Principal of DC Motor


DC Motor

 ” The Function of DC motor is Convert electrical energy in to mechanical energy.”

function of motor
fig: function of motor

Working Principle :

  • The Working Principle of DC motor is when current carrying conductor is kept in a Magnetic field a mechanical force acts on the conductor which tends to rotate it in direction of force.
  • The direction of force is given by Fleming’s left hand rule and the magnitude of this force is given by equation
fleming's left hand rule
fig: Fleming’s left hand rule

 F= BIL   N (Newton)

where , B= flux density in wb/m²

I= current in Ampere

L= length of conductor in meter

  • Fleming’s left hand rule is use to determine direction of force acting on the armature conductor of DC motor


  • “If you keep your left hand in a way that the thumb,first finger and middle finger are at right angle to each other” as shown in fig:
  • “first finger represent direction of magnetic field ,middle finger represent direction of the current in conductor, thumb will represent direction of force act on the conductor.”


fig: working principal of dc motor
fig: working principal of dc motor
  • Take two magnets in position of attraction (North to south) that magnet  that magnet produce the magnetic field between space of each other now keep the conductor or coil  between that two magnets ,When give dc supply to the conductor or coil . Hence current pass through that conductor and conductor will carrying the current . Because of that magnetic field of the magnet mechanical force is produce  on the conductor according to Fleming’s left hand rule. The direction of force is upward. conductor are wound on the rotor hence rotor will rotate in direction of force .motor starts working and its Convert electrical energy to mechanical energy.
  • What is Back EMF :-

When the armature of DC motor rotate in magnetic field its conductor cut by the magnetic field. According Faraday’s law of electromagnetic induction EMF will be induced in armature winding the direction of EMF is reverse to the direction of armature current , its called Back EMF.

             Back EMF:  Eb = øZNP/60A


  •  Z= No of conductor
  • P = No of pole
  • N =Speed of armature
  • ø = Flux
  • A = Parallel conductor for (Lap winding A=P)

(Wave winding A=2)

Construction :-

  • Different parts of  DC machine are following.
  1. Yoke
  2. Pole
  3. Field winding
  4. Armature winding
  5. Armature core
  6. Commutator
  7. Brush & brush holder
  8. Bearing
construction of DC machine
fig: construction of DC machine
Function of the parts:-
  • Yoke:- Its give the mechanical support of pole and field winding, its work as protective cover against mechanical damage. its provide a passage the magnetic flux produced by the poles.yoke are made by cast iron.
  • Pole :- Its consists of lamination of silicon steel or dynamo steel.the main function of pole is to produced the magnetic field.
  • Field winding:- The field coil are placed on the field pole , and its made by copper wire or strips,when current passes through that coil pole get magnetized and produced the flux,
  • Armature core:- Shape of core is cylindrical,its called rotor ,its made by soft iron stamping to reduced eddy current loss. armature conductor are placed in this core(slots of core).
  1. Its provide a path of low reluctance to the magnetic flux
  2. It house the armature winding
  • Armature winding :- Main  function of armature winding is produced the EMF in armature placed in the armature slots. Two types of armature winding are,
  1. Lap winding
  2. Wave winding
  • Commutator:- It is a mechanical rectifier. the main function of the commutator is to convert AC induced EMF in armature in to the DC EMF. its made by copper strips and separated by insulation.
  • Brushes & brush holder:- Function of the brush is to collect the current came through commutator, its made by carbon and its placed in the brush holder.
  • Bearing :- Bearing is give the smooth path to the rotor, and it is used to reduced friction losses.most of roller bearing are used.



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