Electrical Technology (Power Systems) (English) June 2025 Question Paper.pdf

JUNE EXAMINATION JUNIE EKSAMEN GRADE/GRAAD 12 2025 ELECTRICAL TECHNOLOGY: ELEKTRIESE TEGNOLOGIE: POWER SYSTEMS KRAGSTELSELS TIME/TYD: 3 hours/uur MARKS/PUNTE: 200 15 pages + a 2-page formula sheet 15 bladsye + 'n 2-bladsy formuleblad Please turn over Blaai om asseblief

ELECTRICAL TECHNOLOGY: 2 POWER SYSTEMS GR12 0625 INSTRUCTIONS AND INFORMATION 1. This question paper consists of SIX questions. 2. Answer ALL the questions. 3. Sketches and diagrams must be large, neat and FULLY LABELLED. 4. Show ALL calculations and round-off answers correctly to TWO decimal places. 5. Number the answers correctly according to the numbering system used in this question paper. 6. A non-programmable calculator may be used. 7. Calculations must include: 7.1 Formulae and manipulations where needed 7.2 Correct replacement of values 7.3 Correct answer and relevant units where applicable 8. A formula sheet is attached at the end of this question paper. 9. Write neatly and legibly. Please turn over

ELEKTRIESE TEGNOLOGIE: 2 KRAGSTELSELS GR12 0625 INSTRUKSIES EN INLIGTING 1. Hierdie vraestel bestaan uit SES vrae. 2. Beantwoord AL die vrae. 3. Sketse en diagramme moet groot, netjies en VOLLEDIG BENOEM wees. 4. Toon ALLE berekeninge en rond antwoorde korrek tot TWEE desimale plekke af. 5. Nommer die antwoorde korrek volgens die nommering wat in hierdie vraestel gebruik is. 6. Jy mag ʼn nie-programmeerbare sakrekenaar gebruik. 7. Berekeninge moet die volgende insluit: 7.1 Formules en manipulasies waar nodig 7.2 Korrekte vervanging van waardes 7.3 Korrekte antwoord en relevante eenhede waar van toepassing 8. ʼn Formuleblad is aan die einde van hierdie vraestel aangeheg. 9. Skryf netjies en leesbaar. Blaai om asseblief

ELECTRICAL TECHNOLOGY: 3 POWER SYSTEMS GR12 0625 QUESTION 1: MULTIPLE-CHOICE QUESTIONS Various options are provided as possible answers to the following questions. Choose the answer and write only the letter (A – D) next to the question numbers (1.1 to 1.15) in the ANSWER BOOK, e.g. 1.16 A. 1.1 An attempt to numerically determine the probabilities of adverse events and potential losses if they occur is known as … A risk analysis. B quantitative risk analysis. C qualitative risk analysis. D dangerous practices. (1) 1.2 The opposition of a capacitor to the flow of alternating current in an RLC circuit is the … A inductive reactance. B impedance. C capacitive reactance. D inductance. (1) 1.3 When the frequency of an RLC circuit increases, the resistance will … A also increase. B decrease. C remain the same. D double its value. (1) 1.4 In an RLC parallel circuit, the current through a capacitor will … when the frequency increases. A remain the same B decrease C increase D be zero (1) 1.5 The correct method to connect a wattmeter is by connecting the voltage coil in … with the load. A parallel, and the current coil in series B series, and the current coil in series C parallel, and the current coil in parallel D series, and the current coil in parallel (1) Please turn over

