A wide variety of winding types may be used to produce a desired voltage with the desired number of phases and a suitable waveshape. In small generators, “scramble wound” armature windings may be used.

However, in most alternator applications, double-layer, form-wound coils in open slots with 60° phase belts are used. In such a winding, each slot has two conductor bars (often called halfcoils), not necessarily from the same phase winding.

These bars are insulated from ground and secured in the slot, usually by wedges. It is usually necessary for the bar to have the ability to slide axially in the slot to accommodate thermal expansion, but it must not be loose in either the radial or azimuthal directions. This has led to a number of proprietary techniques for armature construction.

Winding Forms
Figure 7-20 shows an example winding diagram. For the purposes of this figure, the machine is shown “rolled out flat,” with the dotted lines on either side representing the same azimuthal location. In this case, the machine has 24 slots, each with two half-coils, as shown in the slot allocation section of the drawing, at the bottom of the figure.

FIGURE 7-20 Armature in 24 slots, 5/16 pitch.

The upper part of the figure shows how one phase of the winding would be laid out. This drawing shows a lap type winding (the most commonly used) with a 5/6 pitch. In a 24-slot, 2-pole winding a full-pitch coil would span 12 slots, while in the 5/6 pitch winding the coils span 10 slots.

Fractional Slot Windings.
Fractional slot windings, in which the number of slots per pole per phase is not an integer, have coil groups that differ from one another. These can be arranged to produce balanced voltages under circumstances that are beyond the scope of this discussion.

Stranding and Transposition
At power frequencies (50 or 60 Hz), the skin depth in copper is on the order of 1 cm so that it is usually necessary to subdivide armature conductors into a number of parallel strands. In form-wound coils, these strands are usually rectangular to allow for good space factor.

To prevent circulating currents between parallel strands, it is necessary to employ transposition to ensure that voltages induced in each strand are approximately the same.

The simplest form of transposition, often used in transformers and sometimes in generators, is to twist the armature conductors at 180° in the end turns. Or sometimes, groups of conductors are connected together in the end turns with a progressive transposition that constitutes a “twist” of the winding from half-coil to half-coil.

Transposition of strands in the end turns is generally not satisfactory in large ac generators. A transposition scheme attributed to Roebel is usually used (see Fig. 7-21). The Roebel transposition is equivalent to a twist of the conductors in the slot. It is usual to carry out the Roebel transposition only within the slot part of the winding.

FIGURE 7-21 Illustration of Roebel transposition: (a) typical offset conductor strand; (b) group of conductor strands composing half the conductor; (c) complementary group; (d) assembly.

A variety of transpositions are used, including 180°, 360°, and 540°. The first two are effective only at eliminating circulating currents due to flux crossing the slot, but do not compensate for flux in the end windings.

The 540° transposition, attributed to Ringland and Rosenberg (1959) is often applied because it filters out most of the circulating currents in a bar. Other more complex Roebel transposition arrangements that more extensively filter out circulating currents are possible but are rarely used because of manufacturing complexity.

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