GRADE 12 SEPTEMBER 2015 ELECTRICAL TECHNOLOGY · mono-stable multivibrator. Draw at least TWO...

NATIONAL

SENIOR CERTIFICATE

GRADE 12

SEPTEMBER 2015

ELECTRICAL TECHNOLOGY

MARKS: 200 TIME: 3 hours

This question paper consists of 11 pages, including formula sheets.

*ELCTDM*

2 ELECTRICAL TECHNOLOGY (EC/SEPTEMBER 2015)

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INSTRUCTIONS AND INFORMATION 1. The question paper consists of SEVEN questions. 2. Answer ALL the questions. 3. Sketches and diagrams must be large, neat and fully labelled. 4. ALL calculations must be shown, and correct to TWO decimal places. 5. Number the answers correctly according to the numbering system used in

this question paper.

6. A formula sheet is provided at the end of the paper. 7. You may use a non-programmable calculator. 8. Show the units for all answers of calculations. 9. Write neatly and legibly.

(EC/SEPTEMBER 2015) ELECTRICAL TECHNOLOGY 3


QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY 1.1 Dangerous practices most commonly refer to processes that have some

form of risk or hazard when performed. State TWO activities or tasks that can be referred to as dangerous practices performed in an electrical workshop. (2)

1.2 An employer has the right to monitor communication within the workplace as

long as the employee is aware of the monitoring before it takes place. Name ONE such form of communication that an employer may monitor. (1)

1.3 Good work ethics require certain character traits or characteristics. Name

THREE characteristics that contribute to good work ethics. (3) 1.4 It is important that employees work as a team. Describe TWO

characteristics that contribute to effective teamwork. (2) 1.5 As a first-aider, you are the first on the scene of an accident. Before giving

first aid, what is the first action that must be taken? (2) [10] QUESTION 2: THREE-PHASE AC GENERATION 2.1 Draw a phasor diagram showing the three voltages in a three-phase

system. (3) 2.2 List THREE advantages that three-phase systems have over single

phase systems. (3) 2.3 A delta-connected alternator generates 220 V per phase. Each phase in

this balanced system has an impedance of 44 Ω. Calculate the line voltage, line current and phase current. (8)

2.4 Explain the difference between active power (also called real or true power)

and reactive power. (2) 2.5 Although three-phase alternators can deliver power to balanced and

unbalanced loads, why is it important that each phase of the alternators be balanced? Justify your answer. (4)

[20]



QUESTION 3: THREE-PHASE TRANSFORMERS 3.1 State whether the following statements are TRUE or FALSE. 3.1.1 The generators in the power stations produce electricity at 22 kV. (1) 3.1.2 This 22 kV is increased by transformers before it is sent out at 275

kV, 400 kV or even 765 kV onto the transmission grid. (1) 3.1.3 11 kV is fed into our homes for domestic use. (1) 3.1.4 Eskom is the first utility in the world to successfully operate

transmission lines at 765 kV at high altitudes above sea level. (1) 3.1.5 380 V and 240 V are available for domestic use. (1) 3.1.6 High voltage overhead conductors are nearly always made of

aluminium alloy. (1) 3.2 A farmer uses a three-phase delta-star transformer with a transformation

ratio of 150 : 3 to reduce a line voltage from 11 kV. He uses both single and three-phase motors on his farm.

3.2.1 Determine the voltage across each phase of the three-phase motor

if it is star-connected. (4) 3.2.2 This three-phase motor has a rating of 12 kW at a power factor of

0,8 lagging. Calculate the line current supplied from the secondary of the transformer. (6)

3.3 3.3.1 Does the size of the transformer affect the methods of cooling? (1) 3.3.2 Name THREE methods or ways in which transformers are cooled. (3) [20]



QUESTION 4: THREE-PHASE MOTORS AND STARTERS 4.1 State THREE advantages that three-phase induction motors have over

single-phase motors. (3) 4.2 Name the THREE main parts of a three-phase induction motor. (3) 4.3 Draw TWO sketches of a three-phase terminal block showing how the

terminals are connected for: 4.3.1 Star (3) 4.3.2 Delta (3) Label the terminals 4.4 A 50 kW three-phase induction motor has 12 poles in total and a rated

laggingpower factor of 0,9. It is connected to a 380 V / 50 Hz supply. Calculate the: 4.4.1 Apparent power (s) (3) 4.4.2 Line current (3) 4.4.3 Synchronous speed of the motor (4) 4.4.4 Slip if the shaft speed is 1 400 rpm (3) 4.4.5 Efficiency of the motor if the power on the shaft is 45 kW (3) 4.5 Draw a fully labelled circuit diagram of the control circuit of an automatic

star-delta motor starter. (12) [40]



