ELEKTRIESE TEGNOLOGIE

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1 NASONALE SENOR SERTFKAT GRAAD ELEKTRESE TEGNOLOGE ME / JUNE 205 PUNTE: 50 TYD: 2 hours HERDE RASTEL BESTAAN UT 9 BLABSYE.

2 2 NSTRUKSES: Beantwoord AL die vrae. Sketse en diagramme moet groot, netjies en ten volle benoem wees. Alle berekeninge moet getoon en korrek tot twee desimale plekke afgerond wees. Antwoorde moet duidelik genommer wees. n Formuleblad word verskaf aan die einde van hierdie vraestel. Nie-programmeerbare sakrekenaars mag gebruik

(2).5 dentifiseer die volgende veiligheidstekens..5..5.2 ().6 Noem EEN belangrike veiligheidsaspek wanneer n multimeter gebruik word om n weerstandslesing te neem. (2).

3 3 RAAG : BEROEPSELGHED EN GESONDHED, GEREEDSKAP EN MEETNSTRUMENTE.. erduidelik die doel van die Beroepsveiligheid en gesondheidswet. (2).2 Noem TWEE verantwoordelike aspekte wat jy as werkgewer moet volhou om veiligheid in die werkswinkel te verseker. (2).3 Noem twee voordele van goeie ergonomie in die werkswinkel. (2).4 Gee TWEE voorbeelde van onveilige kondisies in die werkswinkel. (2).5 dentifiseer die volgende veiligheidstekens ().6 Noem EEN belangrike veiligheidsaspek wanneer n multimeter gebruik word om n weerstandslesing te neem. (2).7 Noem TWEE toetse wat met n megger gedoen kan word. (2).8 Noem TWEE gebruike van n ossiloskoop. (2).9 Noem TWEE veiligheidsaspekte wat jy in ag moet neem wanneer jy met n bankslypwiel werk. (2).0 Noem TWEE belangrike veiligheidsaspekte wat jy moet nakom wanneer jy met n soldeerboud werk. (2) [20] RAAG 2: ENKELFASE WS OPWEKKNG en ENKELFASE TRANSFORMATORS. 2. Teken TWEE golfvorms om die verskil tussen WS en GS aan te dui. (2) 2.2 Noem die DRE faktore wat die grootte van die geïnduseerde emk in 'n enkele geleier wat deur 'n magneetveld beweeg beïnvloed. (3) ()

4 4 2.3 Definieer die term periode met verwysing na n sinusgolf. (2) 2.4 n Spoel met 400 draaie het n deursneeoppervlak van 30 cm² en roteer teen n spoed van 000 r/min in n magneetveld met n vloeddigtheid van 0.8 T. Bereken die volgende: 2.4. Die frekwensie van die opgewekte sein (3) Die EMK geinduseer (3) Die oombliklike spanning teen 30 o (3) 2.5 n Spoel met n nie-magnetiese kern het 300 windings en trek n stroom van 0.7 A Bereken die volgende: 2.5. Die magneto-motoriese krag.(mmk) (3) Die magnetiese veldsterkte indien die lengte van die kern 20 cm is. (3) 2.6 Bestudeer FGUUR 2. en bereken die volgende: 5 t FGUUR 2. ENKELFASE WSSELSTROOM OPWEKKNG 2.6. Die effektiewe waarde. (3) Die gemiddelde waarde. (3) 2.7 Bereken die frekwensie van n agt-pool generator indien dit teen n spoed van 750 r/min draai. (3) 2.8 Beskryf die werksbeginsel van n enkelfase transformator. (5) 2.9 Maak n netjies benoemde skets van n verlagings transformator. Toon ook aan hoe die inset en uitset golfvorms lyk. (6)

5 5 2.0 Beskryf TWEE tipes verliese wat in n transformator voorkom. (4) 2. n Enkelfase transformator is gekoppel aan n 240 toevoer. n 2 nduktiewe las word aan die sekondêre spoel van die transformator gekoppel en trek n stroom van 3 A. Bereken die volgende: 2.. Die primêre stroom (3) 2..2 Die windingsverhouding van die transformator (3) 2..3 Die drywingsaanslag van die transformator (3) 2..4 Die drywing as die las n arbiedsfaktor van 0.8 het (3) [60] RAAG 3: ENKELFASE MOTORS EN BESKERMNGSTOESTELLE. 3. Noem DRE verskillende oorstroom beskermingstoestelle. (3) 3.2 Noem TWEE tipes enkelfase induksiemotors. (2) 3.3 Bestudeer die vervaardigingskets in FGUUR 3. hieronder en benoem nommers 3.3. tot FGUUR 3. ENKELFASE MOTOR (5) 3.4 Noem DRE gebruike van n universele motor. (3) 3.5 Beskryf die werksbeginsel van n kapasitor-aansit en loop motor. (6) 3.6 Noem TWEE meganiese inspeksies wat op n elektriese motor gedoen word. (2)

