Electrical Technology Grade 12 Eastern Cape Mock 2020 QP

Downloaded from hlayiso.com NATIONAL SENIOR CERTIFICATE GRADE 12 SEPTEMBER 2020 ELECTRICAL TECHNOLOGY: POWER SYSTEMS MARKS: 200 TIME: 3 hours This question paper consists of 13 pages, including a 2-page formula sheet.

Downloaded from hlayiso.com 2 ELECTRICAL TECHNOLOGY: POWER SYSTEMS (EC/SEPTEMBER 2020) INSTRUCTIONS AND INFORMATION 1. This question paper consists of SIX questions. 2. Answer ALL the questions. 3. Show ALL calculations and round off answers correctly to TWO decimal places. 4. Number the answers correctly according to the numbering system used in this question paper. 5. Sketches and diagrams must be large, neat and FULLY LABELLED. 6. You may use a non-programmable calculator. 7. Calculations must include: 7.1 Formulae and manipulations where needed 7.2 Correct replacement of values 7.3 Correct answers and relevant units where applicable 8. A formula sheet is provided at the end of this question paper. 9. Write neatly and legibly. Copyright reserved Please turn over

Downloaded from hlayiso.com (EC/SEPTEMBER 2020) ELECTRICAL TECHNOLOGY: POWER SYSTEMS 3 QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY 1.1 Define the term accident with reference to the Occupational Health and Safety Act, 1993 (Act 85 of 1993). (2) 1.2 Explain TWO general duties of employees in the workplace. (2) 1.3 State TWO unsafe conditions in a school workshop that can cause an accident. (2) 1.4 Briefly explain a third-degree burn. (2) 1.5 State TWO functions of a health and safety representative. (2) [10] QUESTION 2: RLC 2.1 Explain the phase relationship between current and voltage in the following AC circuit: 2.1.1 A pure capacitive circuit (2) 2.1.2 A pure inductive circuit (2) 2.1.3 A resistive circuit (2) 2.2 Refer to the circuit diagram FIGURE 2.2 below and answer the questions that follow. FIGURE 2.2: RLC CIRCUIT Calculate the: 2.2.1 Inductive reactance (3) 2.2.2 Capacitive reactance (3) 2.2.3 Impedance of the circuit (3) 2.2.4 Total current flow through the circuit (3) Copyright reserved Please turn over

Downloaded from hlayiso.com 4 ELECTRICAL TECHNOLOGY: POWER SYSTEMS (EC/SEPTEMBER 2020) 2.3 Explain how the value of the capacitive reactance will be affected if the supply frequency is increased. (3) 2.4 A parallel circuit in FIGURE 2.5 consists of a capacitor that draws a current of 4 A, an inductor that draws a current of 6 A and a resistor that draws a current of 5 A. All the components are connected to 240 V / 50 Hz supply. FIGURE 2.5: RLC PARALLEL CIRCUIT Calculate: 2.4.1 The total current through the circuit (3) 2.4.2 The phase angle (3) 2.4.3 The inductive reactance (3) 2.4.4 The capacitive reactance (3) 2.5 A parallel RLC circuit consists of a 10 Ω resistor and a 0,05 µF and a 0,2 H inductor connected across a 120 V supply. Calculate the: 2.5.1 Resonant frequency (3) 2.5.2 Q-factor (4) 2.5.3 Bandwidth (3) 2.6 A series RLC tuned circuit has a resonant frequency of 95 MHz and a bandwidth of 200 kHz. The capacitor is 2,5 pF and the wire used to wind the coil has zero resistance. Calculate: 2.6.1 The Q factor of the circuit (3) 2.6.2 The size of the inductor (2) 2.6.3 The circuit resistance (2) [50] Copyright reserved Please turn over

Downloaded from hlayiso.com (EC/SEPTEMBER 2020) ELECTRICAL TECHNOLOGY: POWER SYSTEMS 5 QUESTION 3: THREE-PHASE AC GENERATION 3.1 Draw a neat, labelled schematic representation of a star connected power system. Show all voltages and phasors. (4) 3.2 State the functions of the following meters: 3.2.1 Kilowatt-hour meter (2) 3.2.2 Wattmeter (1) 3.3 Name TWO disadvantages of a single-phase system when compared to a three-phase system. (2) 3.4 Refer to FIGURE 3.4 below and answer the questions that follow. FIGURE 3.4 3.4.1 Describe how the two watt meters are connected to the star system. (3) 3.4.2 Calculate the total power of the three-phase system. (3) 3.4.3 Calculate the power factor of the system. (5) Copyright reserved Please turn over

