QUESTION BANK ENGINEERS ACADEMY. EE : Electrical Machines Transformers 59

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1 EE : Electrcal Macnes Transormers 59 QUESTION BANK. Two transormers o te same type, usng te same grade o ron and conductor materals, are desgned to work at te same lux and current denstes; but te lnear dmensons o one are two tmes tose o te oter n all respects. Te rato o KA o te two transormers closely equals : Wen a transormer wndng suers a sort-crcut, te adjonng turns o te same wndng experence: an attractve orce a repulsve orce no orce none o te above 3. Supply to one termnal o a -Y connected tree-pase core type transormer wc s on no-load, als. Assumng magnetc crcut symmetry, voltages on te secondary sde wll be : 30, 30, 5 30, 5, 5 345, 5, 5 345, 0, Auto-transormer s used n transmsson and dstrbuton : wen operator s not avalable wen ron losses are to be reduced wen ecency consderatons can be gnored wen te transormaton rato s small 5. Keepng n vew te requrement o parallel operaton, wc o te 3-pase connectons gven below are possble? delta-delta to delta-star delta-delta to star-delta star-star to delta-star delta-star to star-delta 6. Te laws o electromagnetc nducton (Faraday s and Lenz s law) are summarzed n te ollowng equaton d e = R e L dt d e none o tese dt 7. Te ecency o a 00 KA transormer s 0.98 at ull as well as at al load. For ts transormer at ull load te copper loss. s less tan core loss. s equal to core loss. s more tan core loss none o te above 8. Te magnetsng current n a transormer s rc n : 3 rd armonc 5 t armonc 7 t armonc 3 t armonc 9. In a constant voltage transormer (CT), te output voltage remans constant due to : capactor nput nductor saturaton tapped wndngs 0. I an ac voltage wave s corrupted wt an arbtrary number o armoncs, ten te overall voltage waveorm ders rom ts undamental requency component n terms o : only te peak values only te rms values only te average values all te tree measures (peak, rms and average values). A 0/440, 50 KA sngle pase transormer operates on 0, 40 Hz supply wt secondary wndng. Ten : te eddy current loss and ysteress loss o te transormer decrease te eddy current loss and ysteress loss o te transormer ncrease te ysteress loss o te transormer ncreases wle eddy current loss remans te same te ysteress loss remans te same wereas eddy current loss decreases # 00-0, Ram Nagar, Bambala ulya.: , Emal : engneersacademy.org

2 60 Transormers Junor Engneer. Te low voltage wndng o a 400/30, -pase, 50 Hz transormer s to be connected to a 5 Hz, te supply voltage sould be : A 50 Hz transormer avng equal ysteress and eddy current losses at rated exctaton s operated at 45 Hz at 90% o ts rated voltage. Compared to rated operatng pont te core losses under ts condton : reduce by 0% reduce by 9% reduce by 4.5% reman uncanged 4. A 0 KA, 400/00 sngle pase transormer wt 0% mpedance draws a steady sort crcut lne current o : 50A 50A 50A 350A 5. A 400/00, 0 KA two wndng transormer s reconnected as an auto transormer across a sutable voltage source. Te maxmum ratng o suc an arrangement could be : 50 KA 5 KA.5 KA 8.75 KA 6. A 0 KA, 400/00, sngle pase transormer wt a percentage r esstance o 3% and percentage reactance o 6% s supplyng a current o 50A to a resstve load. Te value o te load voltage s : A 0-turn ron-cored nductor s connected to a 00, 50Hz source. Te maxmum lux densty n te core s Wb/m. Te cross-sectonal area o te core s : 0.5m 0.345m 0.05m 0.56m 8. A 0/0 volts, 50Hz transormer as a core lux densty o Wb/m, nd ts lux densty n Wb/m wen connected to 0 volts, 5Hz supply : A sngle-pase transormer ed rom a rated voltage supply operates at no-load. Te core loss o te transormer s assumed to be neglgble. Te current drawn by te transormer rom te supply wll ave te waveorm. 0 t t t t 0. Te equvalent resstance and equvalent reactance o a transormer are respectvely 0.5 and 0.8. Te voltage regulaton wll be zero wen te load power actor s 0.8 laggng laggng zero # 00-0, Ram Nagar, Bambala ulya.: , Emal : engneersacademy.org

