windingstroom. (4)

ELX 311 Exam 2009 5

Q2.2: Determine the energy stored in the magnetic system. (1)

V2.2: Bepaal die energie wat in die magnetise stelsel gestoor is. (1)

Q2.3: For a current of 20 A, what is the maximum load mass in kg the system will be able to pick up? (1)

V2.3” Vir ‘n stroom van 20 A, wat is die maksimum las massa in kg wat die stelsel sal kan optel? (1)

ELX 311 Exam 2009 6


The linear DC machine shown in Fig. Q3 has a magnetic flux density of 0.5 T directed into the page, a resistance of 0.25 Ω, a bar length of 1.0m and a battery voltage of 100V.

Die lineêre GS masjine in Fig. Q3 het ‘n magnetise vloeddigtheid van 0.5T in die bladsy in, ‘n weerstand van 0.25 Ω, ‘n staaf lengte van 1.0m en ‘n batteryspanning van 100V.

Fig. Q3: AFig. Q3: AFig. Q3: AFig. Q3: A linear dc machine. The magnetic field points into the page. linear dc machine. The magnetic field points into the page. linear dc machine. The magnetic field points into the page. linear dc machine. The magnetic field points into the page. Q3.1: What is the initial force magnitude and direction on the bar at starting? (2)

V3.1: Wat is die aanvanklike krag grootte en rigting op die staaf? (2)

Q3.2: What is the no-load stead-state speed of the bar? (1)

V3.2: Wat is die nul-las bestendige toestand spoed van die staaf? (1)

ELX 311 Exam 2009 7

Q3.3: If the bar is loaded with a force of 25N opposite to the direction of motion, what are the new steady state speed and the efficiency of the machine under these conditions? (5)

V3.3: Indien die staaf belas word met ‘n krag van 25N teenoorgesteld aan die rigting van beweging, was sat die nuwe bestendige toestand spoed en die effektiwiteit van die masjien onder hierdie omstandighede wees? (5)

ELX 311 Exam 2009 8


A square magnetic core having 750 windings has a mean length of 2.25m, a cross sectional area of 200 cm2 and the magnetizing curve of Fig. Q4.

‘n Vierkantige magnetiesekern met 750 windings het ‘n gemiddelde lengthe van 2.25m, ‘n deursnit oppervlakte 200 cm

2 en die magnetiseringskurwe in

Fig. Q4.

Fig. Q4: Magnetizing curve for question 4.Fig. Q4: Magnetizing curve for question 4.Fig. Q4: Magnetizing curve for question 4.Fig. Q4: Magnetizing curve for question 4. Q4.1: Calculate the flux in mWb that will result in the core from a winding current of 1.5 A. (3)

V4.1: Bereken die vloed in mWb wat in die kern sal ontstaan in geval van ‘n 1.5 A windingstroom. (3)

Q4.2: What is the relative permeability of the core at this operating point? (1)

V4.2: Wat is die relatiewe permeabiliteit van die kern by hierdie werkspunt? (1)

ELX 311 Exam 2009 9

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ELX 311 Exam 2009 10

Question 5 / Vraag 5 [8]Question 5 / Vraag 5 [8]Question 5 / Vraag 5 [8]Question 5 / Vraag 5 [8]

A three phase Y-Y 50Hz 60 kVA V1/V2 = 11kVrms /400 Vrms transformer has the given per phase equivalent-circuit parameters at rated load having a lagging power factor of 0.85. Use the given equivalent transformer model to answer the questions below.

‘n Driefase 50Hz 60kVA V1/V2 = 11kVrms /400 Vrms transformator het die gegewe per fase ekwivalente stroombaan parameters by vollas met ‘n nalopende drywingsfaktor van 0.85. Gebruik die gegewe ekwivalente model om die onderstaande vrae te beantwoord.

