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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 backor 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.

Back emf in dc motor

A shunt wound dc motor shown in figure below. When dc voltage V is applied across the motor terminals, the field magnets are excited and armature conductors are supplied with current. Therefore, driving torque acts on the armature which begins to rotate. As the armature rotates, back emf Eb is induced which opposes the applied voltage V.

image

The applied voltage V has to force current through the armature against the back emfEb. The electric work done in overcoming and causing the current to flow against Eb is converted into mechanical energy developed in the armature. It follows, therefore, that energy conversion in a d.c. motor is only possible due to the production of back emf.

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