Electrical Technology Power Systems Paper 1 (English)

May-June 2024 — Grade 12 / Matric — Electrical Technology

Past Paper 2024 Grade 12 English

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🎯 20 marks ⏱️ 3h 📄 18 pages 📝 104 items found ∑ 1 math symbols 📚 3 topics

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7.3Formulae and manipulations where needed Correct replacement of values Correct answer and relevant units where applicable 8. 9. A formula sheet is attached at the end of this question paper. Write neatly and legibly.

Q1— Section header —

1.1covering the burnt area with butter. putting ice on the burn wound. puncturing the blisters. running cold water over the burnt area until the pain reduces.

1.2apparent power. active power. reactive power. true power. (1\

1.3> X R = Z

1.4An increase in resistance of a parallel resonant circuit will cause the total double. decrease. increase. be zero.

1.5Efficiency of electrical systems in industries with large inductive loads high power factor with current leading the voltage. low power factor with current leading the voltage. high power factor with current lagging the voltage.

1.6The line current of a star connected system is 10 A. The phase current 30 17,32

1.7The apparent power of a delta connected three phase system with a line

1.8The following statement describes a core type transformer: The core hides the major part of the windings

1.9The fac tors constant overloading. insufficient ventilation. transformer oil may be insufficient.

1.10A continuity test on the windings of a three phase induction motor is carried have been removed.

1.11motor starters. main contacts auxiliary contacts zero

1.12The ratio of the output power to the input power of a three phase motor is power factor. efficiency.

1.13proximity sensors are used only to detect the presence of metal objects. Capacitive Ultrasonic

1.14PLC virtual storage memory which can be used to hold data and behave as relays. Latching contacts

1.15The purpose of a braking resistor in the regenerative braking process is to .. store the excess energy in its electrostatic field. limit the amount of excess current. dissipate the excess energy in the form of heat.

Q2— Section header —

2.1State TWO unsafe acts that would be regarded as dangerous practices by a user when operating machinery. (2\

2.2With reference to the Occupational Health and Safety Act, 1993 (Act 85 of 1993\ give TWO examples that are considered to be offences when reporting to the safety inspector. (2\

2.3Discuss the general duties that manufacturers perform when designing or manufacturing articles used at work. (2\

2.4Define a non critical incident (2\

2.5Describe a dangerous effect that a current of 200 mA has on the human body. (2\ [10] Electrical Technology: Power Systems DBE/May/June 2024 SC/NSC Confidential10

Q3— Section header —

3.1Explain the term reactance with reference to an alternating current circuit. (2\

3.2FIGURE 3.2 below shows the circuit diagram, waveforms and a partial phasor diagram of the voltages in a RLC circuit that is connected to an AC supply. Study the diagrams below and answer the questions that follow.

3.2.1State whether the circuit is predominantly inductive or capacitive. Motivate your answer. (2\

3.2.2Calculate the supply voltage. (3\ 10 V 18 V

3.2.3Calculate the phase angle. (3\

3.2.4Redraw and complete the phasor diagram in the ANSWER BOOK. (3\

3.2.5Explain why it could be assumed that the supply current is leading the supply voltage. (2\

3.3Refer to FIGURE 3.3 below and answer the questions that follow. FIGURE 3.3: RLC PARALLEL CIRCUIT Given: = 62,83 = 200

3.3.1Current flow through the inductor (3\

3.3.2Total current flow (3\

3.3.3Power factor (3\

3.3.4Value of capacitance that would cause resonance when the frequen cy and inductor remain constant (4\ 62

3.4FIGURE 3.4 below shows the current vs frequency response curves of a series resonant circuit with a variable resistor. The inductive reactance of the circuit is 000 resistance value. FIGURE 3.4: FREQUENCY RESPONSE

3.4.1State how a decrease in resistance affects the Q factor of the circuit. (1\

3.4.2(3\

3.4.3Calculate the resonant frequency when = 1 200 Hz and = 2

4.1FIGURE 4.1 below shows a block diagram of the national power grid in South Africa . Answer the questions that follow. FIGURE 4.1

4.1.1State the voltage generated at block (1\

4.1.2Name the standard international colour code for a three phase system with reference to each phase (3\

4.1.3Briefly describe the national grid (2\

4.1.4xplain why the transmission of electricity is done at such high voltages (2\

4.1.5xplain how the voltage for an industrial consumer is different from

4.2Give TWO reasons why an industrial consumer would prefer a three phase supply over a single phase supply

4.3180 kW three phase delta connected load is powered

4.3.1Line current (3\

4.3.2Phase current (3\

4.3.3Power factor (3\

4.3.4Reactive power (5\ Generation Transmission Distribution Industrial consumers Domestic consumers

