how does dc motor work ???

There are two conditions which are necessary to produce a force on the conductor.
The conductor must be carrying current, and must be within a magnetic field.
When these two conditions exist, a force will be applied to the conductor, which will attempt to move the conductor in a direction perpendicular to the magnetic field.
This is the basic theory by which all DC motors operate.
Consider a coil in a magnetic field of flux density B .
When the two ends of the coil are connected across a DC voltage source, current I flows through it.
A force is exerted on the coil as a result of the interaction of magnetic field and electric current.
The force on the two sides of the coil is such that the coil starts to move in the direction of force In an actual DC motor, several such coils are wound on the rotor, all of which experience force, resulting in rotation.
The greater the current in the wire, or the greater the magnetic field, the faster the wire moves because of the greater force created.
At the same time this torque is being produced, the conductors are moving in a magnetic field.
At different positions, the flux linked with it changes, which causes an emf to be induced (e = d/dt) .
This voltage is in opposition to the voltage that causes current flow through the conductor and is referred to as a counter-voltage or back emf.
The value of current flowing through the armature is dependent upon the difference between the applied voltage and this counter-voltage.
The current due to this counter-voltage tends to oppose the very cause for its production according to Lenz’s law.
It results in the rotor slowing down.
Eventually, the rotor slows just Induced emf enough so that the force created by the magnetic field (F = Bil) equals the load force applied on the shaft.
Then the system moves at constant velocity

DC motors are in countless consumer electronic devices from CD players to computers to radio-controlled airplanes.
There are many different kinds of DC motors, but they all work on the same principle.
They turn current into pulses of magnetism, which they use to turn a rotor.
There are many different kinds of electric m Brushed DC Motors Brushed DC Motors have two coils of wire around a rotor in the middle.
Surrounding the coil are two magnets, both facing in the same direction.
When the coils are facing the magnets, electricity flows into them.
When electricity flows into a coil, it creates a magnetic field, and this magnetic field pushes the coils away from their magnets.
As the rotor turns, the current shuts off.
When the rotor has turned 180 degrees, each rotor faces the opposite magnet.
The coils turn on again, this time with the electricity flowing in the opposite direction.
This creates another pulse, pushing the rotor around again.
The rotor has electric contacts on it, and there are small metal brushes that bump against the contacts.
The brushes send in electricity, turning the motor on and off at the right times.
Brushless DC Motors Brushed motors work reasonably well, but they have a few drawbacks.
The brushes create friction, slowing the motor and wasting energy.
They also wear out.
The brushes corrode or get worn away by friction from the rotor.
Brushless motors solve both of these problems.
A brushless motor has a permanent magnet on the inside of the rotor, facing so that its north and south poles are perpendicular to the axis of rotation.
Around the rotor are coils.
The coils work much like they do in a brushed motor.
They give out timed pulses to push the magnet, spinning the rotor.
Because there are no brushes, however, the motor can't control itself.
Instead, it is attached to an electronic speed controller, which gives pulses of electricity at a certain speed to control the motor.
The faster the coils pulse, the faster the motor will spin

The induction of a force in a wire by a current in the presence of a magnetic field is the basis of motor action.
Almost every type of motor depends on this basic principle for the forces and torques which make it move.
The induction of voltages in a wire moving in a magnetic field is fundamental to the operation of all types of generators.
For this reason, it is called generator action.

The principle of operation of a d.c.
motor is that whenever a current-carrying conductor is placed within a magnetic field, a force acts on that conductor which is perpendicular to that field -in other words, the force acts to push the conductor out of the field.
If a pivoted loop of wire is placed within the same magnetic field, the forces on opposite sides of that loop act in opposite directions to each other, and a torque is applied to that loop.
If the relative directions of current and field are maintained, then the loop will continue to rotate -this is done through the use of a split-ring commutator, a type of rotary switch, which also acts to supply the rotating loop from a fixed external circuit.