R1

= 20Ω, R2 = 0.04 Ω, X1 = 210 Ω, X2 = 0.2 Ω, Rc = 250 kΩ, Xm = 40 kΩ

Q5.1:Q5.1:Q5.1:Q5.1: What are the values of 1

V and 1

I (2) V5.1V5.1V5.1V5.1 Wat is die waardes van 1

V en 1

I (2)

Q5.2Q5.2Q5.2Q5.2: Calculate the source voltage required to achieve rated load voltage. (2)

V5.2V5.2V5.2V5.2:::: Bereken die bronspanning benodig om aanslag lasspanning te verseker. (2)

V1 V2 = Vload Vsource

+

+

+

I1 I2

ELX 311 Exam 2009 12

Question 6 / Vraag 6 [6]Question 6 / Vraag 6 [6]Question 6 / Vraag 6 [6]Question 6 / Vraag 6 [6] The following information is given about the simple rotating loop in Fig. Q6.

Die volgende inligting word gegee met betrekking tot die roterende lus in Fig. Q6.

B = 0.8 T VB = 24V l = 0.5m R = 0.4 Ω

r = 0.125m ω = 275 rad/s

Q6.1: Is the machine operating as a motor or as a generator? Motivate. (2)

V6.1: Funksioneer die masjien as ‘n motor of as ‘n generator? Motiveer. (2)

Q6.2: How much energy in kWh does the machine provide or consume over a 1 hour period? (2)

V6.2: Hoeveel energy in kWh verskaf of verbruik die masjien oor ‘n 1 uur periode? (2)

ELX 311 Exam 2009 13

Q6.3: A simplex 2-pole, eight coil lap wound DC machine has a flux density of 1.0 T in the air gap, a coil radius of 0.1m, a coil side length of 0.4m and is rotating at 1800 rpm. What is the magnitude of the voltage between the brushes? (2)

V6.3 ‘n Simpleks 2-pool, agt winding “lap wound” GS motor het ‘n vloeddigtheid van 1.0 T in die luggaping, ‘n winding radius van 0.1 m, ‘n winding sy lengte van 0.4m en roteer teen 1800 rpm. Wat is die grootte van die spanning tussen die borsels? (2)

ELX 311 Exam 2009 14


The following information for a 120V 25 kW DC generator is provided below:

Die volgende inligting met betrekking tot ‘n 120V 25 kW GS Generator word hier onder gegee:

Pole(s) = 8 Duplex Lap Wound 64 Coils 16 turns per coil rated speed 2400rpm

Q7.1: How much flux per pole is required to produce rated voltage under no-load conditions? (2)

V7.1: Hoeveel vloed word per pool benodig om aanslag spanning op te wek onder nul-las toestande? (2)

Q7.2: What is the current per path in the armature of this generator at rated load? (1)

V7.2: Wat is die stroom per pad in die anker van die generator by aanslag las? (1)

Q7.3: What is the induced torque in this machine at rated load? (1)

V7.3: Wat is die geinduseerde wringkrag in die masjien by aanslag las? (1)

ELX 311 Exam 2009 15

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ELX 311 Exam 2009 16


The following information is given for a 120V series dc motor at full load conditions along with the magnetizing curve in Fig. Q8.

Die volgende inligting word gegee vir ‘n 120V GS motor onder vollas toestand tesame met die magnetiseringskurwe in Fig. Q8.

Armature Resistance 0.2 Ohm Anker-weerstand

Series Field Resistance 0.16 Ohm Serie Veldweerstand

Input current 58 A Inset stroom

Speed 1050 rpm Spoed

Core losses (constant) 200 W Kernverliese (konstant)

Mechanical losses, proportional to the n3.

240 W Meganieseverliese, direk eweredig aan n

3.

Stray losses - Diverse verliese

Fig. Q8: Magnetizing curve for the series motor in question 8.Fig. Q8: Magnetizing curve for the series motor in question 8.Fig. Q8: Magnetizing curve for the series motor in question 8.Fig. Q8: Magnetizing curve for the series motor in question 8.

