Working Principle of DC Motor - How a DC Motor Works?

Working Principle of DC Motor – How a DC Motor Works?

Posted on

A motor is an electrical machine which changes over electrical energy into mechanical energy. The principle of working of a DC motor is, it can be said that “whenever a current carrying conductor is put in a magnetic field, it experiences a mechanical force“.

How a DC motor works? The DC motor is of indispensable significance for the business today and is similarly critical for engineers to look into the working principle of DC motor. To investigate the operating principle of DC motor we need to first look into its single loop constructional feature.

Working Principle of DC Motor – How a DC Motor Works?

The very basic construction of a DC motor contains a current carrying armature, associated with the supply end through commutator portions and brushes. The armature is set in between north pole and south pole of a permanent or an electromagnet as appeared in the chart above.

As soon as we supply direct current in the armature, a mechanical force acts on it due to the electromagnetic effect of the magnet on armature conductors.

Operating Principle of DC Motor

Now to go into the details of the operating principle of DC motor it is essential that we have a reasonable comprehension of Fleming’s left-hand guideline to decide the heading of the power following up on the armature conductors of DC motor.

Fleming left hand rule

If a current carrying conductor is set in a magnetic field perpendicularly, at that point the conductor experiences a force in the direction mutually perpendicular to both the commonly opposite to both the course of field and the current conveying conductor.

Fleming’s Left-Hand Rule can decide the bearing of rotation of the motor. This rule says if we extend the index finger, center finger and thumb of our left-hand opposite to each is such a way that center finger is toward current in the conductor, and index finger is along the direction of magnetic field, i.e., north to south pole, then thumb demonstrates the direction of the made mechanical force.

For clear understanding the principle of DC motor we have to determine the magnitude of the force, by considering the diagram below.

See Also: Construction of DC Motor (Parts and Functions)

Operation of DC Motor

We know that when an infinitely little charge dq is made to flow at a velocity ‘v’ affected by an electric field E, and a magnetic field B, at that point the Lorentz Force dF experienced by the charge is given by:

For the operation of DC motor, considering E = 0.

i.e. it’s the cross product of dq v and magnetic field B.


  • dL is the length of the conductor carrying charge q.

From the 1st diagram we can see that the construction of a DC motor is with the end goal that the direction of current through the armature conductor at all occasion is perpendicular to the field.

Hence the force acts on the armature conductor in the direction perpendicular to both uniform field, and current is constant.

So if we take the current in the left-hand side of the armature conductor to be I, and current at the right-hand side of the armature conductor to be -I, because they are flowing in the opposite direction to each other.

Then the force on the left-hand side armature conductor:

Similarly, the force on the right-hand side conductor:

In this way, we can see that at that position the force on either side is equal in magnitude but opposite in direction.

Since the two conductors are isolated by some separation w = width of the armature turn, the two inverse forces produce a rotational force or a torque that outcomes in the pivot of the armature conductor.

Now let’s examine the expression of torque when the armature turn create an angle of α (alpha) with its initial position.

The torque produced is given by:

See Also: Working Principle of Air Conditioner

Principle Behind The Rotation of The Motor

Here α (alpha) is the angle between the plane of the armature turn and the plane of reference or the initial position of the armature which is here along the direction of magnetic field.

The nearness of the term cos α in the torque equation very well signifies that dissimilar to compel the torque at all position isn’t the equivalent. It, in fact, fluctuates with the variety of the angle α (alpha). To clarify the variety of torque and the principle behind the rotation of the motor let us do a stepwise investigation.

working of dc motor

Step 1:

At first considering the armature is in its beginning stage or reference position where the angle α = 0.

Since, α = 0, the term cos α = 1, or the maximum value, consequently torque at this position is greatest given by τ = BILw. This high beginning torque helps in overcoming the initial inertia of rest of the armature and sets it into the turn.


Step 2:

When the armature sets in motion, the angle α between the genuine position of the armature and its initial reference position continues expanding in the way of its rotation until it becomes 90o from its initial position.

Consequently, the term cosα diminishes and also the value of torque. The torque in this situation is given by τ = BILw cos α which is less than BIL w when α is greater than 0o.

principle of dc motor

Step 3:

In the path of the rotation of the armature a point is achieved where the genuine position of the rotor is actually opposite to its initial position, i.e. α = 90o, and as a result the term cosα = 0.

The torque acting on the conductor at this position is given by:

working principle of dc motor

Construction of DC Motor

A DC motor like as we know is a device that deals in the transformation of electrical energy to mechanical energy and this is basically realized about by two major parts required for the construction of DC motor, namely:

  1. Stator – The static part that houses the field windings and receives the supply, and
  2. Rotor – The rotating part that brings about the mechanical rotations.

See Also: Working Principle of AC Motor

Working Principle of DC Motor Conclusion

After going through the above portion of working principle of dc motor we can now establish operating principle of dc motor and construction of DC motor. I hope you enjoy when reading this article, thank you.