GRADE 11 NOVEMBER 2015 ELECTRICAL TECHNOLOGY

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1 NATIONAL SENIOR CERTIFICATE GRADE 11 NOVEMBER 2015 ELECTRICAL TECHNOLOGY MARKS: 200 TIME: 3 hours This question paper consists of 10 pages including a formula sheet.

2 2 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) INSTRUCTIONS AND INFORMATION 1. Answer ALL the questions. 2. Sketches and diagrams must be large, neat and fully labelled. 3. ALL calculations must be shown and correct to TWO decimal places. 4. Answers must be numbered correctly according to the numbering system used in this question paper. 5. A non-programmable calculator may be used. 6. A formula sheet is provided at the end of the question paper.

3 (EC/NOVEMBER 2015) ELECTRICAL TECHNOLOGY 3 QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY, TOOLS AND MEASURING INSTRUMENTS 1.1 An employee is responsible for working according to the safety rules and regulations in an electrical workshop. Give THREE examples of these rules and regulations that the employee must obey, in order to meet his responsibility. (3) 1.2 Oscilloscopes are normally used to measure AC and DC voltages and to examine waveforms. Discuss the procedure that can be used to measure the phase angle between two waveforms. (5) 1.3 Why is it necessary to use an insulation tester when measuring insulation resistance? (2) [10] QUESTION 2: SINGLE-PHASE AC GENERATION SINGLE-PHASE TRANSFORMERS 2.1 What is meant by instantaneous current? (2) Discuss the term RMS value with reference to a sine wave. (2) What is the relationship between radians and degrees? (1) What is meant by magnetic flux (φ), and how is it calculated? (3) 2.3 A coil with 200 turns has an area of 0,03 m 2 and is rotated at rpm about an axis through the centre and parallel with two sides in a uniform magnetic field of 0,6 T. If the frequency is 40 Hz and the period is 0,025 seconds, calculate: The maximum value of the generated EMF (3) The RMS value of the generated EMF (3) The instantaneous value of the generated EMF when the coil is at a position 60 degrees after passing its maximum induced voltage. (3) 2.4 With reference to AC generators, answer the following questions How does the number of windings of the coil affect the generated EMF? (2) How does the number of pole pairs affect the frequency of the generated EMF? (2) Why is it necessary to laminate the core used in generators? (2)

4 4 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) 2.5 How are transformers rated? (1) 2.6 Explain the principle of operation of a transformer. (4) 2.7 If the frequency applied to the primary of a transformer is 50 Hz, what will the secondary frequency be? Explain your answer. (2) 2.8 Give ONE reason why a transformer would overheat. (1) 2.9 A 220/24 Volt transformer can deliver 2 ampere. Calculate: The transformation ratio of the transformer (3) The resistive value of the load so that not more than 2 ampere is drawn from the transformer (3) The primary current (3) 2.10 What is an autotransformer? (2) 2.11 Name TWO types of losses that occur in transformers. (2) 2.12 Where are instrument transformers used? (2) 2.13 Describe how voltage and current (PT s and CT s) transformers are used. (4) [50]

5 (EC/NOVEMBER 2015) ELECTRICAL TECHNOLOGY 5 QUESTION 3: SINGLE-PHASE MOTORS AND PROTECTION DEVICES 3.1 Are the following statements TRUE or FALSE? Induction motors consist of a rotor, stator and stator windings. (1) Induction motors are self-starting. (1) In all single-phase induction motors the rotors are of the squirrel-cage type. (1) Universal motors can run off AC or DC supplies. (1) 3.2 Describe in detail the operation of a single-phase induction motor. (8) 3.3 Describe the operation of the centrifugal switch in an induction motor. (5) 3.4 What is the purpose of the hold-in contacts in the starter control circuit of a single-phase induction motor? (3) 3.5 Before a single-phase motor is put into service, various electrical tests need to be done; a continuity test and insulation test. The insulation test comprises of two tests What is the purpose of the continuity test? (1) Name the TWO tests involved in the insulation test. (2) State which test instrument must be used for the insulation test. (1) What readings are acceptable? (1) 3.6 Name TWO applications where universal motors are used. (2) 3.7 Explain how to change the direction of rotation of a capacitor start inductive run induction motor. (3) [30]