6 6 3.7 Noem TWEE elektriese inspeksies wat op n elektriese motor gedoen word. (2) 3.8 Skryf die minimum aanvaarbare waarde vir die isolasieweerstandstoets neer wanneer n elekrtiese inspeksie gedoen word. () 3.9 Maak n netjies benoemde skets om aan te toon hoe die draairigting van n kapasitor-aansit motor gedoen word (6) 3.0 Bestudeer die stroombaan in FGUUR 3.2 en antwoord die vrae wat volg: FGUUR 3.2 MOTOR AANSTTER 3.0. dentifiseer die bogenoemde stroombaan () erduidelik die doel van die stop knoppie (2) erduidelik die doel van die O/L kontak (2) erduidelik die doel van die parallel kontak MC (N/O) (2) Beskryf hoe die nul volt spoel MC die operateur sal beskerm in geval van n kragonderbreking (3) [40] RAAG 4: RLC 4. Definieer die volgende terme met betrekking tot RLC stroombane. 4.. nduktiewe reaktansie (2) 4..2 Resonante frekwensie (3) 4.2 n Gloeilamp is in serie gekoppel aan n spoel wat op sy beurt aan n wisselstroom toevoer gekoppel is. erduidelik wat met die helderheid van die lamp sal gebeur indien die frekwensie van die toevoer verhoog. (3)

7 7 4.3 Bestudeer die reaktansie en weerstand teenoor frekwensie grafiek van n RLC stroombaan in FGUUR 4. hieronder en beantwoord die vrae wat volg: Ω Xc X L R 60 Hz f / Hz FGUUR 4. REAKTANSE EN WEERSTAND TEENOOR FREKWENSE erduidelik wat sal gebeur indien die toevoer frekwensie op 60 Hz gerstel word (2) erduidelik wat met die kapasitiewe reaktansie sal gebeur indien die toevoer frekwensie bo 60 Hz styg () Skryf die reeks frekwensies neer waar die stroombaan meer kapasitief sal wees () 4.4 Die instemkringbaan van n radio bestaan uit n5 mh spoel, 00 μf kapasitor en 9 Ω weerstand wat almal in serie aan n 8 / 50 Hz toevoer mekaar verbind is. Bereken die volgende: 4.4. Die kapasitiewe reaktansie van die kapasitor (3) Die induktiewe reaktansie van die spoel (3) Die impedansie van die stroombaan (3) Die stroomvloei van die stroombaan (3) Die fase hoek tussen spanning en stroom (3) Die resonante frekwensie (3) [30] TOTAL = 50

8 8 mmf H l mmf N Z e E max sin FORMULEBLAD BA R EMF Blvsin GAN c b f r 2 LC Z EMF 2BAnN sin Z R 2 ( X L X C ) mmf X C 2fC ave 0,637 rms 707 max max f T X L 2fL Q sin P cos p s s p N N p s f np F N Rc cc cc ave 0,637 max rms 707 max c c(max) c(min) cos R Z

9 GERT SBANDE DSTRCT GRADE ELECTRCAL TECHNOLOGY MEMO MAY JUNE 205 MARKS:50 TME: 2 hours

10 2 NSTRUCTONS: Answer ALL the questions. Sketches and diagrams must be large, neat and fully labelled. All calculations must be shown, and correct to two decimal places. Answers must be clearly numbered. A formula sheet is provided at the end of the paper. Non-programmable calculators may be used.

11 3 QUESTON : OCCUPATONAL HEALTH AND SAFETY ANDTOOLS AND MEASURNG NSTRUMENTS. To elliminate, prevent or reduce accidents to make our workshops accident free and safe places to work in. (2).2 Ensure the health and safety of yourself and others by your acts f any situation which is unsafe or unhealthy comes to your attention, report such a situation immediately (2).3 mproved employee comfort Reducing chances for occupational injuries mproved productivity ANY TWO (2).4 nadequate guarding Bad ventilation Rough or slippery floors Disorganised workshop ANY RELEANT ANSWER (2).5.5. Caution electric shock ().5.2 Gas mask must be worn ().6 The circuit MUST be switched off, the component must be removed completely or one lead must be removed to ensure that no external current flow through the component that is measured. (2).7 Earth continuity nsulation resistance between conductors nsulation resistance between conductors and earth Open or Short circuits Polarity of switches ANY TWO (2).8 To measure AC and DC voltages Measure Frequency Analyse waveforms Determine phase relationships between waveforms. ANY TWO (2).9 Always wear eye protection Always use a vice-grip when grinding small objects (2).0 Always solder in a well ventilated room When soldering iron is not in use, switch it off Always clean the tip of the soldering iron. ANY TWO (2) [20]

