Working Principle of DC Motor with Single Loop Model

Before going through the detailed study of the working principle of DC motor, we may recall the expression of force acts in a current carrying conductor in a magnetic field.

Whenever we place a current carrying conductor inside a magnetic field, it experiences a mechanical force. Fleming’s left-hand rule determines the direction of this mechanical force. The equation gives the expression for this mechanical force.

Where B is the flux density of the field.

I is the current flowing through the conductor.

L is the length of the conductor comes under the influence of that magnetic field.

Working Principle of DC Motor with a Single Loop Coil Model

Construction wise, a DC motor is exactly similar to that of a DC generator. For understanding the basic principle of a DC motor let us consider a single loop model of a DC machine. There is one loop (say) ABCD placed inside two opposite magnetic poles as shown in the figure. The end terminals of the single loop coil ABCD come out from the machine through commutator segments. Now we place the brushes on the commutator segments and connect the DC supply to the machine through these brushes. At the initial situation, the conductor AB of the single loop coil is under the north pole. At the same time, the conductor CD is under the south pole.  Please refer to the figure above.

Direction of Force Acting on Conductors

Now, we start supplying DC to this single loop ABCD, through the commutator brush arrangement. At that instant, according to the polarity of supply voltage, the direction of current through the conductor AB is from A to B. On the other side the direction of current in conductor CD is from C to D. Then if we apply Fleming’s left-hand rule, we will get the direction of mechanical force (F) acting on the conductors. We will see that the force acting on conductor AB is straight downward. The same force on the conductor CD is straight upward. These two forces create a torque on the loop. As a result, the loop starts rotating anticlockwise.

No Force Acting on Conductors

After a rotation of 90°, the motion of the conductor AB and CD both become parallel to the direction of the magnetic flux. At that situation, there is no flux cutting by the conductors. Hence, at that position of the conductors, no force acting on them. Although, because of the moment of inertia the loop continues to rotate. Hence, after the next 90° of rotation conductor AB comes in front of S pole. So, the conductor CD comes in front of N pole. Now, we will examine the direction of current in both conductors. Here, in conductor AB, the current flows from B to A and in conductor CD the current flow D to C.

Unidirectional Force Acting on Conductors

Here again, we apply Fleming’s left-hand rule to examine the direction of the mechanical force acting on the conductors. One thing we should remember here that as the motion of the conductor at that horizontal position is exactly perpendicular to the direction of flux from N pole to S pole, the magnitude of mechanical force will be maximum at that horizontal position of the loop. Now we see, the direction of the mechanical force acting on the conductor at that position, is downward for conductor CD and it is upward for conductor AB. At that position again the maximum torque tries to rotate the loop in the same direction as it was before in the previous position of 180° behind.

So we can see that there is unidirectional torque acting on the loop. Because of this unidirectional torque, the loop continues to rotate in the anticlockwise direction as long as we keep the supply on to this system.

So this is the basic working principle of a DC motor. Practically the construction of a DC motor is not so simple but the working principle is the same as we have just explained. Here the function of the commutator is to alter the direction of current in a conductor when it comes from one pole to another pole to maintain unidirectional torque. That means the commutator converts the DC supply to the alternating current in the coil of the motor.

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