ELEKTRIESE TEGNOLOGIE: 3 KRAGSTELSELS GR12 0625 VRAAG 1: MEERVOUDIGEKEUSE-VRAE Verskillende opsies word as moontlike antwoorde op die volgende vrae gegee. Kies die antwoord en skryf slegs die letter (A – D) langs die vraagnommers (1.1 tot 1.15) in die ANTWOORDBOEK neer, bv. 1.16 A. 1.1 ʼn Poging om die waarskynlikhede van nadelige gebeurtenisse en die potensiële verliese numeries te bepaal staan bekend as … A risiko-analise. B kwantitatiewe risiko-analise. C kwalitatiewe risiko-analise. D gevaarlike praktyke. (1) 1.2 Die opposisie van ʼn kapasitor teen die vloei van wisselstroom in ʼn RLC- stroombaan is die … A induktiewe reaktansie. B impedansie. C kapasitiewe reaktansie. D induktansie. (1) 1.3 Wanneer die frekwensie van ʼn RLC-stroombaan toeneem, sal die weerstand ... A ook toeneem. B verminder. C dieselfde bly. D se waarde verdubbel. (1) 1.4 In ʼn RLC-parallelkring sal die stroom deur ʼn kapasitor … wanneer die frekwensie toeneem. A dieselfde bly B afneem C toeneem D nul wees (1) 1.5 Die korrekte metode om ʼn wattmeter te verbind, is om die spanningspoel in … met die las te verbind. A parallel en die stroomspoel in serie B serie en die stroomspoel in serie C parallel en die stroomspoel in parallel D serie en die stroomspoel in parallel (1) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 4 POWER SYSTEMS GR12 0625 1.6 The active current in a three-phase system is: A I Cos θ B I Sin θ C I Tan θ D I (1) 1.7 Self-contained units controlled from main control centres are called … A switchgear. B distribution networks. C substations. D power stations. (1) 1.8 After adding power factor correcting capacitors in parallel with a load, the power factor increases from 0,6 to 0,9. With the load unchanged, the current drawn from the supply will … A increase. B decrease. C remain the same. D double. (1) 1.9 Air Natural (AN) is a cooling method used for … A dry transformers. B oil-immersed transformers. C relays. D contactors. (1) 1.10 The function of transformers in distribution networks is to … A increase the voltage to the required value. B change AC voltages to the required DC voltage. C reduce the voltage to the required value. D generate the required voltage. (1) 1.11 The construction of a three-phase shell-type transformer consists of a core with … A three limbs, with coils wound around all three limbs. B five limbs, with coils wound around the three middle limbs. C five limbs, with coils wound around all five limbs. D three limbs, with coils wound around the two outer limbs. (1) Please turn over

ELEKTRIESE TEGNOLOGIE: 4 KRAGSTELSELS GR12 0625 1.6 Die aktiewe stroom in ʼn driefase-stelsel is: A I Cosø B I Sinø C I Tanø D I (1) 1.7 Selfonderhoudende eenhede wat vanaf hoofbeheersentrums beheer word, word … genoem. A skakeltoerusting B verspreidingsnetwerke C substasies D kragstasies (1) 1.8 Nadat drywingsfaktor-verbeteringkapasitors in parallel met ʼn las gekoppel is, verhoog die drywingsfaktor van 0,6 tot 0,9. Indien die las dieselfde bly, sal die stroomvloei wat vanaf die toevoer getrek word ... A toeneem. B afneem. C dieselfde bly. D verdubbel. (1) 1.9 Natuurlike lug ("Air Natural – AN") is ʼn verkoelingsmetode wat vir … gebruik word. A droë transformators B onderolie transformators C relê D kontaktors (1) 1.10 Die funksie van transformators in verspreidingsnetwerke is om … A die spanning tot die vereiste waarde te verhoog. B die WS-spanning na die vereiste GS-spanning te verander. C die spanning tot die vereiste waarde te verminder. D die vereiste spanning op te wek. (1) 1.11 Die konstruksie van ʼn driefase-dop-tipe transformator bestaan uit ʼn kern met ... A drie bene, met spoele om al drie bene gedraai. B vyf bene, met spoele om die middelste drie bene gedraai. C vyf bene, met spoele om al vyf bene gedraai. D drie bene, met spoele om die twee buitenste bene gedraai. (1) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 5 POWER SYSTEMS GR12 0625 1.12 The purpose of a no-volt relay is to ... A allow a motor to automatically start after power is restored. B increase the voltage of a three-phase motor. C prevent a motor from automatically starting when power is restored. D monitor the current drawn by a motor. (1) 1.13 Power in an induction motor is transferred from the … A rotor to the stator through mutual induction. B stator to the rotor through slip rings. C stator to the rotor through self-induction. D rotor to the stator through self-induction. (1) 1.14 The moment an induction motor starts, the … is at maximum and decreases as the motor speed increases. A slip B rotor speed C full-load speed D momentum (1) 1.15 The stator windings of a three-phase induction motor may be connected … A in star only. B in star or delta. C in delta only. D None of the above-mentioned. (1) [15] QUESTION 2: OCCUPATIONAL HEALTH AND SAFETY 2.1 Name TWO general duties of employees at the workplace. (2) 2.2 Define health and safety equipment. (2) 2.3 State TWO characteristics or moral principles related to work ethics. (2) 2.4 Define the term danger with reference to the Occupational Health and Safety Act, 1993 (Act 85 of 1993). (2) 2.5 Explain the ideal location of the master switch in a workshop and its purpose. (2) [10] Please turn over