12 4 QUESTON 2: SNGLE PHASE AC GENERATON AND SNGLE-PHASE TRANSFORMERS 2. t t AC DC (4) 2.2 Flux density or the strength of the magnetic field Length of the conductor moving through the magnetic field The speed at which the conductor is moving (3) 2.3 Period is the time taken to complete one full cycle of a sinusoidal waveform (2) f f revolutions Hz EMF = 2πBAnN = 2π(0.8)(30 x 0-4 )(6.67)(400) = (3) ei = EMF x sin 30 = x sin 30 = (3) (3) MMF = N = 300 x 0.7 = 20 ampere (3)

13 H RMS mmf l A / m x max x5 (3) (3) ave 0.637x max 0.637x (3) 2.7 f pn 60 4 x Hz (3) 2.8 A single phase transformers primary winding is connected to an alternating current supply. This alternating current sets up an alternating magnetic field in the primary winding. The alternating magnetic field moves through the core material and cuts the secondary winding inducing an alternating emf in the secondary winding. The direction of this induced emf is in such a direction that it attempts to oppose the inducing force. (5)

14 6 2.9 p Np Ns s p 2.0 Copper losses caused by the resistance of the copper conductors in the coils. Hysteresis losses, caused by the resistance of the molecules in the core being magnetized and demagnetized. Eddy current losses, caused by stray currents that are induced in the core material setting up their own small opposing magetic fields. Dielectric losses, caused in the insulating material in very high voltage transformers. ANY TWO (4) (6) x A N N : (3) (3) 2..3 S = 2 x 2 = 2 x 3 (3)

15 7 = 36 A 2..4 P = 2 x 2 x cos θ = 2 x 3 x 0.8 = 28.8 W (3) [60] QUESTON 3: SNGLE-PHASE MOTORS AND PROTECTE DECES 3. Bi-metal strip Electromagnetic overload Electronic digital overload Eutectic alloy overload Fuse ANY THREE (3) 3.2 Split-phase motor Capacitor-run motor Capacitor-start motor Capacitor-start-and-run motor Shaded pole motor ANY TWO (2) Connection box Stator Housing (frame) Stator Windings Rotor End plate (5) 3.4 acuum cleaners Portable drills Drink mixers Sewing machines (3) 3.5 The start winding has a series capacitor and/or a centrifugal switch. The main winding also has a capacitor in series with it which are not disconnected by the centrifugal switch When the supply voltage is applied, current in the main winding lags the supply voltage due to the main winding impedance. At the same time, current in the start winding leads the supply voltage. nteraction between the magnetic fields generated by the main winding and the starting mechanism generates a resultant magnetic field rotating in one direction. The rotor starts rotating in the direction of the resultant rotating magnetic field. Once the motor reaches about 75% of its rated speed, a centrifugal switch opens and disconnects the start winding. (6)

16 8 3.6 nspect the mounting of the motor and that the bolts are tight. nspect the condition of the rotor shaft and key way. nspect the front and back bearings for smooth rotation nspect the condictio of the motor frame for any cracks ANY TWO (2) 3.7 nspect the wires isolation and the cover plates holding the terminals are sealed nspect the motor starter for loose wires or broken terminals Check for burn marks on the commutator nspect the capacitors. ANY TWO (2) 3.8 MΩ () FGURE 6. (6) 3.0. t is a direct on-line starter () The stop button - is to ensure that the supply can be isolated at any time during the operational process. t cuts the supply to the main contactor effectively stopping the motor. (2) Overload protection to disconnect the supply from the load when the current exceeds a preset value. f too much current is drawn due to a mechanical or an electrical fault it will cut the supply to the motor. (2) To allow the operator to release the start button without stopping the motor and to ensure that the motor must be started manually after a power failure has occurred. (2)

17 As soon as a power failure occurs the power to the motor is off, as soon as the power comes back on, the contactor is now open,the motor will not start automatically and the operator must switch the motor on manually. (3) [40] QUESTON 4 :RLC nductive reactance is the resistance that an inductor gives against the flow of current in an alternating current circuit (2) 4..2 Resonance frequency is the frequency at which the capacitive reactance is equal to the inductive reactance. This causes the Resistance to be equal to the impedance and currentflow will be maximum. (3) 4.2 f the frequency of the supply incease, the inductive reactance will increase, causing the current to decrease and the brightness of the lamp will also decrease. (3) Hz is the resonant frequency where XL = Xc, R = Z and current will be maximum. (2) Capacitive reactance will decrease () All frequencies between 0 and 60 Hz. () Xc 2fC 2 x x50 x00 x (3) X L 2fL 2 x x 50 x5 x (3) Z R ( X L - X C ) ( ) 2 2 (3) Z A (3)

18 cos cosθ Z R (3) Hz x x x LC fr [30] TOTAL = 50

FAKULTEIT INGENIEURSWESE FACULTY OF ENGINEERING

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