6 6 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) QUESTION 4: SEMI-CONDUCTOR DEVICES, POWER SUPPLIES AND AMPLIFIERS 4.1 Describe what is meant by the depletion region with reference to diodes. (4) 4.2 Give ONE application where Zener diodes are used. (1) 4.3 Describe with the aid of a circuit diagram, the operation of a SCR. Use the two transistor analogy. (6) 4.4 State TWO ways that an SCR can be switched off. (2) 4.5 In a lamp dimming circuit, what is the purpose of the resistor that is in series with the variable resistor? (2) 4.6 Draw a labelled block diagram of a regulated DC power supply. (4) 4.7 Draw a fully labelled circuit diagram of a shunt regulator that uses a transistor. (4) 4.8 What is the advantage of a bridge rectifier over against using two diodes and a centre-tap transformer? (2) With reference to the transistor load line, what is meant by the Q-point? (3) How is the output of a transistor influenced, when biased in class A? (2) Name TWO applications where Class B amplifiers are used. (2) 4.10 Transistor amplifier circuits are configured in one of three ways. Name the THREE configurations. (3)

7 (EC/NOVEMBER 2015) ELECTRICAL TECHNOLOGY FIGURE 4.10 What is the purpose of each of the components numbered 1 8 in the diagram in FIGURE 4.10? (8) 4.12 Give THREE advantages of negative feedback. (3) With reference to common emitter transistor amplifiers, what is meant by thermal runaway? (2) How can thermal runaway be prevented? (2) [50]

8 8 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) QUESTION 5: RLC SERIES CIRCUITS How is the reactance of a capacitor influenced by an increase in frequency? (1) How is the reactance of an inductor influenced by an increase in frequency? (1) What is meant by the term power factor? (2) State TWO characteristics of an RLC circuit at resonance. (2) 5.2 A series AC circuit consists of an 24 Ω resistor, a 10 mh inductor, and a 470 μf capacitor. The circuit is connected across a 110 V, 60 Hz supply. Calculate: The impedance of the circuit (9) Will this circuit have a leading, or lagging power factor? (1) At what frequency will this circuit resonate? (4) [20] QUESTION 6: LOGIC 6.1 Create a NAND gate using NOR gates. Make use of your knowledge of logic circuits and Boolean expressions. (5) 6.2 The owner of a spaza shop in your area asks you to design a simple alarm for his shop. The shop has one window and one door. You must design an alarm that will sound if either the window or the door is opened Write down the truth table. (3) From the truth table derive the sum-of-products expression. (3) Simplify the sum-of-products expression. (2) Draw the gate network. (2) 6.3 Prove (5) [20]

9 (EC/NOVEMBER 2015) ELECTRICAL TECHNOLOGY 9 QUESTION 7: COMMUNICATIONS 7.1 State whether the following statements are TRUE or FALSE A single repeater system requires one frequency. (1) The receiver of a repeater is tuned to the transmit frequency of the mobile radios. (1) The transmitter of a repeater transmits on the receive frequency of the mobile radios. (1) The wavelength of an electromagnetic signal is the speed of light, multiplied by the frequency of the signal. (1) The gain of an antenna is determined by the radiation of the antenna. (1) Radio propagation is a term used to explain how radio waves behave when they transmit. (1) 7.2 Name THREE agencies or services that use repeater systems. (3) Explain the difference between AM and FM. (4) Describe the advantage that FM has over AM. (1) State TWO advantages that cellular systems have over alternative systems. (2) 7.4 An AM receiver uses a detector. An FM receiver uses a discriminator. Compare the two forms of demodulation. (4) [20] TOTAL: 200

10 10 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) ELECTRICAL TECHNOLOGY GRADE 11 FORMULA SHEET Rs = R1 + R2 + R3 + R = P = P = = R = P = l R a R C R L 2 d a 4 f Cos θ V Vcc RB V B F = o of F = s Emf = 2 BAnNsin Current gain = X X Z I V Z Z L C E E E E ave e EmSin 2 F Em rms wgk gem 2 FL 1 2 FC R I V 2 Em 0,637 Em 0,707 Em ),637 2 R ( 2 R ( X I L ( V X X L X L C I V 1 FR 2 LC Vout Gain Vin Vuit Wins Vin Vcc Ic Rc Ns Vs Ip Np Vp Is S Vp Ip A. B A B 1 T F V V Div Div Vz Iz Z P V. I. Cos P V V S O CE VI V V Zener I V V O basis ) 2 X C X C ) 2 ) 2-1 (R/Z)

11 NATIONAL SENIOR CERTIFICATE GRADE 11 NOVEMBER 2015 ELECTRICAL TECHNOLOGY MEMORANDUM MARKS: 200 This memorandum consists of 10 pages.

12 2 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) 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 recalculate 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.

