Electrical Technology Grade 12 NSC November 2015 QP (Eng)

Downloaded from hlayiso.com GRAAD 12 NATIONAL SENIOR CERTIFICATE GRADE 12 ELECTRICAL TECHNOLOGY NOVEMBER 2015 MARKS: 200 TIME: 3 hours This question paper consists of 13 pages and a 2-page formula sheet. Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 2 DBE/November 2015 NSC INSTRUCTIONS AND INFORMATION 1. This question paper consists of SEVEN questions. 2. Answer ALL the questions. 3. Sketches and diagrams must be large, neat and fully labelled. 4. Show ALL calculations and round off answers correctly to TWO decimal places. 5. Number the answers correctly according to the numbering system used in this question paper. 6. You may use a non-programmable calculator. 7. Show the units for all answers of calculations. 8. A formula sheet is attached at the end of this question paper. 9. Write neatly and legibly. Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 3 DBE/November 2015 NSC QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY 1.1 State ONE unsafe act that may lead to an accident in an electrical technology workshop. (1) 1.2 Define the term dangerous practices with reference to workshop practice. (2) 1.3 Name TWO unsafe conditions that may exist in an electrical workshop. (2) 1.4 Explain the term human rights in the workplace with reference to the protection of a worker. (2) 1.5 Explain why risk management is important for ensuring safety in a workshop. (3) [10] QUESTION 2: THREE-PHASE AC GENERATION 2.1 Describe the term active power. (2) 2.2 Draw a neatly labelled phasor diagram of the voltages generated by a three-phase AC system. Indicate the direction of rotation. (5) 2.3 A three-phase star-connected motor has an output power of 50 kW and draws a current from a 380 V/50 Hz supply at a power factor of 0,95 lagging. Assume there are no losses. Given: P = 50 kW Vl = 380 V Cos Ө = 0,95 lagging Calculate the: 2.3.1 Line current (3) 2.3.2 Apparent power (3) 2.4 State TWO advantages of power-factor correction in AC systems. (2) 2.5 State the function of a wattmeter. (2) 2.6 Two wattmeters that are used to measure the input power are connected to a balanced three-phase circuit and indicate 2 500 W and 500 W respectively. Calculate the total input power. Given: P1 = 2 500 W P2 = 500 W (3) [20] Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 4 DBE/November 2015 NSC QUESTION 3: THREE-PHASE TRANSFORMERS 3.1 Name ONE protection device used in transformers. (1) 3.2 State ONE negative impact of losses in transformers. (1) 3.3 Name THREE methods of cooling transformers. (3) 3.4 Describe the effect on the primary current of a transformer if the load is increased. (3) 3.5 The delta-star-connected transformer in FIGURE 3.1 supplies a factory with 60 kW. The current lags the voltage by 36,87°. The primary line voltage is 11 kV and the secondary line voltage is 380 V. Vl (primary) = 11 kV Vl (secondary) = 380 V L1 L1 F A N C L2 L2 T O L3 L3 R Y FIGURE 3.1: DELTA-STAR-CONNECTED TRANSFORMER Given: Pout = 60 kW Ө = 36,87° lagging Vl (primary) = 11 kV Vl (secondary) = 380 V Calculate the: 3.5.1 Secondary line current (3) 3.5.2 Primary line current (3) 3.5.3 Apparent power (3) 3.5.4 Reactive power (3) [20] Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 5 DBE/November 2015 NSC QUESTION 4: THREE-PHASE MOTORS AND STARTERS 4.1 Name TWO parts of a three-phase squirrel-cage induction motor. (2) 4.2 State TWO advantages of a three-phase motor when compared to a single-phase motor. (2) 4.3 Describe why it is important to carry out electrical inspections on a motor after installation and before energising. (2) 4.4 Refer to the circuit diagram in FIGURE 4.1. L O/L Stop Start Hold in Start Hold in Forward MC1 (N/O) Reverse MC2 (N/O) MC2 MC1 N/C N/C MC1 MC2 Forward Reverse N FIGURE 4.1: CONTROL CIRCUIT OF A FORWARD-REVERSE STARTER 4.4.1 Describe how the contact O/L protects the motor. (3) 4.4.2 Describe how interlocking is used in the starter. (5) 4.5 