NATIONAL
SENIOR CERTIFICATE
GRADE 12
SEPTEMBER 2018
ELECTRICAL TECHNOLOGY
POWER SYSTEMS
MARKS: 200
TIME: 3 hours
This question paper consists of 10 pages, including a 2–page formula sheet.
INSTRUCTIONS AND INFORMATION
1. This question paper consists of SIX questions.
2. Sketches and diagrams must be large, neat and fully labelled.
3. Show ALL calculations and round off answers correctly to TWO decimal places. Show the units for ALL answers of calculations
4. Number the answers correctly according to the numbering system used in this question paper.
5. You may use a non-programmable calculator.
6. A formula sheet is provided at the end of this question paper.
7. Write neatly and legible.
QUESTION 1: OCCUPATIONAL HEALTH AND SAFETY
1.1 Summarise the purpose of the Occupational Health and Safety Act. (2) 1.2 Explain the ‘let-go’ current during an electrical shock. (2)
1.3 State ONE unsafe condition that must be avoided in a workshop. (1) 1.4 Give ONE unsafe act in a school workshop that can cause an accident. (1)
1.5 Describe TWO standard treatments for shock. (2)
[8]
QUESTION 2: RLC
2.1 A series RLC circuit consists of an inductor with an inductive reactance of 24 Ω, a resistor with a resistance of 12 Ω and a capacitor with a capacitive reactance of 18 Ω connected across a 30 V supply.
FIGURE 2.1
Calculate the:
2.1.1 Impedance of the circuit (3)
2.1.2 Current flowing through the circuit (3)
2.1.3 Voltage across each component in the circuit (6) 2.1.4 Prove by calculation that the sum of the three voltages equals the
supply voltage (3)
2.1.5 Circuit phase angle and state whether is it leading or lagging (3) 2.1.6 Draw a fully labelled voltage phasor diagram (5)
2.2 The phasor diagram in FIGURE 2.2 indicates the current values flowing through the components of a parallel circuit connected across a 240 V/50 Hz supply. Refer to the diagram below and answer the questions that follow.
FIGURE 2.2
Calculate the:
2.2.1 Total current flow through the circuit (3)
2.2.2 Inductive reactance (3)
2.2.3 Inductance of the coil (3)
2.3 Define the following terms with reference to RLC circuits:
2.3.1 Resonance (2)
2.3.2 Q-factor in a parallel circuit (2)
2.4 A circuit with a resistor of 4 Ω, an inductor with an inductive reactance of 157 Ω and a variable capacitor set to 120 µF are connected in series to a 100 V/50 Hz supply.
Given: R = 4 Ω XL = 157 Ω Cvar = 120 µF Vs = 100 V f = 50 Hz Calculate the:
2.4.1 Value of the capacitance that will result in resonance at 50 Hz (3)
QUESTION 3: THREE-PHASE AC GENERATION
3.1 Draw a labelled waveform diagram of a three-phase system. (4) 3.2 Describe apparent power with reference to AC generation. (3) 3.3 A three-phase star connected load is supplied by a 40 kVA generator at a
current of 25 A. The power factor of the load is 0,87 lagging.
Calculate:
3.3.1 The line voltage (3)
3.3.2 The phase voltage (3)
3.3.3 The reactive power (4)
3.4 Explain why the power factor of a load is corrected. (3) 3.5 Three loads, with impedance of 45 Ω each, are connected in star to a 415 V
three-phase supply line operating at a power factor of 0, 85 lagging.
Calculate:
3.5.1 The current through each load (impedance) (4)
3.5.2 The real power (3)
3.5.3 The apparent power (3)
3.6 Two wattmeters are connected to a balanced three-phase system. Wattmeter R gives a reading of 3 kW and wattmeter B gives a reading of 8, 5 kW.
Calculate the power factor. (3)
3.7 Define real power with reference to AC generation. (1) [34]
QUESTION 4: THREE-PHASE TRANSFORMERS
4.1 Explain the function of a transformer in a distribution network. (2) 4.2 List THREE requirements that must be satisfied for three single-phase
transformers to be connected as a three-phase transformer. (3) 4.3 Describe how an increase of the load will affect the primary current of a
transformer. (4)
4.4 Explain the purpose of the oil in a transformer. (4)
4.5 Give THREE types of losses that occur in transformers. (3) 4.6 A delta-star connected transformer supplies a factory with 85 kW of electrical
power. The current is lagging the voltage with a phase angle of 36,87o. The total losses in the transformer are 12,5 kW. The primary line voltage is
13,8 kV and the secondary line voltage is 450 V.
