$A$ rectangular coil of a simple $A.C.$ generator rotates about one of its diameters with a constant angular frequency in the presence of a uniform magnetic field. The direction of the induced current reverses for the third time after how many rotations of the coil?

$A$ motor having an armature of resistance $2\,\Omega$ is designed to operate at $220\,V$ mains. At full speed,it develops a back $e.m.f.$ of $210\,V$. When the motor is running at full speed,the current in the armature is: (in $,A$)

$A$ generator has an armature resistance of $0.1 \, \Omega$ and develops an induced emf of $120 \, V$ when driven at its rated speed. Its terminal voltage when a current of $50 \, A$ is being drawn is ................. $V$.

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 $emf$.

$A$ simple electric motor has an armature resistance of $1 \,\Omega$ and runs from a d.c. source of $12 \,V$. It draws a current of $2 \,A$ when unloaded. When a certain load is connected to it,its speed reduces by $10 \%$ of its initial value. The current drawn by the loaded motor is ......... $A$

When the speed of a $dc$ motor increases, the armature current: