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.

GENERATOR STATOR CONSTRUCTION BASIC INFORMATION

The stator core is constructed from a stack of thin steel sheets or laminations which are stamped to shape and insulated electrically from each other, either by a thin coating or by an oxide layer which is produced during heat treatment.

The steel used has a low silicon content; this increases the resistivity of the steel and therefore reduces the losses due to eddy currents. The steel is carefully processed in order to minimize the hysteresis losses because the whole stator core is subjected to alternating magnetic flux.

In large turbogenerators the core is built up in segments and grain-oriented steel is used to reduce the losses further.

The stator windings are located in axial slots in the stator core which are formed by the shape of the laminations. Except in high-voltage machines, the individual coils of the winding are wound with copper wire covered with a layer of polyester/polyamide enamel which is about 0.05 mm thick.

The slots are lined with a tough insulating sheet, usually about 0.25–0.5 mm thick; a popular material is a laminate of Mylar and Nomex.

The coils are impregnated when in place with a resin to give the winding mechanical strength as well as to improve the heat transfer from the copper to the cooling air.

Windings operating at different voltages, such as the three phases, have a further sheet of insulating material separating them in the end-winding region. The mounting of the stator core in its frame differs according to the size of the machine, but for a majority of medium-sized generators, the arrangement is as shown in Fig. 5.18

Fig. 5.18 Cut-away section of an ac generator (courtesy of Newage International)

At either end of the frame are bearing housings which locate the rotor shaft. These housings or end bells are cast in smaller and medium size machines, and fabricated in larger sizes. The generator is often mounted directly onto the engine, and in this case it is usual to eliminate the drive-end bearing, using the rear bearing of the engine to locate the generator shaft.