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Working principle of Dc motor (How a DC motor works?)

November 22, 2017
Working principle of Dc motor (How a DC motor works?)

Working principle of Dc motor

Working principle of dc motorDc motor, Dc generator both descended from the same family Dc machine which means that there isn’t a big difference between Dc motor and Dc generator contrary we will find lots of similarities between them so let’s go.

Dc motor

In a simple definition, Dc motor is also an electrical machine, but it converts electrical power into mechanical power, the direction of the mechanical force produced when we place a current carrying conductor in a magnetic field is determined by Fleming’s left-hand rule and the magnitude of this force is:

F=BIL      Newton



B= magnetic flux density.

L= the length of the conductor.

Fleming’s left hand rule


That’s good and that will lead us to the principle of Dc motor that’s what we need, but we also need to know why Dc motor is very important and where we can find it? So let’s see.

Applications of Dc motor:

Dc motor is the latest electrical machine used to produce mechanical force so we can find it in many applications where constant or low-speed torque is required, those applications include:

  • Conveyors.
  • Turntables.
  • Fractional HP applications.
  • Cement plants.
  • Automotive.
  • Solar airplane.
  • Spacecraft.
  • UAV.
  • Electric aircraft.
  • Speed control.
  • Conditional monitoring.
  • Textile, and so on everywhere around us.

I think as Dc motor has this great importance and include all these applications everywhere around us it must have extra features so;

Advantages of Dc motor:

Dc motor has the many advantages seems like:

  • Low cost.
  • Simple and efficient design.
  • Easy to maintain and service.
  • Small converters and drives.
  • High power density.
  • Full torque at zero speed.
  • Varying speed easily.
  • And less inertia.

That’s very good; now I can start the story of working principle of Dc motor which requires some of your attention.

Working principle of Dc motor:

working of dc motor

The stator of the Dc motor is a permanent magnet and it provides us with a constant magnetic field, and the rotating part is the armature and it’s a simple coil.

We use a pair of commutator rings to connect the armature to a Dc power source, and when a current flow through the armature coil an electromagnetic force induced in the coil so the coil starts to rotate.

working principle of Dc motor

When the coil rotates the commutator rings will connect with the opposite polarity of the power source.

So the electricity on the left side of the coil will always flow away and on the right side, it will flow towards.

So the torque action will also be in the direction of motion so the coil will continue rotating.

But if the coil becomes perpendicular to the magnetic flux the torque action will near zero and if we run this motor there may be an irregular motion of the rotor.

principle of dc motor

To overcome this problem we add one more loop with a separate commutator to the rotor and by that, if there is a loop in the vertical position the other will be connected to the power source and thus ensure continuity produce of motive force in the system.

In the same way the more loops we added the smoother rotation of the motor we get, and here we care to fit the armature loops inside highly permeable steel layers slots to enhance the interaction of the magnetic flux.

good; I think we have interesting knowledge about Dc motor which leads us to search in types of Dc motor and the importance of each type and where we found them; so:

Types of Dc motor:

As Dc motor is much similar to Dc generator we would find the same types as Dc motor has the types:

  • 1- Separately excited Dc motor:

From the name, we deduce that we use a separate Dc source to energize the field coils (field windings).

Separately excited Dc motor
Separately excited Dc motor

Self-excited means that the motor depends on itself to supply the current flows in the windings, and this motor divided into:

In this motor, we connect the field winding in series with the armature winding, and so,

it has a good starting torque but the speed drops drastically with the load.


series excited Dc motor

We can find series wound Dc motors in:

  • Traction system.
  • Cranes.
  • Elevators.
  • Hair drier.
  • Air compressors.
  • Vacuum cleaner.
  • Sewing machines and so on, where high starting torque is required and speed variation are possible.
  • Shunt wound Dc motor:

it is the most common types of Dc motor, and the field winding is connected parallel with the armature, and this motor has the addition that it has a low starting torque but in the same time, it runs at a constant speed.

shunt excited Dc motor

Shunt excited motor can be used with:

  • Lathe machines.
  • Centrifugal pumps.
  • Drills.
  • Boring mills.
  • Shapers.
  • Fans.
  • Blowers.
  • Conveyors.
  • Lifts.
  • Weaving machine.
  • Spinning machines, and everywhere where constant speed is required.
  • Compound wound Dc motor:

In the compound motor, there are a shunt and series field windings, and it can subdivide into a cumulative compound where the flux is produced by both the windings and a differential compound where the flux produced by the series winding opposite the flux produced by the shunt winding.

cumulative compound excited Dc motor

We can use this type at:

  • Presses.
  • Shears.
  • Conveyors.
  • Elevators.
  • Rolling mills.
  • Heavy planners, and in places where high starting torque and constant speed is required.


Finally; we can find DC motors in many portable home appliances, automobiles, and types of industrial equipment.