How it works
The commutator converts the alternating EMF induced in armature conductors into DC at the brushes — this is the key function that separates a DC generator from an AC alternator. In a shunt generator, the field winding is connected across the armature terminals; the machine self-excites from residual magnetism if the field resistance is below the critical value. The external characteristic (terminal voltage vs load current) droops because V = E − IaRa and Ia = IL + If; as load increases, both armature drop and reduced flux lower terminal voltage. Adding a series winding (cumulative compound) compensates this droop.
Key points to remember
The armature winding types matter: lap winding gives number of parallel paths A = P (number of poles), while wave winding gives A = 2 always. Losses in a DC machine include copper losses (I²R in armature and field), core losses (hysteresis + eddy current in armature core), and mechanical losses (friction, windage). Efficiency η = output/(output + losses); maximum efficiency occurs when variable losses equal constant losses, typically at 90–95% of full load. Critical field resistance is the slope of the tangent to the OCC from the origin.
Exam tip
Every Anna University DC machines paper has a question on the EMF equation with lap vs wave winding — remember that A = P for lap and A = 2 for wave, since this single difference changes the calculated EMF significantly.