ELEKTRIESE TEGNOLOGIE: 5 KRAGSTELSELS GR12 0625 1.12 Die doel van ʼn nul-volt-relê is om ... A ʼn motor outomaties te laat begin nadat krag herstel is. B die spanning van ʼn driefase motor te verhoog. C te verhoed dat ʼn motor outomaties aanskakel wanneer krag herstel is. D die stroom wat deur ʼn motor getrek word te monitor. (1) 1.13 Drywing in ʼn induksiemotor word vanaf die … oorgedra. A rotor na die stator deur wedersydse induksie B stator na die rotor deur sleepringe C stator na die rotor deur selfinduksie D rotor na die stator deur selfinduksie (1) 1.14 Die oomblik wat ʼn induksiemotor aanskakel, is die … by maksimum en neem dit af soos wat die motorspoed toeneem. A glip B rotorspoed C vollasspoed D momentum (1) 1.15 Die statorwikkelings van ʼn driefase-induksiemotor kan ... gekoppel word. A slegs in ster B in ster of delta C slegs in delta D Geeneen van die bogenoemde nie. (1) [15] VRAAG 2: BEROEPSGESONDHEID EN VEILIGHEID 2.1 Noem TWEE algemene pligte van werknemers in die werkplek. (2) 2.2 Definieer gesondheids- en veiligheidstoerusting. (2) 2.3 Noem TWEE eienskappe of morele beginsels wat verband hou met werksetiek. (2) 2.4 Definieer die term gevaar met verwysing na die Wet op Beroepsgesondheid en - veiligheid, 1993 (Wet 85 van 1993). (2) 2.5 Verduidelik die ideale ligging van die hoofskakelaar in ʼn werkswinkel en die doel daarvan. (2) [10] Blaai om asseblief

ELECTRICAL TECHNOLOGY: 6 POWER SYSTEMS GR12 0625 QUESTION 3: RLC CIRCUITS 3.1 Name TWO factors that would influence the reactance of a capacitor. (2) 3.2 State the phase relationship between the current and voltage in a pure inductive AC circuit. (2) 3.3 Define the following with reference to RLC circuits connected across an alternating voltage supply: 3.3.1 Reactance (2) 3.3.2 Impedance (2) 3.4 Describe what the term in phase means with reference to the relationship between the applied voltage and current in an RLC circuit connected to an AC supply. (3) 3.5 A series circuit with a resistance of 1 000 Ω; inductance 200 mH and capacitance of 4 μF is connected across a 120 V / 60 Hz supply. Refer to FIGURE 3.5 below and answer the questions that follow. FIGURE 3.5: SERIES RLC CIRCUIT Given: R = 1 000 Ω L = 200 mH C = 4 μF V = 120 V f = 60 Hz Calculate the: 3.5.1 Reactance of the inductor (3) 3.5.2 Reactance of the capacitor (3) Please turn over

ELEKTRIESE TEGNOLOGIE: 6 KRAGSTELSELS GR12 0625 VRAAG 3: RLC-KRINGBANE 3.1 Noem TWEE faktore wat die reaktansie van ʼn kapasitor sal beïnvloed. (2) 3.2 Noem die fase verhouding tussen die stroom en spanning in ʼn suiwer induktiewe WS-stroombaan. (2) 3.3 Definieer die volgende met verwysing na RLC-stroombane wat oor ʼn wisselspanningstoevoer gekoppel is: 3.3.1 Reaktansie (2) 3.3.2 Impedansie (2) 3.4 Beskryf wat die term gelykfasig (in fase) beteken met verwysing na die verwantskap tussen die toegepaste spanning en stroom in ʼn RLC-kringbaan wat aan ʼn WS-toevoer gekoppel is. (3) 3.5 ʼn Seriestroombaan met weerstand van 1 000 Ω; induktansie van 200 mH en kapasitansie van 4 μF word oor ʼn 120 V / 60 Hz-toevoer gekoppel. Verwys na FIGUUR 3.5 hieronder en beantwoord die vrae wat volg. FIGUUR 3.5: RLC-SERIEKRINGBAAN Gegee: R = 1 000 Ω L = 200 mH C = 4 μF V = 120 V f = 60 Hz Bereken die: 3.5.1 Reaktansie van die induktor (3) 3.5.2 Reaktansie van die kapasitor (3) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 7 POWER SYSTEMS GR12 0625 3.5.3 Circuit impedance (3) 3.5.4 Circuit current (3) 3.6 Refer to FIGURE 3.6 below and answer the questions that follow. FIGURE 3.6: RLC PARALLEL CIRCUIT Given: I = 13 A I =4A X = 10 Ω V = 120 V f = 30 Hz 3.6.1 Calculate the current flowing through the capacitor. (3) 3.6.2 Calculate the total current flowing through the circuit. (3) 3.6.3 Calculate the power factor. (3) 3.6.4 State, with a reason, whether the current is leading or lagging the voltage. (2) Please turn over

