higher education & training Department: Higher Education and Training REPUBLIC OF SOUTH AFRICA T490(E\A13)T APRIL EXAMINATION NATIONAL CERTIFICATE ELECTROTECHNICS N4 (8080074) 13 April 2015 (Y-Paper) 13:00-16:00 REQUIREMENTS: Graph paper Calculators may be used. paper consists of § pages and 1 formula sheet of 2 pages. itresored Please turnover(eoe0074) TASO(EA)T DEPARTMENT OF HIGHER EDUCATION AND TRAINING REPUBLIC OF SOUTH AFRICA NATIONAL CERTIFICATE ELECTROTECHNICS N4 TIME: 3 HOURS MARKS: 100 INSTRUCTIONS AND INFORMATION 1, Answer ALL the questions. 2 ‘Read ALL the questions carefully. 3. Number the answers according to the numbering system used in this question Paper. 4. White neatly and legioly Copyright resorved Ploase tum over(eoe0074) 3 THSO(EVAIST ‘QUESTION 1 1.1 The field coll of a motor has a resistance of 300 0 at 75, 12 Calculate the final resistance if the temperature is 200 °C, Take the temperature coefficient of resistance as 0,004 °C at 75 °C. Distinguish between a positive and a negative temperature coefficient of resistance. ‘Acoll having 1 800 tums of conductor with a combined cross-sertional area ‘of 300 mm? and a mean length per turn of 400 mm, has an inductance of 8 H, Calculate the folowing 4.3.1 The resistance of the winding ifthe resistivty (specific resistance) cof the conductor is 4 uc.m 1.3.2 The average value of the EMF induced In the coll when a current of 60 A reversed in 18 seconds Two capacitors connected in series have voltage readings of 40 V and 10 V respectively. The total charge equals 400 UC, Calculate the following 1.41 The total capacitance 1.4.2 The value of each capacitor ‘QUESTION 2 2a 22 23 Copyright reserved Explain Kirchhot's fst law. ‘Two batteries of EMF 65 V and 50 V and intemal resistance of 0,9 9 and 0,6 © respectively, are connected in paralle! to supply a load resistance of 280. Use Kirchhotf's Iams and calculate the following: 22.1 Theccurrent supplied by each battery 222 The voltage across the load Define a farad. 4 @ 6 2 @ a) po @ ” @ e Please tum over(eo80074) ‘Teso(eyatsyt 24 A rsistor of unknown value Fis connected in parallel with a reeistance of 180 Q, This combination is connected in series with a resistance of 25 ‘The circu is then connected across a 280-V DC-suppl. Calculate the following: 2.41 The value of the resistor R when a 4-A current is draun from the supply 6 24.2 The power dissipated in the circuit a po QUESTIONS 3.1 The open-circuit characteristics of a shunt-excited DC motor are as follow: “Terminal wotage (V) [200 [400 [500 [560 _[610_[ 6a0 Field current (A 10 [20 [30 [50 [es [75 Using the above values, plot a graph and determine the following 3.1.1 The voltage which the motor will excite on nodoad when shunt ‘connected ifthe total eld resistance is 10 0. ® 34.2 The ical resistance Q 82 A long-shunt compound-vound DC motor has an armature resistance of 0,3 0, a serieseld resistance of 0,1 0 and a shunt-feld resistance of 48 Q. ‘The motor draws a currant of 210 A from a 480-V DC supply Calculate the EMF generated in the armature. 6) 33 Whatis the purpose of a pole shoe in a DC motor? @ 3.4 Name any THREE types of capacitors. @) bol Copyright resorved Pease turn over(o000074) s TaQ0(EVArIT QUESTION 4 4.1 What can be done to improve the power factor in a circuit? @ 4.2 50 Hz sinusoidal voltage has an RMS value of 424.2 V. Calculate the following 4.2.1 The time for the voltage to reach a value of 800 V from zer0 for the fist ime ” 422 Drawa phasor diagram and show the waveform of his votage, 2 43 Acoil with a resistance of 200 0 and an inductance of 0,3189 His connected In series with a 10,6t-uf capacitor. This circuit 's connected, across a 565/-685 V, 60 Hz supply Calculate the voltage drop actoss the folowing: 431 Thocoll ” 432 The capacitor 0 483 Draw the phasor diagram to represent the distribution of the voltage and the current in the cicut. a) 20) Conia reser Please wen over(2080078) + 4S04eVAt9T ‘QUESTION 5 5.1 The value of a resistor is measured by the voltmeter-ammeter method. 