The rotor and stator (field and armature) of a synchronous machine must have the same number of poles, as the magnetic interaction is between a succession of north-south magnetic-field pole pairs. The number of pole pairs for a machine will be noted as p.
The relationship between electrical frequency fe and mechanical speed N is
fe = pN/60
where P is number of poles (not number of pole pairs).
Synchronous generators are built in two elementary forms:
• Round-rotor machines are constructed with a rotor consisting of a cylinder of magnetic steel. In modern generators, the cylinder is formed from a single forging of vacuum degassed steel. The field winding is contained in radial slots in the surface of the rotor.
Round-rotor machines usually have two or four poles as illustrated in Figs. 7-2 and 7-3 respectively. The diameter of the rotor of a typical 25-MW generator is about 700 mm. The diameter of a 2000-MW generator can approach 2 m.
• Salient-pole machines are constructed with a number of pole pieces mounted to a central rotor shaft. The rotor pole pieces can be solid steel or assemblies of steel plates that are bound together axially with bolts.
The diameter of the rotor can range from less than 1 m in smaller salient pole generators to nearly 20 m in the largest hydroelectric generators.
In both round-rotor and salient-pole generators, the magnetic flux passing through the rotors does not vary in time, and the magnetic flux passing through the stator core does vary periodically in time at the electrical line frequency.
Consequently, the rotors can be made of solid steel, but the stator cores must be made of thousands of thin layers of highly permeable electrical steel. Each layer of stator core steel is coated with a thin layer of electrical insulation.
For electric utility operation, in which generation takes place at 50 or 60 Hz, mechanical speed is inversely proportional to the number of poles. Thus, 2-pole machines, which turn at 3000 or 3600 r/min, are used for most fossil-(fuel)-fired steam turbine generators which require high shaft speeds.
Most nuclear steam turbine generators, which have a lower shaft speed requirement, employ 4-pole designs and therefore turn at 1500 or 1800 r/min. Turbine generators for both fossil and nuclear power plants are typically round-rotor designs.
Hydroelectric generators, which typically have much lower shaft speeds than turbine generators and consequently require a large number of poles, are generally built as salient-pole machines. This is true also for generators intended for operation with large reciprocating engines, such as medium-speed diesels.
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