Calculate:
4.6.1 The secondary line current (3)
4.6.2 The apparent power (2)
4.6.3 The primary phase current (5)
[26]
QUESTION 5: THREE-PHASE MOTORS AND STARTERS
5.1 State THREE advantages of squirrel cage induction motors when compared
to other similar induction motors. (3)
5.2 Give TWO applications of induction motors. (2)
5.3 The stator of a three-phase induction motor driving a conveyor belt has 48 poles and the supply frequency is 50 Hz. Calculate the synchronous speed of
the motor. (4)
5.4 Draw TWO sketches of a terminal box of an induction motor and indicate a star connection on one and a delta connection on the other one. (4) 5.5 Explain how the direction of rotation of a three-phase motor can be reversed (2) 5.6 List ANY of the important information shown on the name plate of a motor. (1) 5.7 Describe why it is necessary to use a starter with induction motors. (4) 5.8 In an electrical test to determine the insulation resistance of the windings,
the megger was connected to the U2 and W1 terminals. The reading on the
megger was 35 Ω. Conclude whether the motor can be activated or not. (3) 5.9 As part of the electrical test, the megger was connected to terminals U1 and
E. If the motor is in a good condition, what sort of reading can be expected? (2) 5.10 The rotor speed of a squirrel cage induction motor is 3 384 rpm and the
synchronous speed is 3 600 rpm. Calculate the slip. (3) 5.11 The losses of a 25 kW squirrel cage induction motor amounts to 2,2 kW.
Calculate the efficiency. (3)
5.12 The following information appears on the nameplate of a squirrel cage motor:
Current = 20 A Voltage = 415 V Power factor = 0, 8 Efficiency = 90 %
Determine the maximum power the motor can deliver. (3) [34]
QUESTION 6: PROGRAMMABLE LOGIC CONTROLLERS
6.1 Name the industrial control system that the PLC was developed to replace. (1) 6.2 Explain TWO disadvantages of the industrial control system used before
PLC’s. (2)
6.3 Give THREE advantages of using a PLC over the older industrial control
system. (3)
6.4 Describe THREE of the safety steps to be considered when using
