What is Back 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 e.m.f. is induced in them
as in a generator. The induced e.m.f. acts in opposite direction to the
applied voltage V (Lenz’s law) and in known as back or counter e.m.f. Eb.
The back emf Eb(= PΦZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.
The back emf Eb(= PΦZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.
Back EMF in DC Motor
In a DC Motor, the
induced EMF of rotation of the armature is known as Back EMF or Counter
EMF. When the current is supplied to the armature conductors placed in
the main magnetic field, the torque develops, and the armature of the motor
rotates. The armature conductors cut the magnetic flux of the main magnetic
field. Therefore, the EMF of rotation Er is induced in the conductors.
The direction of the
induced EMF in the armature conductor is determined by Fleming’s Right Hand
Rule. It is shown in the figure below.
It can be seen in the
figure above that the direction of this induced EMF is opposite to the applied
voltage. This is the reason that this induced EMF in the armature, when the
machine works as a motor, is called Back EMF (Eb).
Since the back Emf is induced
due to the generator action, its magnitude is given by the same expression as
that for the generated EMF in a DC generator. It is expressed by the relation
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 ϕ is the useful flux per pole.
A simple conventional circuit diagram of the machine working as a motor is shown in the diagram
In this case the supply voltage is always
greater than the induced or back emf i.e. (V > Eb).
Therefore, the current is always supplied to the motor from the mains and the
relation among the various quantities will be given as Eb = V – IaRa.
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