The DC motor uses direct current (electrical energy) to create mechanical movement. Rotary motion. When it converts electrical energy into mechanical energy it is called a DC motor, and when it converts mechanical energy into electrical energy it is called a direct current generator. . The functional principle of the DC motor is based on the fact that a current-carrying conductor, when it is brought into a magnetic field, experiences a mechanical force and begins to rotate. Its direction of rotation depends on Fleming’s left-hand rule. DC motors are used in many applications, e.g. B. Robots for motion control, toys, quadrocopters, CD / DVD drives in PCs/laptops, etc.


Construction of Dc Motor
Construction of dc motor

It has mainly two major parts as,

  1. Stator – Static part of the motor.
  2. Rotor – Rotating part of the motor.
Theoretical  diagram of Dc Motor
theoretical diagram of dc motor

The south and north poles of the permanent magnet or electromagnet are the stator part of the DC motor and the armature connected to the commutator is the rotating part of the motor. The south and north poles are used to create a magnetic field as shown in the picture. a conductive material placed between the magnetic field created by the north and south poles. The current (i) shown in the figure flows through the armature. The brushes are used to connect the armature DC power supply across the commutator having segments connected to each end of the conductive armature. Therefore, the commutator also rotates with the armature. The brushes are part of the stator that always stay in contact with the commutator.

Working Principle of DC motor 

A DC motor is an electrical machine that converts electrical energy into mechanical energy. The basic principle of DC motor operation is that whenever a current-carrying conductor is brought into the magnetic field, it experiences a mechanical force.

Fleming’s left-hand rule and its magnitude decide the direction of this force.



B = magnetic flux density,

I = current and

L = length of the conductor within the magnetic field.

When the armature winding is connected to a direct current source, an electric current is generated in the winding. Permanent magnets or field windings (electromagnetism) supply the magnetic field. In this case, the armature conductor through which current flows experience a force due to the magnetic field, according to The commutator is segmented in order to achieve a unidirectional torque. Otherwise, the direction of force would have reversed with each reversal of the direction of movement of the conductor in the magnetic field. This is how a DC motor works!

Speed Controlling

Speed (N) of DC motor is measured in RPM (Rotation Per Minute) and it is given by,

N = 60AE / PZ Φ


                     E = Back EMF

                     A = Parallel paths

                     Z = No. of armature conductors

                     P = No. of poles

                     Φ = Flux

Device Constant      K = 60A/PZ

Back EMF    E = V – IARA

Hence, speed   N = K * (V – IARA) / Φ

Here, we can see that speed of DC motor can be controlled through,

  • Terminal voltage of armature i.e. V
  • External resistance with armature i.e. RA
  • Field flux i.e. Φ

From the above speed control parameters, we can find that V and RA are related with armature circuit and Φ related to the magnetic field, hence they are classified as,

  • Armature control method
  • Field control method

We can vary V across motor terminals by using PWM methods.

Pulse Width Modulation Technique


TON is the time during which the signal is HIGH and TOFF is the time during which it is LOW. Therefore, the terminal voltage applied to the DC motor is only valid for the TON (ON) time of the period. For PWM with 50% duty cycle, as shown in the figure above, an average voltage of ≈50% is supplied to the motor terminal. In this way, we get simple control of the speed of the DC motor using the PWM method. A higher duty cycle provides higher speed and a lower duty cycle provides lower speed. We can precisely vary the pulse width with the microcontroller to have precise control over the DC motor. Now we will see how to change the directions of rotation of the DC motors.

Bidirectional DC Motor Using H-Bridge Configuration

As shown in the picture, there are two terminals ‘A’ and ‘B’ of the DC motor. If we now connect terminal A to supply + Ve and terminal B to supply -Ve or ground, the current from the motor flows from A to B and the motor rotates in one direction, say clockwise (CW) or in a forward direction. Now we change the supply terminals as shown in the second figure. B is connected to + Ve and A to the ground. The current flows from the motor from B to A and the motor rotates in a different direction (counterclockwise, counterclockwise or backwards).