Electrical Trade Theory N3 — Document
August 2019 — N3 — Electrical Trade Theory
N3
2019
Nated Engineering
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Q1— Section header —
1.1A stove that has been wired for three phase supplies must be coupled to a single phase supply. Show, w ith the aid o f a sketch, how the connection to the supply must be made. (2\
1.2Draw a neat, labelled sketch of the bimetal type thermostat a laundry iron. (4\
1.3Briefly explain how the temperature of the laundry iron mentioned in
Q1— Section header —
1.4State TWO possible fault conditions that could l ead to the iron tripping the mains earth leakage protection. (2\ [10]10
Q2— Section header —
2.1FIGURE 1 shows a magnetic type circuit breaker. Identify the parts indicate in the sketch and write only the answer next to the question
2.1.7
2.2Explain how the operation of the overload is achieved in the circuit breaker shown in FIGURE 1 (3\ [10]10
Q3— Section header —
3.1Explain the purpose of the choke in fluorescent lamp circuits, operating with glow starters.
3.2Briefly explain how the safety circuit of an instant start fluorescent lamp reduces the risk of electric shock. (2\
3.3State TWO applications of each of the following lamps:
3.3.1High pressure mercury vapour discharge lamps
3.3.2Sodium vapour discharge lamps
3.3.310
Q4— Section header —
4.1The RMS value of the current
4.2The instantaneous value 8 milliseconds after reading z ero while increasing positively (3\
4.3How long it would take to build up to a value of 130 amperes, after passing through zero (3\
4.4State the meaning of the term Instantaneous Value (1\ [10]10
Q5— Section header —
5.1Calculate the following:
5.1.1The Impedance
5.1.2The total voltage (3\
5.1.2Phase angle (3\
5.2State he nature of the phase angle. [10]10
Q6— Section header —
6.1Draw a three phase waveform to illustrate how a rotating magnetic field i three phase induction motor is produced. (3\
6.2Draw a three phase phasor diagram of the rotating magnetic field mentioned
Q6— Section header —
6.3A three phase squirrel cage motor is connected in delta across a 380 V, three phase supply and draws a line current of 20 A at 0,9 lagging. Calculate the following:
6.3.1Input power to the motor (3\
6.3.2Phase current of the motor [10]10
Q7— Section header —
7.1Draw a diagram to show how three single phase transformers can be connected in star delta for a three phase operation.
7.2A 300 k VA transf ormer has its primary connected in delta to 66 V supply. It has a star connected secon dary line voltage of 11 k Calculate the
Q8— Section header —
8.1State the functions of the following components in a machine:
8.1.1Carbon brushes
8.1.2Field coils
8.1.3
8.2Make use of two neat label ed sketch to show how a DC long shunt motor can be reversed. [10]10
Q9— Section header —
9.1Ind icate whether the following statements are TRUE or FALSE. Choose the answer and write only 'True' or 'False' next to the question number (9.1.1
9.1.5\ in the ANSWER BOOK.
9.1.1The starting current of a squirrel cage motor is high c ompared to a wound rotor motor.
9.1.2A universal motor operates only on AC.
9.1.3A resistance start induction run motor can only be used for driving fans.
9.1.4Shaded pole motors are self starting cheap and robust.
9.1.5
9.2Study the diagram of a starter circuit shown in FIGURE 2 and answer the questions. FIGURE 2
9.2.1For which type of motor would the starter circuit shown in FIGURE 2 be uitable? (1\
9.2.2State the reason for using the starter
Q9— Section header —
9.2.3State the purpose of the no volt coil shown in FIGURE 2 (2\ [10] Start (Start\ O/L Stator windings10
Q10— Section header —
10.1Study FIGURE 3 and answer the questions below: FIGURE 3 Redraw the circuit diagram shown in FIGURE 3 and add measuring instruments that can measure the following:
10.1.1Frequency across the supply
10.1.2The voltage across R2
10.1.3
10.2Study the symbol of ansistor shown in FIGURE 4 Name the parts indicated on the sketch and write only the answer next to the question number (10.2.1
10.2.3\ in the ANSWER BOOK. FIGURE (3\
10.3Identify the type of transist represented by FIGURE 4 (1\ [10] TOTAL:10
Mark Allocation
Total100 marks
Sections10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 = 100 marks
Paper information
TitleDocument
SubjectElectrical Trade Theory
LevelN3
Document typeQuestion paper
Year2019
MonthAugust
Pages8
Downloads4
Examination context
Decade2010s
Years ago7y
SeasonWinter (SH)
Start dayThursday
Exam periodAugust 2019
Days since exam2,629
About Electrical Trade Theory examination preparation
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