# Working Principle of DC Motor

- Link: https://studyelectrical.com/2014/12/working-principle-of-dc-motor.html
- Published: 2014-12-27T12:28:00+00:00
- Author: 

A DC motor is an electrical machine that **converts electrical energy into mechanical
energy**. 

The working of DC motor is based on the principle that when a current carrying conductor
is placed in a magnetic field, it experiences a mechanical force. 

The direction of the mechanical force is given by **Fleming’s Left-hand Rule** and
its magnitude is given by **F = BIL** Newton.

The working of the AC motor ([Induction motor](https://studyelectrical.com/2014/11/how-three-phase-induction-motor-work.html)
and [Synchronous Motor](https://studyelectrical.com/2018/11/starting-methods-of-synchronous-motor.html))
is different from the DC motor.

![dc motor working](https://studyelectrical.com/wp-content/uploads/2014/12/dc-motor-
working.jpg)

There is no basic difference in the [construction of a DC generator and a DC motor](https://studyelectrical.com/2014/06/construction-dc-motor-dc-motor-construction.html).
In fact, the same DC machine can be used interchangeably as a generator or as a 
motor.

Like generators, there are [different types of DC motors](https://studyelectrical.com/2014/12/types-of-dc-motors.html)
which are also classified into **[shunt-wound](http://www.studyelectrical.com/2013/11/speed-control-of-dc-shunt-motors.html),
[series-wound](http://www.studyelectrical.com/2013/11/speed-control-of-dc-series-motors.html)
and compound-wound dc motors**. 

_DC motors are seldom used in ordinary applications because all electric supply 
companies furnish alternating current._

However, for special applications such as in **steel mills, mines, and [electric trains](https://studyelectrical.com/2014/05/how-electric-locomotives-work.html)**,
it is advantageous to convert alternating current into direct current in order to
use dc motors. The reason is that the **speed/torque **[**characteristics** of DC motors](https://studyelectrical.com/2016/09/characteristics-of-dc-series-motor.html)
are much more superior to that of AC motors. 

Therefore, it is not surprising to note that for industrial drives, DC motors are
as popular as [three-phase induction motors](https://www.studyelectrical.com/2014/11/how-three-phase-induction-motor-work.html).

## DC Motor Principle

A machine that converts DC electrical power into mechanical power is known as a 
Direct Current motor. 

**DC motor working is based on the principle that when a current carrying conductor
is placed in a magnetic field, the conductor experiences a mechanical force. **

The direction of this force is given by **Fleming’s left-hand rule** and magnitude
is given by;

**F = BIL Newtons**

_According to Fleming’s left-hand rule when an electric current passes through a
coil in a magnetic field, the magnetic force produces a torque that turns the DC
motor._

The direction of this force is perpendicular to both the wire and the magnetic field.

![Flemings Left Hand Rule](https://studyelectrical.com/wp-content/uploads/2014/12/
flemings-left-hand-rule.jpg)

Flemings Left Hand Rule

Basically, there is no constructional difference between a DC motor and a DC generator.
The same DC machine can be run as a generator or motor.

![Cross-Section of a DC Machine](https://studyelectrical.com/wp-content/uploads/
2014/12/Dc-machine-generator-and-motor-construction.png "Cross-Section of a DC Machine")

Cross-Section of a DC Machine

> [Read in Detail about the Construction of DC machines](http://www.studyelectrical.com/2014/06/construction-dc-motor-dc-motor-construction.html)

## Working of DC Motor

Consider a part of a **multipolar DC motor** as shown in the figure below. When 
the terminals of the motor are connected to an external source of DC supply:

 * the **field magnets** are excited developing alternate North and South poles
 * the **armature conductors** carry currents.

![Part of a Multi-polar DC Motor](https://studyelectrical.com/wp-content/uploads/
2014/12/Working-of-a-DC-motor.png "Part of a Multi-polar DC Motor")

Part of a Multi-polar DC Motor

 All conductors under North-pole carry currents in one direction while all the conductors
under South-pole carry currents in the opposite direction.

The armature conductors under N-pole carry currents into the plane of the paper (
denoted as ⊗ in the figure). And the conductors under S-pole carry currents out 
of the plane of the paper (denoted as ⨀ in the figure).

Since each armature conductor is carrying current and is placed in the magnetic 
field, a **mechanical force** acts on it.

On applying **Fleming’s left-hand rule**, it is clear that force on each conductor
is tending to rotate the armature in the anticlockwise direction. All these forces
add together to produce a **driving torque** which sets the armature rotates.

When the conductor moves from one side of a brush to the other, the current in that
conductor is reversed. At the same time, it comes under the influence of the next
pole which is of opposite polarity. Consequently, the **direction of the force on
the conductor remains the same**.

It should be noted that the **function of a commutator** in the motor is the same
as in a generator. By reversing current in each conductor as it passes from one 
pole to another, it helps to develop a **continuous and unidirectional torque**.

### Video Animation

Watch the working of DC motor by [learnengineering.org](http://learnengineering.org/)
in this video.

 

Next: [Back EMF in DC Motor](https://studyelectrical.com/2015/02/back-emf-and-significance-in-dc-motor.html)

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