Armature Winding of Alternator

The Armature winding of an alternator is the connected arrangement of coils in which the output voltage is induced. In most alternators, the alternator’s armature winding is on the stator, and its phase windings are connected in star or delta.
The following properties describe common winding arrangements.

1. The two sides of a full-pitch coil are one pole pitch, or 180 electrical degrees, apart. A short-pitch coil spans less than one pole pitch.
2. The winding may use a single layer or a double layer of coil sides in each slot.
3. The coils are arranged among the slots to produce an emf waveform that is as close to sinusoidal as practical.

Types of Armature Winding of Alternator

Designers classify the types of armature winding used in alternators in several ways:

1. Single-phase and polyphase armature winding.
2. Concentrated winding and distributed winding.
3. Half-coiled and whole-coiled winding.
4. Single layer and double layer winding.
5. Lap winding, wave winding and concentric or spiral winding.
6. Full-pitch and fractional-pitch coil winding.

In addition, the armature winding of alternator may be an integral-slot or fractional-slot winding, according to the number of slots per pole per phase.

Single Phase Armature Winding

A Single phase armature winding can be concentrated or distributed.

Concentrated Armature Winding

In a concentrated winding, the coil sides for each pole are placed in one slot or a small group of slots. Concentrating the turns can give a high fundamental output voltage, but the resulting waveform contains more harmonics than a suitably distributed winding.

Figure 1 shows a simple single-phase winding with equal numbers of poles, slots and coil sides. One side of a coil occupies a slot under one pole, and the other side occupies a slot under the next pole. The coil connections make the induced emfs add in series.

This arrangement is called a skeleton wave winding. In Figure 1, coil side 1 under a north pole connects at the back to coil side 2 under a south pole, then at the front to coil side 3, and so on.
The induced emf directions alternate with the pole polarity, while the series connections make the coil-side emfs add at the terminals. Although the arrangement is simple, its end connections require considerable space. Multiturn coils reduce the number of separate end connections and provide a higher emf. Coils that occupy only half of the armature periphery form a half-coiled or hemitropic winding, as shown in Figure 2. Coils spread around the complete armature periphery form a whole-coiled winding.

Figure 3 shows a double-layer winding. One side of each coil lies in the upper part of a slot, and the other side lies in the lower part of another slot, as indicated by the dotted lines.

Distributed Armature Winding of Alternator

In a distributed winding, the conductors of each phase are spread across several slots under each pole. The fundamental emf is slightly lower than it would be with the same turns concentrated in one slot, but distribution provides several practical benefits:

1. It reduces selected harmonic components and improves the emf waveform.
2. It produces a more nearly sinusoidal magnetomotive-force distribution, which improves the effect of armature reaction.
3. Spreading the conductors among more slots supports more even cooling.
4. It uses the available slots around the armature periphery more evenly.

Lap Winding of Alternator

Figure 4 shows a full-pitch lap winding for a four-pole, 12-slot alternator with one conductor in each slot.
The back pitch is three conductor positions, equal to the number of conductors per pole, and the front connection advances the winding to the next coil. Sections completed under successive pole pairs are connected in series.

Wave Winding of Alternator

The wave winding in Figure 5 uses the same four-pole, 12-slot arrangement. Its end connections progress around the armature so that coil sides in corresponding positions under successive poles connect in series.

Concentric or Spiral Winding

Figure 6 shows a concentric winding for the same four-pole, 12-slot alternator. The coils share a common centre but have different spans. The outer coil pitch is five slots, the middle pitch is three slots and the inner pitch is one slot.

Poly Phase Armature Winding of Alternator

The following terms help explain a polyphase armature winding.

Coil Group

A coil group contains adjacent coils assigned to one phase under one pole. The total number of phase groups in a complete winding equals the number of poles multiplied by the number of phases.
Number of phase groups = number of poles × number of phases.

Balanced Winding

A winding is balanced when each phase has the same number of turns and the same winding arrangement, with phase axes equally separated in electrical angle. Under balanced conditions, the phase emfs have equal magnitudes and equal phase displacement.

Unbalanced Winding

A winding is unbalanced if corresponding phase windings do not have equal effective turns or equivalent spatial arrangements. In a two-phase alternator, the two phase axes are 90 electrical degrees apart.
In a three-phase alternator, the three phase axes are 120 electrical degrees apart.
The first figure shows a skeleton two-phase, four-pole winding with two slots per pole. Adjacent slots are separated by 180/2 = 90 electrical degrees.

Points a and b mark the starts of the first and second phase windings. Points a’ and b’ mark their respective finishes. The next figure shows a skeleton three-phase, four-pole winding with three slots per pole. Adjacent slots are separated by 180/3 = 60 electrical degrees. Points a, b and c mark the phase starts. Points a’, b’ and c’ mark the phase finishes.

In the illustrated connection, the red phase starts at slot 1 and ends at slot 10, the yellow phase starts at slot 2 and ends at slot 11, and the blue phase starts at slot 3 and ends at slot 12. Reversing the yellow-phase connections changes its voltage polarity. This gives the required 120-degree phase displacement between the terminal emfs of the three phase system.

For a four-pole, 24-slot, single-layer, full-pitch, three-phase distributed winding, the number of slots per pole per phase is 24/(4 × 3) = 2.
One pole pitch occupies 24/4 = 6 slots.
The electrical angle between adjacent slots is 180/6 = 30 degrees.
Hence, the slots are assigned to the phases as follows:

Slots No: 1, 2, 7, 8, 13, 14, 19, and 20 for R phase

Slots No: 5, 6, 11, 12, 17, 18, 23 and 24 for Y phase

Slots No: 3, 4, 9, 10, 15, 16, 21 and 22 for B phase

The figure below shows a three-phase, full-pitch, double-layer lap winding. The axes of the three phase windings are 120 electrical degrees apart. In a full-pitch winding, each coil spans one pole pitch, or 180 electrical degrees.
In a fractional-pitch winding, the coil span is less than 180 electrical degrees. For example, a coil may span 10 slots where a full pole pitch is 12 slots.
With a full-pitch coil, the two coil-side emfs are opposite in physical direction but add in the series-connected coil because the conductors run in opposite directions. The resulting coil emf is the arithmetic sum of the two equal magnitudes.
With a short-pitch coil, the two coil-side emfs are not exactly 180 electrical degrees apart before connection, so their connected emfs add as vectors. The fundamental coil emf is therefore slightly lower than for a full-pitch coil with the same turns. Short pitching also shortens the end connections and can suppress selected harmonics when the coil span is chosen for that purpose.

Integral Slot and Fractional Slot Winding

An integral-slot winding has a whole number of slots per pole per phase. A fractional-slot winding has a non-integer value.
A fractional-slot winding repeats its phase pattern only after several poles. This can limit the number of identical parallel paths because the phase groups within one complete repeating unit must first be connected in series. Fractional-slot arrangements may use single-layer or double-layer windings, depending on the machine design.