Email: [email protected] questions TRADE THEORY N3 · 3.1.2 State why a choke is included in this circuit. 3.1.3 State why a capacitor is included in this circuit. (3 x 1)

Web

site

: w

ww

.eku

rhu

len

ite

ch.c

o.z

a

Emai

l: in

fo@

eku

rhu

len

itec

h.c

o.z

a

Elec

tr

ic

al TR

AD

E TH

EO

RY

N3

Ty

pic

al ex

am

q

uest

io

ns

EKU

RH

ULE

NI T

ECH

CO

LLEG

E.

No

. 3 M

oga

le S

qu

are

, Kru

gers

do

rp.

Web

site

: w

ww

. eku

rhu

len

itec

h.c

o.z

a Em

ail:

info

@ek

urh

ule

nit

ech

.co

.za

TEL:

01

1 0

40

73

43

CEL

L: 0

73

77

0 3

02

8/0

60

71

5 4

52

9

Copyright reserved Please turn over

T460(E)(N23)T NOVEMBER EXAMINATION

NATIONAL CERTIFICATE

ELECTRICAL TRADE THEORY N3

(11041263)

23 November 2016 (X-Paper) 09:00–12:00

This question paper consists of 7 pages and 1 formula sheet.

(11041263) -2- T460(E)(N23)T


DEPARTMENT OF HIGHER EDUCATION AND TRAINING REPUBLIC OF SOUTH AFRICA

NATIONAL CERTIFICATE ELECTRICAL TRADE THEORY N3

TIME: 3 HOURS MARKS: 100

INSTRUCTIONS AND INFORMATION 1. 2. 3. 4. 5. 6. 7. 8.

Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Where applicable, answers must be in accordance with the SABS (SANS) Code of Practice SANS 10142-1:2003 for the Wiring of Premises. Sketches must be neat, labelled and large enough to show the required detail. Formulae used in Electrical Trade Theory N3 can be found at the end of the question paper. Answers must be given to TWO decimal places. Write neatly and legibly.

(11041263) -3- T460(E)(N23)T


QUESTION 1: DOMESTIC APPLIANCES 1.1 Give THREE hints to save electricity in a normal household. (3) 1.2

A stove that has been wired for a three-phase supply must be coupled to a single-phase supply. With the aid of a sketch, show how the connection to the supply must be made.

(3) 1.3

Name THREE maintenance procedures/checks that can be done to maintain a washing machine.

(3)

1.4 Name ONE precaution that should be taken before replacing a geyser

element. (1)

[10] QUESTION 2: PROTECTION 2.1

State THREE safety procedures that can be applied in a household which would eliminate dangerous situations when using electricity.

(3)

2.2 State where the following conductors in house wiring are found: 2.2.1 Earth bar 2.2.2 Earthing lead 2.2.3 Earth electrode

(3 x 1)

(3) 2.3 Describe the operation of a thermal-magnetic miniature circuit breaker under

the following headings:

2.3.1 Overload protection 2.3.2

Short-circuit protection

(2 x 2)

(4) [10]

(11041263) -4- T460(E)(N23)T


QUESTION 3: ILLUMINATION 3.1 A fluorescent lamp has a quick-start starter with preheating. 3.1.1 State what is being preheated. 3.1.2 State why a choke is included in this circuit. 3.1.3

State why a capacitor is included in this circuit.

(3 x 1)

(3) 3.2 Draw a neat, labelled circuit diagram of a fluorescent lamp that uses a glow

starter to start.

(4) 3.3 In a workshop containing rotating machinery, two fluorescent lamps must be

connected to a single-phase supply. Explain how the two lamps should be connected to limit the stroboscopic effect. Do NOT draw a circuit diagram.

(3) [10]

QUESTION 4: ALTERNATING CURRENT THEORY A voltage waveform is represented by e = 381sin(628,32t) volts NOTE: The angle is in radians. Calculate:

4.1 RMS value of the voltage (3) 4.2 Frequency of the wave (3) 4.3

Instantaneous voltage 5 m/s after t = 0 s Round off the answer to the closest whole number.

(3)

4.4 State the meaning of the term instantaneous value with reference to a voltage

waveform.

(1) [10]

(11041263) -5- T460(E)(N23)T


QUESTION 5: SERIES RLC CIRCUITS A farmer wishes to use a 100 W, 220 V lamp on a 380 V, 50 Hz supply. He decides to insert an inductor in series with the lamp to obtain the desired voltage. Assume that the lamp is purely resistive and that the inductor is purely inductive.

5.1

Draw a circuit diagram and insert all the given information at the appropriate places in your diagram.

(3)

5.2 Calculate the current flowing through the inductor. (3) 5.3 Calculate resistance of the lamp. (3) 5.4 State how it is possible to use a coil in an AC circuit to reduce the current

through the lamp

(1) [10] QUESTION 6: THREE-PHASE AC SYSTEMS 6.1 State the angle between the phasors of a three phase voltage waveform. (1) 6.2 A three-phase 380 V supply delivers 50 A to a balanced delta-connected load.

The current lags the voltage by 30°. Calculate:

6.2.1 Total power consumed (3) 6.2.2 Total apparent power (3) 6.2.3 Total reactive power (3)

[10] QUESTION 7: TRANSFORMERS 7.1 A balanced three-phase star-coupled load draws a line current of 100 A from

a transformer. This 200 kVA delta-star transformer is connected to a 6,6 kV supply. Make a neat sketch that represents this circuit. Indicate on your sketch where the following can be measured:

7.1.1 The phase voltages of the transformer windings (2) 7.1.2 The line current on the primary (1) 7.1.3 The 6,6 kV line voltage (1) 7.2

Calculate the turns ratio of the transformer, mentioned in QUESTION 7.1, if the secondary phase voltage is 220 V.

(3)

(11041263) -6- T460(E)(N23)T


7.3 Calculate the apparent power used by the load that is connected to the

transformer with a secondary line voltage of 380 Volts.

(3) [10]

QUESTION 8: DIRECT-CURRENT MACHINES 8.1 Name the functions of the following components of a DC motor: 8.1.1 The pole shoe 8.1.2 The pole core 8.1.3

The yoke

(3 x 2)

(6) 8.2 Draw a freehand graph of the load characteristics of the following DC motors: 8.2.1 A shunt-connected motor 8.2.2 A series-connected motor

(2 x 2)

(4) [10]

QUESTION 9: ALTERNATING-CURRENT MACHINES 9.1 Name TWO types of single-phase AC motors. (2) 9.2 Name TWO types of rotors found in AC motors. (2) 9.3 State the purpose of the no-volt coil in a motor circuit. (2) 9.4 Indicate whether the following statements are TRUE or FALSE. Choose the

answer and write only 'true' or 'false' next to the question number (9.4.1–9.4.4) in the ANSWER BOOK.

9.4.1

Direct-on-line starting of motors places a reduced voltage on the stator coils to limit the starting current.

9.4.2

In star-delta starters there are six stator coils, three for star and three for delta.

9.4.3

In an automatic star-delta starter, the timer relay will activate the circuitry that opens the star contactors and closes the delta contactors.

9.4.4 By the time the motor is running at full speed, the rotor resistances

of a slip-ring motor would have gradually decreased to form a short-circuit.

(4 x 1)

(4) [10]

(11041263) -7- T460(E)(N23)T

Copyright reserved

QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS 10.1 Give the function(s) of the following components of measuring instruments:

10.1.1 Hair springs 10.1.2 Fixed iron 10.1.3 Permanent magnets

(3 x 1)

(3) 10.2 Draw the following: 10.2.1

A neat, fully labelled circuit diagram of a full-wave rectifier using two diodes and a centre-tap transformer

(3)

10.2.2

The waveform across a load connected to the circuit, mentioned in QUESTION 10.2.1, if the supply is sinusoidal

(2)

10.2.3 The waveform across a load connected to the circuit, mentioned in

QUESTION 10.2.1, if the supply is sinusoidal and a smoothing capacitor is connected across the load

(2) [10]

TOTAL: 100

(11041263) T460(E)(N23)T

Copyright reserved


FORMULA SHEET

Z

VI

T

CosII TACTIVE

SinIIREACTIVE T

Lf2πXL

fC2π

1CX

2

CL

2 )X(XRZ

Z

RCos 1

RIV TR

LTXL XIV

CTXC XIV

2

XCXL

2

R )V(VVV

R IP 2

VIS

3 – phase

osIV3P LL C

LL I V3S

DELTA

V FPH /L V

FPHI

/L3I

STAR

V FPH /L V3

FPHI /LI

p

s

s

p

s

p

I

I

N

N

V

V

f2ω

p

fN

60.

n

nns r

The next five formulae are true for voltage:

t)(inIi m S

mrms I 0,707I

mave I0,637I

n

i...iiI

2

n

2

2

2

1rms

n

i..iiI n21

ave

value-AVE

value-RMSfactor Form

value-RMS

value-MAXfactorCrest

SERIES

n21T R...RRR

PARALLEL

n21T R

1...

R

1

R

1

R

1



NOVEMBER EXAMINATION


23 NOVEMBER 2016

This marking guideline consists of 7 pages.

MARKING GUIDELINE

MARKING GUIDELINE -2- T460(E)(N23)T ELECTRICAL TRADE THEORY N3


NOTE: There is not only one answer or one method (approach) of answering

the questions. This marking guideline gives only one answer or one possible method (approach). Examiners must analyse the student's solution to determine if the question has been answered and therefore must not adhere strictly to this marking guideline.

QUESTION 1: DOMESTIC APPLIANCES 1.1 Use energy saving lamps.

Switch off geyser while away.

Boil only enough water for hot drinks. (Any 3 suitable answers) (3 x 1)

(3)

1.2 stove wiring from connection box o o o o o L N E supply conductors

(3)

1.3 Check plug and cord for damages.

Check if on-off switch is in order.

Check if motor is not too dusty. (Any 3 suitable answers)

(3)

1.4 Switch the geyser isolator off. (1) [10]

QUESTION 2: PROTECTION 2.1 Ensure that no-one can insert their fingers into power sockets.

Ensure that children cannot pull electrical appliances from table tops.

Check that the insulation of electrical cords are in a good condition. (3 x 1)

(3)

2.2 2.2.1 In the distribution board 2.2.2 Between the earth electrode and the earth bar 2.2.3 In the ground

(3 x 1)

(3)

2.3 2.3.1 When the current exceeds the design value, the bimetal strip bends far enough to activate the trigger mechanism that operates the trip switch.