Downloaded from hlayiso.com 6 ELECTRICAL TECHNOLOGY: POWER SYSTEMS (EC/SEPTEMBER 2020) 3.5 A 380 V three-phase system is star connected to a lagging load. The input power is 25 kW and the load causes a lagging power factor of 0,8. Given : VL = 380 V PIN = 25 kW cos θ = 0,8 Calculate: 3.5.1 The phase voltage (3) 3.5.2 The line current to the load (3) 3.5.3 The reactive power (3) 3.6 Explain why the power supply to dwellings cannot be connected in delta. (2) 3.7 Describe how a low power factor affects an AC electrical power system. (4) 3.8 Discuss the following stages of three-phase AC generation from the supplier to the consumer. 3.8.1 Generation (2) 3.8.2 Transmission (2) 3.8.3 Mention ONE type of consumer that receives electricity from intermediate substations. (1) [40] Copyright reserved Please turn over

Downloaded from hlayiso.com (EC/SEPTEMBER 2020) ELECTRICAL TECHNOLOGY: POWER SYSTEMS 7 QUESTION 4: THREE-PHASE TRANSFORMERS 4.1 Name TWO types of transformer configurations available when manufacturing three-phase transformers. (2) 4.2 Mention the reasons for mounting the magnetic circuit and windings of power transformers in mineral oil. (2) 4.3 Mention TWO types of protection devices used for the protection of transformers. (2) 4.4 State TWO cooling methods used for oil-immersed transformers. (2) 4.5 Name the type of induction that takes place between the primary and secondary coils of a transformer. (1) 4.6 A 20 kVA delta-star transformer has a primary voltage of 6 kV and a secondary voltage of 1 000 V. The losses amount to 160 W and the power factor is 0,88. Given: S = 20 kVA VLP = 6 kV VLS = 500 V PLOSSES = 160 W cos θ = 0,88 Calculate: 4.6.1 The secondary line current (3) 4.6.2 The transformation ratio (6) 4.6.3 The input power (3) 4.6.4 The efficiency of the transformer (3) 4.7 Explain why transformers have a better efficiency than motors. (3) 4.8 Explain why a transformer only operates with an AC supply. (3) [30] Copyright reserved Please turn over

Downloaded from hlayiso.com 8 ELECTRICAL TECHNOLOGY: POWER SYSTEMS (EC/SEPTEMBER 2020) QUESTION 5: THREE-PHASE MOTORS AND STARTERS 5.1 Define the term slip with reference to a three-phase AC induction motor. (2) 5.2 Give TWO reasons why the rotor of an induction motor is skewed. (2) 5.3 One of the electrical tests performed on the stator of a three-phase motor is the insulation resistance test between windings and earth. 5.3.1 State the reasons for performing this test. (2) 5.3.2 Write down the accepted value for a motor to pass this test. (1) 5.4 State the main function of a three-phase motor starter. (1) 5.5 Refer to the control circuit in FIGURE 5.5 and answer the questions that follow. L STOP O/L START F START R N/O F N/O R N/C R N/C F FORWARD REVERSE CONTACTOR CONTACTOR N FIGURE 5.5: CONTROL CIRCUIT 5.5.1 Identify the starter shown in FIGURE 5.5. (1) 5.5.2 Describe the function of the two normally closed contacts in the circuit. (2) 5.5.3 Explain what happens if the circuit is energised and the start forward button is pressed. (4) 5.5.4 Describe the function of the overload contact in the circuit. (2) 5.5.5 Explain how a faulty N/O R that remains stuck in the closed position, would affect the operation of the starter. (4) Copyright reserved Please turn over

Downloaded from hlayiso.com (EC/SEPTEMBER 2020) ELECTRICAL TECHNOLOGY: POWER SYSTEMS 9 5.6 A manual sequence starter has the following sequence:  Motor 1 must start before Motor 2  Each motor must have independent overload protection  A stop button controls both motors Draw the control circuit of the sequence starter. (6) 5.7 A delta connected three-phase motor has a phase voltage of 380 V with a power factor of 0,866. The line current is 25 A when the motor is connected to the supply. Calculate the input power to the motor. Given: VPH = 380 V cos θ = 0,866 IL = 25 A (3) 5.8 Define the term synchronous speed of a motor. (2) 5.9 A three-phase motor with 24 poles has a synchronous speed of 1 200 rpm. Determine the percentage slip if the rotor speed is 1 140 rpm. Given: Poles = 24 NS = 1 200 rpm NP = 1 140 rpm (3) 5.10 Mention THREE types of information that can be found on the nameplate of a motor. (3) 5.11 A 110 V / 60 Hz three-phase motor is connected to a supply in a factory in South Africa. State with a reason if the motor will operate. (2) [40] Copyright reserved Please turn over

