Question

An electric motor runs a `D.C.` source of e.m.f. `200 V` and draws a current of `10A`. If the efficiency is `40%`, then ressistance of the armature is:
A. `2Omega`
B. `8 Omega`
C. `12 Omega`
D. `16 Omega`

Answer 1

Correct Answer - C
c) Input power, VI=`200 xx 10 = 2000W`
Output power = Efficiency x Input power
`=40/100 xx 2000= 800W`
Power loss in heating the armature
`=2000-800=1200W`
`therefore I^(2)R=1200`
or `R=1200/I^(2) = 1200/(10 xx 10)`
or `R=12Omega`