Back EMF in DC Motor:-
DC motor has four main part - permiianent magnet, armature conductor, commutator and brushes . When providing the external supply to the armature conductor and then this armature conductor are placed in magmatic field the torque induced in the conductor. The torque rotates the armature conductor which cuts the flux of the magnetic field .
According to the electromagmatic phenomenon When The conductor cuts the magmatic field , EMF induces in the conductor. The Fleming right hand rule determine the direction of the induced EMF .
According to Fleming right hand rule
If we spreading a thumb index finger and middle finger of the right hand by an angle 90° then the thumb shows the motor of the conductor the field is given by the index finger and the direction of the back EMF induces in the armature of the conductor is given by the middle finger represent the emf induces on the conductor.
See more : Working principle of DC motor
See more : Working principle of DC motor
On applying this rule in the given image shows below, it is seen that the direction of the induced EMF is completely opposite to the supply voltage . Thereby the emf is known as the counter EMF or back EMF. The back emf is developed in series with the applied voltage bit opposite in direction i,e., the back EMF opposes the current which causes it.
The magnitude of the back EMF is given by the same expression shown below.
Where Eb is the induced emf of the motor known as Back EMF, A is the number of parallel paths through the armature between the brushes of opposite polarity. P is the number of poles, N is the speed, Z is the total number of conductors in the armature and Ï• is the useful flux per pole.
A simple conventional circuit diagram of the machine working as a motor is shown in the diagram below.
In this case, the magnitude of the back emf is always less than the applied voltage. The difference between the two is nearly equal when the motor runs under normal conditions. The current induces on the motor because of the main supply. The relation between the main supply, back emf and armature current is given as Eb = V – IaRa.
Advantages of back EMF :-
The presence of back emf 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.
Armature current (Ia),
When the motor is running on no load, small torque is required to overcome the friction and windage losses. Therefore, the armature current Ia is small and the back emf is nearly equal to the applied voltage.
If the motor is suddenly loaded, the first effect is to cause the armature to slow down. Therefore, the speed at which the armature conductors move through the field is reduced and hence the back emf Eb falls.
The decreased back emf allows a larger current to flow through the armature and larger current means increased driving torque.
Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the armature current is just sufficient to produce the increased torque required by the load.
If the load on the motor is decreased, the driving torque is momentarily in excess of the requirement so that armature is accelerated.
As the armature speed increases, the back emf Eb also increases and causes the armature current Ia to decrease. The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque required by the load.
Therefore, the back emf in a DC motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement.
Ia
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