GRAAD 12 NATIONAL SENIOR CERTIFICATE GRADE 12

Transcription

1 GRAAD 12 NATIONAL SENIOR CERTIFICATE GRADE 12 ELECTRICAL TECHNOLOGY NOVEMBER 2015 MARKS: 200 TIME: 3 hours This quesion paper consiss of 13 pages and a 2-page formua shee.

2 Eecrica Technoogy 2 DBE/November 2015 INSTRUCTIONS AND INFORMATION This quesion paper consiss of SEVEN quesions. Answer ALL he quesions. Skeches and diagrams mus be arge, nea and fuy abeed. Show ALL cacuaions and round off answers correcy o TWO decima paces. Number he answers correcy according o he numbering sysem used in his quesion paper. You may use a non-programmabe cacuaor. Show he unis for a answers of cacuaions. A formua shee is aached a he end of his quesion paper. Wrie neay and egiby.

3 Eecrica Technoogy 3 DBE/November 2015 QUESTION 1: OCCUATIONAL HEALTH AND SAFETY 1.1 Sae ONE unsafe ac ha may ead o an acciden in an eecrica echnoogy workshop. (1) 1.2 Define he erm dangerous pracices wih reference o workshop pracice. (2) 1.3 Name TWO unsafe condiions ha may exis in an eecrica workshop. (2) 1.4 Expain he erm human righs in he workpace wih reference o he proecion of a worker. (2) 1.5 Expain why risk managemen is imporan for ensuring safey in a workshop. (3) [10] QUESTION 2: THREE-HASE AC GENERATION 2.1 Describe he erm acive power. (2) 2.2 Draw a neay abeed phasor diagram of he voages generaed by a hree-phase AC sysem. Indicae he direcion of roaion. (5) 2.3 A hree-phase sar-conneced moor has an oupu power of 50 kw and draws a curren from a 380 V/50 Hz suppy a a power facor of 0,95 agging. Assume here are no osses. V Cos Ө 50 kw 380 V 0,95 agging Cacuae he: Line curren (3) Apparen power (3) 2.4 Sae TWO advanages of power-facor correcion in AC sysems. (2) 2.5 Sae he funcion of a wameer. (2) 2.6 Two wameers ha are used o measure he inpu power are conneced o a baanced hree-phase circui and indicae W and 500 W respecivey. Cacuae he oa inpu power W 500 W (3) [20]

4 Eecrica Technoogy 4 DBE/November 2015 QUESTION 3: THREE-HASE TRANSFORMERS 3.1 Name ONE proecion device used in ransformers. (1) 3.2 Sae ONE negaive impac of osses in ransformers. (1) 3.3 Name THREE mehods of cooing ransformers. (3) 3.4 Describe he effec on he primary curren of a ransformer if he oad is increased. (3) 3.5 The dea-sar-conneced ransformer in FIGURE 3.1 suppies a facory wih 60 kw. The curren ags he voage by 36,87. The primary ine voage is 11 kv and he secondary ine voage is 380 V. V (primary) 11 kv V (secondary) 380 V L 1 L 2 L 3 L 1 N L 2 L 3 F A C T O R Y FIGURE 3.1: DELTA-STAR-CONNECTED TRANSFORMER ou Ө V (primary) V (secondary) 60 kw 36,87 agging 11 kv 380 V Cacuae he: Secondary ine curren (3) rimary ine curren (3) Apparen power (3) Reacive power (3) [20]

5 Eecrica Technoogy 5 DBE/November 2015 QUESTION 4: THREE-HASE MOTORS AND STARTERS 4.1 Name TWO pars of a hree-phase squirre-cage inducion moor. (2) 4.2 Sae TWO advanages of a hree-phase moor when compared o a singe-phase moor. (2) 4.3 Describe why i is imporan o carry ou eecrica inspecions on a moor afer insaaion and before energising. (2) 4.4 Refer o he circui diagram in FIGURE 4.1. L O/L Sop Sar Forward Hod in MC 1 (N/O) Sar Reverse Hod in MC 2 (N/O) MC 2 N/C MC 1 N/C MC 1 Forward MC 2 Reverse N FIGURE 4.1: CONTROL CIRCUIT OF A FORWARD-REVERSE STARTER Describe how he conac O/L proecs he moor. (3) Describe how inerocking is used in he sarer. (5) 4.5 Describe he funcion of a sar-dea sarer. (3) 4.6 Describe he funcion of a no-vo coi in a moor sarer. (3) 4.7 Sae how he direcion of roaion of a hree-phase moor may be reversed. (2) 4.8 The name pae on a hree-phase moor provides imporan informaion abou he moor. Lis THREE ypes of informaion i provides. (3)