ELEKTRIESE TEGNOLOGIE: 7 KRAGSTELSELS GR12 0625 3.5.3 Stroombaan impedansie (3) 3.5.4 Stroom in die stroombaan (3) 3.6 Verwys na FIGUUR 3.6 hieronder en beantwoord die vrae wat volg. FIGUUR 3.6: RLC-PARALLELKRING Gegee: I = 13 A I =4A X = 10 Ω V = 120 V f = 30 Hz 3.6.1 Bereken die stroom wat deur die kapasitor vloei. (3) 3.6.2 Bereken die totale stroom wat deur die stroombaan vloei. (3) 3.6.3 Bereken die drywingsfaktor. (3) 3.6.4 Noem, met ʼn rede, of die stroom die spanning voorloop of naloop. (2) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 8 POWER SYSTEMS GR12 0625 3.7 Refer to FIGURE 3.7 below and answer the questions that follow. FIGURE 3.7: RESONANT RLC CIRCUIT Given: R = 6 kΩ L = 22 mH X = 691,15 Ω V = 600 V f = 5 kHz 3.7.1 Calculate the quality factor of the circuit. (3) 3.7.2 Calculate the bandwidth. (3) 3.7.3 Calculate the value of the capacitor. (3) 3.7.4 Define the term selectivity with reference to resonant circuits. (2) [45] QUESTION 4: THREE-PHASE AC GENERATION 4.1 State TWO advantages of improved power factor correction to the consumer. (2) 4.2 State the difference between a wattmeter and a kilowatt-hour meter. (2) 4.3 State the size of the angles between the phases of a balanced three-phase AC generated waveform. (1) Please turn over

ELEKTRIESE TEGNOLOGIE: 8 KRAGSTELSELS GR12 0625 3.7 Verwys na FIGUUR 3.7 hieronder en beantwoord die vrae wat volg. FIGUUR 3.7: RESONANTE RLC-STROOMBAAN Gegee: R = 6 kΩ L = 22 mH X = 691,15 Ω V = 600 V f = 5 kHz 3.7.1 Bereken die kwaliteitsfaktor van die stroombaan. (3) 3.7.2 Bereken die bandwydte. (3) 3.7.3 Bereken die waarde van die kapasitor. (3) 3.7.4 Definieer die term selektiwiteit met verwysing na resonante stroombane. (2) [45] VRAAG 4: DRIEFASE-WS-OPWEKKING 4.1 Noem TWEE voordele van arbeidsfaktorverbetering vir die verbruiker. (2) 4.2 Noem die verskil tussen ʼn wattmeter en ʼn kilowatt-uurmeter. (2) 4.3 Noem die grootte van die hoeke tussen die fases van ʼn gebalanseerde driefase WS-gegenereerde golfvorm. (1) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 9 POWER SYSTEMS GR12 0625 4.4 Define the following terms: 4.4.1 Neutral (2) 4.4.2 Power factor (2) 4.5 Calculate the phase voltage in a star-connected three-phase system if the line voltage is 380 V. (3) 4.6 With reference to three-phase power generation: 4.6.1 Explain the transmission process with reference to voltages in the national grid. (3) 4.6.2 Explain the advantage of connecting a three-phase alternator in star. (2) 4.7 Explain how copper losses are reduced in overhead transmission. (2) 4.8 A 380 V three-phase system supplies a star-connected inductive load. The input power to the load is 10,53 kW with a lagging power factor of 0,8. Given: V = 380 V P = 10,53 kW Cos θ = 0,8 lagging Calculate the: 4.8.1 Phase voltage (3) 4.8.2 Line current to the load (3) 4.8.3 Phase angle (3) Please turn over

ELEKTRIESE TEGNOLOGIE: 9 KRAGSTELSELS GR12 0625 4.4 Definieer die volgende terme: 4.4.1 Neutraal (2) 4.4.2 Drywingsfaktor (2) 4.5 Bereken die fasespanning in ʼn stergekoppelde driefasestelsel as die lynspanning 380 V is. (3) 4.6 Met verwysing na driefase-kragopwekking: 4.6.1 Verduidelik die transmissieproses met verwysing na spanning in die nasionale netwerk. (3) 4.6.2 Verduidelik die voordeel daarvan om ʼn driefase-alternator in ster te koppel. (2) 4.7 Verduidelik hoe koperverliese in oorhoofse transmissie verminder word. (2) 4.8 ʼn 380 V driefasestelsel voorsien ʼn ster-gekoppelde induktiewe las. Die insetdrywing na die las is 10,53 kW met ʼn nalopende drywingsfaktor van 0,8. Gegee: V = 380 V P = 10,53 kW Cos θ = 0,8 nalopend Bereken die: 4.8.1 Fasespanning (3) 4.8.2 Lynstroom na die las (3) 4.8.3 Fasehoek (3) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 10 POWER SYSTEMS GR12 0625 4.9 Refer to FIGURE 4.9 below and answer the questions that follow. FIGURE 4.9: THREE-PHASE STAR CONNECTION 4.9.1 Name TWO quantities, other than power, that can be determined by using the two-wattmeter method. (2) 4.9.2 Calculate the total power of the load. (3) 4.9.3 Explain why the two-wattmeter method is preferred over the three-wattmeter method. (2) 4.10 Draw a phasor diagram of the system, showing the correct phase sequence of line voltages and the angle between them. (5) [40] QUESTION 5: THREE-PHASE TRANSFORMERS 5.1 Single phase transformers can be used to create a three-phase transformer unit. Answer the questions that follow. 5.1.1 List THREE characteristics of single-phase transformers that must be identical. (3) 5.1.2 Name the connection to the secondary side of a three-phase- transformer that will create a neutral point. (1) 5.2 Name THREE cooling methods for oil-immersed transformers. (3) 5.3 Name TWO factors that can contribute to excessive heating in transformers. (2) Please turn over

