Electrical Technology Power Systems Memo 1 (English)

May-June 2021 — Grade 12 / Matric — Electrical Technology

Memo 2021 Grade 12 English

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🎯 200 marks 📄 4 pages 📝 91 items found ∑ 1 math symbols 📚 2 topics 🔑 1 key info

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2.5All calculations must show the formulae. Substitution of values must be done correctly. All answers MUST contain the correct unit to be considered. Alternative methods could be considered, provided that the correct

Q1— Section header —

1.1Safe means free from any hazard. (1\

1.2Discipline. Sense of teamwork. Emphasis on quality. Integrity. Sense of responsibility (2\

1.3The use of power tools. The handling of hand tools. The use of etching acid and other chemicals. (1\

1.4Poor ventilation reduces the correct amount of oxygen which might lead to drowsiness. Covid 19 protocols also refer to proper ventilation that must be adhered to

1.5To take reasonable care for the health and safety of himself/herself and others who may be affected by his/her act. Cooperate with the employer or persons to enable that any duty given by the employer to the employee shall be performed or complied with according to

1.6Human rights ensure human dignity and that people are treated with respect and not exploited. (2\ [10]10

Q2— Section header —

2.1.1Phase angle is the shift in phase between the supply voltage and the current in a circuit which contain reactances and resistances. (2\

2.1.2Capacitance is the ability of a capacitor to store an electric charge. (1\

2.2It will affect the phase angle. It will oppose the current flow in the circuit. back emf is set up which is opposite to the applied voltage. (2\

2.3.1(3\ Electrical Technology: Power Systems DBE/ 2021 SC/NSC Marking Guidelines Copyright reserved Please turn over

2.3.2(3\

2.3.3Inductive circuit. The inductive reactance is greater than the capacitive reactance. (2\

2.4.1(3\

2.4.2(3\

2.4.3(3\

2.4.4(3\

2.5.1Impedance. (1\

2.5.2Capacitive reactance is greater than the inductive reactance. (2\

2.5.3The inductive reactance is represented by a straight line because it is directly proportional to the frequency of the supply voltage and the capacitive reactance is represented by the curved line because

2.5.4(3\

2.5.5Tuning circuits. Radio circuits. Oscillator circuits. TV circuits. Filter cir cuits. Band pass filter (1\

2.6.1The higher the value of the Q factor the greater the value of the current flow. (1\

2.6.2Half power points are the upper and lower cut off frequencies in a circuit. Point at which the output power has dropped to half of its peak value, at a level of approximately 3db. (1\

2.6.3The value of the series resistor. The inductor/capacitor (L/C\atio. (2\

2.6.4As the Q factor of the circuit is lowered, its selectivity decreases and its bandwidth increases. (2\ [40]40

Q3— Section header —

3.1.2Efficiency of a three phase system is the ratio of the output power to the input power. Power factor correction is the process of introducing reactive elements to improve the power factor

3.2Installations costs are very high. Not available everywhere. Not suitable for most residential applications. Appliance operating with three phase supply are expensive. (3\

3.3.1The value of the phase voltage (V ph \is equal to the value of the line voltage (V (V ph = V

3.3.2NOTE mark for rotation 1 mark for the correct sequence 1 mark for the phase angle (3\

3.4The generated electricity is less at the point of distribution because of the copper losses occurring inside the transmission lines and transformers during power transmission. (2\

3.5.1The current through a capacitor leads the voltage by 90 degrees and it will as a result reduce the inductive lagging current. Capacitors draw leading currents which neutralise the lagging currents. (2\

3.5.2Reduced supply current. Thinner supply conductors are required Reduced cost of thinner conductors Reduced maintenance of supply equipment (2\

3.6.2(3\

3.6.3(5\

3.7The kilowatt hour meter is used to measure the energy consumed by a load. (2\ [30]30

Q4— Section header —

4.1Air orced. (AF\ Air Natural. AN (2\

4.2Losses Transformer loading (1\

4.3Assemble or modify the circuit ONLY when the supply is OFF. Do not switch the circuit ON before a supervisor (teacher\ has inspected and is satisfied. NEVER touch any bare electric wire or terminal. Exercise extreme care during experiments.

4.4.1Delta/star (1\

4.4.2Industry or commercial sites where both single and three phase power is needed. Domestic distribution networks where only single phase power is needed. (2\

4.4.3It is a step down transformer because the turns ratio is 5:1. (2\

4.5.1To supply the same loads, single phase transformers become more expensive than three phase transformers. (1\

4.5.2Three phase transformers are more efficient than single phase transformers. (1\ kVA 109

4.6When a fault occurs inside a transformer the oil heats up and gas forms inside the oil depending on the severity of the fault. The heated oil and gas flows from the main tank though the Buchholz relay into the conservator tank.