Q8.1: What is the efficiency of the motor? (2) V8.1: Wat is die effektiwiteit van die motor? (2)

ELX 311 Exam 2009 17

Q8.2: What will the speed of the motor be if operated at an armature current of 35 A? (5)

V8.2: Wat sal die spoed van die motor wees indien die ankerstroom 35 A is? (5)

Q8.3: Assuming the new speed to be 1330 rpm at an armature current of 35 A, what will the efficiency of the motor be? (3)

V8.3: Aangeneem dat die nuwe spoed 1330 rpm is met ‘n ankerstroom van 35A, wat sal die effektiwiteit van die motor wees? (3)

ELX 311 Exam 2009 18

Question 9Question 9Question 9Question 9 / Vraag 9 [5] / Vraag 9 [5] / Vraag 9 [5] / Vraag 9 [5] This question refers to the following shunt connected dc motor having the magnetizing curve in Fig. Q9 and operating under rated full load conditions.

Die vraag verwys na die volgende parallel gekoppelde GS motor met die magnetiseringskurwe in Fig. Q9 wat onder aanslag vollas toestand werk.

Fig. Q9: Magnetizing curve for Question 9Fig. Q9: Magnetizing curve for Question 9Fig. Q9: Magnetizing curve for Question 9Fig. Q9: Magnetizing curve for Question 9

test

11.9 kW

175 Ω

ELX 311 Exam 2009 19

Q9.1: Calculate the field current IF. (1) V9.1: Bereken die veld stroom IF. (1)

Q9.2: Calculate the armature current IA. (1) V9.2: Bereken die ankerstroom stroom IA. (1)

Q9.3: Calculate the internal generated voltage EA. (1)

V9.3: Bereken interne gegenereerde spanning EA. (1)

Q9.4: Calculate the full load speed of the machine. (2)

V9.4: Bereken die vollas spoed van die masjien (2)

ELX 311 Exam 2009 20


A 440-V, 50 Hz, two-pole Y-connected induction motor is rated at 75 kW operating at a slip of 0.04 resulting in a line current IA of A°−∠ 3.184.149 . The equivalent circuit parameters are provided below.

‘n 440 V, 50 Hz, twee-pool Y-gekoppelde induksie motor het ‘n aanslag drywing van 75 kW en werk by ‘n glip van 0.04 wat ‘n lynstroom van IA =

A°−∠ 3.184.149 tot gevolg het. Die ekwivalente

stroombaan parameters word hieronder gegee.

Q10.1: Calculate the stator and rotor power factors. (2)

V10.1: Bereken die rotor en stator drywingsfaktore. (2)

Q10.2: Calculate the stator copper losses PSCL. (1)

V10.2: Bereken die stator koperverliese PSCL. (1)

Q10.3: Calculate the air gap power PAG. (2)

V10.3: Bereken die luggaping drywing PAG. (2)

ELX 311 Exam 2009 21

Q10.4: Given the PAG is 103 kW, calculate the power converted from electrical to mechanical form Pconv and the induced torque. (1)

V10.4: Gegewe dat PAG 103 kW is, bereken die drywing wat vanaf elektriese na meganiese energie omgeskakel is Pconv en die geinduseerde wringkrag. (1)

Q10.5: Calculate output power Pout, the rotational speed nm and hence finally the output torque. (4)

V10.5: Bereken die uitset drywing Pout, die rotasie spoed nm en dan die uitset wringkrag. (4)

Q10.6: Finally calculate the efficiency of the motor. (1)

V10.6: Bereken laastens die effektiwiteit van die motor. (1)

ELX 311 Exam 2009 22

SECTION B [15] SECTION B [15] SECTION B [15] SECTION B [15] / AFDELING B [15] / AFDELING B [15] / AFDELING B [15] / AFDELING B [15]

Question B1 [3]Question B1 [3]Question B1 [3]Question B1 [3] Even if a transformer’s secondary side is open circuited, there is always a current flowing consisting of two components.

- The first is the core-loss current. Which losses do this current represent?

- What are the second current component and its function?

- What is the total combined current called?

Vraag B1 [3]Vraag B1 [3]Vraag B1 [3]Vraag B1 [3] Selfs al is ‘n transformator se sekondêre kant ‘n opebaan, is daar altyd ‘n stroom wat vloei wat bestaan uit twee komponente.