4.4FIGURE 4.4 below shows the result of power factor correction in a three phase system. The supply voltage and load remain ed

4.4.1State where power fac tor correcting devices may be installed. (1\

4.4.2Identify ONE improvement with reference to FIGURE 4.4 above (1\

4.4.3Explain how power factor correction affected the current drawn from the supply by comparing the power triangle before and after power factor correction. (2\

4.5Differentiate between wattmeters and energy meters with reference to their applications. (2\

4.6wo wa tmeters are used to measure the total power of a three phase system

5.1Refer to FIGURE 5.1 below and answer the questions that follow. FIGURE 5.1 : THREE PHASE TRANSFO MER

5.1.4State whether this is a core type or shell type three phase transformer. State ONE advantage of a shell

5.2With reference to transformers, answer the following questions

5.2.2Describe the effect that an increase in the load will have on the primary current of a transformer. Name ONE function of the dielectric oil used in transformers. (1\

5.2.3Name TWO types of losses other than copper losses that occur in transformers. (2\

5.2.4Name and explain TWO cooling methods used for dry transformers. (4\

5.2.5Compare conductor sizes between a single phase transformer and three phase transformer for the same power rating

5.3FIGURE 5.3 below shows a 300 kW delta connected load with a power factor of 0,87 that is connected to a delta star transformer. The primary and secondary line voltages of the transformer are 8

5.3.1Line current of the load (3\

5.3.2Phase current of the load (3\

5.3.3Primary line current (3\

5.4Protection devices used with three phase transformers are categorised in two categories, relay and switches. Name TWO protection relays used in three phase transformers

6.1Name TWO rotating parts of a three phase induction motor. (2\

6.2FIGURE 6.2 below shows how the coils of a three phase induction motor

6.2.1Determine the angle between coil and coil (1\

6.2.2xplain how a rotating magnetic field is created when a three phase supply is connected to the stator windings (4\

6.2.3State why squirrel cage is less of a fire hazard (explosi on proof\ (1\

6.3A three phase induction motor has 12 poles. When operating from 50 Hz supply the slip is 5%

6.3.1ynchronous speed (3\

6.3.2Roto r speed (3\ Stator housing Stator coils

6.4The following information is available on a delta connected three phase induction motor: Input current = 20

6.4.1Efficiency (3\

6.4.2Apparent power (3\

6.4.3Power factor (3\

6.4.4Output power of the motor (3\

6.5Refer to FIGURE 6.5 below and answer the questions that follow. FIGURE 6.5: MOTOR STARTER CONTROL CIRCUIT AND CONVEYOR SYSTEM

6.5.1Identify the motor starter control circuit in FIGURE 6.5 above (1\ MC N/O STOP

6.5.2Explain the function of MC with refer ence to the main (power\ circuit. (2\

6.5.3State the advantage of having two overload relays in the circuit. (1\

6.5.4Explain the disadvantage of connecting the overloads as shown in the control circuit in FIGURE 6.5 when MC

6.5.5Currently MC can be energised either before or after energising

7.1State THREE advantages of hard wiring. (3\

7.2Refer to FIGURE 7.2 below and answer the questions that follow. FIGURE 7.2 LOGIC GATE

7.2.1Draw the ladder logic diagram. (3\

7.2.2Complete the truth table for the logic gate by writing down only the state of the output for the following TWO input conditions

7.3Refer to analogue and digital inputs on a PLC and answer the questions that follow.

7.3.1Give THREE examples of analogue input devices (3\

7.3.2Explain why an analogue input should be converted to a digital input in a computing device (4\

7.4Explain the concept latching (3\

7.5Refer to FIGURE 7. below and answer the questions that follow. FIGURE 7.5 AUTOMATIC SEQUENCE STARTER

7.5.1Describe the function of (2\

7.5.2State TWO condition that must be met for MC to energise.

7.5.3Differentiate between ON delay and OFF delay timers

7.5.4In the ANSWER BOOK, redraw the PLC ladder logic diagram in FIGURE 7.5.4 below and complete it so as to execute the same function as the circuit

7.6Refer to variable speed drive as a programmable motor controller . Describe the basic principle of operation of a variable speed drive when it controls the speed of an AC induction motor. (3\

7.7FIGURE 7.7 below shows the characterist ic curve of speed versus torque when VSD is used to control the speed of a three phase induction motor. Stu dy the characteristic curve and

Multiple Choice Options

A)The phase current

Mathematical Content

lim

Topics Identified

CircuitsMachinesMaterials

Mark Allocation

Total200 marks

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

Paper information

SubjectElectrical Technology

Year2024

SessionMay-June

TypePast Paper

PaperPaper 1

LanguageEnglish

Pages20

Downloads13

Examination context

Decade2020s

Years ago2y

Season—

Start day—

Exam period—

Days since exam—

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