An electric fan operates on the principle of which of the following?

The back emf in a $DC$ motor is maximum when,

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 $emf$.

$A$ motor starter:

The armature of a $DC$ motor has $20\,\Omega$ resistance. It draws a current of $1.5\,A$ when run by a $220\,V$ $DC$ supply. The value of the back $EMF$ induced in it will be......$V$

$A$ motor operating on $100 \ V$ draws a current of $1 \ A$. If the efficiency of the motor is $91.6 \%$,then the loss of power in units of $cal/s$ is