Armature winding of
alternators is different from that of d.c. machines. Basically three
phase alternators carry three sets of windings arranged in the slots in
such a way that there exists a phase difference of 120o
between the induced e.m.f.s. in them. In a d.c. machine, winding is
brought out. In three phase alternators winding is open i.e. two ends of
each of set of winding is brought out. In three phase alternators, the
six terminals are brought out which are finally connected in star or
delta and then the three terminals are brought out. Each set of windings
represents winding per phase and induced e.m.f. in each set is called
induced e.m.f. per phase denoted as Eph. All the coils used
for one phase must be connected in such a way that their e.m.f.s. help
each other. And overall design should be in such a way that the waveform
of an induced e.m.f. is almost sinusoidal in nature.
1.1 Winding Terminology
1) Conductor : The part of the wire, which is under the influence
of the magnetic field and responsible for the induced e.m.f. is called
active length of the conductor. The conductor are placed in the armature
slots.
2) Turn : A conductor in one slot, when connected to a conductor
in another slot forms a turn. So two conductors constitute a turn. This
is shown in the Fig. 1(a).
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| Fig. 1 |
3) Coil : As there are number of turns, for simplicity the number
of turns are grouped together to form a coil. Such a coil is called
multiturn coil. A coil may consists of single turn coil. The Fig. 1(b)
shows a multiturn coil.
4) Coil side : Coil consists of many turns. Part of the coil in each slot is called coil side of a coil as shown in the Fig. 1(b).
5) Pole Pitch : It is centre to centre distance between the two
adjacent poles. We have seen that for one rotation of the conductors, 2
poles are responsible for 360o electrical of e.m.f., 4 poles are responsible for 720o electrical of e.m.f. and so on. So 1 pole is responsible for 180o electrical of induced e.m.f.
Note : So 180o electrical is also called one pole pitch.
Practically how many slots are under one pole which are responsible for 180o electrical, are measured to specify the pole pitch.
e.g. Consider 2 pole, 18 slots armature of an alternator. Then
under 1 pole there are 18/2 i.e. 9 slots. So pole pitch is 9 slots or
180o electrical. This means 9 slots are responsible to
produced a phase difference of 180 between the e.m.f.s induced in
different conductors.
This number of slots/poles is denoted as 'n'.
6) Slot angle (β) : The phase difference contributed by one slot in degrees electrical is called slot angle β.
As slots per pole contributes 180o electrical which is denoted as 'n', we can write,
... 1 slot angle = 180o/n
In the above example, n = 18/2 = 9, while β = 180o/n = 20o
Note : This means that if we consider an induced e.m.f. in the
conductors which are placed in the slots which are adjacent to each
other, there will exist a phase difference of βo in between
them. While if e.m.f. induced in the conductors which are placed in
slots which are 'n' slots distance away, there will exist a phase
difference of 180oin between them.