(2)

2.3.2 A coil causes a magnetic field close to the iron armature which is immediately attracted and activates the trigger mechanism that operates the trip switch.

(2) [10]



QUESTION 3: ILLUMINATION 3.1 3.1.1 The lamp cathodes 3.1.2 To limit current 3.1.3 To improve the circuit's power factor

(3 x 1)

(3)

3.2

glow starter suppressor tube correctly coupled surge coil

(4)

3.3 The two lamp circuits must be connected to the supply (in parallel). The one lamp circuit must have a capacitor connected in series with the lamp.

(3) [10]

QUESTION 4: ALTERNATING CURRENT THEORY 4.1 RMS = 0,707.Em

= 0,707.381 = 269.37 V

(3)

4.2 2.π.f = 628,32

f=628,32/2 f = 100 Hz

(3)

4.3 e = 381sin(628,32t) = 381 x sin(628,32 x 0,005) = 381 x 7 x 10-6

= 0 V

(3)

4.4 Instantaneous value is the value of the voltage waveform at any given point in time after the wave has commenced. It means the specific value of the wave at a specific moment in time.

(1) [10]



QUESTION 5: SERIES RLC CIRCUITS 5.1 220 V

100 W L

380 V 50 Hz

Lamp in series with L

220 V across lamp

380 V supply

(3)

5.2 I = P/V

= 100/220

= 0,455 A

(3)

5.3 R = V/I

=220/0,455

= 484 Ω

OR

P = V2/R

R = V2/P

= 2202/100

= 484 Ω

(3)

5.4 The inductive reactance of the coil reduces the voltage across the lamp, thus allowing for the higher voltage to be connected across the lamp.

(1) [10] QUESTION 6: THREE-PHASE AC SYSTEMS 6.1 Three rotating coils 120°apart.

(1)

6.2 6.2.1 P = √ .VLILcosФ

= √ x 380 x 50 x cos(30°) = 28,5 kW

(3)

6.2.2 S = √ .VLIL

= √ x 380 x 50 = 32,9 kVA

(3)

6.2.3 VAR = √ x VLIL sinФ

= √ x 380 x 50 x sin(30°) = 16 454 VAR

(3) [10]



QUESTION 7: TRANSFORMERS 7.1 I1.L I2.L = 100 A

(phase voltages 6,6 kV line voltage )

(5) 7.2 N1 / N2 = V1 / V2

= 6 600/220 = 30:1

(3)

7.3 S = √ .VL.IL

= √ .380.100 = 65 818 VA = 66 kVA

(3) [10] QUESTION 8: DIRECT-CURRENT MACHINES 8.1 8.1.1 It keeps the field coil in position and provides a better path for the

magnetic flux. (Any 2 suitable answers)

8.1.2 The field coil is placed over it and provides a path for the

magnetic flux. (Any 2 suitable answers)

8.1.3 It completes the path for the magnetic flux and provides protection

for the inside of the motor. The end plates that house the bearings are bolted to it. (Any 2 suitable answers)

(3 x 2)

(6)



8.2 8.2.1 A shunt-connected motor

speed

8.2.2 A series-connected motor

armature current

(2 x 2) (4) [10]

QUESTION 9: ALTERNATING-CURRENT MACHINES 9.1 Split-phase induction motor, AC series motor, repulsion motor, shaded-pole

synchronous motor

(2) 9.2 Wound rotor and squirrel-cage rotor (2) 9.3 To disconnect the circuit when the voltage falls below the design value of

the no-volt coil

(2) 9.4 9.4.1

9.4.2 9.4.3 9.4.4

False False True True

(4 x 1)

(4) [10]


Copyright reserved

QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS 10.1 10.1.1 They provide a reverse (controlling) torque against the deflecting

torque.

10.1.2 It increases the magnetic attraction force when a coil nearby

carries current.

10.1.3 They provide a second magnetic field which will influence the

moving coil field. (3 x 1)

(3)

10.2 10.2.1 A full-wave rectifier using two diodes and a centre-tap transformer

OR

(3) 10.2.2

shape time

(2) 10.2.3

V or I

shape can also be a straight line

(2) [10] TOTAL: 100


T460(E)(A5)T

APRIL EXAMINATION



(11041263)

5 April 2016 (X-Paper) 09:00–12:00


(11041263) -2- T460(E)(A5)T





INSTRUCTIONS AND INFORMATION 1. 2. 3. 4. 5. 6. 7.

Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Where applicable, answers must be in accordance with the SABS (SANS) Code of Practice SANS 10142-1:2003 for the Wiring of Premises. Sketches must be neat, labelled and large enough to show the required detail. Answers must be given to TWO decimal places. Write neatly and legibly.

(11041263) -3- T460(E)(A5)T


QUESTION 1: DOMESTIC APPLIANCES

1.1 A stove using 40 A must be installed in a new house. Explain the procedure of installing a stove, type of materials used and placement of parts. Start from the distribution board to the stove's screw connectors.

(4)

1.2

Explain how you would test each circuit (plate number 1 and oven) of the above stove for continuity with the aid of an ohm-meter.

(3)

1.3 Draw the single phase sub-circuit that adheres to wiring code regulations that would supply the stove with power.

(3) [10]

QUESTION 2: PROTECTION 2.1

Name the conductors in a single-phase domestic installation that the earth-leakage relay disconnect when the earth-leakage protection is activated.

(2)

2.2 State what value of 220 V AC current is it considered dangerous to humans (2) 2.3 FIGURE 2.1 shows a Balanced Core Earth Leakage Relay.

Explain the operating principle of a core balance earth-leakage relay.

(6)

FIGURE 2.1: BALANCED CORE EARTH LEAKAGE RELAY [10]

(11041263) -4- T460(E)(A5)T


e = 310 sin(18 000.t°) Volts

QUESTION 3: ILLUMINATION 3.1 Indicate whether the following statements are TRUE or FALSE. Choose the

answer and write only 'true' or 'false' next to the question number (3.1.1–3.1.5) in the ANSWER BOOK.

3.1.1

A fluorescent tube emits light because of a continuous electric arc in the tube.

3.1.2

Gasses used in discharge lamps have a positive temperature coefficient of resistance and the current needs to be limited as the temperature increases.

3.1.3 Phosphor powder is used in lamps because it has a long afterglow. 3.1.4

Rotating machinery give a stroboscopic effect under incandescent lamps.

3.1.5 When compared with discharge lamps, tungsten halogen lamps

emit bright light but consume a higher amount of energy. (5 x 1)

(5)

3.2 Draw a labelled circuit diagram of a fluorescent lamp that is started with the

aid of a glow starter. (You will lose marks if the components are not labelled.)

(5) [10]

QUESTION 4: ALTERNATING CURRENT THEORY 4.1 FIGURE 4.1 shows a sine wave. NOTE: The angle is in degrees.

FIGURE 4.1: SINE WAVE 4.1.1 Calculate the waveform's effective value. (3) 4.1.2 Redraw the waveform indicating the RMS, peak and Average

Values on the waveform correctly. (3)

(11041263) -5- T460(E)(A5)T


220V 50Hz

R=5Ω

Xc=27,65 Ω

L=0.1 H

4.2 Draw a phasor diagram that will represent a balanced three phase voltage

supply. Indicate on the diagram the following:

Correct order of Phases if the reference point is at 0°

Correct Direction of Rotation

Correct phase angle between phases

Current in each phase lagging the voltage – Exact scale is not required.

(4) [10] QUESTION 5: SERIES RLC CIRCUITS 5.1 FIGURE 5.1 shows an RLC Circuit. Answer the following questions.

FIGURE 5.1: RLC CIRCUIT 5.1.1 Calculate the inductive reactance (3) 5.1.2 Calculate the circuit impedance. (3) 5.1.3 Sketch a fully labelled phasor diagram. (4) [10] QUESTION 6: THREE-PHASE AC SYSTEMS

6.1 Calculate the power in balanced 3-phase delta connected system with the following information:

Line Voltage = 420 Volt

Line Current = 23A

Φ=9°

Load = inductive

(3)

6.2

Calculate the line voltage between two phases in a star connected system if the phase voltage between one fase and neutral is 220 V.

(3)

6.3

Figure 6.1 shows a block diagram representing the losses in a 3-phase motor. Calculate the output power of the motor from the given information.

(11041263) -6- T460(E)(A5)T


Input

VL=380 Volt

IL=42.2A

Δ Connected

Output

Cosθ=0.8

ŋ=90% efficient

Losses in a motor

(4)

FIGURE 6.1: LOSSES IN A MOTOR [10] QUESTION 7: TRANSFORMERS

7.1 The circuit in FIGURE 7.1 below consists of three single-phase transformers connected together. This circuit is connected to a three-phase 380 V supply. The primaries are labelled with capital letters.

A

B

C

4

3

2

1

550:1

FIGURE 7.1: 3-PHASE TRANSFORMER

7.1.1 Identify the configuration in which the transformer is connected from primary to secondary.

(2)

7.1.2 State whether this is a step up / step down transformer. (1)

7.1.3 Identify the Neutral connector/s on the transformer (1) 7.2 Calculate the output line voltage of a delta/delta step up transformer if the

transformer has a turns ratio of 1:10 and an input line voltage of 380 Volts.

(3) 7.3 A current of 2 A at a power factor of 0,8 is drawn from the 380 V supply when

a transformer is connected to a balanced three-phase load. Calculate the total power consumed.

(3) [10]

(11041263) -7- T460(E)(A5)T


QUESTION 8: DC MACHINES 8.1 Name the functions of the following components of a DC motor circuit:

8.1.1

A variable resistor placed in series with the armature of a shunt motor

8.1.2 A no-volt release coil

8.1.3 An overload relay (3 x 1)

(3)

8.2 Name any TWO types of field-coil connections used in DC motors. (2)

8.3

Name any TWO methods to reduce armature reaction with reference to DC motors.

(2)

8.4 Explain how you would do a field volt-drop test on a four-pole DC motor. (3) [10]

(11041263) -8- T460(E)(A5)T


QUESTION 9: AC MACHINES

9.1

Explain why the running winding of a split-phase motor has a lower resistance and lower inductive reactance than the starting winding

(2)

9.2

Draw a circuit diagram of a split-phase motor that has one capacitor which is permanently connected during operation.

(3)

9.3 State the code of practice on motor circuits regarding the starter circuit of a large DC motor and loss of supply.

(1)

9.4

Explain how the slip-ring motor in FIGURE 9.1 below is started and how it reaches full speed.