Describe the function of a star-delta starter. (3) 4.6 Describe the function of a no-volt coil in a motor starter. (3) 4.7 State how the direction of rotation of a three-phase motor may be reversed. (2) 4.8 The name plate on a three-phase motor provides important information about the motor. List THREE types of information it provides. (3) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 6 DBE/November 2015 NSC 4.9 A three-phase motor is connected to a 380 V/50 Hz supply. The motor has six pole pairs and a slip of 4%. Given: Vs = 380 V f = 50 Hz S = 4% p = 6 Calculate the: 4.9.1 Synchronous speed (3) 4.9.2 Rotor speed (3) 4.10 A three-phase delta-connected motor is connected to a 380 V/50 Hz supply. The motor develops 29,7 kW at full load. It has a power factor of 0,85 and an efficiency of 90%. Given: Pout = 29,7 kW Vl = 380 V Cos Ө = 0,85 ŋ = 90% Calculate the: 4.10.1 Input power (3) 4.10 2 Input line current (3) 4.11 Explain what will happen to the current drawn by a motor if the power factor of the motor is improved. (3) [40] Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 7 DBE/November 2015 NSC QUESTION 5: RLC 5.1 Calculate the capacitive reactance of a 120 µF capacitor when it is connected to a 240 V/50 Hz supply. Given: C = 120 µF Vs = 240 V f = 50 Hz (3) 5.2 Describe why the current in an RLC series circuit is at maximum at resonant frequency. (4) 5.3 Refer to the circuit diagram in FIGURE 5.1 and answer the questions that follow. Ir= 2,5 A Il = 3,6 A Ic = 6 A It t Vs = 240 V/50 Hz FIGURE 5.1: PARALLEL RLC CIRCUIT Given: Ir = 2,5 A Il = 3,6 A Ic = 6A Vs = 240 V f = 50 Hz 5.3.1 Calculate the total current in the circuit. (3) 5.3.2 Calculate the inductive reactance of the coil. (3) 5.3.3 Describe what will happen to the current through the coil if the frequency of the supply is increased. (3) 5.4 Define the term Q-factor with reference to a parallel circuit. (4) [20] Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 8 DBE/November 2015 NSC QUESTION 6: LOGIC 6.1 Study FIGURE 6.1 which shows a typical PLC scan cycle and answer the questions that follow. STEP 1 STEP 2 STEP 3 Input Process Output FIGURE 6.1: PLC SCAN CYCLE 6.1.1 Name TWO input devices that may be connected in Step 1. (2) 6.1.2 List THREE programming languages that may instruct the PLC in Step 2. (3) 6.1.3 Describe how EACH step in FIGURE 6.1 is used to run a PLC program. (6) 6.1.4 Explain why heavy-current devices cannot be connected directly to the output of a PLC that uses a transistor output. (3) 6.1.5 Describe how the problem in QUESTION 6.1.4 is solved in industrial applications. (3) 6.2 Describe why relay control systems (hard-wired systems) are replaced by PLC control systems. (2) 6.3 Refer to the following Boolean equation to answer the questions that follow: X  A B CD  A B CD  A B CD  A B CD  A B CD 6.3.1 Draw a Karnaugh map and show the grouping. (9) 6.3.2 Write down the simplified expression that can be extracted from the Karnaugh map. (3) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 9 DBE/November 2015 NSC 6.4 Refer to the circuit in FIGURE 6.2. L Stop O/L L Start Hold in Start Hold in 1 MC1 (N/O) 2 MC2 (N/O) MC1 MC2 (Lamp) (Fan) N FIGURE 6.2: CONTROL CIRCUIT OF A SEQUENCE STARTER 6.4.1 MC1 switches on a lamp and MC2 closes a contactor that energises a fan. Draw the PLC ladder diagram that would execute the same function of the relay control circuit in FIGURE 6.2. (8) 6.4.2 State what programming function must be included in the ladder diagram to automate the starting of the fan (MC2). (1) [40] QUESTION 7: AMPLIFIERS 7.1 List THREE characteristics of an ideal operational amplifier (op amp). (3) 7.2 Describe the term bandwidth in relation to op amps. (3) 7.3 Give TWO reasons why negative feedback is used in op-amp circuits. (2) 7.4 Describe the term positive feedback. (3) 7.5 Describe the term stability in relation to op amps. (2) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 10 DBE/November 2015 