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

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......$V$

An electric motor operates on a $50\,V$ supply and a current of $12\,A$. If the efficiency of the motor is $30\%$,what is the resistance of the winding of the motor in $\Omega$?

The armature of a generator of resistance $1 \,\Omega$ is rotated at its rated speed and produces $125 \,V$ without load and $115 \,V$ with full load. The current in the armature coil is ......... $A$.

In an $A.C.$ generator,when the plane of the coil is perpendicular to the magnetic field:

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 $emf$.