Electrical Technology Digital Electronics Paper 1 (English)

November 2022 — Grade 12 / Matric — Electrical Technology

Past Paper 2022 Grade 12 English

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🎯 200 marks ⏱️ 3h 📄 6 pages 📝 94 items found ∑ 2 math symbols 📚 3 topics

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7.1Formulae and manipulations where needed

7.2Correct replacement of values

7.3Correct answer and relevant units where applicable 8. A formula sheet is attached at the end of this question paper. 9. Write neatly and legibly. Electrical Technology: Power Systems

Q1— Section header —

1.1A/An is an occurrence when a serious or unexpected dangerous situation occurs that requires immediate attention. vacuation procedure non

1.2The total opposition against the flow of alternating current in an RLC circuit is the .. inductive reactance. impedance.

1.3When decreasing frequency to below resonance in a series RLC resonance circuit impedance increases and the circuit becomes inductive voltage drop across the inductor and capacitor increases

1.4The opposition to AC current flow caused by a capacitor will increase when the .. capacitance is decreased

1.5An advantage of three phase system it is available everywhere.

1.6Power that is transferred backwards and forwards between the supply and the inductor or capacitor without doing any work is known as .. power apparent

1.7In an electric power system, a load with a low power factor draws .. current from the supply compared with a load with a high power factor. less

1.8When comparing three phase transformer s to single phase transformer s,

1.9A Buchholz relay will activate and isolate the transformer from the supply when .. a severe fault causes a large amount of gas forming inside the oil of a transformer. the

1.10An advantage of a three phase motor over a single phase motor is that a

1.11Refer to the typical speed versus the torque characteristic curve of a three phase induction motor

1.12A three phase motor with 18 poles ha .. . pole pairs per phase.

1.13n example of an output device i n a PLC system that could automatically switch on a high current motor.

1.14In pulse width modulation (PWM\ onger ON times create

1.15Braking that occurs when the load on the motor rotates fas ter than the motor is known as .. braking regenerative

2.5State TWO types of risk analysis reports done by the health and safety representative. (2\ [10] Electrical Technology: Power Systems10

3.1Explain the term inductance with reference to RLC circuits connected to an AC supply. (2\

3.2Draw the phasor diagrams for FIGURE

3.2.1and 3.2.2 in the ANSWER BOOK

3.2.1FIGURE 3.2.1: VOLTAGE AND CURRENT WAVEFORMS (2\

3.2.2FIGURE

3.2.2: VOLTAGE AND CURRENT WAVEFORMS (2\

3.3series RLC and a capacitive reactance of 13 150 V/60 Hz AC supply. Answer the questions that follow. Given:

3.3.1Calculate the impedance of the circuit (3\

3.3.2Calculate the phase angle of the circuit. (3\

3.3.3Calculate the value of the inductor. (3\

3.3.4Explain what is meant by a lagging power factor (1\

3.3.5Explain why the current and the voltage waveforms are in phase in a series RLC resonance circuit. (2\

3.4Refer to FIGURE 3.4 and answer the questions that follow FIGURE 3.4: PARALLEL RLC PHASOR DIAGRAM Given:

3.4.1Calculate the total current. (3\

3.4.2Calculate the power factor. (3\

3.4.3Calculate the total power. (3\

3.4.4State with a reason whether the circuit has a leading or lagging power factor. (2\

3.5Refer to FIGURE 3.5 and answer the question that follow. FIGURE 3.5 IMPEDANCE AND CURRENT RESPONSE CURVES

3.5.1Name the circuit that produces the response at and in FIGURE 3.5 (2\

3.5.2Discuss the difference between the impedance and current at resonant frequency. (2\

3.5.3Describe what happens to impedance when the frequency increases in FIGURE 3.5 (2\ [35]35

4.1Refer to FIGURE 4.1 and answer the questions that follow FIGURE 4.1: THREE PHASE PHASOR DIAGRAM

4.1.1State whether the phasor diagram represent positive phase sequence or negative phase sequence. Motivate your answer.