52 53 54 585 Copyi The internal resistance of the voltmeter is 800 O. When the voltmeter is connected directly across the resistor to be measured, the ammeter reads 4125.8 and the voltmeter 200 V. Calculate the value ofthe unknown resistor as follows: 5.1.1 Approximately 512 Accurately 5.1.3 The percentage error in the value ofthe resistor The no-load current of a 3 000/150-V single-phase transformer is 25 A at a power factor of 0,3. The primary winding has 150 tums and the supply Frequency is 60 Kz. Calculate the folowing: 52.1 The maximum value of the fixin the core 522 The powerloss on no-load 5.2.3 The value of the magnatising current Name THREE types of power stations used to generate electricity Why isthe rotor bars ofan induction motor skewed? Name the THREE main parts of an induction motor. reseed 2x3) TOTAL: a) @ @ 6) @ @ °) 100(2080074) 4904197 ELECTROTECHNICS N4 FORMULA SHEET Any applicable formula may also be used, 1. Principles of electcty at 2. ++ G=aee hae; Atta, ira 2. Ditacteuriont motors R=Rileey(t-0) ere onze peau 8, Atemating-curent motos ety Ly =2a0ANn E=Bw e= Eysin (2af. 157.3" poll rn 1et8 oy a Copii reonved Pease turn over(2080074) Np 0 L039 Vatsing 4, Transformers Fass ON ™ Satna 5. Measuring instruments R, Ron 1 Re copycigtrsorved ‘TesO(ENAtOIThigher education & training Department: Higher Education and Training REPUBLIC OF SOUTH AFRICA MARKING GUIDELINE NATIONAL CERTIFICATE APRIL EXAMINATION ELECTROTECHNICS N4 13 APRIL 2015 ‘This marking guideline consists of 7 pages.MARKING GUIDELINE a THSOENAIT ELECTRGTECHNICS NA ‘auesrion 1 14 Ry=Raalt + ag tO) 300[1 +0,004(200-75°)] =300[1+0,004(425")] “) 1.2 «Postive temperature coeficent of resistance refers to matofals whose resistance rises when the temperature increases. «Negative temperature coefficient of resistance rofers to materials! whose resistanco fli when the temperature increases. ® 18 184 L=1800x0.4=600my mew ar 4x10°Sx 600 © 132 @ 1444s @ 142 a) (20) Copyright reserved Please turnoverMARKING GUIDELINE TesO(EYArayT * ELECTROTECHNICS NA QUESTION 2 2.1 The sumo the currents flowing towards a junction is equal tothe sum of the ‘currents flowing away from that junction YW @ 22 A 8 8 c +, i av a F E > q 221 Consider loop/Vanat lus:ABEFA Joopilus ACDFA 3E-pIR= 3E-SIR=0 (EE) “GR HERa). (E-GuR,+ Roly “Ta)=¢ (65-50)-(0,31,+0,61,)=0 V 65-(0,31,+2,81,-2,81, 15-0,31,-0,61,=Onn(1) 65-3,11,+2/81,, EqNorg (14667 = 70-14% -2.8%=0 sn) Eqiverg(2) + (9) 195= 451,501 Thus/Dus: i= 195/45 = 208 SubstuteVervang f= 30 inion Ea.Verg. (1 15-0,3(30) =0,61, B= 15-9/0,6=-6/0,6 = 10.8 I-1h=30-10=20 ‘Current across 65-V Battery = 30 AY Currant across 50-V Battery = 10 AY a 2.22 Voltage across the load = IR = 20 x 2 SEW @ 2.3 One farad is that capacitance which wil accumulate a charge cf 1 coulomb when connected across a voltage of 1 volt WY @ opi reserved Please tun overNARIENG GUIDELINE ‘TAa0(EKAIOIT ELECTROTECHNICS NA 24 241 Vaq = IRgg= 4x25 = 100 6 242 (a) (20) Questions 34 Wintyn= = €0x 10800, CURENT APERES STROM AGP (19) 32 coe V—TyRy+TRee TE itty y= 480-[(200 x 0,3)+(200%0,4)1 -480-(60+20) =400V 6 Copysight reseed Please tum overMABKING GUIDELINE 5. ELECTROTECHNICS NA 33 To hold the fold windings in place and to increase the cross-sectional area WW a4 Ain’ Paper’ Mica Coramic Polycarbonate Electrolytic (Any 3x1) QUESTION 4 4.1 Making the circuit more capacitive To run a synchronous motor with litle or no load with rotor over-excited by a high direct currentY By use of suitable ‘corrective apparatus (machines) Y (Any 2x1) 4242.1 Vemsywole =0,707Ven 424,2=0.707Vm 4242” 422 Copyright reserved Plaase tum over T490(EA19)T 2 @) (20) @ ® QMARKING GUIDELINE 6. THSKEVAISTT ELECTROTECHNICS Ne 43° 43.4 My =2efl =2950 x 0,3183-1000. 2 t xe-ste 008 ©" Fa 2550xi, osixio > Zeon = \R2+%12 = 12002 +1002 —— 2=JR2 +212, = f200? + (200-200) Viz = 565,695/282,84 23,607 21 ® 432 o 433 aoov © s6s.69v @ (20) ‘QUESTION 5 BI B44 0 512 @ 513 2096 @ Copyright eserves Please tun overMARKING GUIDELINE ‘soge\AtayT ELECTROTECHNICS NA 521 Ey = 4.440 INy ©, =E,/4,440N, mivbW 3 000/4,44 x 60 x 150 =0,075075 = 75,075 522 — Powerloss=V,l,Cost = 3000x25x0,3=22,5Kw W 523 Coreloss _ 22500 SAY) ¥, 3000 1y=YPo-Po = 28" x2 @) 5. © Coabtired power station + Hydro-electric power station + Nuclear power station ° 4 To reduce magnetic noise and to eliminate variation in stating torcue at diferent positions of the rotor @ 55» Rotor * Stator + End plates @ (20) TOTAL: 100 Copyright reserved