ELEKTRIESE TEGNOLOGIE: 10 KRAGSTELSELS GR12 0625 4.9 Verwys na FIGUUR 4.9 hieronder en beantwoord die vrae wat volg. FIGUUR 4.9: DRIEFASE-STERVERBINDING 4.9.1 Noem TWEE hoeveelhede, behalwe drywing, wat bepaal kan word deur die tweewattmeter-metode te gebruik. (2) 4.9.2 Bereken die totale drywing van die las. (3) 4.9.3 Verduidelik waarom die tweewattmeter-metode verkies word bo die driewattmeter metode. (2) 4.10 Teken ʼn fasordiagram van die stelsel wat die korrekte fasevolgorde van lynspannings en die hoek tussen hulle aandui. (5) [40] VRAAG 5: DRIEFASE-TRANSFORMATORS 5.1 Enkelfase-transformators kan gebruik word om ʼn driefase- transformatoreenheid te skep. Beantwoord die vrae wat volg. 5.1.1 Noem DRIE kenmerke van enkelfase-transformators wat identies moet wees. (3) 5.1.2 Noem die verbinding met die sekondêre kant van ʼn driefase- transformator wat ʼn neutrale punt sal skep. (1) 5.2 Noem DRIE verkoelingsmetodes vir oliegevulde-tipe transformators. (3) 5.3 Noem TWEE faktore wat tot oormatige verhitting in transformators kan bydra. (2) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 11 POWER SYSTEMS GR12 0625 5.4 Explain why transformers have better efficiency compared to other machines. (3) 5.5 A three-phase transformer with a turns ratio of 30 : 1 is connected in delta- star. Answer the questions that follow. 5.5.1 Determine whether the transformer is a step-down or step-up transformer. (1) 5.5.2 Describe why the transformer can be used for distributing electrical power to domestic and industrial loads. (3) 5.6 Explain the principle of mutual induction with reference to transformers. (3) 5.7 A 100 kVA three-phase delta-star transformer with a transformer ratio of 42 : 1 delivers power to an industrial load. The primary line voltage is 10 kV, and the load operates at a power factor of 0,9. Answer the questions that follow: Given: S = 100 kVA TR = 42 ∶ 1 VL = 10 kV pf = 0,9 Calculate the: 5.7.1 Primary line current (3) 5.7.2 Secondary phase voltage (3) Please turn over

ELEKTRIESE TEGNOLOGIE: 11 KRAGSTELSELS GR12 0625 5.4 Verduidelik waarom transformators beter rendement het in vergelyking met ander masjiene. (3) 5.5 ʼn Driefase-transformator met ʼn draaiverhouding van 30 : 1 is in delta-ster gekoppel. Beantwoord die vrae wat volg. 5.5.1 Bepaal of die transformator ʼn verlagings of ʼn verhogingstransformator is. (1) 5.5.2 Beskryf waarom die transformator gebruik kan word vir die verspreiding van elektriese krag na huishoudelike en industriële laste. (3) 5.6 Verduidelik die beginsel van wedersydse induksie met verwysing na transformators. (3) 5.7 ʼn 100 kVA driefase delta-ster-transformator met ʼn transformatorverhouding van 42: 1 lewer drywing aan ʼn industriële las. Die primêre lynspanning is 10 kV en die las werk teen ʼn drywingsfaktor van 0,9. Beantwoord die vrae wat volg: Gegee: S = 100 kVA TR = 42 ∶ 1 VL = 10 kV df = 0,9 Bereken die: 5.7.1 Primêre lynstroom (3) 5.7.2 Sekondêre fasespanning (3) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 12 POWER SYSTEMS GR12 0625 5.8 FIGURE 5.8 below shows a three-phase transformer. Answer the questions that follow. FIGURE 5.8: THREE-PHASE TRANSFORMER Given: V = 25 kV V = 8,66 kV I = 40 A P = 450 kW Calculate the: 5.8.1 Secondary phase voltage (3) 5.8.2 Transformer ratio (3) 5.8.3 Apparent power (3) 5.8.4 Efficiency if the copper losses are 1 kW and the core losses are 500 W (3) 5.8.5 Power factor (3) [40] QUESTION 6: THREE-PHASE MOTORS AND STARTERS 6.1 Name THREE mechanical inspections that could be carried out on the rotor and bearings of a motor. (3) 6.2 Name TWO applications of squirrel-cage induction motors where constant speed and torque is essential. (2) 6.3 Describe why it is necessary to have protective devices as part of motor control. (2) Please turn over