Downloaded from hlayiso.com 10 ELECTRICAL TECHNOLOGY: POWER SYSTEMS (EC/SEPTEMBER 2020) QUESTION 6: PROGRAMMABLE LOGIC CONTROLLERS (PLC’s) 6.1 Choose a description from COLUMN B that matches with the term in COLUMN A. Write only the letter (A–E) in COLUMN B next to the question number (6.1.1–6.1.5) in COLUMN A, for example 6.1.6 G. COLUMN A COLUMN B 6.1.1 Input device A Brain of the PLC 6.1.2 Hardware B Disadvantage of hardwiring 6.1.3 Processor C Physical parts and components 6.1.4 Output D Sensor 6.1.5 Bulky E Motor (5 x 1) (5) 6.2 Explain why software is installed. (3) 6.3 Refer to FIGURE 6.3 below and answer the questions that follow. INPUT A INPUT B OUTPUT (F) 0 0 1 0 1 0 1 0 0 1 1 0 FIGURE 6.3: TRUTH TABLE 6.3.1 Draw the ladder diagram that this truth table represents. (3) 6.3.2 Draw the logic gate symbol that this truth table represents. (3) 6.4 Define an opto-isolator. (3) 6.5 State TWO applications of light sensors. (2) 6.6 Describe the function of the timer used in PLC programs. (2) Copyright reserved Please turn over

Downloaded from hlayiso.com (EC/SEPTEMBER 2020) ELECTRICAL TECHNOLOGY: POWER SYSTEMS 11 6.7 Refer to FIGURE 6.7 and draw a logic ladder diagram that executes the same function as the one in FIGURE 6.7. L O/L STOP N/O N/C N/O START MC1 MC1 MC1 MOTOR LAMP 2 MC1 LAMP 1 N FIGURE 6.7 NOTE: The circuit is energised. (9) [30] TOTAL: 200 Copyright reserved Please turn over

Downloaded from hlayiso.com 12 ELECTRICAL TECHNOLOGY: POWER SYSTEMS (EC/SEPTEMBER 2020) FORMULA SHEET RLC-CIRCUITS THREE-PHASE AC GENERATION 1 XL = 2πfL and XC = 2πfC STAR 1 fr = 2π√LC VL = √3 × VPH and VPH = IPH × ZPH 𝑃 = 𝑉𝐼 cos 𝜃 IL = IPH SERIES DELTA IT = I R = IL = IC VL = VPH and VPH = IPH × ZPH Z = √R2 + (X L − XC )2 VL = I × XL and VC = I × XC IL = √3 × IPH VT = IZ and 𝑉𝑇 = √𝑉𝑅 2 + (𝑉𝐿 − 𝑉𝐶 )2 POWER R VR S = √3VL IL cos θ = and cos θ = Z VT Q = √3VL IL sin θ XL XC VL VC 1 L Q= = = = = √ Z Z VS VS R C P = √3VL IL cos θ PARALLEL P = S cos θ VS = VR = VL = VC P cos θ = S VR IR = R TWO-WATTMETER METHOD VL VC IL = and IC = PT = W1 + W2 XL XC W1 − W2 IT = √IR 2 + (IL − IC )2 tanθ = √3 ( ) W1 + W2 I cos θ = R IT XL XC VL VC 1 L Q= = = = = √ Z Z VS VS R C Copyright reserved Please turn over

Downloaded from hlayiso.com (EC/SEPTEMBER 2020) ELECTRICAL TECHNOLOGY: POWER SYSTEMS 13 FORMULA SHEET THREE-PHASE TRANSFORMERS THREE-PHASE MOTORS AND STARTERS STAR STAR VL = √3 × VPH and IL = IPH VL = √3 × VPH and IL = IPH DELTA DELTA VL = VPH and IL = √3 × IPH VL = VPH and IL = √3 × IPH POWER POWER 𝑆 = √3𝑉𝐿 𝐼𝐿 𝑆 = √3𝑉𝐿 𝐼𝐿 𝑄 = √3𝑉𝐿 𝐼𝐿 𝑠𝑖𝑛 𝜃 𝑄 = √3𝑉𝐿 𝐼𝐿 𝑠𝑖𝑛 𝜃 𝑃 = √3𝑉𝐿 𝐼𝐿 𝑐𝑜𝑠 𝜃 𝑃 = √3𝑉𝐿 𝐼𝐿 𝑐𝑜𝑠 𝜃 𝑃 = 𝑆 𝑐𝑜𝑠 𝜃 𝑃 = 𝑆 𝑐𝑜𝑠 𝜃 𝑃 𝑃 𝑐𝑜𝑠 𝜃 = 𝑐𝑜𝑠 𝜃 = 𝑆 𝑆 𝑃𝐼𝑁𝑃𝑈𝑇 −𝑃𝐿𝑂𝑆𝑆𝐸𝑆 𝑃𝐼𝑁𝑃𝑈𝑇 −𝑃𝐿𝑂𝑆𝑆𝐸𝑆 ŋ= × 100 ŋ= × 100 𝑃𝐼𝑁𝑃𝑈𝑇 𝑃𝐼𝑁𝑃𝑈𝑇 VPHP NP I T. Ratio = = = PHS MOTOR SPEED VPHS NS IPHP 60×f ns = p ns −nr Slip = × 100% ns Copyright reserved Please turn over