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.
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ReplyDeleteA 6 pole alternator has 36 slots, calculate the pole pitch if the machine runs at 375rpm
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