3 EE : Electrcal Macnes Transormers leadng leadng I te secondary wndng o te deal transormer o te crcut sown n te gven gure as 40 turns, ten or maxmum power transer to te -om resstor, te number o turns requred n te prmary wndng wll be : 8 Avodng overloads Setonalzng and nterleavng te prmary and secondary wndngs 6. In an deal transormer, K s some constant, ten te supply voltage, n terms o ts magnetzng current I m can be expressed as jki m jk. I m g Ideal Transormer # 00-0, Ram Nagar, Bambala ulya.: , Turns. A transormer as at ull-load ron loss 900W and copper loss o,600w. At wat % o load, transormer wll ave maxmum ecency 33% 5% 75% 66.6% 3. A transormer s so desgned tat prmary and secondary ave... Hg leakage reactance Large resstance Tgt magnetc couplng Good electrc couplng 4. Wc o te ollowng 3-pase connectons o transormer create dsturbances n communcaton systems? Star/Delta Star/Star Delta/Star Delta/Delta 5. Leakage luxes o a transormer may be mnmzed by Keepng te magnetzng current o te mnmum Reducng te reluctance o te ron core to te mnmum - jki m - jk. I. 7. CRGO lamnatons n a transormer are used to mnmse : Eddy - Current loss Hysteress loss Bot eddy - current and ysteress loss Omc loss 8. Te useul lux o a transormer s Wb. Wen t s loaded at 0.8 p lag, ten ts mutual lux May decrease to 0.8 Wb May ncrease to.0 Wb Remans constant May decrease to 0.99 Wb 9. I, n a transormer c = core loss and sc = ull load omc loss, ten maxmum ka delvered to te load at maxmum ecency s equal to rated ka multpled by c sc c sc 30. Te ysteress and eddy - current losses o -pase transormer workng on 00, 50 Hz supply are and e respectvely. Te percentage decrease n tese losses wen operated on a 60, 40 Hz Emal : engneersacademy.org c sc m sc c

4 6 Transormers Junor Engneer supply would respectvely be Increase by a actor o. 3, 36 0, 36 5, 50 40, In a transormer prmary leakage mpedance s neglected, ten. Magnetzng current lags te appled voltage by Core loss current lags by Exctng current lags by Core loss current s n pase wt 5. Exctng current lags by about Magnetzng current statements are rom tese te correct statement are, 4, 5 3, 4, 6, 4,, 6 3. A 50 Hz transormer avng equal ysteress and eddy current losses at rated exctaton s operated at 45 Hz at 90% o rated voltage compared to rated operatng pont, te core loss under ts condton reduces by 90 % reduces by 9 % reduces by 4.5 remans uncanged 33. Wen compared wt power transormer, a dstrbuton transormer as Low % age mpedance and g I R loss to core loss rato Hg % age mpedance and g I R loss to core loss rato Hg % age mpedance and low I R loss to core loss rato Low % age mpedance and low I R loss to core loss rato 34. A transormer desgned or operaton o 60 Hz supply s worked on 50 Hz supply system wtout cangng ts voltage and current ratngs. Wen compared wt ull load ecency at 60 Hz, te transormer ecency on ull load at 50 Hz wll be reman unaltered decrease margnally 35. A bank o tree dentcal sngle pase 50 KA k / 30 transormer s used to provde 400 Low tenson supply rom a k, 3 pase substaton te eectve KA ratng o te bank wll be wen a sort crcut test s conducted on a sngle pase transormer, 30% o te rated voltage s requred to allow ull load current. Te sort crcut power actor s ound to be 0.. Te percentage regulaton o UF s A auto transormer as, I as nput quanttes and, I as output quanttes wt < te A conducted rom nput to output s I I I I I 38. A auto transormer as, I as nput quanttes and, I as output quanttes wt < te A transormed rom prmary to secondary s I I I I I 39. A 400 / 00 transormer as a ull load voltage regulaton o x p.u. at 0.8 p laggng ts transormer s used as on auto transormer wt voltage ratng 400 / 600 or 00 / 600, ten ts voltage regulaton would be Increase margnally # 00-0, Ram Nagar, Bambala ulya.: , Emal : engneersacademy.org