6 Eecrica Technoogy 6 DBE/November A hree-phase moor is conneced o a 380 V/50 Hz suppy. The moor has six poe pairs and a sip of 4%. V s f S p 380 V 50 Hz 4% 6 Cacuae he: Synchronous speed (3) Roor speed (3) 4.10 A hree-phase dea-conneced moor is conneced o a 380 V/50 Hz suppy. The moor deveops 29,7 kw a fu oad. I has a power facor of 0,85 and an efficiency of 90%. ou V Cos Ө ŋ 29,7 kw 380 V 0,85 90% Cacuae he: Inpu power (3) Inpu ine curren (3) 4.11 Expain wha wi happen o he curren drawn by a moor if he power facor of he moor is improved. (3) [40]

7 Eecrica Technoogy 7 DBE/November 2015 QUESTION 5: RLC 5.1 Cacuae he capaciive reacance of a 120 µf capacior when i is conneced o a 240 V/50 Hz suppy. C V s f 120 µf 240 V 50 Hz (3) 5.2 Describe why he curren in an RLC series circui is a maximum a resonan frequency. (4) 5.3 Refer o he circui diagram in FIGURE 5.1 and answer he quesions ha foow. I r 2,5 A I 3,6 A I c 6 A I V s 240 V/50 Hz FIGURE 5.1: ARALLEL RLC CIRCUIT I r I I c V s f 2,5 A 3,6 A 6 A 240 V 50 Hz Cacuae he oa curren in he circui. (3) Cacuae he inducive reacance of he coi. (3) Describe wha wi happen o he curren hrough he coi if he frequency of he suppy is increased. (3) 5.4 Define he erm Q-facor wih reference o a parae circui. (4) [20]

8 Eecrica Technoogy 8 DBE/November 2015 QUESTION 6: LOGIC 6.1 Sudy FIGURE 6.1 which shows a ypica LC scan cyce and answer he quesions ha foow. STE 1 Inpu STE 2 rocess STE 3 Oupu FIGURE 6.1: LC SCAN CYCLE Name TWO inpu devices ha may be conneced in Sep 1. (2) Lis THREE programming anguages ha may insruc he LC in Sep 2. (3) Describe how EACH sep in FIGURE 6.1 is used o run a LC program. (6) Expain why heavy-curren devices canno be conneced direcy o he oupu of a LC ha uses a ransisor oupu. (3) Describe how he probem in QUESTION is soved in indusria appicaions. (3) 6.2 Describe why reay conro sysems (hard-wired sysems) are repaced by LC conro sysems. (2) 6.3 Refer o he foowing Booean equaion o answer he quesions ha foow: X ABCDABCDABCDABCDABCD Draw a Karnaugh map and show he grouping. (9) Wrie down he simpified expression ha can be exraced from he Karnaugh map. (3)

9 Eecrica Technoogy 9 DBE/November Refer o he circui in FIGURE 6.2. L Sop O/L L Sar 1 Hod in MC 1 (N/O) Sar 2 Hod in MC 2 (N/O) N MC 1 (Lamp) MC 2 (Fan) FIGURE 6.2: CONTROL CIRCUIT OF A SEQUENCE STARTER MC 1 swiches on a amp and MC 2 coses a conacor ha energises a fan. Draw he LC adder diagram ha woud execue he same funcion of he reay conro circui in FIGURE 6.2. (8) Sae wha programming funcion mus be incuded in he adder diagram o auomae he saring of he fan (MC 2 ). (1) [40] QUESTION 7: AMLIFIERS 7.1 Lis THREE characerisics of an idea operaiona ampifier (op amp). (3) 7.2 Describe he erm bandwidh in reaion o op amps. (3) 7.3 Give TWO reasons why negaive feedback is used in op-amp circuis. (2) 7.4 Describe he erm posiive feedback. (3) 7.5 Describe he erm sabiiy in reaion o op amps. (2)