(4)

FIGURE 9.1: SLIPRING MOTOR STARTER [10]

(11041263) -9- T460(E)(A5)T

Copyright reserved

QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS 10.1 Explain how you would increase the range of the following instruments: 10.1.1 A DC ammeter (1) 10.1.2 A DC voltmeter (1) 10.1.3 An AC wattmeter (3) 10.2 Define each of the following terms: 10.2.1 Electron current flow 10.2.2 Semi-conductor 10.2.3 Positive ions 10.2.4 P-type doping 10.2.5 Pentavalent atom

(5 x 1)

(5) [10]

TOTAL: 100

(11041263) -10- T460(E)(A5)T

Copyright reserved


FORMULA SHEET

Z

VI

T

CosII TACTIVE

SinIIREACTIVE T

Lf2πXL

fC2π

1CX

2

CL

2 )X(XRZ

Z

RCos 1

RIV TR

LTXL XIV

CTXC XIV

2

XCXL

2

R )V(VVV

R IP 2

VIS

3-phase

osIV3P LL C

LL I V3S

DELTA

V FPH /L V

FPHI

/L3I

STAR

V FPH /L V3

FPHI /LI

p

s

s

p

s

p

I

I

N

N

V

V

f2ω

p

fN

60.

n

nns r

The next five formulae are true for voltage

t)(inIi m S

mrms I 0,707I

mave I0,637I

n

i...iiI

2

n

2

2

2

1rms

n

i..iiI n21

ave

value-AVE

value-RMSfactor Form

value-RMS

value-MAXfactorCrest

SERIES

n21T R...RRR

PARALLEL

n21T R

1...

R

1

R

1

R

1

MARKING GUIDELINE -1- T...(E)(A…)T ELECTRICAL TRADE THEORY N3

Copyright reserved


APRIL EXAMINATION


5 APRIL 2016


MARKING GUIDELINE

MARKING GUIDELINE -2- T460(E)(A5)T ELECTRICAL TRADE THEORY N3


Live

Neutral

Earth

Liveü Neutral

Earth

Stove

Stove

Disconnectorü

MCB

for

Stoveü

Distribution Board

NOTE: There is not only one answer or one method (approach) of answering the

questions. This memorandum gives only one answer or one possible method (approach). Examiners must analyse the student's solution to determine if the question has been answered and therefore must not adhere strictly to this memorandum.

QUESTION 1: DOMESTIC APPLIANCES 1.1 25 mm conduit installed from D.B to stove.

Isolator box not more than 0,5 m from stove.

Flexible conduit between wall and stove.

6 mm2 L, N and E conductors from D.B to stove. (4 x 1)

(4) 1.2 Make sure the disconnector is off.

Place meter on ohms scale and measure between live and neutral.

Switch the stove plate or the oven on.

Ohms value should be low. (Any 3 x 1)

(3) 1.3

(3) [10] QUESTION 2: PROTECTION 2.1 Live üand neutral. ü (2) 2.2 Approximately 20 mA ü and higher. ü (2) 2.3 When the live and neutral wire current is the same,ü the magnetic fields

cancel each other since the current flows in opposite directions.ü

No EMF will be induced in the secondary.ü

If an earth fault occurs, the live will carry more current than the neutralü and

at 20 mAü the resultant field will be strong enough to activate the relay.ü

(6) [10]



RF Capacitor ü

Chokeü

Glow Starter ü

PF Correcting Capacitor ü

Flourescent Tube Filamentsü

V(Maximum)ü V(RMS) = V(Maximum)X 0,707 ü

V(Average) = V(Maximum)X 0,637 ü

Over ½ cycle

QUESTION 3: ILLUMINATION 3.1 3.1.1

3.1.2 3.1.3 3.1.4 3.1.5

False False True False True

(5 x 1)

(5) 3.2

(5) [10] QUESTION 4: ALTERNATING CURRENT THEORY 4.1 4.1.1

(3)

4.1.2

(3)



0°L1ü

L2

L3

I1üI2

I3

120°ü

120°

120°

ü

4.2

(4)

[10]



Vzü

Θ=0.8 Laggingü

VXL

VXC

VX=VXL-VXC

VX=6,25ü

IZ VRü

QUESTION 5: SERIES RLC CIRCUITS 5.1 5.1.1 XL = 2.π.f.L ü

XL = 2.π.50.0,1 ü XL = 31,4 Ωü

(3) 5.1.2

ü

ü

ü

(3) 5.1.3

(4) Note to Marker

Must show lagging p.f OR XL XC] Vz leads IZ Φ = angle between VZ and VR

[10]



QUESTION 6: THREE-PHASE AC SYSTEMS 6.1 ü

°ü

(3) 6.2 VL = .xVph ü

= x 220ü = 381 Vü

(3) 6.3 Pin = VLILcos Ф x ŋü

= .380 X 42,2 X 0,8 X 0.9ü = 19 998.12 Watt ü

η = 90% = 0.9ü

If candidate can show 90% efficiency = 0.9 as a factor of 1 = fourth mark is awarded.

(4) [10] QUESTION 7: TRANSFORMERS 7.1 7.1.1 Deltaü: Starü connected transformer

Note: Order must be correct

(2) 7.2 Step down transformerü. 550:1 indicates 550 turns for every 1 turn

on the secondary

(1) 7.3 Neutral = 4ü

(1) 7.4 Va1a2 = VA1A2 ÷ T.Rü

= 380 ÷ 1/10ü =3 800 Vü

(3) 7.5 P = x VL x IL cosøü

= x 380 x 2 x 0,8 ü = 5,77 A ü

(3) [10]



QUESTION 8: DC MACHINES

8.1 8.1.1 For speed control

8.1.2 To disconnect the motor when the supply voltage is too low

8.1.3 To trip when the motor draws too much current (3 x 1)

(3)

8.2 Shunt

Series

Combination of series and parallel (Any 2 x 1)

(2)

8.3 Brush shifting

Interpoles

Compensating windings, add a series field. (Any 2 x 1)

(2)

8.4 Connect the 4 windings in series to a low voltage supply.

Use a voltmeter to measure the voltage across each winding.

The readings should be the same across all the poles. (3 x 1)

(3) [10] QUESTION 9: AC MACHINES

9.1 The running winding has less turnsü in the coils than the starter winding, but it is of a thicker wire, thus having less inductance and less resistanceü, ergo less reactance.

(2)

9.2

Note to marker:

Must show Centrifugal switch in series with starter capacitor and labelled.

Run Cap must be permanently connected in circuit and labelled.

Main Winding / run Winding in parallel with supply

(3)


Copyright reserved

9.3 The starting arm must move back to the OFF position during loss of supply. (1) 9.4 The motor will only start up if the resistors are in circuit

and the brushes are on (short-circuit rings open).

As the motor gains speed the resistors are gradually removed from the circuit.

Once removed, the slip rings are short-circuited.

(4) [10]

QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS 10.1 10.1.1 Insert a shunt resistor (parallel to meter) into the circuit. (1) 10.1.2 Insert a resistor in series with the meter. (1) 10.1.3 Use instrument transformers.

The secondary of a current transformer supplies the current coil of the meter and

the secondary of a potential transformer supplies the voltage coil of the meter

(3)

10.2 10.2.1 Flow of electrons in the external circuit (not in the battery) from

negative pole to positive pole.

10.2.2 Material with special properties and electrical resistance between a

conductor and an insulator.

10.2.3 Atoms with fewer electrons than protons. 10.2.4 An impurity added to Si or Ge that has only 3 valence electrons. 10.2.5 An atom that has 5 valence electrons (electrons available for

bonding). (5 x 1)

(5)

[10] TOTAL: 100


T470(E)(M30)T

APRIL EXAMINATION



(11041263)

30 March 2015 (Y-Paper) 13:00–16:00


(11041263) -2- T470(E)(M30)T






Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Where applicable, answers must be in accordance with the SABS (SANS) Code of Practice SANS 10142-1:2003 for the Wiring of Premises. Sketches must be neat, labelled and large enough to show the required detail. Answers must be given to TWO decimal places. Write neatly and legibly.

(11041263) -3- T470(E)(M30)T


QUESTION 1: DOMESTIC APPLIANCES 1.1

Draw a neat, fully labelled sketch of a bi-metal thermostat that is used to regulate the temperature of a laundry iron.

(5)

1.2

Figure 1.1 shows an immersion type thermostat that is used to regulate the temperature of water in a geyser. Identify parts A, B, C, D and E

D

AScrew Terminals

C

Heating Element

L

N

B

E

Joined

mechanically

(5)

FIGURE 1.1: IMMERSION TYPE THERMOSTAT [10]

(11041263) -4- T470(E)(M30)T


QUESTION 2: PROTECTION 2.1 Draw a neat, labelled sketch that shows how earthing is achieved from a supply

point in a domestic installation to the general mass of the earth. Your sketch must include the following labels:

Distribution board

Earth electrode

Earth bar

Earth lead

Earth continuity conductor (5 x 1)

(5)

2.2 Explain the operation of the thermal-magnetic-type circuit breaker on:

2.2.1 2.2.2

Overload Short circuit

(3) (2) [10]

QUESTION 3: ILLUMINATION 3.1 Indicate whether the following statements are TRUE or FALSE. Choose the

answer and write only ‘true’ or ‘false’ next to the question number (3.1.1–3.1.5) in the ANSWER BOOK.

3.1.1

3.1.2 3.1.3 3.1.4 3.1.5

A fluorescent lamp circuit was started. Removing the glow starter at this point will switch the lamp off. A choke will limit the current in a D.C. supply lamp circuit. A glow starter actually starts the lamp by opening its contacts. Cold cathode neon lamps emit a white glow. Vaporised mercury emits a blue-green light.

(5 x 1)

(5)

(11041263) -5- T470(E)(M30)T


3.2

Study the circuit in FIGURE 3.1. State why this type of circuit is used and explain its operation.

(3)

L

N

Flourescent Tubes

Capacitor

Supply – 220 V / 50Hz

FIGURE 3.1: FLOURESCENT LIGHT CIRCUIT

3.3 Name TWO types of lamps that are ideal for street lighting. (2)

[10]

QUESTION 4: ALTERNATING CURRENT THEORY A voltage waveform is represented by e = 381sin(628,32t) volts NOTE: The angle is in radians. Calculate the following:

4.1 Average value (3) 4.2 Frequency of the wave (4) 4.3 Instantaneous voltage 10 ms after t = 0 s (3) [10]

(11041263) -6- T470(E)(M30)T


QUESTION 5: SERIES RLC CIRCUITS 5.1 The series RLC circuit in FIGURE 5.1 has a lagging power factor of 0,8. The

component values for R, L and C are 5Ω, 0.02H and 1 258μF respectively. The circuit is supplied with 220 V at 50 Hz.

