Electrotechnics N6 — Document

April 2021 — N6 — Electrotechnics

N6 2021 Nated Engineering

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🎯 100 marks ⏱️ 3h 📄 5 pages 📝 36 items found 📚 1 topics

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Q1— Section header —

1.1A three wire DC system has two operati ng voltages of 440 V and 220 V. Show by means of

1.2An 8 kW shunt wound motor takes a current of 20 A f rom a 450 V mains at full load. The armat and the fiel d circuit

1.2.1The combined iron and friction losses (8\

1.2.2The efficiency at full load (2\ [14]14

Q2— Section header —

2.1Differentiate be tween the electrical quantities active power and reactive power (2\

2.2A 400 V, unbalanced, three phase, delta connected load consists of the phase currents: RY = 30 A at a power factor of 0,8

Q3— Section header —

3.1Explain with the aid of diagrams, how the efficiency of a single phase transformer can be calculated fro m the results obtained from open

3.2A 12 kVA transformer has an iron loss of 240 W and a full load copper loss of 340 W. Calculate:

3.2.1Th e efficiency at 25% of the full load and unity power factor (2\

3.2.2The efficiency at 75% of full load and 0,8 lagging power factor (2\

3.2.3The kVA load at which the efficiency will be maximum and the value of this efficiency at a power factor of 0,8 lagging (4\ [15]15

Q4— Section header —

4.1Explain, with the aid of a neat diagram, how a short circuit test is carried out on a three phase, star connected alternator.

4.2A 12 kV, three phase, 2 MVA, star connected alternator operates at a power factor of 0,85 lagging and has a synchro Ca

4.2.1The open circuit line voltage to which the machine must be excited to deliver full load (7\

4.2.2The percentage voltage regulation of the alternator (2\

4.2.3The load angle at which the machine operates (1\ [15] 808009615

Q5— Section header —

5.1Give TWO uses of synchronous motors. (2\

5.2A 750 kVA, 3,5 kV, three phase, star connected synchronous motor is fully loaded and takes 600 kW at leading power factor. The machine has a percentage impedance of (5 + j60\ percent. Calculate

5.2.1The power factor (1\

5.2.2The resistive and reactive volt drops (6\

5.2.3The emf to which the machine is excited (4\

5.2.4The load angle in electrical degrees (1\ [14]14

Q6— Section header —

6.1Define the term slip as it is used in induction motors. (2\

6.2Name the TWO main parts of an AC motor. (2\

6.3The input of a six pole, three phase, 50 Hz induction motor is 42 kW at a speed of 900 r/min. The stator losses of the motor are 2 kW and the friction and windage losses are 2,5 kW. Calculate

6.3.1he slip (3\

6.3.2he rotor copper loss (4\

6.3.3he output of the motor (2\

6.3.4he full load efficiency (1\ [14]14

Q7— Section header —

Topics Identified

Business Plan

Mark Allocation

Total100 marks

Sections14 + 14 + 15 + 15 + 14 + 14 + 14 = 100 marks

Paper information

TitleDocument

SubjectElectrotechnics

LevelN6

Document typeQuestion paper

Year2021

MonthApril

Pages9

Downloads6

Examination context

Decade2020s

Years ago5y

SeasonAutumn (SH)

Start dayThursday

Exam periodApril 2021

Days since exam2,020

About Electrotechnics examination preparation

This TVET examination resource can be used as part of a structured revision programme. Past papers are most useful when learners attempt questions independently, work within a realistic time limit, and review their answers afterwards.

How to use this resource for revision

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For stronger examination practice, complete the paper without unnecessary interruptions and keep track of the questions that take the most time. Afterwards, review the memorandum and separate knowledge gaps from mistakes caused by interpretation, calculation, structure, or examination technique.

understanding question requirements

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A practical revision approach

Start by completing the resource without looking at the memorandum. Review incorrect and incomplete answers afterwards and make a short revision list based on the areas that caused difficulty.

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