Power Plant Engineering

ELECTRICAL SYSTEM 389

12.2.2 Stators

Large stator housings comprise a series of annular rings flame-cut from steel plate, joined by
tubes and longitudinal bars, and carrying ribs to take the stator core laminations. Fig. 12.6 shows a
simple stator housing requiring two end plates and four intermediate plates, held apart by tie bars. The
core stampings are built up in the frame, the end plates places in position, and the whole stator clamped
together by bolts. The frame is then covered with sheet steel.

Assembly End Plate Centre Plate

Fig. 12.6. Method of Fabrication Turbo-Generator Stator Housing.
(a) Core. The active part of the stator consists of segmental lamina-tions of low-loss alloy steel.
The slots, ventilation holes and dovetails or dovetail keyholes, are punched out in one operation. The
stamp-ings are rather complicated on account of the number of holes and slots that have to be produced.

The use of cold-rolled grain-oriented steel sheet has possibili-
ties in machines as well as in transformers, most particularly in two -
pole machines where the major loss occurs in the annular part of the
core external to the slotting. Here the flux direction is mainly circum-
ferential, and by cutting the core- plate sectors in such a way that the
pre-ferred flux direction is at right-angles to their central radial axis,
Fig. 12.7, a sub-stantial reduction in core-loss can be secured.

It is of great importance that the assembled stator laminations
are uniformly compressed during and after building, and that the slots
are accurately located. The core plates are assembled between end
plates with fingers projecting between the slots to support the flanks
of the teeth. The end plates are almost invariably of non-magnetic
material, for this greatly reduces stray load loss. The end packets of
core plates may be stepped to a larger bore for the same reason.

(b) Windings. The windings of two-pole machines are comparatively straightforward. The number
of slots must be a multiple of 3 (or 6 if two parallel circuits are required). Single-layer concentric or two-
layer short-pitched windings may be used.

The single-layer concentric winding is readily clamped in the overhang, but causes a higher load
loss because the end-connections run parallel to the stator end-plates. Chording is not possible so that
flux harmonics have full effect.

The two-layer winding is more common, chorded to about a 5/6 pitch which practically elimi-
nates 5th and 7th harmonics from the open-circuit e.m.f. wave. The end windings are packed, and
clamped or tied with glass cord.

It is the invariable practice with two-layer windings to make the coils as half turns and to joint the
ends. The conductors must always be transposed to reduce eddy-current losses. The conductors are

Flux

path

Core

segment

Grain

orientation

Fig. 12.7. Use of Grain-Oriented

Steel.