0,02 H

5Ω C

AC

220V / 50Hz

1.258uF

FIGURE 5.1: RLC CIRCUIT Calculate the: 5.1.1 Inductive reactance (3) 5.1.2 The circuit impedance (3) 5.2

Draw the phasor diagram.

(4) [10]

QUESTION 6: THREE-PHASE AC SYSTEMS 6.1 List TWO advantages of three-phase distribution over single-phase distribution. (2) 6.2 A three-phase delta-connected motor draws 250 A from a 380 V supply at a

lagging power factor of 0,86. Calculate the:

6.2.1

6.2.2 6.2.3

Input power Apparent power Phase current of the motor winding

(3) (3) (2) [10]

(11041263) -7- T470(E)(M30)T


QUESTION 7: TRANSFORMERS 7.1

Explain the operation of a transformer under the following headings:

7.1.1 7.1.2 7.1.3

Type of supply used with this device

Method of power transfer from primary to secondary Method of reduction of eddy currents

(3 x 1)

(3)

7.2 The three-phase step-down star-delta transformer in FIGURE 7.1 has a primary line

voltage of 132 kV and a secondary line voltage of 11 kV.

L1

N

L2

L3

L1

L2

L3

VLP=132kV VLS=11kV

FIGURE 7.1: TRANSFORMER

Ignore losses and calculate the following:

7.2.1 Magnitude of primary phase voltages. Show all manipulations of formulae. (4)

7.2.2 The turns ratio (3) [10]

(11041263) -8- T470(E)(M30)T


QUESTION 8: DC MACHINES 8.1 Indicate whether the following statements are TRUE or FALSE. Choose the

answer and write only ‘true’ or ‘false’ next to the question number (8.1.1–8.1.3) in the ANSWER BOOK.

8.1.1 8.1.2 8.1.3

Circuit conductors shall have a current carrying capacity at least equal to the full load rated current of the motor they supply. For a motor carrying uniform loads, the rated current of the overload protection must exceed the current carrying capacity of the supply cables. In addition to the over-current protection, a motor must have an integral thermal protector with an accessible reset button.

(3 x 1)

(3)

8.2 Draw a neat, fully labelled circuit diagram of a face-plate starter for a series DC

motor. (7)

[10] QUESTION 9: AC MACHINES 9.1 Name TWO types of rotors found in AC-motors. (2) 9.2 State the main purpose of the following components of an induction motor: 9.2.1

9.2.2 9.2.3

Stator frame Stator core Stator coils

(3 x 1)

(3)

9.3 Name TWO properties that the starter circuit of a large 3-phase motor should

possess. (2)

(11041263) -9- T470(E)(M30)T


9.4 Study the diagram in FIGURE 9.1 of a starter circuit below and answer the following

questions:

MC

TR

STOP

START

MC1

TR1MC2

TR2

O/L

MC

STATOR WINDINGS

O/L

L1 L2 L3

1

1

FIGURE 9.1: MOTOR STARTER CIRCUIT

9.4.1

9.4.2 9.4.3

State how the relay of the mains contactor (MC) is energised. State why the motor does not stop when you release the start button. State how the simultaneous energising of the Star and Delta contactors are prevented. (3 x 1)

(3)

[10]

(11041263) -10- T470(E)(M30)T


QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS 10.1

Study the circuit in FIGURE 10.1. Redraw the circuit but add a wattmeter to the diagram to measure the power consumed by one of the 20 Ω loads. (Do not add any calculations.)

L1

L2

L3

20Ω 20Ω

20Ω

38

0 V

38

0 V

FIGURE 10.1: DELTA CIRCUIT

(4)

10.2 Explain the following terms in semiconductor theory:

10.2.1 10.2.2 10.2.3 10.2.4

Holes Junction Depletion layer Forward bias

(4 x 1)

(4)

(11041263) -11- T470(E)(M30)T


10.3 Study the circuit in FIGURE 10.2 of a common emitter amplifier using an NPN

transistor. Answer the following questions.

R1

T1

RE

RL

R2

CE

0V

+VCC

CO

CI

FIGURE 10.2: Common Emmittor Amplifier 10.3.1

10.3.2

State the purpose of capacitor Ce connected across resistor RE. A voltage divider circuit can be used to forward bias the base-emitter region. Identify the components in the diagram that serve this purpose.

(2 x 1)

(2)

[10]

TOTAL: 100

(11041263) -12- T470(E)(M30)T


FORMULA SHEET/FORMULEBLAD

2

p p s

s s p

V N I

V N I

f

.60

r

fN

p

n ns

n

The next five formulae are also true for voltage/Die volgende vyf formules geld ook vir spanning

/

/

2 2 2

1 2/

1 2/

( )

0,707

0,637

......

......

m

rms wgk m

ave gem m

nrms wgk

nave gem

i I Sin t

I I

I I

i i iI

n

i i iI

n

Form factor/Vormfaktor

/

/

RMS value WGK waarde

AVE value GEM waarde

Crest factor/Kruinfaktor

/

/

MAX value MAKS waarde


Series/Serie

1 2 .....T nR R R R

Parallel

1 2

1 1 1 1.....

T nR R R R

/

/

2 2

1

2 2

2

/

2

1

2

/

( )

( )

3 / 3

3

3

L

C

L C

T

ACTIVE AKTIEWE T

REACTIVE REAKTIEWE T

L

C

L C

R T

X T L

X T C

R X X

L L

L L

L PH F

VI

Z

I I Cos

I I Sin

X fL

XfC

Series Serie

Z R X X

RCos

Z

V I R

V I X

V I X

V V V V

P I R

S VI

phase fase

P V I Cos

S V I

Delta

V V

/

/

/

3

/

3

L PH F

L PH F

L PH F

I I

Star Ster

V V

I I

-1-



APRIL EXAMINATION


30 MARCH 2015


MARKING GUIDELINE

MARKING GUIDELINE -2- T490(E)(M30)T ELECTRICAL TRADE THEORY N3


QUESTION 1: DOMESTIC APPLIANCES NOTE: There could be more than one answer/method/approach to the questions. Examiners must use their discretion with regard to students’ answers. 1.1

Bi-Metal Stripü

Snap Action Contactsü

Control Knobü

Spring SteelTemperature Adjustment CAMü

Heating Elementü

L

N

(5)

1.2

Contactsü

Adjustment ScrewüScrew Terminals

Brass Tube

with different

temperature

related

expansion

coefficient

than INVARü

Heating Element

L

N

INVAR Rod

able to extend

in length

inside Brass

Tubeü

Magnet for snap

action switchingü

INVAR Rod and

Brass Tube joined

machanically

(5)

[10]




Earth Bar

Earth Rod

Earthing Lead

Earth

Continuity

Conductors

Distribution Board

Domestic Installation

Earth

Steel /

Copper

Waterpipes

Bonding

(One mark per label)(5 x 1) (5) 2.2 2.2.1 The bi-metal is heated by the overload current. ü This bends the

bi-metal strip and the mechanical coupling opens the contacts. ü This breaks the circuit and current stops. ü (3 x 1)

(3)

2.2.2 The magnetic field becomes strong enough to attract the

mechanical coupling without the aid of the bi-metal bending. ü This breaks the circuit immediately and current stops. ü (Any 2 x 1)

(2)

[10]



QUESTION 3: ILLUMINATION 3.1 3.1.1 False ü 3.1.2 False ü 3.1.3 True ü 3.1.4 False ü 3.1.5 Trueü

(5 x 1)

(5) 3.2 REASON: To minimise the stroboscopic effect ü

EXPLANATION: The capacitor causes the current in the one lamp circuit to lead the current in the other. ü The brightest moments of the two lamps therefore occur at different times. ü

(3)

3.3 High-pressure mercury lampsü and sodium vapour lamps ü (2) [10] QUESTION 4: ALTERNATING CURRENT THEORY 4.1 VAVG = 0,637x VMAXü

VAVG = 0,637x 381 ü VAVG = 242,7 Vü

(3)

4.2

- given therefore

(4)

4.3 e = 381sin(628,32 t) ü

= 381x sin(628,32 x 0,01) ü = 381 x 0,000015 = 0,00559 Vü = 0,01 V

(3)

[10]



QUESTION 5: SERIES RLC CIRCUITS 5.1 5.1.1

(3) 5.1.2

Cos Φ = R/Z ü OR Z= ü

Z = r/cos Φü = ü = 6,25 Ωü = 6,25 Ωü

(3) 5.2

IZ VRü

V

L

Vc

Vxü

Vzü

Θ=0.8 Laggingü

(4) [10] QUESTION 6: THREE-PHASE AC-SYSTEMS 6.1 Less copper per MVA ü

Two voltages available ü (2 x 1) (2)

6.2 6.2.1 P = .VLILcosФ ü

= x 380 x 25 x 0,86 ü = 14,15 kW ü

(3)

6.2.2 S = .VLILü

S = x 380 x 25ü S = 16,45 kVA ü

(3)

6.2.3 Iph = IL ü

Iph = 25 Aü

(2) [10]



QUESTION 7: TRANSFORMERS 7.1 7.1.1 Only alternating currentü 7.1.2 Occurs magnetically through inductionü 7.1.3 Iron core consists of laminated platesü

(3 x 1)

(3) 7.2 7.2.1

ü

ü

ü

(4)

7.2.2 T.R = N1 ÷ N2 = V1 ÷ V2 ü

= 76 210 ÷ 11 000 ü = 6,93:1 ü

(3) [10] QUESTION 8: DC-MACHINES 8.1 8.1.1 True ü 8.1.2 False ü 8.1.3 False ü

(3 x 1)

(3) 8.2

üresistors

in series ücontact arm removes resistors from circuit ütension spring returns arm to start position

(Sketch = 4 marks; Any 3 labels = 3 marks)

(7)

[10]



QUESTION 9: AC MACHINES 9.1 A squirrel cage induction motor. ü

A wound rotor induction motor. ü

(2) 9.2 9.2.1 Supports/protects the stator core ü 9.2.2 Has slots that house the insulated windings/provide metal path for

magnetic fieldü

9.2.3 Produces the main magnetic field ü

(3 x 1)

