Assertoion : An electric motor will have maximum efficiency when back emf becomes equal of half of applied emf.
Reason. Power factor of series `R_L` circuit is given by `cos theta = (2R)/(sqrt(R^(2) + omega^(2) L^(2)))`

Options

A. A
B. B
C. C
D. D

Answer

C

Explanation

Power factor of series `R -L` circuit is given by
`cos phi = (R )/(Z) = (R )/(sqrt(R^(2) + omega^(2) L^(2)))`
when frequency becomes double, then
`cos phi' = (R )/(sqrt(R^(2) + 4 omega^(2) L^(2)))`
So `cos phi' lt cos phi`
Thus Assertion is true. Here, Reason is false.