Three Phase Winding in A.C. Machines

Three Phase Winding in A.C. Machines

In a three phase alternators, three different sets of windings are arranged in the armature (stator) slots in such a way that there exists a phase difference of 120° between the induced e.m.f.s in them. Each set of windings is for each phase of three phase induced e.m.f. Thus the three phase induced e.m.f has three different e.m.f.s given by>

e_R      =       Em    sin(rot)        ...       R phase

e_Y      =       Em    sin(rot-120°)        ...       Y phase

e_B      =       Em    sin(rot-240°)        ...       B phase

Induced e.m.f. in each set of winding is called phase e.m.f. denoted as Eph- All the coils used for one phase are connected in such a way that their e.m.f.s help each other. And overall arrangement of three phase windings is such that the waveform of three phase induced e.m.f. is sinusoidal in nature.

1. Winding Terminologies

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 conductors 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 Fig. 1.10.1 (a).

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 multitum coil. A coil may consist of single turn called single turn coil. The Fig. 1.10.1 (b) shows a multitum 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. 1.10.1 (b).

5) Pole Pitch : It is center to center distance between the two adjacent poles. We have seen that for one rotation of the conductors, 2 poles are responsible for 360° electrical of e.m.f., 4 poles are responsible for 720° electrical of e.m.f. and so on. So 1 pole is responsible for 180° electrical of induced e.m.f. So 180° electrical is also called one pole pitch. Practically how many slots are under one pole which are responsible for 180° 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 180° electrical. This means 9 slots are responsible to produce a phase difference of 180° between the e.m.f.s induced in different conductors.

This number of slots/pole is denoted as 'n’.

Pole pitch = 180° electrical

= Slots per pole (number of slots/P) = n

6) Slot Angle (3) : The phase difference contributed by one slot in degrees electrical is called slot angle β

As slots per pole contributes 180° electrical which is denoted as 'ri, we can write,

1 slot angle = 180° / n   i.e. β =         180° / n

In the above example,

n = 18/2 = 9,

while β = 180° / n = 20°

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 β° 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 180^o in between them.

Review Question

1. Define turn, coil side, pole pitch and slot angle for armature winding of an alternator.