programmable controlled equipment. (3)
6.5 Describe the method used to convert an analogue signal into a ‘standardised’
digital form ready for use by a PLC. (4)
6.6 Explain what is meant by Binary Code Decimal (BCD). (3) 6.7 Convert the following decimal numbers into BCD code:
6.7.1 0110 0011 0100 0111 (2)
6.7.2 1001 0010 0100 1000 (2)
6.8 Explain why an opto-isolator is used in a PLC circuit. (3) 6.9 List FIVE types of sensors, and also explain the uses of each sensor. (10) 6.10 Draw a ladder logic circuit for a PLC as a motor starter using forward, reverse
and stop. (5)
6.11 Describe the basic principle of operation of a Variable Speed Drive (VSD)
motor controller. (2)
6.12 Draw the ladder-logic circuit for an exclusive OR gate (XOR gate). (5) 6.13 Use a fully labelled circuit diagram to describe how the output of a PLC can
be connected in order to drive a large motor. (7)
6.14 Give TWO methods currently used for motor speed control. (2) 6.15 Explain how an application uses regenerative braking. (2) [56]
TOTAL: 200
FORMULA SHEET
THREE-PHASE AC GENERATION RLC CIRCUITS 𝑋𝐿 = 2𝜋𝑓𝐿 and 𝑋𝐶 = 1
2𝜋𝑓𝐶
𝑓𝑅 = 1 2𝜋√𝐿𝐶 SERIES
𝐼𝑇 = 𝐼𝑅 = 𝐼𝐶 = 𝐼𝐿 𝑍 = √𝑅2+ (𝑋𝐿− 𝑋𝐶)2
𝑉𝑇 = √𝑉𝑅2+ (𝑉𝐿~𝑉𝐶)2 𝑉𝐿 = 𝐼𝑋𝐿 and 𝑉𝐶 = 𝐼𝑋𝐶 and 𝑉𝑇 = 𝐼𝑍
cos 𝜃 =𝑅 𝑍 cos 𝜃 =𝑉𝑅 𝑉𝑇 𝑄 =𝑋𝐿
𝑍 = 𝑋𝐶 𝑍 = 𝑉𝐿
𝑉𝑆 =𝑉𝐶 𝑉𝑆 = 1
𝑅√𝐿 𝐶 STAR
𝑉𝐿 = √3𝑉𝑃𝐻 and 𝑉𝑃𝐻 = 𝐼𝑃𝐻× 𝑍𝑃𝐻 𝐼𝐿= 𝐼𝑃𝐻
DELTA
𝑉𝐿 = 𝑉𝑃𝐻 and 𝑉𝑃𝐻 = 𝐼𝑃𝐻× 𝑍𝑃𝐻
𝐼𝐿 = √3𝐼𝑃𝐻 POWER
𝑆 (𝑃𝐴𝑃𝑃) = √3𝑉𝐿𝐼𝐿 𝑄 (𝑃𝑅) = √3𝑉𝐿𝐼𝐿sin 𝜃
cos 𝜃 =𝑃 𝑆 𝑃 = √3𝑉𝐿𝐼𝐿cos 𝜃 TWO-WATTMETER METHOD
𝑃 = 𝑃1+ 𝑃2
THREE-PHASE TRANSFORMERS STAR
𝑉𝐿 = √3𝑉𝑃𝐻 and 𝐼𝐿 = 𝐼𝑃𝐻 DELTA
𝑉𝐿 = 𝑉𝑃𝐻 and 𝐼𝐿 = √3𝐼𝑃𝐻 POWER
𝑆 (𝑃𝐴𝑃𝑃) = √3𝑉𝐿𝐼𝐿 𝑄 (𝑃𝑅) = √3𝑉𝐿𝐼𝐿sin 𝜃
PARALLEL
𝑉𝑇 = 𝑉𝑅 = 𝑉𝐿 = 𝑉𝐶 𝐼𝑅 =𝑉
𝑅 and 𝐼𝐿 = 𝑉
𝑋𝐿 and 𝐼𝐶 = 𝑉
𝑋𝐶
𝐼𝑇 = √𝐼𝑅2 + (𝐼𝐿− 𝐼𝐶)2
𝐶𝑂𝑆𝜃 =𝐼𝑅 𝐼𝑇
𝑄 =𝑋𝐿 𝑍 = 𝑋𝐶
𝑍 = 𝑉𝐿 𝑉𝑆 =𝑉𝐶
𝑉𝑆 = 1 𝑅√𝐿 cos 𝜃 =𝑃 𝐶
𝑆 𝑃 = √3𝑉𝐿𝐼𝐿cos 𝜃 𝑉𝑃𝐻𝑃
𝑉𝑃𝐻𝑆 = 𝑁𝑃
𝑁𝑆 = 𝐼𝑃𝐻𝑆 𝐼𝑃𝐻𝑃
THREE-PHASE MOTORS AND STARTERS
STAR
𝑉𝐿 = √3𝑉𝑃𝐻 and 𝐼𝐿 = 𝐼𝑃𝐻 DELTA
𝑉𝐿 = 𝑉𝑃𝐻 and 𝐼𝐿 = √3𝐼𝑃𝐻 POWER
𝑆 (𝑃𝐴𝑃𝑃) = √3𝑉𝐿𝐼𝐿 𝑄 (𝑃𝑅) = √3𝑉𝐿𝐼𝐿sin 𝜃
cos 𝜃 =𝑃 𝑆 𝑃 = √3𝑉𝐿𝐼𝐿cos 𝜃 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦(ƞ) =𝑖𝑛𝑝𝑢𝑡−𝑙𝑜𝑠𝑠𝑒𝑠
𝑖𝑛𝑝𝑢𝑡 × 100
MOTOR SPEED
𝑛𝑠 =60𝑓 𝑝 𝑆𝐿𝐼𝑃 =𝑛𝑠−𝑛𝑟
𝑛𝑠