(3) 9.3 Allows motor to start at a decreased supply current ü

Disconnects motor on overcurrent ü

(2) 9.4 9.4.1 When the start button is pressed, it energises the coil of the MC,

thus closing the contacts.✓

9.4.2 The MC contactor has extra contacts called MC1 and MC2. MC1 is

bypassing the start button contacts.✓

9.4.3 The Δ contactor has extra normally closed contacts called Δ1.

When the Δ contactor is energised, the star contactor de-energises

because Δ1 is in series with it.✓

(3 x 1)

(3)

[10]



QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS 10.1

Voltage Transformer ü

Current Transformer ü

Current Coilü

Voltage

Coilü

L1

L2

L3

20Ω 20Ω

20Ω

38

0 V

38

0 V

(4) 10.2 10.2.1 Holes are places where electrons can move to or places that the

electrons left behind. ü

10.2.2 A junction is the surface where P-type and N-type material come

together. ü

10.2.3 Depletion layer lacks holes and free electrons OR

Depletion layer is the area where ions have been formed. ü

10.2.4 When a PN junction is forward-biased, current flows through the

semiconductor material OR When a PN junction is forward-biased, there is a decreased depletion region.ü

(4 x 1)

(4)

10.3 10.3.1 Ce is an AC bypass capacitor. ü 10.3.2 R1 and R2 are the voltage dividers. ü

(2 x 1)

(2) [10]

TOTAL: 100


T490(E)(N13)T NOVEMBER EXAMINATION



(11041263)

13 November 2014 (Y-Paper)

13:00–16:00


11041263) -2- T490(E)(N13)T






Answer ALL the questions. Read ALL the questions carefully. Number the answers correctly according to the numbering system used in this question paper. Where applicable, answers MUST be in accordance with the SABS (SANS) Code of Practice SANS 10142-1:2003 for the Wiring of Premises. Sketches MUST be neatly labelled and large enough to show the required detail. Answers MUST be given to TWO decimal places. Write neatly and legibly.

11041263) -3- T490(E)(N13)T


QUESTION 1: DOMESTIC APPLIANCES 1.1

Study the sketch of an oven thermostat in FIGURE 1. Identify the labelled parts and write only the name of each part next to the question number (1.1.1–1.1.5) in the ANSWER BOOK. Do NOT redraw the sketch.

1.1.1

1.1.5

Oven Element (top) Oven element (bottom)

1.1.2 1.1.3 1.1.4

FIGURE 1 (5) 1.2 Explain the operation of the oven thermostat as shown in QUESTION 1.1. (5)

[10] QUESTION 2: PROTECTION 2.1 State whether the following household appliances/components need „to BE

earthed‟ or „NOT earthed‟:

2.1.1

Metal light fittings for fluorescent tubes that are 2,4 m or above the floor.

2.1.2

The metal taps that are connected by PVC water pipes to the water supply.

2.1.3 Metal conduits used for house wiring.

(3 × 1)

(3) 2.2 Name TWO methods of checking if the earth-leakage relay is operating

correctly.

(2) 2.3 State the approximate value of the fault current that should activate the earth

leakage.

(1)

11041263) -4- T490(E)(N13)T


2.4

Describe the operation of a thermal-magnetic miniature circuit breaker under the following headings:

2.4.1 Overload protection 2.4.2 Short circuit protection

(2 × 2)

(4) [10]

QUESTION 3: ILLUMINATION 3.1 State TWO advantages of sodium-vapour discharge lamps. (2) 3.2 State ONE disadvantage of sodium-vapour discharge lamps. (1) 3.3 Explain how the glow starter in a fluorescent lamp circuit starts the lamp. (2) 3.4 Study the circuit below in FIGURE 2.1.Label the parts numbered (a) – (e) in

your answer script.

(a)

(b)

(c)

(d)

(e)

FIGURE 2.1: LIGHT FITTING (5) [10]

11041263) -5- T490(E)(N13)T


QUESTION 4: ALTERNATING CURRENT THEORY 4.1 Draw a sine wave on which you indicate the following: 4.1.1 The RMS Value 4.1.2 Amplitude 4.1.3 Peak to Peak Value 4.1.4 1 Cycle (4 x 1) (4) 4.2 With reference to FIGURE 4.1 calculate the instantaneous value of e at

2.5 mS.

(3)

EM=100V

T= 0.02s

F= 50 Hz

t= 2.5 mS e

FIGURE 4.1: SINE WAVE 4.3 Calculate the RMS value of a sine wave with the following equation:

(3)

[10] QUESTION 5: SERIES RLC CIRCUITS 5.1

Study the circuit in FIGURE 5.1 and answer the following questions.

2KΩ

628Ω

318Ω

220 V / 50 Hz

FIGURE 5.1: SERIES RLC CIRCUIT 5.1.1 Calculate the impedance of the circuit (3) 5.1.2 Calculate the total current in the circuit (3)

11041263) -6- T490(E)(N13)T


5.2 Draw the phasor diagram of the circuit in Figure 5.1 showing IR, IXL, IXC and

IZ. The drawing need not be to scale.

(4) [10] QUESTION 6: THREE-PHASE AC-SYSTEMS 6.1 FIGURE 6.1 shows a 3-phase system. Calculate the line voltage if the voltage

across ONE phase and neutral is 220 V.

N

L1

L2

L3

V

220 V

(3)

FIGURE 6.1: 3-PHASE AC SYSTEM 6.2 A 3-phase motor rated at 25kW draws a full-load current of 42,2 A from a

380 V supply at a power factor of 0,8. Calculate the:

6.2.1 Output Power of the motor (3) 6.2.2 Efficiency of the motor. (3) 6.2.3 State ONE type of loss that would explain why the output of a motor

is lower than the power supplied to it

(1) [10]

11041263) -7- T490(E)(N13)T


QUESTION 7: TRANSFORMERS FIGURE 7.1 shows a three Phase Transformer. The circuit is connected to a 3-phase 380 V supply at contact points A, B and C. Also, a current of 2 kA, at a power factor of 0,8 is drawn from the supply when the transformers are connected to a balanced 3-phase load at points 1, 2, 3 and 4.

A

B

C

4

3

2

1

A1

A2

FIGURE 7.1: THREE PHASE TRANSFORMER

7.1 Identify the configuration of the PRIMARY windings (1) 7.2 Identify the configuration of the secondary windings (1) 7.3 Determine the voltage between A1 and A2 (1) 7.4 Calculate the output voltage at between 2 and 3 if the transformer has a turns

ratio of 1 : 10.

(3) 7.5 Calculate the total power consumed. (3) 7.6 Identify the Neutral point on the secondary winding of the transformer (1) [10] QUESTION 8: DC MACHINES 8.1 With reference to DC motors, explain the term “armature reaction” (2) 8.2 8.2.1

Draw a graph of the load characteristics of a cumulatively compounded DC motor as compared to a shunt motor to show how speed changes as the load increases. Label all aspects of the graph.

(4)

8.2.2 With reference to its torque characteristics, explain what happens

to a series motor if the speed increases. (1)

8.3 Briefly explain, during starting, how you would couple a very large DC motor

so that it is NOT damaged by overcurrent. (3)

[10]

11041263) -8- T490(E)(N13)T


QUESTION 9: AC MACHINES 9.1

Calculate the speed of a four pole induction motor that is connected to 380 V/50 Hz

(3)

9.2 Draw a neat, fully labelled circuit diagram of a single phase resistance-start

split-phase motor.

(5) 9.3 With reference to FIGRE 9.1, show how the direction of rotation of a 3-Phase

motor will be changed. Redraw the figure in your ANSWER BOOK and indicate the changes made.

L1

L2

L3

L1

L2

L3

FIGURE 9.1: DIRECTION OF ROTATION (2) [10] QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS 10.1

Study the circuit in FIGURE 10.1 and answer the following questions.

B

AL

N N

C

D

To LoadL

FIGURE 10.1: POWER MEASUREMENT 10.1.1 Identify the Parts A, B, C and D (4) 10.1.2 State the purpose of parts A and B (1)

11041263) -9- T490(E)(N13)T


10.2 In FIGURE 10.2 below, study the sketch of the circuit and answer the

questions that follow.

R1

T1

RE

RL

R2

CE

+VCC

CO

A

B

C

D

E

F

FIGURE 10.2: AMPLIFIER 10.2.1 Identify the type of transistor being used 10.2.2 State in which configuration is the transistor coupled 10.2.3 Identify the point where the input signal should be connected 10.2.4 Identify the point where the amplified signal appears 10.2.5 In order for this circuit to operate it needs a D.C-source. Identify

the point where the negative of the battery should be coupled. (5 × 1)

(5) [10]

TOTAL: 100

11041263) -10- T490(E)(N13)T


FORMULA SHEET

Z

VI

T

CosII TACTIVE

SinIIREACTIVE T

Lf2πXL

fC2π

1CX

2

CL

2 )X(XRZ

Z

RCos 1

RIV TR

LTXL XIV

CTXC XIV

2

XCXL

2

R )V(VVV

R IP 2

VIS

3-phase

osIV3P LL C

LL I V3S

DELTA

V FPH /L V

FPHI

/L3I

STAR

V FPH /L V3

FPHI /LI

p

s

s

p

s

p

I

I

N

N

V

V

f2ω

p

fN

60.

n

nns r

The next five formulae are true for voltage.

t)(inIi m S

mrms I 0,707I

mave I0,637I

n

i...iiI

2

n

2

2

2

1rms

n

i..iiI n21

ave

valueAVE

valueRMSfactor Form

valueRMS

valueMAXfactorCrest

SERIES

n21T R...RRR

PARALLEL

n21T R

1...

R

1

R

1

R

1



NOVEMBER EXAMINATION


13 NOVEMBER 2014


MARKING GUIDELINE



INSTRUCTIONS AND INFORMATION NOTE: There is not only one answer or one method (approach) of answering the

questions. This memorandum gives only one answer or one possible method (approach). Examiners are advised to analyse the student’s solution in order to determine if the question has been answered and they must not adhere so strictly to the solutions in this memorandum as to disadvantage the candidate.

QUESTION 1: DOMESTIC APPLIANCES 1.1 1.1.1 Gas filled capillary tube 1.1.2 Micro-switch 1.1.3 Temperature adjusting cam 1.1.4 Control (adjust) knob 1.1.5 Expansion bellows (5) 1.2 When oven cold, bellows contracted and micro-switches closed.

Current flows in upper and lower heating element.

