Let in a d.c. motor (See Fig. 4.3), 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; Ia = (V – Eb)/Ra or V = Eb + IaRa ……………………………..(i) This is known as voltage equation of the d.c. motor. Power Equation If Eq.(i) above is multiplied by Ia throughout, we get, VIa = EbIa +I2aRa VIa= electric power supplied to armature (armature input) EbIa = power developed by armature (armature output) I2aRa = electric power wasted in armature (armature Cu loss) Thus out of the armature input, a small portion (about 5%) is wasted as a I2aRa read more

If you enjoyed this post, make sure you subscribe to my RSS feed!

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 e.m.f.  Eb (= P Φ ZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions. Consider a shunt wound motor shown in Fig. (4.2). When d.c. 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 e.m.f. Eb is read more

If you enjoyed this post, make sure you subscribe to my RSS feed!