5 EE : Electrcal Macnes Transormers 63 x x, 3 3 x x, 3 3 x x, 3 3 x x, Wc o te ollowng statement are ncorrect?. Maxmum voltage regulaton o transormer occurs at leadng power actor. oltage regulaton o a transormer s maxmum wen load power actor (laggng) angle as te same value as te angle o equvalent mpedance 3. oltage regulaton o a transormer can be negatve at leadng power actor. 4. oltage regulaton o a transormer at zero power actor s always zero. and 3 and 3 and 4 and 4 4. In an auto transormer o voltage rato / and >, te racton o power transerred nductvely s 4. A sngle pase transormers wt KA ratng k as voltage ratng o ts transormer can be connected as an auto transormer to get two possble voltage ratngs o and te respectve KA ratngs as an auto transormer are K, K K, K K, K K, K 43. Two transormer o dentcal voltage but o derent capactes are operatng n parallel or satsactory load sarng. Impedance must be equal per unt mpedance must be equal per unt mpedance and /R rato must be equal Impedance and /R rato must be equal 44. Two transormer to be operatng n parallel ave ter secondary no load ems Ea or transormer A and Eb or transormer B. As Ea s somewat more ten Eb, a crculatng current IC s establsed at no load wc tends to boost bot Ea and Eb wt Ic = E a ea E E boost Ea and buck Eb wt Ic = a E b E Buck Ea and boost Eb wt Ic = a E b Buck bot Ea and Eb wt Ic = ea E ea b ea b eb eb E 45. Sort crcut test an a sngle pase transormer gave te ollowng date 30 at 50 Hz, 0 A = 0. laggng I S.C. test s perormed on 30, 5 Hz, ten sort crcut current decrease at < 0. ncreases at a.. < 0. ncreases at a p > 0. decreases at a p > wen a transormer wndng suers a sort crcut, eb a eb # 00-0, Ram Nagar, Bambala ulya.: , Emal : engneersacademy.org

6 64 Transormers Junor Engneer te adjonng turns o te same wndng experence. at attractve orce a repulsve orce no orce May be attractve or repulsve dependng upon te current drecton. 47. Transormer maxmum ecency, or a constant load current occurs at at any p zero p laggng zero p leadng unty p 48. In a transormer re jxe equvalent leakage mpedance e maxmum voltage regulaton s equal to r e to x e to e to e to r e e x e e r e r e e e laggng p laggng p leadng p laggng p. 49. Te relatve current drectons troug te prmary () and secondary (s) o a l-pase transormer connected to a resstve load on te secondary sde, are ndcated n te varous cross secton vews gven n gure wc o tese are correct representatons? () () (3) (4) Bot & Only Bot & 4 None o tese 50. In a transormer tappng are provded on.v. sde because. t as larger number o turns wc allow smooter control o output voltage. It as to andle low value o currents 3. It s easly accessble or repars. 4. t requres less nsulaton rom tese te correct answer s,, 3, 4, 3, 4,, 4,, 3 5. Two transormers o derent KA ratng workng n parallel sare te load n proporton to ter ratngs wen ter. per unt leakage mpedance on te same KA base are equal.. per unt leakage mpedances based on ter respectve KA ratngs are equal 3. Omc values o te leakage mpedances are nversely proportonal to ter ratngs 4. Omc values o te magnetzng reactance are te same rom tese te correct answer s, 3, 4, 3, 3, 4, 4 5. Two wndngs o a transormer are ndcated by termnals AB and CD as sown n g. wen a voltage o 00 s appled across AB wt BD sort crcuted, a voltage o 00 appears across # 00-0, Ram Nagar, Bambala ulya.: , Emal : engneersacademy.org