4.7.2(3 4. .3 (3\

4.7.4(3\ [30] 400 00030

98.9100 500 45000 45000 losses OUT OUT

Q5— Section header —

5.1.1End ring Bearing (2\

5.1.2Less maintenance due to no brushes or slip rings. They are less prone to explosions because any sparks are eliminated due to the absence of brushes and slip rings. (1\

5.1.3It helps in the reduction of magnetic hum. It helps to avoid cogging. (locking tendency of the rotor\ It increases the effective ratio of transformation between stator and rotor. It causes an increased rotor resistance due to

5.2Slip is the difference between the synchronous speed and the rotor speed of an induction motor exp ressed in a percentage

5.2.2Commissioning is when the electric motor is connected to the power supply and the load after all inspections (electrical and mechanical\ave been completed. The motor is certified and ready to be used.

5.3Inspect the: Mounting bolts to check if it is properly tightened. Endplates to check if it is securely fastened. Frame for any cracks. Shaft for any play on it. Shaft if it turns freely by hand. Cooling fan if it is intact.

5.4.2(3\ pairs pole (p\ phase per

5.4.4(3\

5.5.1Forward reverse control circuit. (1\

5.5.2(a\ (b\ OLN/C will open the moment the current surpasses the pre set rated current , stopping the current flow to MC

5.5.3MC C is used as an interlocking contact. When MC (forward\ is activated MC

5.6(3\ [30] rpm 1455 100 1500 1500 slip30

Q6— Section header —

6.1(3\

6.2A PLC system would use fewer relays, cam controllers, timers, and counters. Control panels would not have to be re wired when production models were changed. (Hardware\ A PLC system physically takes up less space than Hardwired systems.

6.3The PLC wiring and connections must be checked before switching on to ensure that: There is no i ncorrect wiring There are no loose connections (2\

6.4All processing is done at low voltages and therefore safer. The normal operation of a plant can be observed electronically on a monitor keeping the observer away from dangerous machines. All operations are

6.5.1The unit that performs all logic operations internally. It runs the ladder program. (2\

6.5.2Soft wired systems perform many functions digitally to reduce the amount of wiring in a circuit. (2\

6.5.3PLC software is a program that is written into the unit to control its functions. PLC software is the program used to compile the ladder logic program (1\

6.6An analogue signal is a continuously changing signal. A digital signal is a signal with a number of discret steps. (2\

6.7.1Markers are used to hold data in the PLC program. They can be switched on or off and control other programs or output devices. They can be used to indicate when a certain stage in a program is complete.

6.7.2The contactor on the program drives the coil on the relay contact. It can be programmed to be normally closed or normally open contacts. They can control single phase or three phase contactors. (1\

6.8(3\ Output Output OR Electrical Technology: Power Systems

6.9.1A sensor is a device that detects and converts an environmental condition into an electrical signal that can be used by another device for a particular purpose. (2\

6.9.2Temperature sensor Light sensor Overload sensor. Level sensor. (2\

6.9.3To detect the proximity of an object in relation to its distance Used to measure rotating speed (2\

6.10(10\ Start MC Start MC

6.11ON delay OFF delay ON/OFF delay

6.12When the start button is pressed, coil MC will be energised, Contact MC will close (Retention\

6.13Transistor Relay (2\

6.14.1Regenerative braking block diagram. VSD control system with a load (1\

6.14.2The braking resistor is connected to the circuit to dissipate excessive regenerated energy The resister heats up and emits heat. It protects the DC capacitors of a VSD from being over loaded

6.15The regenerated energy can be converted into another form of energy where it is stored and used when necessary. (3\

6.16.1AC to DC Converter Rectifier (1\

6.16.2Capacitors (1\

6.16.3The inverter, inverts the DC voltage back to AC voltage at a different frequency by switching the Insulated Gate Bipolar Transistor ON and OFF at a high frequency. The ON and OFF times (width of the pulses\are carefully

6.16.460

Mathematical Content

trig

Topics Identified

CircuitsMachines

Mark Allocation

Total200 marks

Sections10 + 40 + 30 + 30 + 30 + 60 = 200 marks

Key Information

Contact Number0 45000 4500

Paper information

SubjectElectrical Technology

Year2021

SessionMay-June

TypeMemo

PaperPaper 1

LanguageEnglish

Pages14

Downloads4

Examination context

Decade2020s

Years ago5y

Season—

Start day—

Exam period—

Days since exam—

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