QUESTION 5: RLC CIRCUITS 5.1 In a series or parallel circuit, describe the effect that the frequency has on: 5.1.1 Resistance (1) 5.1.2 Inductive reactance (1) 5.1.3 Capacitive reactance (1) 5.2 Define the term impedance. (3) 5.3 A parallel RLC consists of a 47 Ω resistor, an inductor with an inductive

reactance of 70 Ω, and a capacitor with a capacitive reactance of 80 Ω. The parallel circuit is connected to a 110 V / 60 Hz supply.

Calculate the : 5.3.1 Total current drawn from the supply (8) 5.3.2 Impedance of the circuit (2) 5.4 In a radio tuned circuit where it is desired to draw the least current, contrast

series and parallel RLC circuits. Decide which would be the better one to use. Justify your choice. (4)

[20] QUESTION 6: LOGIC 6.1 What does the abbreviation PLC stand for? (1) 6.2 Give TWO examples of the following with reference to PLC’s: 6.2.1 Inputs (2) 6.2.2 Outputs (2) 6.3 Name ONE industry where PLC’s are used. (1) 6.4 Draw the ladder diagram symbols for the following: 6.4.1 Normally open contact (input) (1) 6.4.2 Normally closed contact (input) (1) 6.4.3 Output (coil) (1)



6.5 A PLC program is a set of instructions within a specific programming

language for a PLC to perform a specific function. The execution of each instruction requires THREE basic steps. Describe these steps. (3)

6.6 Simplify the following Boolean expression using a Karnaugh map.

(8)

6.7 Draw the ladder diagram for a sequence starter without a timer. Refer to the

following wiring diagram.

(8) 6.8 Refer to the following truth table. A B SUM CARRY 0 0 0 0 0 1 1 0 1 0 1 0 1 1 0 1 6.8.1 Write the Boolean expressions for the given truth table. (4) 6.8.2 Design and draw the gate network for the expressions in QUESTION

6.8.1. (8) [40]



QUESTION 7: AMPLIFIERS 7.1 Draw a neat sketch showing the layout of the dual-in-line IC package of an

LM 741 op-amp. Show especially the inputs, the output and the supply voltage. (5)

7.2 With reference to an ideal op-amp, what is meant by infinite bandwidth? (2) 7.3 State THREE advantages of negative feedback. (3) 7.4 Name TWO applications of the op-amp as a comparator. (2) 7.5 An inverting op-amp using open loop gain and a 5 V reference voltage on the

non-inverting input is joined to a supply voltage of (+) and (–) 15 volts. What would the output voltage be if:

7.5.1 5 V were placed on the inverting input? (1) 7.5.2 3 V were placed on the inverting input? (1) 7.5.3 6 V were placed on the inverting input? (1) 7.6 Draw the circuit diagram of a non-inverting operational amplifier. Label the

components and show input and output waveforms. (8) 7.7 The feedback resistor used in a non-inverting amplifier is 100 kΩ and the

input resistor is 10 kΩ. Calculate the amplitude of the output signal if the amplitude of the input signal is 3 V. (3)

7.8 Draw a fully labelled circuit diagram of an op-amp integrator circuit. (5) 7.9 Draw the input and output waveforms of the op-amp integrator and explain

the mathematical function that the circuit performs. (4) 7.10

FIGURE 7.10 7.10.1 Identify the circuit in FIGURE 7.10. (1) 7.10.2 Name TWO applications for the circuit in QUESTION 7.10.1. (2)



7.11 7.11.1 What is a mono-stable multivibrator? (2) 7.11.2 Draw fully labelled waveforms of the input pulse and the output of a

mono-stable multivibrator. Draw at least TWO cycles of these waveforms. (6)

7.12

FIGURE 7.12 7.12.1 Identify the circuit in FIGURE 7.12. (1) 7.12.2 Calculate the oscillating frequency for an RC oscillator making use

of three RC networks. Assume all resistor values are the same and all capacitor values are the same. The value of each resistor is 10 kΩ and that of each capacitor 250 pF. (3)

