Assertion : An electric motor will have maximum efficiency when back emf becomes equal to half of applied emf.
Reason : Efficiency of electric motor depends only on magnitude of back emf.

Options

A. If both asseration and reason are true and reason is the correct explanation of assertion.
B. If both asseration and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If assertion is false but reason is true.

Answer

C

Explanation

Since the efficiency of an electric motor is given by
`eta=("output power")/("input power")`
From the above relationn, it is quite clear that maximum output power corresponds maximum efficiency of motor.
`eI=(e(E-e))/(R )` …..(i)
To obtain maximum output power differentiating Eq. (i)with respect to `e` which will be equal to zero.
so, `(d)/(de)[(e(E-e))/(R )]=0impliese=(E)/(2)`
Thus, when back emf becomes equal to half of the applied emf, the efficiency of motor will be maximum.