Motor-Basic Principle of DC Motor

DC Motor Principle

A machine that converts dc power into mechanical energy is known as dc motor. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. The direction of the force is given by Fleming’s left hand rule.

Fleming Left Hand Rule:

Back or Counter EMF :

When the armature of a d.c. motor rotates under the influence of the driving torque, the armature 

conductors move through the magnetic field and hence an e.m.f. is induced in them. The induced e.m.f. acts in opposite direction to the applied voltage V(Lenz’s law) and is known as back or 

counter e.m.f. Eb

.

Significance of Back E.M.F :

The presence of back e.m.f. makes the d.c. motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load.Back e.m.f. in a d.c. motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement.

DC Motor Types :

• Shunt Wound

• Series Wound

• Compound wound

Shunt Wound Motor :

In shunt wound motor the field winding is connected in parallel with armature. The current through the shunt field winding is not the same as the armature current. Shunt field winding's are designed to produce the necessary m.m.f. by means of a relatively large number of turns of wire having high resistance. Therefore,shunt field current is relatively small compared with the armature current

Series Wound Motor :

In series wound motor the field winding is connected in series with the armature. Therefore, series field winding carries the armature current. Since the current passing through a series field winding is the same as the armature current, series field winding's must be designed with much fewer turns than 

shunt field winding's for the same mmf. Therefore, a series field winding has a relatively small number of turns of thick wire and, therefore, will possess a low resistance.

Compound wound motor :

Compound wound motor has  two field winding's; one connected in parallel with the armature and the other in series with it. There are two types of compound motor connections 

1) Short-shunt connection

2) Long shunt connection

When the shunt field winding is directly connected across the armature terminals it is called short-shunt connection. 

Compound Wound Motor :

When the shunt winding is so connected that it shunts the series combination of armature and series field it is called long-shunt connection. 

Parts of a DC Motor :

A simple motor has six parts :

• Armature/ Rotor 

• Commutator

• Brushes

• Axle

• Permanent Magnet

• DC Power supply

Commutation in DC Motor :

In order to produce unidirectional force (or torque) on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times.The function of commutator and brush gear in a dc motor is to cause the reversal of current in a conductor as it moves from one side of a brush to the other.

Voltage Equation of Motors :

Let in a d.c. motor 

V = applied voltage

Eb= back e.m.f.

Ra= armature resistance

Ia= armature current

Since back e.m.f. Eb

acts in opposition to the applied voltage V, the net voltage across the armature circuit is V-Eb. The armature current Ia is given by 

 

This is known as voltage equation of the d.c. motor