[50] TOTAL: 200



FORMULA SHEET THREE-PHASE AC GENERATION Star VL = √

IL =

Delta IL = √ VL =

S = √ Q = √ VL IL Sin θ Cosθ =

Two wattmeter method PT = P1 + P2

THREE-PHASE TRANSFORMERS Star VL = √ VPH IL = IPH Delta IL = √ IPH

VL = VPH

P = √ VL IL Cos θ

S = √ VL IL

Q = √ VL IL Sin θ

Cosθ =

=

=

RLC CIRCUITS XL = 2πfL

Xc =

Series IT = IR = IC = IL Z = √

VL = I XL

VC = I XC

VT = I Z

VT = √

IT =

Cosθ =

Cosθ =

Parallel VT = VR = VC = VL

IR =

IC =

IL =

IT = √

Cosθ =



THREE-PHASE MOTORS AND STARTERS Star

VL = √

IL =

Delta IL = √ VL =

Power P = √ VL IL Cos θ S = √ VL IL Q = √ VL IL Sin θ

Efficiency (η)=

Speed

ns =

Slip =

AMPLIFIERS Gain AV =

= [

]

Gain AV =

= [

]

=

√

=

√

APPARENT POWER PAPP =

NATIONAL SENIOR CERTIFICATE

GRADE 12

SEPTEMBER 2015

ELECTRICAL TECHNOLOGY MEMORANDUM

MARKS: 200

This memorandum consists of 11 pages.



INSTRUCTIONS TO MARKERS 1. All questions with multiple answers imply that any relevant, acceptable

answer should be considered. 2. Calculations: 2.1 All calculations must show the formula(e). 2.2 Substitution of values must be done correctly. 2.3 All answers MUST contain the correct unit to be considered. 2.4 Alternative methods must be considered, provided that the same

answer is obtained. 2.5 Where an erroneous 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 the incorrect answer from the first calculation. If correctly used, the learner should receive the full marks for subsequent calculations.

3. The memorandum is only a guide with model answers. Alternative

interpretations must be considered and marked on merit. However, this principle should be applied consistently throughout.



QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY 1.1 x The etching of printed circuit boards. √

x The use of power tools. √ x Incorrect use of hand tools. √ (Any 2 x 1) (2)

1.2 x E-mails √

x Internet access √ x Telephone calls √ x Data √ x Images √ (Any 1 x 1) (1)

1.3 Honesty, √ self-discipline, √ reliability √ and loyalty √ are desirable traits or

characteristics. (Any 3 x 1) (3) 1.4 x Respecting the rights of others. √

x Being a team worker. √ x Being co-operative. √ x Being assertive. √ x Displaying a positive customer service attitude. √ x Seeking opportunities for continuous learning. √ x Demonstrating polite behaviour. √ x Respecting confidentiality. √ (Any 2 x 1) (2)

1.5 Assess hazards √ and make sure the area is safe. √ (2) [10] QUESTION 2: THREE-PHASE AC GENERATION 2.1

(3) 2.2 x Can be connected in star or delta.

x Two voltages available (VL and VPH). √ x For three-phase and single-phase alternators of similar physical sizes,

three-phase will generate more power. √ x Three-phase supply power to single and three-phase loads. √ x Three-phase is cheaper to generate. √ x Three-phase requires less maintenance. √

(Any 3 relevant advantages x 1) (3)



2.3 VL = VPH √ = 220 V √

IPH =

√ =

√ = 5 A√

IL = √3IPH √ = √3 √ = 8,66 A √ (8) 2.4 Active power that is effectively being used by the load or the circuit. √

Reactive power is the power that is wasted and not used to do work on the load. √ (2)

2.5 The copper windings of the alternators have both resistance and

inductance √ and each phase will have a certain impedance. √ It is of utmost importance that the rotating coils generate the same voltages and currents in each phase, √ and that is only possible if the impedance of all three phases are matched. √ (4)

[20] QUESTION 3: THREE-PHASE TRANSFORMERS 3.1 3.1.1 T √ (1) 3.1.2 T √ (1) 3.1.3 F √ (1) 3.1.4 T √ (1) 3.1.5 T √ (1) 3.1.6 T √ (1) 3.2 3.2.1 VL = VPH = 11 Kv √

I

=

√

VSEC (PH) = 3

√ = 220 V √ (4)