NSC 7.6 Study FIGURE 7.1 and answer the questions that follow. Rin = 5 kΩ Rin Rf Rf = 15 kΩ - Vin = 1,5 V Vout Vin __ + 0V FIGURE 7.1: OP-AMP CIRCUIT 7.6.1 Identify the op-amp circuit in FIGURE 7.1. (1) 7.6.2 Draw the input and output waveforms on the same y-axis and label the waveforms. (3) 7.6.3 Calculate the voltage gain if the feedback resistor has a value of 15 kΩ and the input resistor has a value of 5 kΩ. (3) 7.6.4 Calculate the output voltage if an input signal of 1,5 V is applied to the op amp. (3) 7.7 Study FIGURE 7.2 and answer the questions that follow. Vin - Vout -+_ Rf _ R1 FIGURE 7.2: INVERTING SCHMIDT TRIGGER 7.7.1 State TWO applications of the op-amp circuit in FIGURE 7.2. (2) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 11 DBE/November 2015 NSC 7.7.2 Redraw the input waveform shown in FIGURE 7.3 and directly below it draw the output waveform. +Vcc Upper trigger level Input 0 V voltage Lower trigger level -Vcc Output voltage FIGURE 7.3: INPUT WAVEFORM TO AN INVERTING SCHMIDT TRIGGER (6) 7.8 Calculate the resonant frequency of a Hartley oscillator consisting of two coils of 40 mH each and a capacitor of 0,65 µF. The tank circuit of the Hartley oscillator is shown in FIGURE 7.4. L1 L2 C FIGURE 7.4: TANK CIRCUIT OF A HARTLEY OSCILLATOR Given: C = 0,65 µF L1 = 40 mH L2 = 40 mH (3) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 12 DBE/November 2015 NSC 7.9 Study FIGURE 7.5 and answer the questions that follow. C1 Trigger input C2 R1 Vout R2 FIGURE 7.5: OP-AMP CIRCUIT Given: C1 = 1 µF C2 = 22 nF R1 = 1,5 kΩ R2 = 550 kΩ 7.9.1 Identify the op-amp circuit in FIGURE 7.5. (1) 7.9.2 State ONE practical application of this type of op amp. (1) 7.9.3 Calculate the time the multivibrator will remain in the set state after it has received a trigger pulse. (3) 7.9.4 Redraw the input waveform shown in FIGURE 7.6 and directly below it draw the output waveform. Show the time calculated in QUESTION 7.9.3 on the output waveform. Trigger pulse 0V Output waveform FIGURE 7.6: INPUT TRIGGER PULSE FOR FIGURE 7.5 (5) Copyright reserved Please turn over

Downloaded from hlayiso.com Electrical Technology 13 DBE/November 2015 NSC 7.10 Study FIGURE 7.7 and answer the questions that follow. C1 Trigger input R1 R3 Vout R2 0V FIGURE 7.7: BI-STABLE MULTIVIBRATOR 7.10.1 State ONE application of a bi-stable multivibrator. (1) 7.10.2 If the input trigger pulse is negative, describe what value the output would move to. (2) 7.11 Describe the function of the dual DC supply in op-amp circuits. (3) [50] TOTAL: 200 Copyright reserved

Downloaded from hlayiso.com Electrical Technology DBE/November 2015 NSC FORMULA SHEET THREE-PHASE AC GENERATION RLC CIRCUITS Star Xl  2fL Vl  3 Vp Xc  1 2fC Il  Ip 1 fr  2 (LC) Delta Il  3 Ip Series Vl  Vp It  Ir  Ic  Il Z  R 2  X l ~  Xc  2 P  3VpIp cos θ Vl  I X l P  3Vl  Ilcos θ Vc  I X c S  3 Vl Il Vt  I Z Q  3 Vl Il sinθ Vt  Vr  Vl ~  Vc  2 2 P Vt Cos θ  It  S Z Vp R Zp  Cos θ  Ip Z Vr Two wattmeter method Cos θ  Vt Pt  P1  P2 Xl Q R Parallel OPERATIONAL AMPLIFIERS Vt  Vr  Vc  Vl Vout R  V Ir  r Gain A v     f  inverting op amp Vin  R in  R Vout R Vc Gain A v   1 f non-inverting op amp Ic  Vin R in Xc 1 V fr  Hartley oscillator Il  l 2 L t C Xl 1 Il  Ir  Il  2 ~ Ic  2 frc  RC phase-shift oscillator 2 6 RC Vout (V 1  V2  ...Vn ) I Cos θ  r It X Q l R Copyright reserved

Downloaded from hlayiso.com Electrical Technology DBE/November 2015 NSC THREE-PHASE MOTORS AND STARTERS THREE-PHASE TRANSFORMERS Star Star Vl  3 Vp Vl  3 Vp Il  Ip Il  Ip Delta Delta Il  3 Ip Il  3 Ip Vl  Vp Vl  Vp Power Power P  3VpIp cos θ P  3VpIp cos θ P  3 Vl Il cos θ P  3 Vl Il cos θ S  3 Vl Il S  3 Vl Il Q  3 Vl Il sin θ Q  3 Vl Il sin θ P Cos θ  S Vp primary  Np Ip secondary    Vp secondary  Ns Ip primary  Pout Pout Efficiency η  Efficiency η  Pin Pin Pout  PIn  losses Pout  Pin  losses  S  cos θ  η  S  cos θ  η 60  f ns  p n  nr Slipper unit  s ns nr  ns 1  Sper unit  n s  nr %slip   100% ns Copyright reserved