ELEKTRIESE TEGNOLOGIE: 12 KRAGSTELSELS GR12 0625 5.8 FIGUUR 5.8 hieronder toon ʼn driefase-transformator. Beantwoord die vrae wat volg. IL2 = 40 A FIGUUR 5.8: DRIEFASE-TRANSFORMATOR Gegee: V = 25 kV V = 8,66 kV I = 40 A P = 450 kW Bereken die: 5.8.1 Sekondêre fasespanning (3) 5.8.2 Transformatorverhouding (3) 5.8.3 Skyndrywing (3) 5.8.4 Rendement as die koperverliese 1 kW is en die kernverliese 500 W is (3) 5.8.5 Drywingsfaktor (3) [40] VRAAG 6: DRIEFASEMOTORS EN -AANSITTERS 6.1 Noem DRIE meganiese inspeksies wat op die rotor en laers van ʼn motor uitgevoer kan word. (3) 6.2 Noem TWEE toepassings van kourotor-induksiemotors waar konstante spoed en wringkraglewering noodsaaklik is. (2) 6.3 Beskryf waarom dit nodig is om beskermende toestelle as deel van motorbeheer te hê. (2) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 13 POWER SYSTEMS GR12 0625 6.4 Define the following terms with reference to three-phase motors. 6.4.1 Synchronous speed. (2) 6.4.2 Rotor speed. (2) 6.5 FIGURE 6.5 below shows the rotor of an induction motor. Answer the questions that follow. End bell Rotor Bearing Wing cover A Stator B End bell FIGURE 6.5: INDUCTION MOTOR 6.5.1 Name parts A and B. (2) 6.5.2 Identify where heat losses would be found in the motor. (1) 6.5.3 Provide ONE reason why the rotor bars are skewed. (1) 6.6 TABLE 6.6 below shows the nameplate of a three-phase induction motor. Answer the questions that follow. MOTOR MANUFACTURER SPECIFICATION Phase 3 Voltage 380 V Current 52 A Speed 1 500 r/min Power 30 kW Frequency 50 Hz Cos θ 0,82 Frame nr. 286T TABLE 6.6: NAMEPLATE OF A THREE-PHASE INDUCTION MOTOR Please turn over

ELEKTRIESE TEGNOLOGIE: 13 KRAGSTELSELS GR12 0625 6.4 Definieer die volgende terme met verwysing na driefasemotors: 6.4.1 Sinchrone spoed (2) 6.4.2 Rotorspoed (2) 6.5 FIGUUR 6.5 hieronder toon die samestelling van ʼn induksiemotor. Beantwoord die vrae wat volg. Entplaat Rotor Laer Aansluitkas A Stator B Entplaat FIGUUR 6.5: INDUKSIEMOTOR 6.5.1 Benoem onderdele A en B. (2) 6.5.2 Identifiseer waar hitteverliese in die motor gevind sal word. (1) 6.5.3 Verskaf EEN rede waarom die rotorstawe skuins is. (1) 6.6 TABEL 6.6 hieronder toon die naamplaatjie van ʼn driefase-induksiemotor. Beantwoord die vrae wat volg. MOTORVERVAARDIGER SPESIFIKASIES Fase 3 Spanning 380 V Stroom 52 A Spoed 1 500 o/min Krag 30 kW Frekwensie 50 Hz Cos θ 0,82 Raam nr. 286T TABEL 6.6: NAAMPLAATJIE VAN ʼn DRIEFASE-INDUKSIEMOTOR Blaai om asseblief

ELECTRICAL TECHNOLOGY: 14 POWER SYSTEMS GR12 0625 6.6.1 Is the motor suitable for use in South Africa? Give a reason for your answer. (2) 6.6.2 State what the 30 kW on the nameplate indicates. (1) 6.7 Describe the function of a star-delta starter. (2) 6.8 Explain the function of the zero-volt coil in a contactor. (2) 6.9 The rotor speed of an induction motor is 1 400 rpm. The synchronous speed is 1 500 rpm and the motor has 2 pole pairs. Given: N = 1 500 rpm N = 1 400 rpm p=2 Calculate the: 6.9.1 Frequency of the motor (3) 6.9.2 Percentage slip of the motor (3) 6.10 A delta connected three-phase motor with an input power of 12 kW draws 20,99 A when connected to the supply. The motor has a power factor of 0,8 and an efficiency of 90%. Given: P = 12 kW I = 20,99 A Cos ø = 0,8 n = 90% Calculate the: 6.10.1 Line voltage of the motor (3) 6.10.2 Output power of the motor (3) 6.10.3 Reactive power at the phase angle of 36,87° (3) Please turn over

