GENERATOR ROTOR CONSTRUCTION BASIC INFORMATION

It has already been noted that the construction of a turbogenerator is very specialized and the rotor for these machines are not dealt with here. However, even within the class of salient-pole generators, quite different forms of rotor construction are used, depending upon the size.

Generators rated up to about 500 kW use rotor laminations which are stamped in one piece. In larger machines the poles are made separately from stacks of laminations, and each pole is keyed using a dovetail arrangement onto a spider which is mounted on the rotor shaft.

In large high-speed machines the poles can be made from solid steel for extra strength and to reduce mechanical distortion; these solid poles are screwed to the shaft, as shown in the large 4-pole machine in Fig. 5.23.

Fig. 5.23 Large salient-pole rotor (courtesy of Brush Electrical Machines)

The nature of the rotor coils also depends upon the size of the machine. Because the ratio of surface area to volume is larger in the coils of small generators, these are easier to cool.

Generators rated above about 25 kW therefore use a ‘layer-wound’ coil in which each layer of the coil fits exactly into the grooves formed by the layer below.

Rectangular cross section wire can be used to minimize the coil cross section. The simplest and cheapest way to make the coils, often used in smaller machines, is to wind them in a semi-random way.

In either case, the coils are impregnated after winding like the stator windings to give extra mechanical strength and to improve the heat transfer by removing air voids within the coil. The coils are under considerable centrifugal stress when the rotor turns at full speed, and they are usually restrained at both ends of the pole by bars, and by wedges in the interpole spaces, as shown in Fig. 5.23.