D.C. Generator

A magnetic field is produced by afield coil supplied with a D.C. current and a rotor is placed between shaped pole pieces and wound with an armature winding. The diagram above shows only one turn of the armature winding, but in practice there will usually be more than one. The ends of the armature winding are connected to two slip rings that rotate with the armature and the voltage generated is 'picked off' the rings by brushes that bear against the slip rings

The emf generated may be calculated either using the equation for the emf induced in a conductor of length l moving

in an electric field of flux density B webers/m ²

e=B.l.v

with a velocity v m/sec or by considering the rate of change of flux linkage with the armature coil. We shall use the latter method. Due to the shaping of the pole pieces, the flux threading through a particular armature  coil  is  constant  at  +  max   as  it turns from O₄   to  O₁ and decreases linearly from +Ø_max  to  -Ø_min  while  the  rotor  is turning from  O₁ to O₂ . It is constant again during O₂ to   O₃ and then increases linearly during  O₃to O₄. (ref graph)