7 EE : Electrcal Macnes Transormers 65 AC. Te turn rato rom CD and AB s A B 3 3 or or 53. A pase transormer as p.u leakage mpendance on j0.04 ts regulaton at p 0.8 laggng and 0.8 leadng are respectvely 4 %, 0.8 % 4 %, %.4 %, -0.8 % 4 %, - % 54. A 00/ 00, 50 Hz transormer s to be excted at 40 Hz rom 00 sde or te exctng current to be te same, te appled voltage sould be I te load p.. o a transormer s 0.8 laggng ten ts prmary p.. s 0.9 p 0.7 p 0.8 p 4 p 56. A transormer as ysteress loss o 30 watts at 40, 60 Hz supply., te ysteress loss at 00, 50 Hz supply s C D 30 W 60 W 5 W 00 W 57. A transormer as ron loss o 90 watts at 60 Hz supply and 5 watts at 40 Hz supply. Bot losses beng measured at same peak lux densty. Te total ron loss n te transormer at 50 Hz supply wll be 70 W 50 W 90 W 60 W 58. A 0, 60 Hz sngle pase transormer as ysteress loss o 340 watts and eddy current o 0 watts. I te transormer s operated rom 00, 50 Hz supply mans, te total Iron loss n te transormer wll be 55 w 35 w 375 w 45 w 59. A 30, 50Hz sngle pase transormer as eddy current loss o 30 watts. te transormer s excted wt Dc source o same magntude wll be 30 w 0 w 5 w None o tese 60. A 50 Hz sngle pase transormer draws a sort crcut current o 35A at 0.5 p lag, wen connected to 6, 50 Hz source. Wat wll be te p t s energzed rom 6,75 Hz source? 0.7 lag 0.5 lag 0.37 lag 0.9 lag # 00-0, Ram Nagar, Bambala ulya.: , Emal : engneersacademy.org

8 66 Transormers Junor Engneer ANSWER SHEET. Ans. 3. Ans. KA E I, Gven te two Transormers are o same current densty. I = J A, were J s current densty wc s gven as constant : I A (cross sectonal area o conductor) As te lnear dmensons are doubled (radus o conductor becomes double) Cross sectonal area o conductor becomes 4 tmes and ence te current carryng capablty. I I = A 4A 4 A A E = 4.44 Bm An Tp. Ans. E A n # 00-0, Ram Nagar, Bambala ulya.: , [A n = net area o core materal] E KA A 4A 4 E = A A KA = EI [ A = L b L b = E I 6E I E I 4L b 4A ] 6 Accordng to Lorentz s prncple, two conductors carryng current n te same drecton wll experence orce o attracton and te conductors carryng current n opposte drecton wll experence orce o repulson. I te prmary appled voltage s 30 ten or a core type 3-pase Y transormer wt one prmary supply termnal dsconnected, secondary sde pase oltages are o te orm 30, 5, Ans. Advantages o auto transormer wll be more wt % tappng nearer to 00% (or) transormaton rato nearer to unty.e., smaller. In autotransormer transormaton rato s always [(L.)/(H.)]. 5. Ans. Y & Y transormers bot belong to ± 30º pasor group ence tey can be used or parallel operaton. 6. Ans. Faraday s Law: nduced em a rate o cange o lux lnkages e dn dt Lenz s Law: nduced em opposes te basc cause wc produces t e = e = d N dt d dt Emal : engneersacademy.org

9 EE : Electrcal Macnes Transormers Ans.. Ans. Ecency = Let.. = unty, # 00-0, Ram Nagar, Bambala ulya.: , kaout. ka.. W W 00kA ull load = 00kA WE W al load = 50kA 50kA W W 4 out cu cu cu...()...() Solvng () & () W cu = W 8. Ans. To get snusodal lux te magnetzng current must be a peak wave wc s domnant wt 3 rd armonc. 9. Ans. Due to saturaton o termnal voltage o constant voltage transormer mantanted. 0. Ans. I te waveorm conssts armoncs ten ts peak voltage s not same as tat o undamental requency component ence te average and rms values der.. Ans. B max W.6 B W max constant As requency s reduced Hysteress Loss ncreases W e W e.6 eddy current Loss s ndependent o requency.[.e., no cange] B max, as s reduced te B max demand ncreases. Hence te magnetsng component current demand ncreases, wc may damage te Transormer due to ncreased resultant current. Hence must be mantaned constant. = = Ans. Gven ntally te ysteress loss w and eddy current loss w are equal, so So, Total ron loss e w = k 50 w e = k e (50) w = w e k 50 = k e (50) k = 50 k e w = k (45) w e = k e (45) W = W W W Wen requency = 45 Hz & appled voltage = 90% o rated voltage B max = 0.9 constant 0.9 W, W e W = W + W e W = 0.9W W e = 0.8 W e Emal : engneersacademy.org