13 (EC/NOVEMBER 2015) ELECTRICAL TECHNOLOGY 3 QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY, TOOLS AND MEASURING INSTRUMENTS 1.1 No horseplay in the workshop. No eating and drinking in the workshop. Wear protective clothing and equipment when using dangerous tools and machines. Never operate machines without supervision and permission. Never use any tools or machines unless properly trained. (Any 3 x 1) (3) 1.2 Set the time per division control. Read of the number of divisions between the two waveforms. One cycle equals 360 degrees. Calculate the phase angle (e.g. if one cycle uses 10 divisions then one division equals 36 degrees). (5) 1.3 An insulation tester uses +/- 500 V when testing and insulation is more likely to break down at the higher voltage. (2) [10] QUESTION 2: SINGLE-PHASE AC GENERATION SINGLE-PHASE TRANSFORMERS 2.1 Instantaneous current is the current measured at an instant in time. (2) The RMS value of a sine wave is the value that will deliver the same power as an equivalent Direct Current Value. (2) There are 2 radians in 360º. (1) Magnetic flux through a surface refers to the number of magnetic field lines which pass through a given cross-sectional area. It can be calculated using the formula φ = BA, where φ is the number of flux lines measured in webers (Wb), B is the magnetic field strength measured in telsa (T), and A is the cross-sectional area measured in meters squared (m 2 ). (3) EMF = 2 BAnNsinθ = 2 x 0,6 x 0,03 x 40 x 200 x sin 90º = 904,78 V (3) E RMS = E max x 0,707 = 904,78 x 0,707 = 639,68 V (3) e = E max sin(90º + 60º) = 639,68 x sin150º = 319,84 V (3)

14 4 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) The generated EMF is directly proportional to the number of windings. (2) If more pole pairs are added then for each revolution more cycles will be generated increasing the frequency. (2) Laminated cores reduce Eddy currents induced in the core thereby making the coil more efficient. (2) 2.5 Transformers are rated according to their apparent power (VA). (1) 2.6 When an alternating emf is applied to the primary winding, an alternating magnetic field is set up around the primary winding. This alternating magnetic field induces an alternating emf in the secondary winding. The magnitude of this induced emf depends upon the transformation ratio of the transformer. (4) Hz Transformers cannot change frequency. (2) 2.8 If too much current is drawn from the secondary. (1) N P /N S = V P /V S = 220 / 24 = 9,17 : 1 (3) R = V / I = 24 / 2 = 12 Ω (3) V P / V S = I S / I P I P = (I S x V S ) / V P = (2 x 24) / 220 = 218 ma (3) 2.10 An autotransformer is a transformer that does not have separate primary and secondary windings. (The secondary side is tapped off the primary side.) (2) 2.11 Copper losses (I 2 R losses) Eddy Current losses (Heat losses) Dielectric losses Iron losses (Hysteresis losses) (Any 2 x 1) (2) 2.12 Instrument transformers are used to drive panel meters on distribution boards. (2) 2.13 Voltage Transformers (PT s) are used where high voltages need to be measured by low voltage movements in volt meters. Current Transformers (CT s) are used where high currents need to be measured by low current movements in amp. metres. (4) [50]

15 (EC/NOVEMBER 2015) ELECTRICAL TECHNOLOGY 5 QUESTION 3: SINGLE-PHASE MOTORS AND PROTECTION DEVICES True (1) False (1) True (1) True (1) 3.2 A single phase induction motor consists of a stator with two windings, namely a start winding and a run winding. The two windings have different impedances. When an AC supply is applied to the stator, a rotating magnetic field is set up because of the different impedances. This rotating magnetic field induces an EMF in the conductors of the rotor. This EMF sets up magnetic fields that work in conjunction with the rotating magnetic fields of the stator resulting in torque applied to the rotor. The rotation of the rotor is in the same direction as the rotation of the stator field. (8) 3.3 The centrifugal switch is placed in series with the start winding. When the rotor has run up to speed, the centrifugal switch opens, disconnecting the start winding. The motor therefore begins to slow down until the centrifugal switch closes again. So the centrifugal switch is used to control the speed of the motor. (5) 3.4 When the main contactor is energised, the hold-in circuit (Normally Open) closes, and remains closed when the start button is released. When power is removed, the hold-in circuit opens, there by removing power from the main contactor. The starter controlled circuit therefore has to be manually restarted. (3) The purpose of the continuity test is to measure the resistance of the windings. (1) Insulation test between conductors Insulation test between conductors and earth. (Any 2 x 1) (2) Insulation tester (or megger) (1) MΩ or greater (1) 3.6 Vacuum cleaners, portable drills, drink mixers, sewing machines, etc. (Any 2 x 1) (2) 3.7 Change the polarity of the start winding or run winding but not both. (3) [30]

