Why do many DC generators have four to eight field poles?

Direct-current (DC) generators are typically constructed with four to eight field poles for two primary design reasons:

• Efficiency in construction: Using multiple poles helps to shorten the end connections on the rotor coils, which simplifies the physical design of the machine.1
• Material optimization: This configuration reduces the total amount of magnetic iron required in the stator, making the generator more efficient in terms of material usage.1

In these machines, the number of stationary brushes bearing on the commutator generally corresponds to the number of poles, though some designs may utilize only two.1