Electrical Technology Power Systems Memo 1 (English)

November 2018 — Grade 12 / Matric — Electrical Technology

Memo 2018 Grade 12 English

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🎯 200 marks 📄 13 pages 📝 85 items found ∑ 1 math symbols 📚 2 topics

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2.1All calculations must show the formulae.

2.2Substitution of values must be done correctly.

2.3All answers MUST contain the correct unit to be considered.

2.4Alternative methods must be considered, provided that the correct answer is obtained.

2.5Where an incorrect answer could be carried over to the next step, the first answer will be deemed incorrect. However, should the incorrect answer be carried over correctly, the marker has to re calculate the values, using

2.6Markers should consider that learner answers may deviate slightly from the guideline; depending on how and where in the calculation rounding off was used. marking guidelines are only a guide with model answers. Alternative

Q1— Section header —

1.1'Major incident 'is an occurrence of catastrophic proportion resulting from the use of plant and machinery from activities in the workplace. Severe or serious injury or great damage to machinery and persons resulting

1.2Manufacturers must ensure that products are safe Not permitting any employee to do any work or to produce any artefact unless precautionary measures are in place. Safe to use / without risk to health Safety feature

1.3'Horseplay 'is an unsafe act because it is an inappropriate behaviour by learners in the workshop that would compromise the safety of themselves and others. It distracts others that could lead to accidents or incidents.

1.4Do not touch the person. Switch off the supply. Help the person by removing him/her with a type of i lation material. (2\

1.5Qualitative risk analysis defines the levels of threat and devises counter measures to eliminate possible risk. (2\ [10]10

Q2— Section header —

2.1The total opposition to the flow of alternating current in a circuit comprising of resistance and reactance. (2\

2.2.1

2.2.2

2.3.173,68 (3\ NOTE. Note if the formula is copied directly from the formula sheet a mark must be awarded, there after the substitution, manipulation and the answer must be correct to receive 3 marks.

2.3.258,35 (3\ NOTE. Note if the formula is copied directly from the formula sheet a mark must be awarded, there after the substitution, manipulation and the answer must be correct to receive 3 marks.

2.3.318,44 (36 (3\

2.3.43,25

18.44(3\

2.3.5149,38 Sin 3,25 Sin (3\

2.4The value of the inductive reactance will double /increase because the inductive reactance is directly proportional to the frequency of the supply voltage. NOTE. If only the formula is given, no marks.

2.5The resonant frequency is the frequency at which the inductive reactance is equal to the capacitive reactance. NOTE All characteristics of resonance explained correctly will be accepted. (2\

2.6(3\ 16,67 (3\ (3\ Note: If the learner sets one or a difference in reactive currents, the (3\

2.6Phase angle is leading (1\

2.7A low value of resistance produces a high Q factor which results in a low bandwidth and high selectivity. (2\ [40]40

Q3— Section header —

3.1(1\

3.2.1Apparent power is the product of the current and the voltage not considering reactance in an AC circuit. Apparent power is the total power drawn from the supply. (2\

3.2.2Power factor is the ratio of the active to the apparent power consumed in an AC circuit. (2\

3.3Thinner supply cables will be required. It will minimise the supply current. Cost of maintenance will be less (3\

3.4.1

3.4.2When the coils of a three phase alternator are connected in star, a neutral point is created and both line as well as phase voltages of different values are available. The neutral point can be earthed.

3.5The transmission voltage is inversely proportional to the current therefore by increasing the transmission voltage, the line current is decreased and the copper losses are reduced. (2\

3.6.1Phase voltage 219,39 (3\

3.6.2Line current to the load. 34,19 18000 Cos (3\

3.6.3Apparent power 22,50 34,19 22,50 34,19 219,4 (3\ 22,50 18000

3.7.1The total input power to the motor. 2300 1200 (3\

3.7.2The power factor can be determined. The total input power can be measured in a star or delta connected system. Only two wattmeters are required instead of three. using an ammeter. Power in a balanced and unbalanced system can be measured.

