Salient pole vs cylindrical rotor alternators

The frequency of an alternator is $f = \frac{PN_s}{120}$. For $f = 50$ Hz the speed is $N_s = \frac{6000}{P}$. A low speed therefore needs many poles, and a high speed needs few poles.

Salient-pole alternator (low speed)

• Used with hydraulic turbines (100 to 400 rpm), engines, and wind turbines, which need many poles (e.g. $P = 24$ at 250 rpm).
• Many poles must fit on the rotor, so it has a large diameter and short axial length, with projecting poles.
• At low speed, the centrifugal force and windage loss are small, so the projecting poles are mechanically safe.

Cylindrical (non-salient) alternator (high speed)

• Used with steam or gas turbines at 1500 or 3000 rpm, with 2 or 4 poles.
• At high speed, the large-diameter projecting poles would suffer large centrifugal force and high windage loss and noise.
• A smooth cylindrical rotor of small diameter and long axial length (slotted forged steel with distributed winding) withstands the centrifugal stresses and gives low windage loss and a better sinusoidal flux distribution.

So the number of poles needed and the mechanical limits decide the type: salient poles for low-speed, many-pole machines, and cylindrical rotors for high-speed, 2- or 4-pole turbo-alternators.