10 68 Transormers Junor Engneer W = 0.9 W 0.8 W e.7 W % reducton core loss = 4. Ans. W W e W.7W 00 =4.5% W Steady sort crcut current s te current correspondng to rated appled voltage durng sort crcut test on Transormer. To get rated sort crcut current we ave to apply 0% o rated voltage. [ % = 0%] 5. Ans. 00 Rated current = 400 I 00A 00 I I = 3 I 5 = I = 50A 0KA 5A 5A 400 Te current ratng o wndng n bot cases remans same 00A A 5A Ratng o autotransormer Ans. = A = 50 KA % Regulaton o Transormer As te load s resstve, % Regulaton Load voltage 7. Ans. 8. Ans.. Ans.. Ans. 9. Ans. 0. Ans. 3. Ans. 4. Ans. 5. Ans. 6. Ans. 7. Ans. 8. Ans. 9. Ans. 30. Ans. 3. Ans. In a transormer = (%R) cos ± (%) sn (+ve) Laggng Load ( ve) Leadng Load cos =, sn = 0 = % R = 3% = 00 (3% o 00) = 94 I prmary leakage mpedance s neglected, ten Magnetzng current lag te appled voltage by 90 0 Core loss current s n pase wt # 00-0, Ram Nagar, Bambala ulya.: , Emal : engneersacademy.org

11 EE : Electrcal Macnes Transormers 69 Exctng current lags by about Ans. or ts case e / = constant 0.9 Bm = constant e become e e 0.9 become ere e Bm Bm.6 K = voltage rato o te auto transormer 40. Ans. Maxmum voltage regulaton o a transormer occurs at laggng power actor. oltage regulaton o a transormer at zero power actor s not zero. 4. Ans. 4. Ans. 43. Ans. K, K core loss reduced 33. Ans. 34. Ans. # 00-0, Ram Nagar, Bambala ulya.: , = 4.5 % Because wen requency s decreased losses wll ncreases so ecency reduces. 35. Ans. 36. Ans..u mpedance = rated voltage requred to allow ull load current = 30 % = 0.3 sort crcut power actor = R / = 0. So,R = = 0.06 oltage regulaton 0.06 x % 37. Ans. Te A conducted rom nput to output = I 38. Ans. Te A transerred rom nput to output 39. Ans. oltage regulaton o auto transormer at same. K were = voltage regulaton o - wdg (transormer) er unt mpedance and R 44. Ans. 45. Ans. 46. Ans. 47. Ans. 48. Ans. r e eat laggng p e 50. Ans. (a & b) 49. Ans. rato must be equal. In a transormer, tappngs are provded on te.v. sde because t as larger number o turns wc allow smooter control o output voltage It as to andle low value o current It s easly accessble or repars. 5. Ans., Ans. 5. Ans. % Regulaton R cos sn 4 %, % pu pu Emal : engneersacademy.org

12 70 Transormers Junor Engneer 54. Ans Ans. rato s constant or laggng load te prmary power actor s less tan te load power actor. 56. Ans W 58. Ans. 340w 0w * Total Iron loss e Hz and 00, 50Hz 4 constant 57. Ans W 60 90w 60Hz 5w 40Hz Bm = constant K K e Ke K 90 K K e by solvng above two equaton K = 0.9 and Ke = W 59. Ans. No em n core eddy current loss = 0W 60. Ans. Gven data SC test - : 6, 50 Hz cos sc = 0.5 lag Isc =35 A SC test - : 6, 75 Hz, cos sc =? For SC test - 0 R cossc sc I 35 R sc For SC test new 0old 0 R cossc 0.7l ag # 00-0, Ram Nagar, Bambala ulya.: , Emal : engneersacademy.org