3.2.2 PACT = 3 VL IL cosθ √

IL = A T

√3 cosθ √

VL = 3 VPH = 3 √ = 381,7 √ IL = 12 000 √ 3 x 381,7 x 0,8 = 22,65 A (6)

3.3 3.3.1 Yes √ (1) 3.3.2 x Air cooled √

x Oil cooled √ x Water cooled √ (3)

[20]



QUESTION 4: THREE-PHASE MOTORS AND STARTERS 4.1 x Self-starting √

x Robust √ x Simple construction √ x Low maintenance √ x Reduced fire risk √ x Reduced running cost √ x High starting torque √ x Wide range of applications √ x Easy to change direction of rotation √ x Can be star or delta connected √ x High efficiency √ x Higher power factor √ x Wide range of power ratings √ (Any 3 x 1) (3)

4.2 Rotor, √ Stator, √ tator windings (or endplates) √ (3) 4.3 4.3.1

(3) 4.3.2

(3) 4.4 4.4.1

PAPP = A TI

os θ √ =

√= 55,5556 kVA √ (3) 4.4.2 IL = PACTIVE √

√3 os θ = 50000 √ √3 (38 ) ( ,9) = 84,41 A √ (3)

4.4.3 NS = 60 x f √

P = 60 x 50 √ 2 (4 poles/ phase = 2 Pole Pairs) √ = 1500 RPM √ (4)

4.4.4 SLIP = (NS – NR) x 100 √ = (1500 – 1400) x 100 √ = 6 67% √

NS 150 (3)



4.4.5 η = POUT x 100% √

PIN = 45 kW x 100 √ 50 kW = 90% √ (3)

4.5

(12) [40] QUESTION 5: RLC CIRCUITS 5.1 5.1.1 No effect √ (1) 5.1.2 Inductive Reactance is proportional to the frequency (XL = πf ) √ (1) 5.1.3 Capacitive Reactance is inversely proportional to frequency

(XC = /( πf ) √ (1) 5.2 Impedance is the total opposition √ to the flow of current √ in an

A circuit. √ (3) 5.3 5.3.1

(8) 5.3.2 Z = VS = 110 √ = 6,8 3 Ω √

IT 2,35 (2)



5.4 At resonance a parallel circuit draws minimum current √ whereas a

series circuit draws ma imum current. √ The parallel circuit would be better √ because the circuit would be tuned to the resonant frequency for longer. √ (4)

[20] QUESTION 6: LOGIC 6.1 Programmable Logic Controller (1) 6.2 6.2.1 x Push buttons √

x Contacts √ x Limit switches √ x Sensors √ (Any 2 x 1) (2)

6.2.2 x Solenoids √

x Contactors √ x Alarms √ (Any 2 x 1) (2)

6.3 Manufacturing, √ machining, √ food/beverage, √ metal, √ power, √

mining, √ petrochemical/chemical √ (Any 1 x 1) (1) 6.4 6.4.1

(1) 6.4.2

(1) 6.4.3

(1) 6.5 x Checking the inputs – (INPUT)

x Execution of instructions – (PROCESS) x Updating the outputs – (OUTPUT) (3)

6.6

= A √ (8)



6.7

(8) 6.8 6.8.1

(4) 6.8.2

(8) [40]



QUESTION 7: AMPLIFIERS 7.1

(5) 7.2 Amplifies both A and D √ without any loss in Gain. √ (2) 7.3 x Increase in bandwidth. √

x Increased stability. √ x Reduces distortion and noise. √ x Improved input and output impedances. √ x Allow design for a specific gain. √ (Any 3 x 1) (3)

7.4 x Zero Level Detector √

x Threshold detector √ x Null detectors √ (Any 2 x 1) (2)

7.5 7.5.1 0 V (1) 7.5.2 Approximately +15 V (1) 7.5.3 Approximately -15 V (1) 7.6

(8)



7.7

(3) 7.8

(5) 7.9

As in mathematical integration √

The circuit determines area by the gradient √ (4) 7.10 7.10.1 Bi-stable multi-vibrator √ (1) 7.10.2 contact bounce eliminator √

ela ation oscillator √ (2)



7.11 7.11.1 A mono-stable multi-vibrator is a timing circuit that changes state

once triggered, √ but returns to its original state after a certain time delay. √ (2)

7.11.2

(6) 7.12 7.12.1 RC phase shift oscillator (1) 7.12.2

(3) [50] TOTAL: 200

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