Dec 8, 2014

Back EMF or Counter EMF in DC Motor

As soon as the armature of a DC motor revolves under the impact of the driving torque, the armature conductor’s move over the magnetic field and thus e.m.f. is induced in them as in a generator. The induced e.m.f. acts in opposite way to the applied voltage V (Lenz’s law) and in referred to as back or counter e.m.f. Eb. The back e.m.f. Eb = (PΦZN/60A) is all the time a smaller value than the applied voltage V, though this distinction is little, once the motor is running under normal conditions.

Think through a shunt wound motor, which is shown in Fig. When DC voltage V is applied across the motor terminals, the field magnets are excited and armature conductors are supplied with current. Consequently, driving torque acts on the armature that begins to rotate. Because the armature rotates, back e.m.f. Eb is induced that opposes the applied voltage V. The applied voltage V has to force
current through the armature against the back e.m.f. Eb. The electrical work done in overcoming and inflicting the current to drift against Eb is converted into mechanical energy developed within the armature. It follows, as a result, that energy conversion in a DC motor is barely potential owing to the production of back e.m.f. Eb.
Net Voltage across armature circuit = V-Eb
If Ra is the armature circuit resistance, then Ia = V-Eb/Ra
Since V and Ra are typically mounted, the value of Eb can determine the current drawn by the motor. If the motor speed is high, then back e.m.f. Eb = (PΦZN/60A) is giant and thus the motor can draw less armature current and vice- versa.


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