ELEKTRIESE TEGNOLOGIE: 14 KRAGSTELSELS GR12 0625 6.6.1 Is die motor geskik vir gebruik in Suid-Afrika? Gee ʼn rede vir jou antwoord. (2) 6.6.2 Noem wat die 30 kW op die naamplaatjie aandui. (1) 6.7 Beskryf die funksie van ʼn ster-delta-aansitter. (2) 6.8 Verduidelik die funksie van ʼn nul-voltspoel in ʼn kontaktor. (2) 6.9 Die rotorspoed van ʼn induksiemotor is 1 400 opm. Die sinchrone spoed is 1 500 opm en die motor het 2 poolpare. Gegee: N = 1 500 opm N = 1 400 opm p=2 Bereken die: 6.9.1 Frekwensie van die motor (3) 6.9.2 Persentasie glip van die motor (3) 6.10 ʼn Delta-gekoppelde driefasemotor met ʼn insetdrywing van 12 kW en trek 20,99 A wanneer dit aan die toevoer gekoppel is. Die motor het ʼn drywingsfaktor van 0,8 en ʼn rendement van 90%. Gegee: P = 12 kW I = 20,99 A Cos ø = 0,8 n = 90% Bereken die: 6.10.1 Lynspanning van die motor (3) 6.10.2 Uitsetdrywing van die motor (3) 6.10.3 Reaktiewe drywing by die fasehoek van 36,87° (3) Blaai om asseblief

ELECTRICAL TECHNOLOGY: 15 POWER SYSTEMS GR12 0625 6.11 The circuit in FIGURE 6.11 below is used to control TWO three-phase motors. Answer the questions that follow. , L1 Stop MC1 button N/O2 O/L1 O/L2 N/C N/C B T A N/O MC1 T C N FIGURE 6.11: AUTOMATIC SEQUENCE STARTER CONTROL CIRCUIT 6.11.1 Label the symbols marked A, B and C. (3) 6.11.2 Identify the error in the circuit. (1) 6.11.3 Describe how this error affects the operation of the circuit. (4) 6.11.4 Explain the purpose of the timer contact in the circuit. (2) 6.11.5 Determine the setting of the overload in the circuit if the motor nameplate has a rated current of 6 A. (3) [50] TOTAL: 200 END

ELEKTRIESE TEGNOLOGIE: 15 KRAGSTELSELS GR12 0625 6.11 Die kring in FIGUUR 6.11 hieronder word gebruik om TWEE driefasemotors te beheer. Beantwoord die vrae wat volg. L1 Stop- HK1 knoppie N/O2 O/B1 O/B2 N/T N/T B T A N/O HK1 T C N FIGUUR 6.11: OUTOMATIESE SEKWENSIËLE AANSITTERBEHEERKRING 6.11.1 Benoem die simbole gemerk A, B en C. (3) 6.11.2 Identifiseer die fout in die kring. (1) 6.11.3 Beskryf hoe hierdie fout die werking van die kring beïnvloed. (4) 6.11.4 Verduidelik die doel van die tydreëlaarkontakte in die stroombaan. (2) 6.11.5 Bepaal die instelling van die oorlading in die stroombaan as die motornaamplaat ʼn nominale stroom van 6 A het. (3) [50] TOTAAL: 200 EINDE

