DC motor is a electrical machine which converts electrical energy into mechanical. There are three types of dc motors which are employed at various industrial applications. The dc motors are classified on the basis of connection of field winding with respect to armature. Most important application of dc motor found in traction system. Dc series motors are most suitable for traction system.
Working Principle of DC Motor
When a current carrying conductor placed in a magnetic field a mechanical force experienced by the conductor. The twisting movement of shaft about its axis due to force produced by interaction of field produced by current carrying conductor in armature and field produced by field winding is called torque. This type of toque is called electromagnetic torque.
Pole-N and S are field magnets. It may be electromagnet or permanent magnet. The armature conductor is carrying current and placed in the magnetic field. Interaction between field magnet and field produce by armature conductor producing a driving torque.
The e.m.f induced in armature conductor due to flow of direct current which opposes the applied voltage is called back e.m.f or counter e.m.f.
Consider a shunt wound motor connected to the dc supply. A dc current start flowing through the armature conductor. Field magnets are also energized by dc supply which produces a magnetic field. When the interact armature field and main field driving torque is produced. As the armature rotates, back e.m.f is produced which opposes the applied voltage V.
Net voltage across armature circuit = V – Eb
Armature current Ia = V-Eb/Ra
Significance or Importance of Back EMF
The presence of back e.m.f makes the dc machine self regulating machine. The armature resistance is very small. If it carries the large current it will burn out. So the back e.m.f makes the motor to draw as much armature current as it just adequate to develop the torque required by the load
Armature current, Ia = V-Eb/Ra
Types of DC Motors
The DC motors are divided into three types according to their connections of field winding in relation to the armature.
- DC series motor
- DC shunt motor
- DC compound motor
DC compound motor further into two types
- Cumulative compound motor
- Differential compound motor
DC Series Motor OR Series Wound Motor
In DC series motor the armature and field winding connected in series. The current flowing through the field winding is the same as that in the armature, if the mechanical load on the shaft increase the armature current also increases. The resultant increase in magnetic flux causes reduction in speed.
DC Shunt Motor or Shunt Wound Motor
DC shunt motor in which the field winding is connected in parallel with the armature. The shunt winding has fine and large number of coil. The shunt winding carries the constant current from the supply main.
Armature current, Ia = Ia – Ish
Shunt current, Ish = V/Rsh
DC Compound Motor or Compound Wound Motor
There are two types of DC compound motor or compound wound motors. Compound motors are those motors which has two field winding. One winding is connected in series with the armature and the other winding is connected in parallel with the armature series winding has less number of turns having large cross sectional area. Where as the shunt winding has more turns having small cross sectional area. There are two types of Compound Wound Motors.
- Cumulative compound wound motor
- Differential compound wound motor
Commulative Compound Wound Motor
When the shunt field winding directly connected across the armature terminals is called Cumulative Compound Motor.
Differential Compound Wound Motor
When the shunt field winding is connected in such a way that it shunts the series combination of armature and series field, it is called Differential Compound Motor.