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.