Explain the Cylindrical or Round Rotors?

All synchronous machines have a similar stator design and it is actually the rotor construction that makes them different. The rotor is also made of punched laminations that are stacked together for the same reasons as in the stator case. There are the two main types of rotor design. For high-speed operation, the 2-pole or 4-pole cylindrical or round rotors are commonly used with slots on their periphery to house the field windings (for 50-Hz supply, the respective speeds are 3000-rpm and 1500-rpm as indicated by the above expression). Figs.2 and 3 show the construction and winding arrangement of such rotors.

Figure: 4-pole round rotor with a field winding

Figure: Finite Element (FE) models and structure of 8-pole diesel generator (left) and 2-pole turbo-generator (right)

For this machine type the air-gap is approximately constant (ignoring slotting effects). If operated as a generator, the prime mover is normally a steam or gas turbine rotating at 22000-rpm coupled through a speed reduction gearbox or, for smaller machines, a direct-coupled diesel engine. Due to high operating speeds and consequent relatively large centrifugal forces, there is normally a design limit imposed on the rotor diameter. Hence rotor length to diameter ratios of, say, 6:1 are typical for round rotors with the axis of rotation being always horizontal.