10 Eecrica Technoogy 10 DBE/November Sudy FIGURE 7.1 and answer he quesions ha foow. R in - R f R in 5 kω R f 15 kω V in 1,5 V V in +_ V ou FIGURE 7.1: O-AM CIRCUIT Idenify he op-amp circui in FIGURE 7.1. (1) Draw he inpu and oupu waveforms on he same y-axis and abe he waveforms. (3) Cacuae he voage gain if he feedback resisor has a vaue of 15 kω and he inpu resisor has a vaue of 5 kω. (3) Cacuae he oupu voage if an inpu signa of 1,5 V is appied o he op amp. (3) 0 V 7.7 Sudy FIGURE 7.2 and answer he quesions ha foow. V in - +_ R f V ou R 1 FIGURE 7.2: INVERTING SCHMIDT TRIGGER Sae TWO appicaions of he op-amp circui in FIGURE 7.2. (2)

11 Eecrica Technoogy 11 DBE/November Redraw he inpu waveform shown in FIGURE 7.3 and direcy beow i draw he oupu waveform. +V cc Upper rigger eve Inpu voage 0 V Lower rigger eve -V cc Oupu voage FIGURE 7.3: INUT WAVEFORM TO AN INVERTING SCHMIDT TRIGGER (6) 7.8 Cacuae he resonan frequency of a Harey osciaor consising of wo cois of 40 mh each and a capacior of 0,65 µf. The ank circui of he Harey osciaor is shown in FIGURE 7.4. L 1 L 2 C FIGURE 7.4: TANK CIRCUIT OF A HARTLEY OSCILLATOR C L 1 L 2 0,65 µf 40 mh 40 mh (3)

12 Eecrica Technoogy 12 DBE/November Sudy FIGURE 7.5 and answer he quesions ha foow. Trigger inpu C 1 C 2 R 1 V ou R 2 FIGURE 7.5: O-AM CIRCUIT C 1 1 µf C 2 22 nf R 1 1,5 kω R kω Idenify he op-amp circui in FIGURE 7.5. (1) Sae ONE pracica appicaion of his ype of op amp. (1) Cacuae he ime he muivibraor wi remain in he se sae afer i has received a rigger puse. (3) Redraw he inpu waveform shown in FIGURE 7.6 and direcy beow i draw he oupu waveform. Show he ime cacuaed in QUESTION on he oupu waveform. Trigger puse 0 V Oupu waveform FIGURE 7.6: INUT TRIGGER ULSE FOR FIGURE 7.5 (5)

13 Eecrica Technoogy 13 DBE/November Sudy FIGURE 7.7 and answer he quesions ha foow. Trigger inpu C 1 R 3 R 1 V ou R 2 0 V FIGURE 7.7: BI-STABLE MULTIVIBRATOR Sae ONE appicaion of a bi-sabe muivibraor. (1) If he inpu rigger puse is negaive, describe wha vaue he oupu woud move o. (2) 7.11 Describe he funcion of he dua DC suppy in op-amp circuis. (3) [50] TOTAL: 200

14 Eecrica Technoogy DBE/November 2015 FORMULA SHEET THREE-HASE AC GENERATION RLC CIRCUITS Sar V 3 V p X X c 2fL 1 2fC I I p 1 f r 2 (LC) Dea I 3 Series I p V V p I Ir Ic I X X 2 2 Z R ~ 3VpI pcosθ V I X 3V Icos θ Vc I X c S 3 V I V I Z Q 3 V I sinθ 2 V V V ~ V 2 Cosθ Z p V I p p S Two wameer mehod 1 2 OERATIONAL AMLIFIERS Gain Gain f r A A v v 1 2 L C Vou R V in R Vou Rf 1 V R in in f in Harey osciaor inveringop amp non-inveringop amp I V Z r Cosθ Cosθ R Z Vr V X Q R arae V V V I r I c Vr R V X r c V I X c c V c c f rc 1 2 RCphase-shif osciaor I 2 Ir I ~ Ic 2 6 RC V ou (V 1 V 2...V n ) Cos θ X Q R I I r

15 Eecrica Technoogy DBE/November 2015 THREE-HASE MOTORS AND STARTERS THREE-HASE TRANSFORMERS Sar Sar V 3 V p V 3 Vp I I p I Ip Dea Dea I 3 I p I 3 Ip V V p V Vp ower ower 3V p I p cos θ 3V p I p cos θ 3 V I cos θ 3 V I cos θ S 3 V I S 3 V I Q 3 V I sinθ Q 3 V I sinθ Cos θ S Vp primary Np Ip Vp secondary Ns Ip ou ou Efficiency η Efficiency η ou In in osses S cos θη 60 f n s p ns nr Sipper uni n n r n s n %sip 1 S s s per uni n n s r 100% ou in in osses S cos θ η secondary primary