Why do cylindrical Alternators operate with steam turbines?

Steam turbines are found to operate at fairly good efficiency only at high speeds. The high speed operation of  rotors tends to increase mechanical losses and so the rotors should have a smooth external surface. Hence, smooth cylindrical type rotors with less diameter and large axial length are used for Synchronous generators driven by steam turbines with either 2 or 4 poles.

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1. Why almost all large size Synchronous machines are constructed with rotating field system type?

The following are the principal advantages of the rotating field system type construction of Synchronous machines:

· The relatively small amount of power, about 2%, required for field system via slip-rings and brushes.

· For the same air gap dimensions, which is normally decided by the kVA rating, more space is available in the stator part of the machine for providing more insulation to the system of conductors, especially for machines rated for 11kV or above.

· Insulation to stationary system of conductors is not subjected to mechanical stresses due to centrifugal action.

· Stationary system of conductors can easily be braced to prevent deformation.

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It is easy to provide cooling arrangement for a stationary system of conductors.

· Firm stationary connection between external circuit and system of conductors enable he machine to handle large amount of volt-ampere as high as 500MVA.

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