ELECTRICAL TECHNOLOGY: 1 POWER SYSTEMS GR12 0625 FORMULA SHEET RLC CIRCUITS THREE-PHASE AC GENERATION 𝑃 = 𝑉 × 𝐼 × 𝐶𝑜𝑠Ɵ STAR 𝑋 = 2𝜋𝑓𝐿 𝑉 = √3𝑉 𝑋 = 𝑉 =𝐼 ×𝑍 𝑓 = OR 𝑓 = 𝐼 =𝐼 √ 𝐵𝑊 = OR 𝐵𝑊 = 𝑓 − 𝑓 DELTA SERIES 𝑉 =𝑉 𝑉 = 𝐼𝑅 𝑉 =𝐼 × 𝑍 𝑉 = 𝐼𝑋 𝐼 = √3 𝐼 𝑉 = 𝐼𝑋 𝐼 = OR 𝐼 =𝐼 =𝐼 =𝐼 POWER 𝑍= 𝑅 + (𝑋 − 𝑋 ) 𝑆 (𝑃 ) = √3 𝑉 𝐼 𝑉 = 𝑉 + (𝑉 − 𝑉 ) OR 𝑉 = 𝐼𝑍 𝑄 (𝑃 ) = √3 𝑉 𝐼 𝑆𝑖𝑛Ɵ 𝐶𝑜𝑠Ɵ = OR 𝐶𝑜𝑠Ɵ = 𝑃 = √3 𝑉 𝐼 𝐶𝑜𝑠Ɵ 𝑄= = = = = 𝐶𝑜𝑠Ɵ = PARALLEL EFFICIENCY 𝑉 =𝑉 =𝑉 =𝑉 𝜂= × 100 𝐼 = 𝐼 = TWO-WATTMETER METHOD 𝐼 = 𝑃 =𝑃 +𝑃 𝐼 = 𝐼 + (𝐼 − 𝐼 ) tanƟ=√3( ) 𝑍= 𝐶𝑜𝑠Ɵ = THREE-WATTMETER METHOD 𝑄= = 𝑃 =𝑃 +𝑃 +𝑃

ELEKTRIESE TEGNOLOGIE: 1 KRAGSTELSELS GR12 0625 FORMULEBLAD RLC-KRINGE DRIEFASE-WS-OPWEKKING P = V × I × CosƟ STER X = 2πfL V = √3V X = V = I ×Z f = OF f = I =I √ BW = OF BW = f − f DELTA SERIE V =V V = IR V =I × Z V = IX I = √3 I V = IX I = OF I =I =I =I DRYWING Z= R + (X − X ) S (P ) = √3 V I Q (P ) = √3 V I SinƟ V = V + (V − V ) OF V = IZ CosƟ = OF CosƟ = P = √3 V I CosƟ Q= = = = = CosƟ = PARALLEL RENDEMENT V =V =V =V η= × 100 I = I = TWEEWATTMETERMETODE I = P =P +P tanƟ=√3( ) I = I + (I − I ) Z= CosƟ = DRIEWATTMETERMETODE Q= = P =P +P +P

ELECTRICAL TECHNOLOGY: 2 POWER SYSTEMS GR12 0625 THREE-PHASE MOTORS AND THREE-PHASE TRANSFORMERS STARTERS STAR STAR 𝑉 = √3𝑉 𝑉 = √3𝑉 𝐼 =𝐼 𝐼 =𝐼 DELTA DELTA 𝑉 =𝑉 𝑉 =𝑉 𝐼 = √3𝐼 𝐼 = √3𝐼 POWER POWER 𝑆 (𝑃 ) = √3 𝑉 𝐼 𝑆 (𝑃 ) = √3 𝑉 𝐼 𝑄 (𝑃 ) = √3 𝑉 𝐼 𝑆𝑖𝑛Ɵ 𝑄 (𝑃 ) = √3 𝑉 𝐼 𝑆𝑖𝑛Ɵ 𝑃 = √3 𝑉 𝐼 𝐶𝑜𝑠Ɵ 𝑃 = √3 𝑉 𝐼 𝐶𝑜𝑠Ɵ 𝑃 = √3 𝑉 𝐼 𝐶𝑜𝑠Ɵ 𝜂 𝐶𝑜𝑠Ɵ = 𝐶𝑜𝑠Ɵ = ( ) ( ) = = ( ) ( ) 𝑇𝑅 = EFFICIENCY 𝜂= × 100 𝜂= × 100 𝜂= × 100 SPEED 𝑝= х 𝑛 = 𝑃𝑒𝑟 𝑈𝑛𝑖𝑡 𝑆𝑙𝑖𝑝 = % 𝑆𝑙𝑖𝑝 = х 100 𝑛 = 𝑛 (1 − %𝑠𝑙𝑖𝑝) 𝑆𝑙𝑖𝑝 = 𝑛 − 𝑛

ELEKTRIESE TEGNOLOGIE: 2 KRAGSTELSELS GR12 0625 DRIEFASETRANSFORMATORS DRIEFASEMOTORS EN -AANSITTERS STER STER V = √3V V = √3V I =I I =I DELTA DELTA V =V V =V I = √3I I = √3I DRYWING DRYWING S (P ) = √3 V I S (P ) = √3 V I Q (P ) = √3 V I SinƟ Q (P ) = √3 V I SinƟ P = √3 V I CosƟ P = √3 V I CosƟ P = √3 V I CosƟ η CosƟ = CosƟ = ( ) ( ) = = ( ) ( ) TR = RENDEMENT η= × 100 η= × 100 η= × 100 SPOED p= х n = Per Eenheid Glip = % Glip = х 100 n = n (1 − %glip) Glip = n − n