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Copyright reserved Please turn over NATIONAL CERTIFICATE ELECTROTECHNICS N5 (8080085) 22 November 2022 (X-paper) 09:00–12:00 Nonprogrammable calculators may be used. This question paper consists of 5 pages and a formula sheet of 2 pages. 128Q1E2222
(8080085) -2- Copyright reserved Please turn over DEPARTMENT OF HIGHER EDUCATION AND TRAINING REPUBLIC OF SOUTH AFRICA NATIONAL CERTIFICATE ELECTROTECHNICS N5 TIME: 3 HOURS MARKS: 100 INSTRUCTIONS AND INFORMATION 1. 2. 3. 4. 5. 6. Answer all the questions. Read all the questions carefully. Number the answers according to the numbering system used in this question paper. Start each section on a new page. Use only a black or a blue pen. Write neatly and legibly.
(8080085) -3- Copyright reserved Please turn over QUESTION 1 1.1 Name two characteristics of DC machines. (2) 1.2 Can a series motor be started without some mechanical load? Substantiate your answer. (2) 1.3 Explain the term, commutating plane. (1) 1.4 The resistance of the armature current of a 250 V shunt motor is 0,2 Ωand its full-load speed is 1 000 r/min. Calculate: 1.4.1 The resistance required in series with the armature to reduce the speed with full-load torque to 800 r/min. The full-load armature current is 50 A. (5) 1.4.2 What speed will the motor run if the load torque is halved? Neglect the effect of armature reaction. (5) 1.5 A 450 kW, 380 V, six-pole DC generator has a lap connected winding with 180 armature conductors. Calculate the number of turns per pole required for the commutating pole, assuming that the compole ampere turns per pole to be about 1,3 times the armature ampere turns per pole and the brushes to be in the geometric neutral axis. (5) [20] QUESTION 2 2.1 The voltage applied to a single element load is v(t) = 84,86 sin (1 000t + 45) V. The resulting current flow is i(t) + 14,14 sin (1 000t – 45) A. Calculate the nature and magnitude of the element. (5) 2.2 An impedance Z1 = (2 – j1) Ω is connected in parallel with another impedance Z2 = (3 – j5) Ω. This circuit is connected in series with another impedance Z3 = (1 + j2) Ω. The combination is then connected in parallel to an impedance of Z4 = (3 + j4) Ω. Z2 is connected across points AB. Z4 is connected across points CD and Z3 across points AC. I1 flowing through Z1 is 10 A. I2 flowing through Z2, I3 through Z3 and I4 through Z4. The circuit is then connected to a supply across Z4. 2.2.1 Draw the circuit diagram clearly showing all its elements (impedances and supply source). (2) 2.22 Calculate the input supply voltage and current. (13) [20]
(8080085) -4- Copyright reserved Please turn over QUESTION 3 3.1 Each phase of a three-phase balanced load consists of a coil, of resistance of 10 Ω and inductance of 0,02 H in parallel with a capacitor of 31,8 μF. The supply voltage is 415 V, 50 Hz. Calculate the line current when connected in star. (5) 3.2 A 500 kVA, 33/3,3 kV single-phase transformer with a resistance voltage drop of 1,5% and the reactance voltage drop of 6% is connected in parallel with a 1 000 kVA, 33/3,3 kV single-phase transformer with a resistance voltage drop of 1% and a reactance voltage drop of 6,2%. Calculate the kVA loading and operating power factor of each transformer when the total load is 1 200 kVA at a power factor of 0,8 lagging. (7) 3.3 The primary and secondary windings of a 30 kVA, 6 000/230 V transformer has a resistance of 12 Ω and 0,018 Ω respectively. The total resistance reactance of the transformer referred to the primary is 26 Ω. Calculate the percentage regulation of a transformer when supplying full-load current at a power factor of 0,8 lagging. (5) 3.4 State the THREE most essential requirements that must be adhered to before transformers are connected in parallel. (3) [20] QUESTION 4 4.1 The two-wattmeter method is applied to a three-phase three-wire 120 V system, with the meters connected to lines R and B, WR = 820 W and WB = –480 W. Find the impedance of the balanced delta connected load. (5) 4.2 A three-phase induction motor develops 20 kW when running at 90% efficiency, at a power factor of 0,45 lagging. Calculate the readings on each of the two watt meters connected to read the input power. (5) 4.3 A load of 2 500 kVA at 11 kV and a power factor of 0,8 lagging is supplied by a three-phase transmission line with a resistance of 3 Ω per phase and a reactance of 6 Ω per phase. Determine the following: 4.3.1 Percentage regulation of the line (7) 4.3.2 Efficiency of the line (3) [20]