Statement I: An electric motor will have maximum efficiency when back emf becomes equal to half of applied emf. |
Statement II: Efficiency of electric motor depends only on magnitude of back emf. |
A) Statement I is true; Statement B is true; Statement B is not a correct explanation for Statement I,
B) Statement I is true; Statement II is false.
C) Statement I is false; Statement II is true.
D) Statement I is true; Statement II is true; Statements is the correct explanation for Statement I.
Correct Answer: B
Solution :
Since, the efficiency of an electric motor is given by \[\eta \frac{\text{output power}}{\text{input power}}\] From the above-relation, it is quite clear that maximum output power corresponds maximum efficiency of motor. Now, output power is given by \[=ei=\frac{e(E-e)}{R}\] To obtain maximum output power differentiating Eq. (i) with respect to e which will be equal to zero. So, \[\frac{d}{de}\left[ \frac{e(E-e)}{R} \right]=0\Rightarrow e=\frac{E}{2}\] Thus, when back emf becomes equal to half of the applied emf, the efficiency of motor will be maximum.You need to login to perform this action.
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