Gas in tube expands with temperature.

Cam adjusts contacts so that a larger movement is required to open them.

When desired temperature reached, both contacts opened by bellow movement.

As oven cools, the bottom element’s switch closes and keeps oven at temperature. (Any 5 × 1)

(5) [10]

QUESTION 2: PROTECTION 2.1 2.1.1 Need to be earthed. 2.1.2 Need not be earthed. 2.1.3 Need to be earthed.

(3 × 1)

(3) 2.2 Use the test button on the relay.

Connect a tester at an outlet between earth and live that draws no more than 20 mA.

(2)

2.3 Above 15 to 20 mA. (1)



2.4 2.4.1 When the current exceeds the rated value, the current will heat the

bi-metal to a high enough temperature to activate the thermal mechanism that opens the contacts.

(2)

2.4.2 When the current is very large, the magnetic flux around the coil causes the circuit to trip before the bi-metal bends, thus relying on magnetism for fast reaction. (Any 2 × 1)

(2) [10]

QUESTION 3: ILLUMINATION 3.1 Efficiency is high.

Does not need to be cool before restarting.

(2)

3.2 Light is yellow. (1)

3.3 The glow starter heats up and thus opens. This causes the field around the choke to collapse and a voltage surge occurs across the tube.

(2)

3.4 (a) Filaments (b) Choke (c) Capacitor (d) Autotransformer (e) Earthed Enclosure

(5)

[10] QUESTION 4: ALTERNATING CURRENT THEORY

4.1 Amplitude =EM

T= Period

1 x Cycle

ERMS = 0.707 x EM

Peak to Peak Value (4)

4.2

(3)



4.3

(3)

[10] QUESTION 5: SERIES RLC CIRCUITS 5.1 5.1.1

(3)

5.1.2

(3)

5.2

IX

C

IR I

IX

=IX

L-I

XC

IZ

IXL

(4)

[10]



QUESTION 6: THREE-PHASE AC SYSTEMS 6.1 VL = . x Vph

VL = x 220 VL = 381 V

(3) 6.2 6.2.1 POUT = VL IL cos Ф

POUT = .380.42,2.0,8

POUT T = 22,22 kW

(3) 6.2.2

(3) 6.2.3 Mechanical Losses

Phase Angle Copper and Iron Losses I2R Losses

(1) [10] QUESTION 7: TRANSFORMERS 7.1 Delta (1) 7.2 Star (1) 7.3 VP phase = VP L

= 380 V

(1) 7.4

(3)

7.5

(3)

7.6 4 = Neutral (1)

[10]



QUESTION 8: DC MACHINES 8.1 Armature reaction is the effect of the armature ampere-turns upon the value

and distribution of the magnetic flux entering and leaving the armature core. The result is a distorted magnetic field.

(2)

8.2 8.2.1

Sp

ee

d

LOAD

SHUNT Motor

Compound Motor

(4)

8.2.2

To

rqu

e

Speed

Series motor –

Torque reduces

with speed.

(1)

8.3 Insert a resistor bank in series with the armature and gradually reduce this

resistor value as the back EMF increases. (3)

[10]



QUESTION 9: AC MACHINES 9.1 f = np

n = f/p = 50 ÷ 2 = 25 r.p.s or 1 500 r.p.m

(3)

9.2

L

N

Disconnect SwitchRotor

Run Winding

Starter Winding

Starting Resistor

(5)

9.3

L1

L2

L3

L1

L2

L3

L1

L2

L3

(2) [10]



QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS

10.1 10.1.1

Voltage Transformer

Current Transformer

L

N N

Earth Connection

Wattmeter

To LoadL

(4)

10.1.2 Instrument transformers reduce the voltage to a safe value for

measurement.

(1) 10.2 10.2.1 NPN. 10.2.2 Common emitter. 10.2.3 Between B and C. 10.2.4 Between E and F. 10.2.5 On the CF line or at C or at F.

(5 × 1)

(5) [10]

TOTAL: 100


T450(E)(J23)T

AUGUST EXAMINATION



(11041263)

23 July 2014 (Y-Paper) 13:00–15:00


(11041263) -2- T450(E)(J23)T






Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Where applicable, answers must be in accordance with the SABS (SANS) Code of Practice SANS 10142-1:2003 for the Wiring of Premises. Sketches must be neat, labelled and large enough to show the required detail. Formulae used in Electrical Trade Theory N3 can be found at the end of the question paper. Answers must be given to TWO decimal places. Write neatly and legibly.

(11041263) -3- T450(E)(J23)T


QUESTION 1: DOMESTIC APPLIANCES

1.1 Name the alloy that is normally used for manufacturing heating elements. (1)

1.2 Briefly describe the type of thermostat you would expect to find in a/an:

1.2.1 1.2.2 1.2.3 1.2.4

Laundry iron Oven Fridge or freezer Geyser

(4 x 1)

(4)

1.3 Draw a neat, labelled wiring diagram that illustrates the stove terminal connection to

the distribution board. (5)

[10] QUESTION 2: PROTECTION 2.1 Identify the error made in the following statements: Write only the answer next to

the question number (2.1.1–2.1.4) in the ANSWER BOOK.

2.1.1

2.1.2 2.1.3 2.1.4

Touch voltage occurs because the equipment you are touching is at the same potential as the ground you are standing on. A floating earth is connected to the earth pin of the three-pin plug top. The earth leakage protection will disconnect the circuit if the load current is too high. An isolating transformer is a transformer that has no exposed metal parts. (4 x 1)

(4)

2.2 2.3 2.4

Name TWO safety precautions which would reduce the danger when using a washing machine. Name TWO advantages that circuit breakers have over HRC fuses. Show by means of a neat, labelled sketch how an earth leakage tester is connected to a socket outlet, when testing the effectiveness of an earth leakage relay.

(2) (2) (2)

[10]

(11041263) -4- T450(E)(J23)T


Starter

Current

Limiting

Resistor

Supply

Reversing

Switch

Surge Coil

On Off On

QUESTION 3: ILLUMINATION 3.1 3.2 3.3

Give TWO advantages of incandescent lamps. Give ONE disadvantage of incandescent lamps. Explain luminous efficiency.

(2) (1) (2)

3.4 Study the circuit in FIGURE 3.1 and answer the questions.

FIGURE 3.1: LIGHTING CIRCUIT

3.4.1

3.4.2 3.4.3 3.4.4 3.4.5

Identify the type of supply used with this circuit. Identify the function of the reversing switch. State the approximate voltage over the current limiting resistor during operation. State the purpose of the surge coil. Identify the type of lamp used in this circuit. (5 x 1)

(5) [10]

(11041263) -5- T450(E)(J23)T


120°

120°

120°

Phase 1 Phase 2

Phase 3

QUESTION 4: ALTERNATING-CURRENT THEORY 4.1

FIGURE 4.1 shows a 3-phase generator. There are three identical single-phase armature windings wound on the same core. The windings are displaced by 120° and rotating in the same magnetic field. A 380-V EMF is generated in each winding, in the form of a sine wave. Illustrate the difference in the three generated waves by drawing it on a single set of axes. (Use an appropriate scale on the X- and Y-axes). (3)

FIGURE 4.1: 3-PHASE GENERATOR 4.2 A sine wave form has the following characteristics:

EM = 310 Volt F= 1432 Hz

FIGURE 4.1: SINE WAVE

Calculate the following: 4.2.1 RMS value (3) 4.2.2 Instantaneous value of e when t = 1 mS(millisecond) (4) [10]

(11041263) -6- T450(E)(J23)T


0,01 H

5Ω C

AC

VL

VC

VL-VC

35°

VR=60V

VT

73,25V / 100Hz

QUESTION 5: SERIES RLC CIRCUITS 5.1 Study the circuit in FIGURE 5.1 and answer the questions.

FIGURE 5.1: RLC CIRCUIT

Calculate: 5.1.1 Inductive Impedance (3) 5.1.2 The value of VL-VC (3) 5.1.3 The value of C if XC=9,78 Ω (3) 5.2 State whether the magnitudes of the true and apparent power are equal for this

circuit. (1)

[10]

(11041263) -7- T450(E)(J23)T


QUESTION 6: THREE-PHASE AC SYSTEMS Three identical loads are connected to a 3-phase 380-V supply. Each load is purely resistive and the total consumption is 3,8 kW. 6.1

Draw a circuit diagram that shows the loads connected in delta and connected to the supply.

(4)

6.2 Write the voltage values between all conductors in the diagram. (1) 6.3 Calculate the value of the phase current flowing in Line 1. (3) 6.4 State what will happen to the nature of the current flowing in the circuit if the

resistive load is changed to an inductive load. (1) [10] QUESTION 7: TRANSFORMERS 7.1 A 20 MVA 3-phase 132 kV/11 kV delta-star transformer has 100 turns on the

secondary. Calculate the following:

7.1.1 Turns Ratio of the transformer (3) 7.1.2 The number of turns on the primary side of the transformer. (2) 7.1.3 Output power at a power factor of 0,8 (3) 7.2 Name TWO uses (applications) of transformers. (2) [10] QUESTION 8: DC MACHINES 8.1 Explain the principle of operation of the direct-current motor. (3) 8.2 Explain the principle of operation of the direct-current generator. (3) 8.3 Explain how a large DC motor can be started without damaging the motor or

motor circuit.

(2) 8.4 While the motor is turning, there is a power cut. Explain how the protective

circuitry will prevent the motor from automatically starting up as soon as the supply voltage returns.

(2) [10]

(11041263) -8- T450(E)(J23)T


R1 R2

R3 R4

QUESTION 9: AC MACHINES 9.1 Name TWO types of single-phase AC motors. (2) 9.2 Name TWO tests that can be performed on a motor's stator coils. (2) 9.3 State the purpose of the overload relay in a motor circuit. (1) 9.4 Identify the error in the statements below. Write only the answer next to the

question number (9.4.1.–9.4.4) in the ANSWER BOOK.

9.4.1

9.4.2 9.4.3 9.4.4

Direct-on-line starting places a voltage on the stator coils that is times

smaller than the supply voltage. In rotor resistance starters there are resistors connected to the stator coils during starting. The starting winding of a split-phase motor is placed closer to the yoke of the motor than the running winding. A centrifugal switch will automatically disengage when the current is high. (4 x 1)

(4)

9.5 State how the direction of rotation of a three-phase motor can be changed. (1) [10] QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS 10.1 Study the circuit below.

