A.C. Armature Windings of Alternator
A.C. Armature Windings:
A.C. armature windings are always of the non-salient-pole type and are usually symmetrically distributed in slots around the complete circumference of the armature. A.C. armature windings are generally open-circuit type i.e., both ends are brought out. An open-circuit winding is one that does not close on itself i.e., a closed circuit will not be formed until some external connection is made to a source or load. The following are the general features of a.c. armature windings:
(i) A.C. armature windings are generally distributed windings i.e., they are symmetrically distributed in slots around the complete circumference of the armature. A distributed winding has two principal advantages. First, a distributed winding generates a voltage wave that is nearly a sine curve. Secondly, copper is evenly distributed on the armature surface. Therefore, heating is more uniform and this type of winding is more easily cooled.
(ii) A.C. armature windings may use full-pitch coils or fractional-pitch coils. A coil with a span of 180° electrical is called a full-pitch coil. In this case, the two sides of the coil occupy identical positions under adjacent opposite poles and the e.m.f. generated in the coil is maximum. A coil with a span of less than 180° electrical is called a fractional-pitch coil. For example, a coil with a span of 150° electrical would be called a 5/6 pitch coil. Although e.m.f. induced in a fractional-pitch coil is less than that of a full-pitch coil, fractional-pitch coils are frequently used in a.c. machines for two main reasons. First, less copper is required per coil and secondly the waveform of the generated voltage is improved.
(iii) Most of a.c. machines use double layer armature windings. In a double layer winding, one coil side lies in the upper half of one slot while the other coil side lies in the lower half of another slot spaced about one-pole pitch from the first one. This arrangement permits simpler end connections and it is economical to manufacture.
(iv) Since most of a.c. machines are of 3-phase type, the three windings of the three phases are identical but spaced 120 electrical degrees apart.
A group of adjacent slots belonging to one phase under one pole pair is known as phase belt. The angle subtended by a phase belt is known as phase spread. The 3-phase windings are always designed for 60° phase spread.
Armature Winding of Alternator
- With very few exceptions, alternators are 3-phase machines because of the advantages of 3-phase service for generation, transmission and distribution. The windings for an alternator are much simpler than that of a d c. machine because no commutator is used.
- Fig. (1) shows a 2-pole, 3-phase double-layer, full pitch, distributed winding for the stator of an alternator. There are 12 slots and each slot contains two coil sides. The coil sides that are placed in adjacent slots belong to the same phase such as a1, a3 or a2, a4 constitute a phase belt. Note that in a 3-phase machine, phase belt is always 60° electrical. Since the winding has double-layer arrangement, one side of a coil, such as a1, is placed at the bottom of a slot and the other side - a1 is placed at the top of another slot spaced one pole pitch apart.
- Note that each coil has a span of a full pole pitch or 180 electrical degrees. Therefore the winding is a full-pitch winding. Note that there are 12 total coils and each phase has four coils. The four coils in each phase are connected in series so that their voltages aid. The three phases then may be connected to form Y or D-connection. Fig. (2) shows how the coils are connected to form a Y-connection.