Electrical Technology - Electronics (English)

Exemplars 2018 — Grade 12 / Matric — Electrical Technology

Past Paper 2018 Grade 12 English

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🎯 200 marks ⏱️ 3h 📄 11 pages 📝 97 items found ∑ 1 math symbols 📚 2 topics

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Q1— Section header —

1.1Define the term workplace with reference to the Occupational Health and Safety Act, 1993 (Act 85 of 1993\ (2\

1.2Name TWO general duties for employees at the workplace. (2\

1.3Explain why insufficient ventilation is an unsafe condition in the workshop. (2\

1.4State TWO functions of a health and safety representative. (2\

1.5Explain quantitative risk analysis (2\ [10]10

Q2— Section header —

2.1Explain the phase relationship between current and voltage in the following AC circuits

2.1.1Resistive circuit (2\

2.1.2Pure capacitive circuit (2\

2.1.3Pure inductive circuit (2\

2.2FIGURE 2.2 below shows a RLC series circuit which consists of a

2.2.1Inductive reactance (3\

2.2.2Capacitive reactance (3\

2.2.3Impedance (3\

2.2.4Total current (3\

2.2.5Power factor (3\

2.2.6State whether the phase angle is leading or lagging (1\

2.3Refer to FIGURE 2.3 and answer the questions that follow. FIGURE 2.3 : FREQUENCY VERSUS IMPEDANCE

2.3.1Describe how an increase in the frequency of the supply voltage will affect the: (a\ Inductive reactance (2\ (b\ Capacitive reactance (2\

2.3.2Explain why the response of line R is parallel to line F (2\

2.3.3Name the electrical quantity that is equal to R at point D (1\

2.4

2.4.1Resonant frequency (3\

2.4.2factor (5\

2.4.3Bandwidth (3\ [40] Electrical Technology: Power Systems DBE/2018 CAPS Grade 12 Exemplar Copyright reserved40

Q3— Section header —

3.1Name THREE advantages of a three phase AC generation over a single phase AC generation. (3\

3.2Draw a fully labelled waveform to represent the generation of a three phase voltage system. (6\

3.3Explain the following terms:

3.3.1Efficiency (2\

3.3.2Power factor (2\

3.4Refer to the losses occurring during the transmission of electrical power from the power station to the distributing point and answer the following questions:

3.4.1Name the main type of losses occurring in the transmission line (1\

3.4.2Describe how these losses can be reduced (2\

3.5A 380 V/50 Hz three phase AC delta connected motor has an output power of 12,75 kW and operates at a lagging power factor of 0,77

3.5.1Input power (3\

3.5.2Line current (3\

3.5.3Phase current (3\

3.6State the function of a kWh meter. (2\

3.7A two wattmeter method is used to measure the input power in a balanced three phase load. If the wattmeter readings are 8 kW and 4 kW respectively calculate

Q4— Section header —

4.1Name TWO construction types of transformer s. (2\

4.2State THREE factors that may contribute to the excessive heating of transformers. (3\

4.3Explain how the following losses occur in the transformers

4.3.1Hysteresis losses

4.3.2Eddy current losses transformer (3\

4.4Describe how the phase current is induced in the secondary winding of the transformer. (4\

4.512 kVA three phase transformer is connected in delta star and has a turns ratio of 5

4.5.1Primary phase voltage (2\

4.5.2Secondary line voltage (6\

4.5.3Active power if the transformer has a lagging power factor of 0,9 (3\

4.6Refer to QUESTION

4.5and answer the questions that follow

4.6.1Name TWO applications of this type of transformer. (2\

4.6.2State with reason whether the transformer is a step down transformer or

Q5— Section header —

5.1Define the term slip with reference to three phase AC squirrel cage

5.2State TWO mechanical inspection that must be conducted after installation and before commissioning (2\

5.3Give TWO reasons why the rotor of the squirrel cage induction motor is skewed. (2\

5.4Explain how the torque is developed in a squirrel cage induction motor. (3\

5.5State TWO applications of a squirrel cage induction motor. (2\

5.6Refer to FIGURE 5.6 below and answer the questions that follow FIGURE 5.6: CHARACTERISTIC CURVE OF SPEED VS TORQUE

5.6.1Identify the torque developed at the following points: (a\ (1\ (b\ (1\

5.6.2Explain why the torque developed at (c\ is zero (2\ (a\ (b\ (c\

5.7FIGURE 5.7 below shows a control circuit Study the diagram and answer the questions that follow. FIGURE 5.7: CONTROL CIRCUIT

5.7.1Identify the control circuit diagram in FIGURE 5.7 (1\

5.7.2Describe what will happen to the motor if the normally closed contact of the overload is permanently closed due to a fault.

5.7.3Describe the function of the following components as used in the circuits (a\ Stop button (2\ (b\ MC1 (N/O

5.7.4State why the N/C contact of MC3 is connected in series with star contactor. (2\

5.8A three phase 15 kW induction motor is connected in delta to a 380 V/50 Hz supply . The motor has laggin

5.8.1Line current drawn from the supply (3\

5.8.2Apparent power of the motor (3\ 30

Q6— Section header —

6.1State THREE safety precautions to be observed when connecting external cables to a PLC. (3\

6.2Name TWO types of hardware components of the PLC other than the central processing unit (CPU\. (2\

6.3State THREE advantages of the PLC over the hardwired system. (3\

6.4State why an interface is connected at the inputs of the CPU. (2\

6.5Explain the programmed scan cycle under the following headings:

6.5.1Input scan (3\

6.5.2Process scan (3\

6.5.3Output scan (2\

6.6Refer to analogue and digital input as used in the PLC and answer the following questions

6.6.1Explain the difference between an analogue input and a digital input (4\

6.6.2Give TWO examples of the digital inputs (2\

6.6.3State THREE applications of the inductive proximity sensor as an analogue input (3\

6.7State when a maker/flag function is used in the PLC. (2\

6.8Refer to FIGURE 6.8 on the next page and draw the PLC ladder logic diagram that would execute the same function. Electrical Technology: Power Systems 11 DBE/2018 CAPS Grade 12 Exemplar

6.8Refer to FIGURE 6.8 below and draw the PLC ladder logic diagram that would execute the same function. FIGURE 6. CONTROL CIRCUIT (10\

6.9State THREE basic applications of the variable speed drive (VSD\. (3\

6.10Name THREE methods in which the speed of the motor can be controlled with the exception of pulse width modulation (3\

6.11Describe how the pulse width modulation as a method of varying the speed of the motor achieves its function. (4\

6.12Describe how the VSD controls the speed of motors under the following subcircuits:

6.12.1Diode bridge rectifier (2\

6.12.2Filtering circuit

6.12.3Inverting circuit (3\

6.13Describe the concept of regenerative braking (3\ [60] TOTAL 20060

Mathematical Content

trig

Topics Identified

CircuitsMachines

Mark Allocation

Total200 marks

Sections10 + 40 + 30 + 30 + 60 = 170 marks

Paper information

SubjectElectrical Technology

Year2018

SessionExemplars

TypePast Paper

PaperSee title

LanguageEnglish

Pages13

Downloads12

Examination context

Decade2010s

Years ago8y

Season—

Start day—

Exam period—

Days since exam—

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