Electrotechnology N3 — Document
August 2019 — N3 — Electrotechnology
N3
2019
Nated Engineering
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Q1— Section header —
1.10\ in the ANSWER BOOK. lap winding; air gap; laminated plates; field pole; commutation; bedding; pole shoe; yoke; bearing ; brushes; commutator; wave winding
1.1Used to make an electrical connection be tween a stationary circuit and a rotating commutator
1.2process whereby a strip of emery paper is placed around the commutator
1.3ype of armature winding that is suitable for a high current, low voltage application
1.4The space between t he stator and rotor of any machine
1.5sed to minimise eddy current losses in a machine
1.6art of the field system used to evenly distribute the main magnetic flux over the air gap or to increase the efficiency of the magnetic path
1.7rmature winding of which t he ends are connected to commutator segments that are far apart
1.8The outer frame of a DC machine
1.9sed to support a rotating armature
1.10A process of reversing current flow in a DC generator (10 [10]10
Q2— Section header —
2.1Separately excited generators are commonly used in DC motor speed control. State the reason why these generators are used for this particular application. (2\
2.2Draw on the same system of axes characteristic curves of a compound und generator, clearly showing terminal voltage against load current for the following machines: (a\ (b\ (c\ over compounded level
2.3A shunt wound generator having a shunt field resistance of 175 supplies 90 A at a terminal voltage of 300 V to a bank of batteries. Calculate the following
2.3.1Armature current of the generator (2\
2.3.2Magnitude of the generated emf (1\
2.3.3Power deliver ed to the load. Express your answer in kW (1\ [10]10
Q3— Section header —
3.1Indicate whether the following statements are TRUE or FALSE. Choose the answer and write only ' rue' or ' alse' next to the question number (3.1.1
3.1.3\ in th e ANSWER BOOK. Correct the statement if it is FALSE.
3.1.1DC series motors are constant speed motors.
3.1.2DC series motors produce reasonably low starting and operating torque.
3.1.3DC series motors are used when the motor is permanentl y coupled to the load. (5\
3.2The armature of a certain eight pole DC series motor has 44 slots and conductors per slot. The armature is lap wound and has a resistance of 0,3 . The motor draws 32 A from a 40 0 V supply. Calculate the speed at which the armature is rotating if the useful flux per pole
Q4— Section header —
4.1Certain applications require DC motors to rotate in both directions. Show by means of neat fully labelled circu it diagrams TWO methods used to reverse the direction of rotation of a series wound motor. Begin by drawing a
4.2A 20 kW shunt wound generator supplies a load at 400 V f the generator is operatin g at an efficiency of 92%. alculate the following
4.2.1Mechanical power input to the generator (1\
4.2.2Total losses occurring in the machine (1\
4.2.3Armature current if the shunt field current is 2,7 A (2\ [10]10
Q5— Section header —
5.1Alternating quantities such as voltage and current are sinusoidal in nature and can be represented on a vector diagram. Represent the following vectors on the s ame system of axes: Use V as the reference vector.
5.2A non sinusoidal current waveform was plotted over half a cycle and the following mid ordinates were extracted from t he waveform: = 3 A = 9 A = 22 A = 37 A
5.2.1The effective (rms\ value of the wave
5.2.2The average value of the wave
5.2.3The form factor of the wave (3 (6\ [10]10
Q6— Section header —
Q7— Section header —
7.1Phase voltage of the motor when its windings are connected in star (2\
7.2Phase current of the motor when its windings are connected in s tar (3\
7.3Phase voltage of the motor when its windings are connected in delta (1\
7.4Phase current of the motor when its windings are connected in delta (2\
7.5Reactive power of the motor (2\ [10]10
Q8— Section header —
8.1The efficiency of a transformer is very high because it has no moving parts and therefore no frictional losses. Name TWO types of losses that occur in a transformer and state exactly in which part of the transformer each occurs.
8.2ingle phase transformer has 1 250 primary turns and 400 secondary turns. he supply voltage is 240 V. Calculate the following
8.2.1econdary voltage
8.2.2econdary current if the transformer draws a current of 12 A
8.2.3ating of the t ransformer in kVA (3 (6\ [10]10
Q9— Section header —
9.1Indicating type instruments measure the effective or root mean square values of alternating quantities. Name the THREE mechanisms of these indicating type instruments (3\
9.2There are t wo types of moving iron instruments, namely attraction types and repulsion types.
9.2.1State TWO advantages of moving iron instruments.
9.2.2State TWO disadvantages of moving iron instruments. (2 (4\
9.3Potential transformers a re used to step down high voltages to values that are safe and practical to measure. Sketch a neat fully labelled circuit diagram to show how a voltmeter is connected in a high voltage system.
10.1Electronic logic gates form the basic building blocks of digital systems. Draw the switching circuit for a NOR gate. (4\
10.2Every decimal number can be represented by its binary equivalent. Convert 111011,011 into its decimal eq uivalent. (4\
10.3A diode will conduct electrical current when it is forward biased. Explain what is meant by a forward biased diode (2\ [10] TOTAL:10
Mark Allocation
Total100 marks
Sections10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 = 100 marks
Paper information
TitleDocument
SubjectElectrotechnology
LevelN3
Document typeQuestion paper
Year2019
MonthAugust
Pages11
Downloads4
Examination context
Decade2010s
Years ago7y
SeasonWinter (SH)
Start dayThursday
Exam periodAugust 2019
Days since exam2,627
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
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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.
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A practical revision approach
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