Mod EM1 Practice Exam
Why cdoes cSwinburne's ctest cgive ca chigher cefficiency cthan
cHopkinson's ctest?
Answer:
Swinburne’s ctest cis ca cno-load ctest cperformed con cDC cmachines cto cdetermine
cconstant closses c(iron cand cmechanical), cassuming cthem cto cbe cthe csame cat
cfull cload. cHowever, cit cdoesn't cconsider cload-dependent closses, clike:
● Increase cin carmature ccopper closses cdue cto carmature creaction cand
cdistortion.
● Increase cin cstray cload closses.
Hopkinson’s ctest, con cthe cother chand, cis ca cfull-load ctest cwith cboth cmachines
cunder coperating cconditions, cso:
● It cconsiders call creal closses cduring cactual coperation.
● Hence, cit cshows ca clower cefficiency cthan cSwinburne’s ctest, cwhich
cslightly coverestimates cefficiency.
2. Given:
● Supply cVoltage c= c220V
● Armature cresistance c(Ra) c= c0.2Ω
● Field cresistance c(Rsh) c= c110Ω
(i) Starting ccurrent cwithout ca cstarter:
, At cstarting, cEb=0E_b c= c0Eb=0
Istart=VRa=2200.2=1100 cAI_{\text{start}} c= c\frac{V}{R_a} c= c\frac{220}
{0.2} c= c1100 c\text{
A}Istart=RaV=0.2220=1100 c A
(ii) Running carmature ccurrent cat cfull cload:
Assuming cback cEMF cEbE_bEb cis cgiven c(not creadable cin cthe cscan, cassuming
cEb=210VE_b c= c210VEb=210V):
Ia=V−EbRa=220−2100.2=50 cAI_a c= c\frac{V c- cE_b}{R_a} c= c\frac{220 c-
c210}{0.2} c= c50 c\text{ cA}Ia=RaV−Eb=0.2220−210=50 cA
Why cdoes cSwinburne's ctest cgive ca chigher cefficiency cthan
cHopkinson's ctest?
Answer:
Swinburne’s ctest cis ca cno-load ctest cperformed con cDC cmachines cto cdetermine
cconstant closses c(iron cand cmechanical), cassuming cthem cto cbe cthe csame cat
cfull cload. cHowever, cit cdoesn't cconsider cload-dependent closses, clike:
● Increase cin carmature ccopper closses cdue cto carmature creaction cand
cdistortion.
● Increase cin cstray cload closses.
Hopkinson’s ctest, con cthe cother chand, cis ca cfull-load ctest cwith cboth cmachines
cunder coperating cconditions, cso:
● It cconsiders call creal closses cduring cactual coperation.
● Hence, cit cshows ca clower cefficiency cthan cSwinburne’s ctest, cwhich
cslightly coverestimates cefficiency.
2. Given:
● Supply cVoltage c= c220V
● Armature cresistance c(Ra) c= c0.2Ω
● Field cresistance c(Rsh) c= c110Ω
(i) Starting ccurrent cwithout ca cstarter:
, At cstarting, cEb=0E_b c= c0Eb=0
Istart=VRa=2200.2=1100 cAI_{\text{start}} c= c\frac{V}{R_a} c= c\frac{220}
{0.2} c= c1100 c\text{
A}Istart=RaV=0.2220=1100 c A
(ii) Running carmature ccurrent cat cfull cload:
Assuming cback cEMF cEbE_bEb cis cgiven c(not creadable cin cthe cscan, cassuming
cEb=210VE_b c= c210VEb=210V):
Ia=V−EbRa=220−2100.2=50 cAI_a c= c\frac{V c- cE_b}{R_a} c= c\frac{220 c-
c210}{0.2} c= c50 c\text{ cA}Ia=RaV−Eb=0.2220−210=50 cA
