Electrical Technology Memo 1 (English)

Feb-March 2015 — Grade 12 / Matric — Electrical Technology

Memo 2015 Grade 12 English

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🎯 200 marks 📄 14 pages 📝 85 items found ∑ 1 math symbols 📚 2 topics

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2.1All calculations must show the formula(e\.

2.2Substitution of values must be done correctly

2.3All answers MUST contain the correct unit to be considered.

2.4Alternative methods must be considered, provided that the same answer is obtained.

2.5Where an erroneous answer could be carried over to the next step, the first answer will be deemed incorrect. However, should the incorrect answer be carried over correctly, the marker has to re calculate the values, using the incorrect answer from the first

Q1— Section header —

1.1Working on a live system with exposed conductors. Working with portable electric equipment that is not insulated correctly. (2\

1.2Do not touch the person with your bare hands. Switch off the supply immediately se the emergency stop button if the workshop has one. se an insulated item to free the person. Apply the correct medical procedure.

1.3South Africans should provide the same work and economic opportunities for capable people without any discrimination due to gender No person should be exposed to sexual harassment irrespective of their

1.4Do all machines have the required guards Are walkways clearly indicated. Does the workshop have sufficient lighting Are signage clearly and correctly put up. Answers must relate to preventing accidents to be considered.

Q2— Section header —

2.1For alternators of similar frame size, three phase machines produce more power than single phase machines. The same size of machinery is necessary to drive three phase and single phase alternators Three

2.2

2.3(3\

2.4(3\

2.4.2(3\

2.4.3(3\

2.4.4cos cos (3\ [20]20

Q3— Section header —

3.1Core type and Shell type (2\

3.2It improves the insulation of the transformer and conducts the heat away from the windings and core of the cooling surface (2\

3.3Insufficient ventilation oose connection verloading Insulation Breakdown Short circuits (2\

3.4Copper losses Iron losses (2\

3.5Bucholtz relay Over voltage relay Under voltage relay Over current relay Pressure control device (2\

3.6(2\

3.6219,40V (3\

3.6.3219,40 PH(S\ PH(P\ (3\ he voltage ratio shows that the transformer is a step down transformer

Q4— Section header —

4.1Stator Rotor End plates Fan Frame

4.2They require less maintenance as they do have as many parts as single phase motor For the same size frame as a single phase motor they deliver a higher torque.

4.3Power factor Voltage rating Frequency Speed Type of connection Phase displacement (3\

4.4Vs = 400 V F = 60 Hz No Poles = 12 poles per phase pole pairs Slip (Per Unit\

4.4.1rpm (3\

4.4.2PerUnit (3\

4.5It is important to carry out a mechanical inspection to determine before starting if there are any mechanical faults that my cause damage to the motor or operator on starting. (2\

4.6Continuity of each winding Insulation resistance between each winding Insulation resistance between windings and earth Exposed windings (2\

4.7The overload relay unit offers protection to the motor and operator under fault conditions. (3\

4.8.1The function of a star delta starter is to reduce the starting current of a motor at start (1\

4.8.2Delta (1\

4.8The retain ( hold in is the contact that will keep power on the main contactor

4.8When the timer coil is energised the timer will begin to time through. Contact T (N/C\ will only when the timer has timed through a preset time. (2\

4.8Contacts MC (N/C\ & MC (N/C\ in series with the star and delta contactor

4.9.1cos (3\

4.9.2The active power of the motor will increase

4.9.3(1\

4.9If the power factor of the motor improved the moto will draw lower current as it will still deliver the same output power. (2\

Q5— Section header —

5.1Connect an ammet er in the circuit and adjust th e frequency of the power supply until the reading on the ammeter is at a maximum. Connect a volt meter across the coil and capacitor and then

5.2Install a p ower factor correcting capacitor ake use of s ynchronous motors (1\

5.3When the frequency is decreased, XL will decrease and Xc will increase. IXL will then increase and IXC will decrease. IZ will increase as the phase angle changes. Alternative nswer: At resonance XL = XC and the resultant current in the circuit IZ=IR and this

5.4.1At resonance Z=R (3\

4.3Calculation of X (4\

5.4.41000 (3\ [20]20

Q6— Section header —

6.1.1The interface is the connection (platform\ between real world devices and the CPU (controller\ his allows for level conversion of the input information rendering it suitable to fed into the PLC

6.1.2Relay contacts ushbuttons Limit switches nalogue senso Selector switches TWO (2\

6.1.3L.E.Ds Power Transistors Thyristors Any TWO (2\ 10

6.1.4STEP 1 check input status the controller examines the status of the input devices and checks whether it is at a HIGH or LOW OR STEP 2

6.2.111

6.2.2Grouping (6\

6.2.3(7\

6.2.4The Set/Reset function can be used in motor control he reset function takes priority over the set function This could be used to simulate the start and stop function of irect ine tarter (or any other type of starter\

6.2.5An external Stop Button provides the operator with a physical means of disconnecting supply to the No Volt Coil. On a PLC system the Start and Stop functions can be programmed to th e memory as Set Reset functions.

6.3Contacts activate after a pre set time. (2\ [40] 1240

Q7— Section header —

7.1A 741 amp is an integrated circuit which is a circuit that consists of many electronic components situated on a silicon wafer chip. The chip is placed in dual in line package which allows for connection in

7.2Infinite bandwidth means that the amp can amplify an input signal over a range of infinite frequencies.

7.3Open loop voltage gain is infinite Input impedance is nfinite Output impedance is zero Unconditional stability (2\

7.4.1

7.4.2Positive feedback is when a portion of the output signal is f back to the input signal The feedback is in phase with the input signal

7.5The bandwidth is increased. The level of noise (hiss\ is decreased The gain is decreased The deformation of the input signal is reduced ( Any TWO (2\

7.6Output must show Amplification Same frequency Correct phase The input and output frequency must relate/ be the same If open loop gain =

7.7.1Output must show No inversion Square wave At saturation points (3\

7.7.2Zero level detector Threshold detector (1\

7.8.1Non inverting op amp (1\

7.8.2Non inverted o utput must show Amplification Same frequency (2\

7.8.3(3\

7.8.4Vout Vout (3\

7.8.5The value of R determines how much feedback is obtained. If the value of Rf was decreased the amount of negative feedback obtained is increased. The result is that the voltage gain of the circuit will be decreased.

7.8.6The inverting terminal is connected via R to the 0 V rail therefore the value of R sets the voltage at that input. (2\

7.9As a buffer between stages to provide impedance matching from one stage to the other of an amplifier circuit. (2\ 14

7.10.1Output must show Triangular waveform (3\

7.10.2and C form a time constant which controls the rate at which the output voltage increases which determines the time the voltage reaches saturation. (3\

7.11.1Hartley oscillator (1\

7.11.250

Mathematical Content

trig

Topics Identified

CircuitsMachines

Mark Allocation

Total200 marks

Sections10 + 20 + 20 + 40 + 20 + 40 + 50 = 200 marks

Paper information

SubjectElectrical Technology

Year2015

SessionFeb-March

TypeMemo

PaperPaper 1

LanguageEnglish

Pages14

Downloads4

Examination context

Decade2010s

Years ago11y

Season—

Start day—

Exam period—

Days since exam—

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