4.1.2Determine whether phasor RN represent a line vol tage or a phase

4.2Explain the term active power

4.3Explain the effect of stepping up the voltage in transmission lines. (2\

4.4Draw a diagrammatic representation of a four wire three phase

4.5Refer to voltages and e xplain what happens in the distribution stage of the national power grid. (3\

4.6A balanced three phase load is connected in delta to a three phase star connected alternator. The load draws a current of 15

4.6.1Phase current of the load (3\

4.6.2Impedance of the load (3\ RY YB BR 120

4.6.3Phase angle (3\

4.6.4Active power (3\

4.6.5Draw a diagram of a power facto r correcting capacitor bank connected

4.7FIGURE

4.7shows three wattmeters connected to a balanced three phase load. Answer the questions that follow. FIGURE 4.7: THREE WATTMETER METHOD

4.7.1State TWO advantages of using the three wattmeter method. (2\

4.7.2State ONE disadvantage of using the three wattmeter method. (1\

4.7.3Calculate the total power if the reading on W = 450 W. (3\ [35]35

5.1Explain the principle of mutual induction with reference to transformers. (3\

5.2Single phase transformers can be used to create a three phase transformer unit. Answer the questions that follow.

5.2.1List THREE characteristics of single phase transformer that must be identical. (3\

5.2.2Name the connection on the secondary side of three phase transformer hat will create a neutral point.

5.4Describe how insulation failure is controlled in dry type transformers. (2\ 5. Differentiate between shell

6.1Name the TWO types of rotor construction used in induction motors. (2\

6.2Refer to the speed of induction motors and answer the questions that follow.

6.2.1Explain the difference between rated speed and full load speed (2\

6.2.2Calculate the synchronous speed of a three phase motor with four pole pairs per phase that are connected to a 400 V/50 Hz supply. (3\

6.2.3Calculate the percentage slip if the rotor turns at 725 /min ( rpm (3\

6.3Name TWO mechanical inspections that could be carried out on the rotor and bearings of a motor. (2\

6.4

6.4.1Input power (3\

6.4.2Reactive power (3\

6.4.3Output powe r if the motor is 90% efficient (3\

6.5FIGURE 6.5 shows the terminal box of a three phase motor. Redraw the terminal box in the ANSWER BOOK and indicate how the motor can be connected in delta. FIGURE 6.5: TERMINAL BOX

6.6FIGURE 6.6 shows the control circuit of an automatic star delta starter. Answer the questions that follow. FIGURE 6.6: AUTOMATIC STAR DELTA STARTER

6.6.1Identify component (1\

6.6.2Explain the purpose of MC N/O

6.6.3Explain the importance of interlocking in this circuit. (2\

6.6.4Describe the operation of the circuit after the timer has timed through. (5\ [35] STOP35

7.1Easy fault finding is one of the advantages of hard wiring Explain why.

7.2Refer to sensors as input devices to a PLC and answer the questions that follow

7.2.1Explain the term sensor (2\

7.2.2Explain what the PLC does after receiving data from a temperature sensor as an input device. (2\

7.2.3State TWO applications of temperature sensors. (2\

7.3Refer to FIGURE 7.3 and answer the questions that follow. FIGURE 7.3: CONTROL CIRCUIT

7.3.1Identify the control circuit in FIGURE 7.3. (1\

7.3.2Explain latching with reference to motor control circuits. (3\ STOP START MC

7.3.3Redraw and complete the ladder logic diagram in FIGURE 7.3.3 in he ANSWER BOOK to execute the same function as in FIGURE 7.3. FIGURE 7.3.

7.4Draw a ladder logic diagram with two outputs, each controlled by two inputs. Input 1 and input 2 must be closed for output 1 to be high. Either input 3 or input 4 or both must be closed for output 2 to be high.

7.5Refer to PLCs and explain the concept marker (2\ 7. FIGURE 7 .6

7.6.3 Describe the principle of operation of the inverter stage.

7.6.4 State TWO advantages of using VSDs over conventional motor drive s. (2\

7.7Give TWO examples where regenerative energy can be reused. (2\ [40] TOTAL: 200 Stop40

Mathematical Content

f(x)trig

Topics Identified

CircuitsMachinesMaterials

Mark Allocation

Total200 marks

Sections15 + 10 + 35 + 35 + 30 + 35 + 40 = 200 marks

Paper information

SubjectElectrical Technology

Year2022

SessionNovember

TypePast Paper

PaperPaper 1

LanguageEnglish

Pages18

Downloads18

Examination context

Decade2020s

Years ago4y

SeasonSpring (SH)

Start daySaturday

Exam periodNovember 2022

Days since exam1,450

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