16 6 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) QUESTION 4: SEMI-CONDUCTOR DEVICES, POWER SUPPLIES, AND AMPLIFIERS 4.1 When P-type and N-type materials are joined, holes of the P-type and electrons of the N-type combine to form covalent bonds. The electrons diffuse and occupy the holes in the P-type material. A small region of the N- type near the junction loses electrons and behaves like intrinsic semiconductor material. In the P-type, a small region get filled up by holes and behaves like an intrinsic semiconductor material. This thin intrinsic region is called the depletion region, since it is depleted of charge and offers high resistance. (4) 4.2 In simple voltage regulators. (1) on anode, - on cathode + pulse on g switches TR2 on causing collector of TR2 to become more negative. This switches TR1 on, its collector going positive which keeps TR2 on even though the + pulse on gate has been removed. Current will continue to flow until either the supply is removed between the anode and cathode, or the current falls below the holding current. 4.4 Remove supply from anode and cathode. Reduce the current to below the holding current. (2) 4.5 The series resistor prevents the applied voltage from appearing across the capacitor when the variable resistor is set to 0 Ω. (2) (6) 4.6 AC INPUT OUTPUT REGULATOR TRANS- FORMER RECTIFIER FILTER (4) 4.7 (4)

17 (EC/NOVEMBER 2015) ELECTRICAL TECHNOLOGY The output from a bridge rectifier is almost twice that of a circuit using two diodes and a centre-tap transformer. (2) The Q-point on the load line is the point at which DC bias is provided to the transistor to ensure that it operates, depending upon the class of the transistor amplifier. (3) The complete input signal is amplified without distortion. (2) Push-pull amplifiers (audio power amplifiers), RF power amplifiers. (2) 4.10 Common-base, common-collector, common-emitter (3) &2 R1 and R2 form a voltage divider to set the base voltage. 3 R3 limits the current drawn by the transistor. 4 R4 provides a load for the transistor so that V OUT = V CC V RC 5 The transistor amplifies the input signal. 8 C1 acts as a coupling capacitor. 6 C2 acts as a coupling capacitor to the next stage. 7 CE stabilises the bias of the amplifier. (8) 4.12 Reduces noise and distortion at the output. Enables one to design for a specific gain. Stabilises voltage gain. Increases bandwidth. (Any 3 x 1) (3) When current flows through a transistor it heats up and can overheat if too much current is allowed to flow. (2) A resistor is placed in series with the emitter, to limit the flow of current through the transistor. (2) [50]

18 8 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) QUESTION 5: RLC SERIES CIRCUITS Reactance decreases (1) Reactance increases (1) Power factor is the cosine of the phase angle between the applied voltage and the total current. (2) X L = X C Z = Minimum Power factor = 1 Phase angle = 0 I = maximum (Any 2 x 1) (2) X L = 2 fl = 2 x 60 x 0,01 = 3,77 Ω 1 X C = 2 fc, = 1 2 x 0 x 470 x 10 - = 5, 4 Ω Z = R 2+ 2 ( C ) = (5, 4 3,77) = 24,07 Ω (9) Leading (1) fr = 1 2 C = 1 2 0,01 x 470 x 10 - = 73,41 Hz (4) [20]

19 (EC/NOVEMBER 2015) ELECTRICAL TECHNOLOGY 9 QUESTION 6: LOGIC 6.1 = A B X A + B = X A B X (3) SOP = A.B + A.B + A.B (3) A.B + A.B + A.B = B(A + A ) + A.B = B + A.B = A + B (2) (5) A(Window) B(Door) 1 A + B X 6.3 LHS = A.B.C + A.B.C + A.B.C = A.B(C + C) + A. B.C = A.B(1) + A.B.C = A (B + B.C) = A (B + C) = A.B + A.C LHS = RHS (5) [20]

20 10 ELECTRICAL TECHNOLOGY (EC/NOVEMBER 2015) QUESTION 7: COMMUNICATIONS FALSE (1) TRUE (1) TRUE (1) FA SE (1) TRUE (1) TRUE (1) 7.2 Forestry, ambulance, utilities, police, etc. (Any 3 x 1) (3) With AM the amplitude of the carrier is modulated according to the information signal whereas with FM the frequency of the carrier is modulated according to the information signal. (4) FM is not susceptible to interference. (1) Increased capacity. Reduced power use. Larger coverage area. Reduced interference from other signals. (Any 2 x 1) (2) 7.4 A detector is simply a circuit where half of the AM signal is rectified and the RF is removed, leaving the audio signal. The discriminator compares the FM signal with a reference signal and the difference between the two signals is the original audio signal. (4) [20] TOTAL: 200

NATIONAL SENIOR CERTIFICATE GRADE 11

NATIONAL SENIOR CERTIFICATE GRADE 11 ELECTRICAL TECHNOLOGY EXEMPLAR 007 MARKS: 00 TIME: hours This question paper consists of 16 pages and a 1-page formula sheet. Electrical Technology INSTRUCTIONS AND