D.C MOTOR

Introduction :

When electrical energy is supplied,the machine rotates and produces the mechanical torque capable of driving other machines.Hence D.C motor converts the electrical energy into mechanical energy.

A D.C motor is similar in construction to a D.C Generator.D.C generator will run as a motor when its field and armature winding are connected to the source of direct current.The field winding produces the necessary magnetic field.

Though the essential construction of D.C motor is identical to that of a generator,the external appearance of a motor may be somewhat different from that of generator.

Working principle :

Whenever current carrying conductor is placed in a magnetic field,it experiences a force whose direction is given by the Fleming's left hand rule. 

The magnitude of the force is given by 
                                          F=BLI newton

where   F = force produced on the conductor in newton

            B = magnetic flux density in web/m^2

            L = The length of the conductor in the magnetic field

            I = The current flowing through the conductor in ampere.

To under stand the principle of operation of D.C motor .Let us consider two pole motor.

From above fig shows uniform magnetic field in which a straight conductor carrying no current is placed.The direction magnetic flux lines is moving from north to south pole.Now assuming no exciting current flow through the field winding and D.C current is sent through the conductor Let the conductor carry away from the observer .It produces magnetic flux lines around that in clockwise direction .That there is no movement of the conductor during the above two conditions.

The current carrying conductor is placed  in the magnetic field.The field due to the current in the conductor aids the main field above the conductor,but oppose the main field below the conductor.Hence flux strengthens above the conductor and weakens below the conductor.It found force act on a conductor,trying to push the conductor downwards shown by the narrow.

If conductor is reversed (current towards the observer ),the strengthening of flux lines occurs below the conductor and the conductor will be pushed upward.

Back emf or counter emf :

When dc supply is given to the D.C motor its armature starts rotating.The armature rotates and cuts the static magnetic flux produced by the field magnets.There fore emf induced in the armature conductors.As per lens law this induced emf opposes the supply voltage.Hence the emf induced in the armature conductor is called Back emf or counter emf .

Back emf  induced in the armature of a D.C motor

Where 

 = Flux per pole in web

     Z     =  no of conductors in armature

     P     = no of poles

     A    = no of parallel paths

    V    = supply voltage to the motor

    I a   = Armature current 

    Ra   = armature resistance in ohms

Ia Ra is the voltage drop in the armature circuits

Types of D.c Motor:

depending upon the way in which the field winding is connected with armature winding the D.C motor are classifieds 

1.D.C shunt motor

2.D.C series motor

3.D.C compound motor.

Applications of D.C motor :

1.D.C shunt motor :

Shunt motor are used to drive centrifugal pumps,light machine tools,reciprocating pumps,wood working machines,paper mills,drilling machines.

2.D.C series motor:

series motor are used where high starting torque is required,such as electric trains,cranes,lifts and conveyors.

3.D.C compound motor :

This motor is intermittent high starting torque is required as rolling mills,presser,printing machines,punches shears,conveyors,elevators etc,.