D.C motor can be classified into three types:

  1. Series motor

  2. Shunt motor

  3. Compound wound motor


1. Series Motor 


In the case of series motors, field windings are series connected with armature windings. As field winding has more cross sectional area and less number of turns, hence series field resistance is less than armature resistance.

Load current is equal to field current and armature current. 

IL=Iaor Ise

Since there is a load drop on armature and field windings, the voltage equation becomes 

V=Eb+Ia.(Rse+Ra)

Flux is directly proportional to the series current.


In case of serie generator the armature current is reversed hence the equation will be:

V=Eb-Ia(Rse+Ra)


2. Shunt motor 


In case of shunt motors, field winding is connected with armature winding in parallel. As the field winding has less cross section area and more turn than armature coil, the shunt field resistance is more than the armature field resistance. 

Load current is equal to sum of shunt current and armature current. 

IL=Ish+Ia

Same voltage develops between the armature and shunt winding. The voltage equation becomes,

V=Eb+IaRa

The flux produced in the field winding is directly proportional to the current passing through the field winding resistance. 



Compound motors


Compound motors have both series and parallel field windings connected to the armature windings. They are basically classified into long shunt and short shunt compound motors.


Principle of DC motor 


When a current carrying conductor is placed perpendicular to a magnetic field, a force is created which is perpendicular to the magnetic field and current direction of the wire. The greater the current in the wire, or greater the magnetic field the faster the wire moves because of the greater force created.  The force can be found using Fleming's left hand rule.


Force acting on the conductor varies directly with 

  • The strength of the magnetic field 

  • The strength of the current in the conductor

  • The length of the conductor in the magnetic field. 


Force = strength of field X current X length of the conductor


F=BIL,


B= Flux density

I= Current 

L= Length of the conductor