Correct option is D
The emf induced in the armature is AC as its direction changes.
By using the split-ring type commutator, the current flow is made unidirectional, and a DC supply is obtained.
When the current-carrying conductor placed in a magnetic field, the torque induces on the conductor, the torque rotates the conductor which cuts the flux of the magnetic field.
According to the Electromagnetic Induction Phenomenon "when the conductor cuts the magnetic field, EMF induces in the conductor".
According to Fleming Right Hand Rule:
if we hold our thumb, middle finger and index finger of the right hand by an angle of 90∘, then the index finger represents the direction of the magnetic field.
The thumb shows the direction of motion of the conductor and the middle finger represents the emf induces on the conductor.
On applying the right-hand rule in the figure shown below, it is seen that the direction of the induced emf is opposite to the applied voltage.
Thereby the emf is known as the counter emf or back emf.
The back emf is developed in series with the applied voltage, but opposite in direction, i.e., the back emf opposes the current which causes it.
Formula:
