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Consider a D.C generator whose field coil is excited to produce a flux density distribution along the air gap and the armature is driven by a prime mover at constant speed as shown in figure

**EMF
Equation**

Consider
a D.C generator whose field coil is excited to produce a flux density
distribution along the air gap and the armature is driven by a prime mover at
constant speed as shown in figure

Let us
assume a p polar d.c generator is driven (by a prime mover) at n rps. The
excitation of the stator field is such that it produces a φ Wb flux per pole. Also
let z be the total number of armature conductors and a be the number of
parallel paths in the armature circuit. In general, as discussed in the earlier
section the magnitude of the voltage from one conductor to another is likely to
very since flux density distribution is trapezoidal in nature. Therefore, total
average voltage across the brushes is calculated on the basis of average flux
density Bav. If D and L are the rotor diameter and the length of the machine in
meters then area under each pole is . Hence average flux density in the gap is
given by

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