- Die eerste is die kern-verlies stroom. Watter verliese word deur die stroom verteenwoordig?

- Wat is die tweede component en die se funksie?

- Wat word die totale gekombineerde stroom genoem?

B1.1: The core loss current represents the following losses: (1) B1.1: Die kernverliese stroom verteenwoordig die volgende verliese: (1) ___ ______________________ & _______________________

B1.2: The second component and its function are: (1) Die tweede component en die se funksie is: (1)

B1.3: The total combined current is called: (1) The totale gekombineerde stroom word genoem: (1)

ELX 311 Exam 2009 23

Question B2 [2]Question B2 [2]Question B2 [2]Question B2 [2]

Refer to the machine winding shown in Fig. B2. How many parallel paths are there through the rotor and how wide should the brushes be? Motivate. [2]

Vraag B2 [2]Vraag B2 [2]Vraag B2 [2]Vraag B2 [2] Verwys na die masjienwinding in Fig. B2. Hoeveel parallel paaie is daar deur die rotor en hoe wyd moet die borsels wees? Motiveer. [2]

Question B3 [2]Question B3 [2]Question B3 [2]Question B3 [2] Name the two primary problems caused by Armature Reaction and briefly mention the end result of both these problems inside the machine

Vraag B3 [2]Vraag B3 [2]Vraag B3 [2]Vraag B3 [2] Noem die twee primêre probleme wat deur Ankerreaksie veroorsaak word en noem kortliks die eind resultaat van beide die probleme in die masjien.

ELX 311 Exam 2009 24

Question B4 [6]Question B4 [6]Question B4 [6]Question B4 [6] Fill in the missing words in the following sentences describing how torque is developed in a 400V, 50 Hz, 4 pole 3-phase squirrel cage induction machine operating at a slip of 5%. No calculations need to be shown.

When a 3-phase voltage is applied to the __________________ of the

induction machine it sets up a _________________ rotating at (value) _______

rpm. This induces a ___________________ at a frequency of

(value)__________ Hz that flows in the ____________________ of the

machine, which in turn causes a rotating ____________________ on the

rotor rotating at (value)__________ rpm. The _____________________ on the

____________________ is attracted by the ____________________ on the

____________________ thus resulting in the induced torque causing the

rotor to rotate at (value)____________ rpm.

Vraag B4 [6]Vraag B4 [6]Vraag B4 [6]Vraag B4 [6] Vul die ontbrekende woorde en waardes in die volgende sinne in wat beskryf hoe die wringkrag in ‘n 400V. 50Hz, 4 pool 3-fase ‘squirrel cage’ induksie masjien ontwikkel word wat werk by ‘n glip van 5%. Geen berekeninge hoef getoon te word nie.

Wanneer ‘n 3-fase spanning gekoppel word aan die ______________ van die

induksiemasjien skep dit ‘n _____________________ wat roteer teen

(waarde)____________ rpm. Dit induseer ‘n ___________________ teen ‘n

frekwensie van (waarde)_________ Hz wat in die _______________________

van die masjien vloei wat op sy beurt ‘n roterende

_______________________ skep op die rotor wat roteer teen

(waarde)_______________ rpm. Die _______________________ op die

______________________ word dan aangetrek deur die

___________________________ op die _______________________ wat

der halwe wringkrag teweeg bring wat veroorsaak dat die rotor roteer teen

(waarde)____________ rpm.

ELX 311 Exam 2009 25

Question B5 [3]Question B5 [3]Question B5 [3]Question B5 [3] Draw the basic torque vs speed curve for an induction machine and indicate the following thereon: Starting and pullout torque as well as the normal operating region.

Vraag B5 [3]Vraag B5 [3]Vraag B5 [3]Vraag B5 [3] Skets die basiese wringkrag teen spoed kurwe vir ‘n induksie masjien en toon die volgende daarop aan: Begin en uittrek wringkrag asook die normale werksgebied.

eksamenvraestel examination question paper module … · the following information for a 120v ... =...

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