What is the Principle of DC Motor?

In this article, we shall discuss the working principle of a DC Motor (Direct Current Motor). So let us start with the basic introduction of the DC Motor.

Introduction to DC Motor

A DC machine that converts direct current electrical energy into rotational mechanical energy is known as a DC motor. Hence, the DC motor is an electromechanical energy conversion device. It has two major parts namely, the field system and armature. The field system generates the required working magnetic flux in the machine and the armature is the part that is free to rotate and it is a system of conductors.

DC motors are used to drive many types of mechanical loads in industries such as drive fans, pumps, cranes and hoists, etc.

Working Principle of DC Motor

The dc motor works on the principle of electromagnetic interaction. According to this principle, whenever a current-carrying conductor is placed in a magnetic field, the conductor feels an electromagnetic force acting on it.

The magnitude of this electromagnetic force is,

• Directly proportional to the magnetic flux density (B).
• Directly proportional to the current through the conductor (I).
• Directly proportional to the length of the conductor (l).

Therefore, the magnitude of the electromagnetic force is given by,

`\F=BIl" "…(1)`

Explanation:

To understand the working principle of the dc motor, consider the simple single-coil motor as shown in the following figure.

When an electric supply is given to the coil, an electric current I starts flowing through the coil. As we can see that the coil sides are placed under the influence of a magnetic field of flux density B weber per sq. meter. Consequently, each coil side experiences a force that is given by,

`\F=BIl`

Since the two coil sides are under the influence of opposite magnetic poles of the same pole strength, therefore, the force acting on them will be equal and opposite.

Now, if d is the distance between two coil sides, then the developed torque in the coil is given by,

`\τ=F.d`

`\⇒τ=BIld`

`\∴τ=BIA" "…(2)`

Where, A = l.d, area of the coil.

This torque developed in the coil rotates the coil and therefore, we obtain mechanical energy in the form of rotation of the coil.

This is how a dc motor works and converts dc electrical energy input into mechanical energy output.

Direction of Rotation of DC Motor

Now, let us know how to determine the direction of rotation of the dc motor.

The direction of force acting on a current-carrying conductor placed in a magnetic field is identified by Fleming’s Left-Hand Rule (FLHR).

According to Fleming’s left-hand rule, if the thumb, forefinger, and middle finger of the left hand are kept mutually perpendicular to each other, and if the forefinger points to the direction of the magnetic flux, the middle finger points to the direction of electric current through the conductor, then the thumb will point to the direction of electromagnetic force acting on the conductor.

The graphical representation of Fleming’s left-hand rule is shown in the figure below.

Hence, this is all about the operating principle of the dc motor. In conclusion, a dc motor converts dc electrical energy into mechanical energy according to the law of electromagnetic interaction, i.e. interaction between a current-carrying conductor and a magnetic field.