Now, we are going to discuss about working principle of DC motor
The working principle of DC motor is very simple. We know that when a current carrying conductor is placed inside a magnetic field, it experiences
mechanical force towards a particular direction. Let's take a simple elementary model of a DC motor
In this model, one single turn of conductor is placed between two opposite poles i.e
north pole and south pole as shown here. Now, if we start supplying DC through a commutator system to the single turn, current starts
flowing through it. At that position as we see, the positive terminal of the battery is connected with left conductor
of the turn and negative terminal of the battery is connected with right conductor of the turn
Moreover, we see here in the model, the south pole of the magnet is placed at left side
and north pole of the magnet is placed at right side. Current in this left side conductor flows inward and current in right side conductor
of the turn flows outward. As we see, the conductors are carrying current and placed inside a magnetic field
Both of them experience mechanical force acting on them. The direction of the mechanical force can easily be determined by applying Fleming's
left hand rule. For that, we have to stretch forefinger, second finger and thumb of our left hand at right
angle to one another. With that, if the forefinger is aligned along the direction of magnetic field i.e. from
north pole to south pole and second finger or middle finger is aligned along the direction
of current in the left side conductor, then thumb indicates the direction of mechanical force
This is clearly upward here. Similarly, again if forefinger is aligned along the direction of magnetic field i.e
from north pole to south pole and second finger or middle finger is aligned along the direction
of current in the right side conductor, then thumb indicates the direction of mechanical force
This is clearly downward here. Due to these upward and downward forces on the turn, one torque is produced which tends
to rotate the turn in clockwise direction as shown. After clockwise rotation of 90 degrees, the turn comes to a vertical position in respect
of the magnetic field. At that position there is no current in the conductors of the turn as the conductor and
brushes rest in between two commutator segments. Hence there is no force acting on the conductors
But due to moment of inertia, the turn continues to rotate and comes horizontal again
But position of the conductors has been interchanged here. That means the conductor which was previously at left position comes to right position and
which was previously at right position comes to left position. At that position again mechanical force acts on the conductors whose direction can again
be determined by applying same Fleming's left hand rule. Let's do that
For that again we have to stretch the forefinger, second finger and thumb of our left hand at
right angle to one another. With that, if forefinger is aligned along the direction of magnetic field i.e. from
north pole to south pole and second finger or middle finger is aligned along the direction
of current in the left side conductor, then thumb indicates the direction of mechanical force
This is clearly upward here. Similarly, again if forefinger is aligned along the direction of magnetic field i.e
from north pole to south pole and second finger or middle finger is aligned along the
direction of current in the right side conductor, then thumb indicates the direction of mechanical force
This is clearly downward here. Due to these upward and downward forces, the turn tends to rotate in clockwise direction
as shown. Now from that explanation, we can come to the conclusion that, here in this model, whichever
conductor comes under south pole experiences a upward mechanical force and whichever conductor
comes under north pole experiences a downward mechanical force. And due to these two forces, the single turn continues to rotate until the battery is disconnected
A practical DC motor rotates with same principle as of this elementary single turn model
Practically, in a DC motor, in place of single turn, there is multi-turns armature coil and
in place of two poles, there may be more than two, even number of poles
Hope you understood. Thank you