FIGURE 10.1: ELECTRONIC CIRCUIT

Without performing any calculations, redraw the circuit in your ANSWER BOOK and

add THREE measuring instruments in the diagram that will measure the current through resistor R4, the voltage across resistor R2 and the power consumed by resistor R3.

(6)

(11041263) -9- T450(E)(J23)T


10.2 10.3 10.4

Name TWO basic configurations in which transistors can be connected in an amplifier circuit. What is the purpose of a smoothing capacitor? Give ONE use of a diode.

(2) (1) (1) [10]

TOTAL: 100

(11041263) -10- T450(E)(J23)T


FORMULA SHEET

2

p p s

s s p

V N I

V N I

f

.60

r

fN

p

n ns

n

The next five formulae are also true for voltage / Die volgende vyf formules geld ook vir spanning

/

/

2 2 2

1 2/

1 2/

( )

0,707

0,637

......

......

m

rms wgk m

ave gem m

nrms wgk

nave gem

i I Sin t

I I

I I

i i iI

n

i i iI

n

Form factor/Vormfaktor

/

/



Crest factor/Kruinfaktor

/

/



Series/Serie

1 2 .....T nR R R R

Parallel

1 2

1 1 1 1.....

T nR R R R

/

/

2 2

1

2 2

2

/

2

1

2

/

( )

( )

3 / 3

3

3

L

C

L C

T

ACTIVE AKTIEWE T


L

C

L C

R T

X T L

X T C

R X X

L L

L L

L PH F

VI

Z

I I Cos

I I Sin

X fL

XfC

Series Serie

Z R X X

RCos

Z

V I R

V I X

V I X

V V V V

P I R

S VI

phase fase

P V I Cos

S V I

Delta

V V

/

/

/

3

/

3

L PH F

L PH F

L PH F

I I

Star Ster

V V

I I



AUGUST EXAMINATION


23 JULY 2014


MARKING GUIDELINE

MARKING GUIDELINE -2- T450(E)(J23)T ELECTRICAL TRADE THEORY N3


Neutral Barü

Earth Barü

Stove

L 3

N

E

Chassis

Line

MCBü

Lineü Neutral

Distribution Board

L1

L 2 ü

QUESTION 1

1.2 1.2.1

1.2.2 1.2.3 1.2.4

Bimetal strip that bends and opens contacts ü Capillary tube with bellows ü Capillary tube with bellows ü Submersible type with brass that expands ü

(4 x 1)

(4)

1.3

(a) Earth bar ü (b) Circuit breaker ü (c) Live bar ü (d) Disconnector ü (e) 3-phase stove adapted for single-phase supply ü

(5) [10]

1.1 Nichrome.ü (1)



QUESTION 2 2.1 2.1.1

2.1.2 2.1.3 2.1.4

Will not occur OR should be at different potential ü Need not be earthed OR will not be floating ü Leakage current OR the MSB ü No connection between primary and secondary ü

(Any 4 x 1)

(4)

2.2 Wear rubber soled shoes while touching the washing machine. ü Ensure that

the power cords and plug tops are in good condition. ü (Any 2 x 1)

(2) 2.3 (a) Not destroyed ü

(b) Quick to reset ü (c) Can be used as a switch (d) Cannot tamper with rating (Any 2 x 1)

(2)

2.4

(2) [10]

mA

Rvar

Earth ü

Liveü



QUESTION 3 3.1 3.2 3.3

Cheap to replace, ü bright light, ü white light, etc. (Any 2 x 1) Efficiency is low, produces heat, etc. ü (Any 1) High luminous efficiencyü if most of the electrical energy is converted to light energy. ü

(2) (1) (2)

3.4 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5

DC ü To prevent darkening on one side, mercury does not migrate to one side, etc. ü Half of supply voltage ü To start the lamp ü Fluorescent lamp ü (5 x 1)

(5) [10]




(3)

4.2 4.2.1

RMS = 0,707.Emü = 0,707x310 ü = 219,17 Vü

(3)

4.2.2

t=1mS=0,001Sü

ü

ü

ü

(4)

[10]

ü ü ü



QUESTION 5 5.1 5.1.1 XL = 2πfL ü

= 2πx100x0,01 ü = 6,28 Ω ü

(3)

5.1.2

ü

ü

ü

(3)

5.1.3

ü

ü

ü

(3)

5.2 Noü (1) [10]



L1 / Redü

L2 / Yellow ü

L3 / Blue ü

VL1

=3

80V

VL

2=

38

0V

VL

3=

38

0V

IL=√3 x VP

Delta Connection ü

QUESTION 6

6.1

(4)

6.2 VL1 = VL2 = VL3ü = 380 V ü (2) 6.3

ü

ü

ü

(3) 6.4 Making the load inductive will cause the total current to lag behind the supply

voltage.

(1) [10]



QUESTION 7 7.1 7.1.1 ü

(3) 7.1.2 T.R = Np/Ns

Np = 20,78 x 100 ü Np = 2 078 turnsü

(2) 7.1.3 Output power

P = Sx cosФ ü = (20x106)x 0,8 ü = 16 x 106 W =16 MWü

(3)

7.2 To step voltages down or up.ü To isolate primary from secondary. ü (Any 2 x 1) (2)

[10] QUESTION 8 8.1 8.2 8.3 8.4

A moving (changing) magnetic field (cutting the conductor) ü plus a current carrying conductor. üThe conductor will experience a force on it. ü A force on a conductor (conductor is moving) ü cutting magnetic field-lines at an angleü. An EMF will be induced in the conductor. ü A large DC motor can be started without damaging the motor or motor circuit by inserting resistors in the armature circuit üand switching them out as the speed picks up. ü The return spring does not allow the motor to be started with the full supply voltage across the motor üunless the contact arm is forced to remain in the run position.ü OR The NCV will release the stinting arm back to the off-position when the power is cut. The motor will have to be manually started again.

(3) (3) (2) (2) [10]



QUESTION 9 9.1 9.2 9.3

Split-phase induction motor, AC series motor, repulsion motor, shaded pole synchronous motor üü (Any 2 x 1) Resistance between windings, resistance between windings and earth, stator coils open and short-circuit tests.ü ü (Any 2 x 1) To disconnect the circuit when the current exceeds the design value of the circuit. ü

(2) (2) (1)

9.4 9.4.1

9.4.2 9.4.3 9.4.4

Star-delta starting places a voltage on the stator coils that is

times smaller than the supply voltage. OR direct-on-line starting places the supply voltage on the stator coils. ü In rotor resistance starters there are resistors connected to the rotor coils during starting. ü The starting winding of a split-phase motor is placed closer to the rotor of the motor than the running winding. ü A centrifugal switch will automatically disengage when the speed becomes too high OR an overload relay will disengage when the current becomes too high. (4 x 1)

(4)

9.5 Swap the connection of two of the supply lines to the motor. ü (1) [10]



R1 R2

R3

R4

V

A

ü

ü

ü

ü

ü

ü

QUESTION 10 10.1

(6)

10.2 Common- baseü, -emitterü, -collector (2) 10.3 To smoothen the voltage, to store charge that can be used if the voltage

drops, etc. ü (Any 1)

(1) 10.4 To rectify ACü, stop any reverse voltage. (Any 1) (1) [10]

TOTAL: 100


T450(E)(M26)T

APRIL EXAMINATION



(11041263)

26 March 2014 (Y-Paper) 13:00–16:00

This question paper consists of 8 pages and a formula sheet.

(11041263) -2- T450(E)(M26)T






Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Where applicable, answers must be in accordance with the SABS (SANS) Code of Practice SANS 10142-1:2003 for the Wiring of Premises. Sketches MUST be neat, labelled and large enough to show the required detail. Formulae used in Electrical Trade Theory N3 can be found at the end of the question paper. Answers MUST be given in up to TWO decimal places. Write neatly and legibly.

(11041263) -3- T450(E)(M26)T


QUESTION 1: DOMESTIC APPLIANCES 1.1 Indicate how a stove-plate that contains TWO heater elements can be

connected to produce the following:

1.1.1 Low heat 1.1.2 Medium heat 1.1.3 High heat

(3 × 1)

(3) 1.2

Explain how you would test the heating elements of the above stove-plate for continuity.

(3)

1.3

Draw the single phase sub-circuit that adheres to wiring code regulations for a geyser.

(4) [10]


Explain the operation of the magnetic (NOT thermal-magnetic) type circuit breaker on:

2.1.1 Overload (2) 2.1.2 Short circuit (2) 2.1.3 Time delay achieved for over-current protection (1) 2.2 Draw a circuit diagram of a single-phase core balance earth-leakage relay. (5)

[10]

(11041263) -4- T450(E)(M26)T


QUESTION 3: ILLUMINATION 3.1 Make use of FIGURE 3.1 below to answer the following: 3.1.1 State the reason for using spring-loaded circuit disconnecting

contacts.

(1) 3.1.2 State what happens to the voltage across the secondary winding of

the ballast once the lamps strike (switch on).

(1) 3.1.3 Explain the sequence of current flow through the circuit to activate

the lamps.

(3)

FIGURE 3.1: ILLUMINATION CIRCUIT 3.2 State TWO reasons why high-pressure mercury vapour lamps would be

preferred above incandescent lamps.

(2) 3.3 What built-in design in fluorescent tubes limits the stroboscopic effect? (1) 3.4 State TWO reasons for inserting a choke into a fluorescent lamp circuit. (2)

[10]

(11041263) -5- T450(E)(M26)T


QUESTION 4: ALTERNATING CURRENT THEORY 4.1 Write down the equation of a waveform that will represent the following: 4.1.1 A sine wave with a maximum value of 300 V and a periodic time of

20 m/s

4.1.2 A sine wave with an R.M.S values of 220 V and a frequency of

50 Hz

4.1.3 A 50 Hz sine wave with an average current of 2 A

(3 × 2)

(6) 4.2 Explain, with the aid of the sketch how a single-phase alternating EMF is

generated.

(4) [10]

QUESTION 5: SERIES RLC CIRCUITS A series RLC circuit is supplied with a 50 Hz, 220 V supply. The component values are 1 kΩ, 10 mH and 1 000 μF. Calculate the following:

5.1 Inductive reactance (3) 5.2 Capacitive reactance (3) 5.3 Impedance (3) 5.4 State what will happen with the value of the inductive reactance on an

inductor if the frequency of the supply is increased.

(1) [10]

QUESTION 6: THREE-PHASE AC SYSTEMS Three identical loads are connected to a three-phase, 380 V 4-wire supply. 6.1 Draw a circuit diagram that will illustrate the following: 6.1.1 The load connected in star (3) 6.1.2 The neutral point (1) 6.2 Calculate the value of the phase voltage if the line voltage is 380 volt. (3) 6.3 Calculate the total apparent power consumed, if each load draws 10 A. (3)

[10]

(11041263) -6- T450(E)(M26)T


QUESTION 7: TRANSFORMERS 7.1 Explain the operation of a transformer under the following headings: 7.1.1 Generation of magnetic flux 7.1.2 Mutual inductance 7.1.3 Link between primary and secondary winding 7.1.4 Efficiency

(4 × 1)

(4) 7.2

A three-phase step-down transformer has a star-connected primary and a delta-connected secondary. The primary line voltage is 275 kV and the secondary line voltage 66 kV. Ignore the losses and calculate the following:

7.2.1 Turns ratio 7.2.2 Primary line current if the secondary line current is 100 A

(2 × 3)

(6) [10]

QUESTION 8: DC-MACHINES 8.1

Explain the generator principle (Faraday's Law) in which mechanical energy is converted to electrical energy.

(4)

8.2 Draw a graph of the load characteristics of a series connected DC-motor. (3) 8.3 Name TWO methods of changing the running speed of a DC-motor. (2) 8.4 State the type of test that is performed with a growler. (1)

[10]

(11041263) -7- T450(E)(M26)T


QUESTION 9: AC-MACHINES 9.1 Study the diagram of a starter circuit shown below and answer the questions.

FIGURE 9.1: THREE-PHASE STARTER

9.1.1 Identify the type of motor used with the starter in FIGURE 9.1. (1)

9.1.2 Name the principle on which the starter in FIGURE 9.1 operates. (1)

9.1.3 State the purpose of the no-volt coil used in the three-phase starter in FIGURE 9.1.

(1)

9.1.4 Explain the sequence of events that happen if the current drawn from the supply exceeds the rating of the overload protection.

(2)

9.1.5 Describe how the starter would react if one of the supply phases is disconnected

(2)

9.2 Indicate whether the following statements are TRUE or FALSE. Choose the answer and write only 'true' or 'false' next to the question number (9.2.1–9.2.3) in the ANSWER BOOK.

9.2.1 Some motors with very high impedance can stall without overheating.

9.2.2 Over-current protective devices in a motor shall prevent multi-phase motors from single phasing.

9.2.3 A direct-on-line motor will start up automatically when the supply voltage is reset.

(3 × 1)

(3) [10]

(11041263) -8- T450(E)(M26)T


QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS 10.1 It is possible to measure power in a three-phase circuit using only TWO watt

meters.

10.1.1 Draw a circuit diagram to show how TWO watt meters are coupled

to a three-phase supply.

(4) 10.1.2 Write the formula used to calculate the total power of the above

circuit when the load is unbalanced.

(2) 10.2 Identify the base material used to make a PN diode. (1) 10.3 Write the name of the dope material used for a P-Type semiconductor. (1) 10.4 Draw a circuit diagram of a half-wave rectifier. (2)

[10] TOTAL: 100

(11041263) -9- T450(E)(M26)T


2

p p s

s s p

V N I

V N I

f

.60

r

fN

p

n ns

n

FORMULA SHEET

The next five formulae are also true for voltage.

/

/

2 2 2

1 2/

1 2/

( )

0,707

0,637

......

......

m

rms wgk m

ave gem m

nrms wgk

nave gem

i I Sin t

I I

I I

i i iI

n

i i iI

n

Form factor:

/

/



Crest factor:

/

/



Series:

1 2 .....T nR R R R

Parallel:

1 2

1 1 1 1.....

T nR R R R

/

/

2 2

1

2 2

2

/

2

1

2

/

( )

( )

3 / 3

3

3

L

C

L C

T

ACTIVE AKTIEWE T


L

C

L C

R T

X T L

X T C

R X X

L L

L L

L PH F

VI

Z

I I Cos

I I Sin

X fL

XfC

Series Serie

Z R X X

RCos

Z

V I R

V I X

V I X

V V V V

P I R

S VI

phase fase

P V I Cos

S V I

Delta

V V

/

/

/

3

/

3

L PH F

L PH F

L PH F

I I

Star Ster

V V

I I



APRIL EXAMINATION


26 MARCH 2014


MARKING GUIDELINE



NOTE: There is not only one answer or one method of (approach to) answering the

questions. This memorandum/marking guideline gives only one answer or one possible solution (or approach). However, the Examiners must attempt to analyse the candidate’s answer/solution to determine if the question has been answered. Thus the Examiners and/or Markers must not adhere so strictly to the answers given in this document that creative answers are ignored to the candidate’s disadvantage.

QUESTION 1 1.1 1.1.1 The two elements are in series with the supply. 1.1.2 Only one element is connected to the supply. 1.1.3 The two elements are connected in parallel to the supply. (3 x 1) (3) 1.2 (Mains supply must be disconnected)

Set multi-meter on resistance (Ohms scale) and measure between the element leads. Ohms (resistance) value should be low.

(3)

1.3

(4)

[10]

Double Pole Isolator Geyser

Distribution Board

Neutral Bar

Live

Earth

MCB

Thermostat

T Element &




The overload current attracts the piston and the oil is pushed through small holes as the piston moves. This takes time to break the circuit since the piston must increase the field strength to activate the tripping mechanism.

(2) 2.1.2

Short circuit. The magnetic field becomes strong enough to attract the mechanical coupling without the aid of the piston. This breaks the circuit immediately and current stops.

(2) 2.1.3 The time it takes for the oil to be pushed through.

It is afforded hydraulically by the time taken for the oil to be forced past the piston as it is pulled inwards. OR The time it takes for the oil to be pushed through.

(1)

2.2

Test circuit Straight primaries Toroidal core (laminated)

Tripping relay

(5)

[10] QUESTION 3 3.1 3.1.1 To ensure good contact with the lamp pins when the lamp is

inserted

(1) 3.1.2 The current in the secondary will be low once the lamps are on. OR

When the tube strikes the voltage across the auto-transformer falls.

(1) 3.1.3 Current moves through top lamp, then trafo-secondary, then

capacitor C2, then bottom lamp.

(3) 3.2 Emit brighter light.

Longer life-span.

(2) 3.3 The tube is coated inside with a phosphorous powder. (1) (1)



3.4 To provide the voltage surge for starting.

For current limiting.

(2) [10]

QUESTION 4 4.1 4.1.1 f = 1/T = 50 Hz Thus e = 300 sin (2.π.50.t)

= 300 sin (314t) if angle is in radians OR e = 300 sin (2.π.50.57,3.t) = 300 sin (18 000t) if angle is in degrees.

4.1.2

R.M.S = 0,707.Vmax Thus Vmax = 220 ÷ 0,707 = 311V e = 311 sin (2.π.50.t) = 311sin (314t) if angle is in radians OR e = 311 sin (2.π.50.57,3.t) = 311 sin (18 000t) if angle is in degrees.

4.1.3 AVE = 0,637.Imax Thus Imax = 2 ÷ 0,637

= 3.14A i = 3.14 sin (2.π.50.t) = 3.14sin (314t) if angle is in radians OR i = 3.14 sin (2.π.50.57,3.t) = 3.14 sin (18 000t) if angle is in degrees.

(3 x 2) (6)

4.2

A conductor moving in a magnetic field will have an e.m.f induced in it depending on the speed and angle that it cuts the magnetic flux lines. (Accept the sketches as an answer OR Any FOUR facts.). Sketches MUST show the following: magnets rotation conductors angles

(4)

[10]



QUESTION 5 5.1 XL = 2.π.f.L

= 2.π. 50.0,01 = 3,14 Ω

(3)

5.2 XC = 1/ 2.π.f.C

= 1/ 2.π.50. 0,001 = 3,18Ω

(3)

5.3

(3)

5.4 The inductive reactance will increase if the frequency of the supply is

increased

(1) [10]


(3)

6.1.2 Neutral (1) 6.2

(3)

6.3 P = 3.VphIph OR 3S = LL IV

= 3 x 219,39 x 10 3= 10380 ××

= 6,58 Kva KVA58,6=

(3)

[10]

L1

L2

L3



QUESTION 7 7.1 7.1.1 The AC in the primary winding causes a changing magnetic flux

that is intensified by the iron core.

7.1.2 A changing magnetic flux in the iron core cuts the secondary coil

windings, inducing an EMF in the secondary coil.

7.1.3 There exists only a magnetic link between primary and secondary

winding. Electrically the coils are isolated from each other (Except auto-transformers) .

7.1.4 Efficiency of transformers is very high ( %97± ).

(4 x 1)

(4)

7.2 7.2.1 The turns ratio. = Vpph Vsph

= (VpL ) Vsph

= (275 ) 66

= 2,41: 1

7.2.2 Second

(2 x 3)

(6) [10]

QUESTION 8: D.C-MACHINES 8.1 When a conductor moves at speed through a magnetic field (not in line

with the flux lines), an EMF will be induced between the conductor ends

(4)

8.2

(3)

8.3 Vary the voltage across the armature.

Vary the current through the field coil. OR varying the flux per pole

(2) 8.4 Short circuit test of armature windings. (1) [10]

Sp

eed

RP

M

Current Drawn I

Series Motor: Change in speed in relation to current drawn as loading increases




9.1.2 9.1.3 9.1.4 9.1.5

A squirrel cage induction motor.

A reduced voltage on the stator windings during starting. To stop the motor if the supply voltage is too low.

When the current drawn from the supply exceeds the rating of the overload relay, it will be activated. This opens the no-volt coil circuit and the run switch opens, thereby stopping the motor. No, the no-volt coil is only supplied from two of the supply lines. OR Yes, provided that the motor now draws enough current from one of the supply lines to activate the overload relay.

(1) (1) (1) (2) (2)

9.2 9.2.1

9.2.2 9.2.3

True True True

(1) (1) (1) [10]

QUESTION 10 10.1 10.1.1

(4) 10.1.2 PT = P1 + P2 (2) 10.2 Silicon or Germanium

Tetravalent semiconductor element

(1) 10.3 Adding acceptor impurity trivalent atoms such as boron, gallium or indium (1)



10.4 Symbol correct Load or Output

(2)

[10]

TOTAL: 100

Documents

Email: [email protected] questions TRADE THEORY N3 · 3.1.2 State why a choke is included in this circuit. 3.1.3 State why a capacitor is included in this circuit. (3 x 1)