ARMATURE SLOTS AND COIL DESIGN BASIC INFORMATION

The depth of an armature slot is limited by several factors, including the tooth density, eddy losses in the armature conductors, available core depths, and commutation. For reasonable frequencies (up to 50 Hz on medium and large dc machines), slots about 2 in deep can ordinarily be used.

Acceptable slot pitches range from 0.75 to 1.5 in. Small machines have shallower slots and a lower range of slot pitches. For medium and large machines, a reasonable tooth density usually results if the ratio of slot width to slot pitch is about 0.4.

Eddy losses in the conductors can be large compared with their load I 2R losses. Sometimes these must be reduced by making each armature conductor from several strands of insulated copper wire.

The number of strands and their size depend on the frequency and the total depth of the conductor. An approximate formula for reasonable eddy losses is

No. of strands = (0.168) (f ^0.83)(de^ 0.4)

where f is the frequency in hertz, (r/min # poles)/120, and dc is the total depth of a conductor.

The insulation space required depends on the type used. Typical conductor strands have about 0.018 in of glass strands and varnish total. Mica wrappers, binding tapes, and varnish and slot finish allowance (0.010 in) total about 0.085 in on the coil width.

If the space for the wedge and its retainer is included, the two coils depth wise total about 0.315 in.

Approximate Slot Design

Width [see text preceding and 0.563 in

following Eq. (8-17)] 0.4 # 1.407

Depth 2.0 in

Approx. total cond. depth 0.875 in

Frequency 42.8 Hz

No. of strands/conductor [Eq. (8-17)] 3

Slot width, in Depth, in

Approx.

Size 0.563 in 2.000 in

Insulation 0.139 (0.085 # 0.054) 0.423 (0.315 # 0.108)

Bare copper 0.424 in 1.577 in

Strand size 0.141 in 0.263 in

Use 3 (0.144 0.289) in strands/conductor

Use available slot 0.570 in 2.250 in

ARMATURE AND FIELD WINDING INSULATION OF ALTERNATING CURRENT (AC) GENERATORS

Electrical insulation is used to isolate field conductors from each other and ground, and in the armature winding to isolate strands and turns from each other and the whole winding from ground. Proper

application of electrical insulation constitutes much of the art of ac machine design, particularly in

the larger generator sizes.

Armature-Winding Insulation

Armature voltages range from about 220 V to about 27 kV. With such a wide range, different techniques are employed. In the armature, insulation is for strands, turns, and ground wall.

Strand insulation is required to prevent circulating currents within a conductor bar. The voltage levels are not high so mechanical integrity is the important feature of strand insulation. This is usually a layer of served fabric or film coating.

Turn insulation is used in multiturn coils, generally applicable only in small-size generators. This insulation is required to withstand turn-turn voltage, although in some cases large transient spikes of voltage may be incident on the winding.

Ground wall insulation must withstand full voltage to ground. Typically, the whole of an armature winding is insulated for full voltage, even though some of the coils, located near the neutral end of the winding, see lesser voltage.

In high-voltage armatures (above ~5 kV), some measures must be taken to control the effects of corona and partial discharge. In the slot portion of the coil, it is necessary to prevent discharges due to capacitive coupling through the insulation, from the surface of the insulation to the grounded stator core.

These discharges are prevented by coating the outer surface of the insulated conductor with a conductive (sometimes called semiconducting) coating (paint or tape). To prevent discharges along the surface of the conductors in the end windings, those sections are sometimes coated with very weakly conducting coatings that are called grading coatings (paint of tape).

It is important to prevent electrical discharges in the vicinity of the winding because such discharges through air and in the presence of any water vapor will produce nitrous and nitric acid and ozone, substances corrosive to the materials of the winding.

Field-Winding Insulation

Field windings operate at much lower voltages (usually less than ~800 V). Some transient conditions, such as interruption of field current, can lead to much higher voltages.

Field windings are subject to the centrifugal forces due to rotation, and this presents special challenges. Dimensional stability is required of the field winding to prevent dynamic rotor imbalance.

It is also necessary, in larger machines, to allow the field winding to expand thermally with respect to the rotor steel. The resulting “creepage” surfaces must allow slip in the axial direction but not movement in the other directions.

STRANDING AND TRANSPOSITION OF AC GENERATORS ARMATURE WINDINGS BASIC AND TUTORIALS

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.