Q4— Section header —

4.1Copper losses R losses / heat losses Iron losses eddy current losses / Hysteresis losses / heat losses Stray losses Magnetic field losses Dielectric losses. (3\

4.2Delta star is used for Power distribution in Commercial Power distribution in Industries power distribution for d omestic step down 380 V to 220 V

4.3It improves the insulation of the transformer. Conducts the heat away from the windings. (2\

4.4When an alternating voltage is applied to the primary windings an alternating flux is created in the windings. This flux will link with the secondary windings through the magnetic path of the iron core of the transformer

4.5Transformers have moving parts as a result less losses develops and it delivers a better power output. Transformers are designed efficiently. (3\

4.6To detect abnormal gas formation in the transformer and isolate it from the supply to prevent it from being damaged. (2\

4.7.1219,39 (3\

4.7.2219,39 2200 (3\

4.7.3149,58 2200 219,39 1500 (3\

4.8.1Is a step down transformer because N (1\

4.8.2The secondary windings of the transformer are connected in star thereby creating a neutral point to supply single phase and three phase system (3\ [30]30

Q5— Section header —

5.1.11,3 A (1\

5.1.2The motor can be used in South Africa because of the supply voltage of 380 V (2\

5.1.3The 7, 5 kW signifies the rated output power the motor can deliver to drive the load. (1\

5.1.4poles poles number Total phase pairs pole 1500 (5\

5.1.51200 7500 losses 86,21 8700 7500 (5\ Alternative answer 86,21

5.2volt protection prevents a motor from restarting after a power failure to protect the operator. If the supply voltage drops below a prescribed value, it will switch off the supply (3\

5.3By changing any two of the three supply lines. (2\ NOTE mentioning the colour of the lines are accepted.

5.4Check if the: End plates are securely fastened. Shaft turns freely. Bearing works smoothly when the shaft is turned. Cooling fan / fin (2\

5.5.1(1\

5.5.2In elevators to change the direction of the motor. Conveyor belts Lathe (1\ 10

5.5.3The purpose of the overload is to disconnect the supply from the motor , when the current exceeds the pre set value. (2\

5.5.4When the start button is pressed, current will flow in the circuit When /NO closes and MC /NC opens and the motor will run forward. When the stop button is pressed MC energised opening contact MC

Q6— Section header —

6.1Require regular maintenance. It takes time to locate a fault. It takes longer to re wire the system when compared to the reprogramming of the PLC. (3\

6.2Input scan/input module . (checks input device state\ Process (complete instruction\ Output module (update\ (3\

6.3Scan time is the time the PLC takes to complete one scan cycle, processing in each of the three most important steps. (2\

6.4.1(3\ 11

6.4.2(4\ TABLE 6.4.2

6.5A PLC is a industrial computing device that is programmed to perform a number of tasks as per instruction and to control various types of machines and processes. (3\ A PLC is a programmable device capable of interpreting logic for the purpose

6.6.1Light sensor. Level sensor. Overload sensor. Temperature sensor. (3\

6.6.2A computing device (microprocessors, computers, and logic units\ can only interpret the elect ronic digital/discrete language instructions for process ing, monitoring and controlling other than an analogue languages,

6.7The output of the PLC sends a positive signal (high logic\ to its internal output relay. to drive the motor. (3\

6.8The purpose of the timer function is to activate or deactivate a device after or before a pre set interval of time. (2\ The purpose of the timer function is to run an operation for a predetermined period of time.

6.9The concept of latching makes it possible for circuit to be triggered remain regardless of whether the activating trigger has been removed. (3\

6.10.1Automatic star delta starter. Star delta starter (1\

6.10.2(13\ ternative 13

6.10.3The function of the MC /NO in ladder logic control circuit is to Latch the circuit and keep the program running even after the start button is released. (2\

6.10.4The purpose of MC NC is to interlock and prevents from engaging when MC is engaged in delta. (2\

6.11Induction motors. AC synchronous motor AC asynchronous motors. DC motors (3\

6.12A process of changing the frequency of the input voltage to change the speed and torque of the motor (2\

6.13The purpose of the braking resistor is to stop or slow down the motor through absorption of excess energy and converting excess energy to heat while in motion by transforming kinetic energy back into electrical

6.14The function of the VSD is to control the speed of an AC motor by varying the frequency of the voltage supplied to the motor. The function of the VSD is to control the speed of a DC motor by varying the

Mathematical Content

trig

Topics Identified

CircuitsMachines

Mark Allocation

Total200 marks

Sections10 + 40 + 30 + 30 + 30 + 60 = 200 marks

Paper information

SubjectElectrical Technology

Year2018

SessionNovember

TypeMemo

PaperPaper 1

LanguageEnglish

Pages13

Downloads5

Examination context

Decade2010s

Years ago8y

SeasonSpring (SH)

Start dayMonday

Exam periodNovember 2018

Days since exam2,911

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