Electrotechnology N3 — Document
April 2020 — N3 — Electrotechnology
N3
2020
Nated Engineering
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Q1— Section header —
1.1Why are t he field poles and the armature cores of DC machines laminated? (2\
1.2Friction losses occur in the bearings that support the rotating armature. Name TWO types of bearings commonly used in DC machines. (2\
1.3State the function of each of the following components that make up the field system of a DC machine
1.3.1Field coils
1.3.2
1.4Choose the correct word(s\ from those given in brackets. Write only the ord (s\ next to the question number (1.4.1
1.4.2\ in the ANSWER BOOK.
1.4.1(Wave wound, Lap wound, Armature winding\ is for heavy current machines
1.4.210
Q2— Section header —
2.1State Faraday s first law of electromagnetic induction (2\
2.2Name TWO methods used to improve commutation and to reduce sparking. (2\
2.3A 200 V DC, shunt wound generator produces an armature current of 42, Calculate the following
2.3.1nitude of the generated EMF (2\
2.3.2fficiency of the generator if it is driven by a prime mover rated at 9, 5 kW (4\ [10]10
Q3— Section header —
3.1Briefly explain the term armature reaction as applicable DC machines. (2\
3.2A 400 V, 15 kW, six pole DC shunt motor has a lap wound armature. The armature has 100 slots and there are four conductors per slot. The motor operates at a full load efficiency of 85 Calculate the useful flux per pole if the motor develops a full
Q4— Section header —
4.1Name TWO places in a DC machine where iron losses occur. (2\
4.2An overload relay is a protective device used to guard a motor against the harmful effects of an overload. Make a neat fully labelled sketch of a simpl e thermal type overload relay. Show and l abel the following part
4.3During a brake test on a DC motor the following information was recorded: Effective load on the brake drum Pulley diameter 250 mm Speed of shaft 875 r/min Current drawn from the supply
Q5— Section header —
5.1State what is meant by the following terms with regards to a sinusoidal waveform
5.1.1Cycle
5.1.2
5.2An alternating current is represented by the following equation: i = 13, 5 sin (376,991t\ Determine the following:
5.2.1Fre quency of this current
5.2.2Average value of this current
5.2.310
Q6— Section header —
6.1Current drawn by each component (5\
6.2Total current flowing through the circuit (2\
6.3Power factor of the circuit (1\
6.4Reactive power of the circuit (2\ [10]10
Q7— Section header —
7.1An alternating current circuit has three powers associated with it. Copy the following table into your ANSWER BOOK and fill in the blank spaces. SYMBOL QUANTITY FORMULA UNIT True power
Q8— Section header —
8.1The losses occurring in a transformer can be classified as either constant or variable losses. Name TWO types of constant losses that occur in a transformer. (2\
8.2A three phase distribution transformer has 800 turns on the primary side and 200 turns on the secondary side. For a primary voltage of 11 kV, calculate the secondary line voltage on open circuit if the transformer is connected in:
8.2.1Delta star
8.2.210
Q9— Section header —
9.1A moving coil instrument gives a full scale deflection if 20 mA flows through it. The internal resistance of coil is 85 Determine the following:
9.1.1The resistance required to use the instrument as an ammeter for a 6,2 A current flow. (3\
9.1.2The series resistance required to use it as a voltmeter up to 150 V. (3\
9.2Give the value of the following measuring instruments:
9.2.1The usual f ull scale deflection of an ammeter connected to a current transformer.
9.2.2Output voltage of a potential transformer.
9.3Name TWO mechanisms which are found in measuring instruments. [10]10
Q10— Section header —
10.1Draw a fully labelled IEC symbol for a silicon controlled rectifier (SCR\. (4\
10.2Determine t he number of inputs required so that the gate can produce 16 outputs.
10.3Convert 15,8125 into its binary equivalent. [10] TOTAL:10
Mark Allocation
Total100 marks
Sections10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 = 90 marks
Paper information
TitleDocument
SubjectElectrotechnology
LevelN3
Document typeQuestion paper
Year2020
MonthApril
Pages9
Downloads1
Examination context
Decade2020s
Years ago6y
SeasonAutumn (SH)
Start dayWednesday
Exam periodApril 2020
Days since exam2,384
About Electrotechnology 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
A TVET past examination paper is most useful when it is treated as an active revision exercise. Try the questions yourself before checking the corresponding memorandum. Where you make an error, identify the reason — was it a knowledge gap, a calculation mistake, an interpretation issue, or an examination technique problem?
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
applying subject knowledge
working under time constraints
checking answers
identifying revision priorities
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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