What are Armature Windings?
Armature windings of alternators are 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 120° between the induced e.m.f.s in them.In a dc machine, winding is closed while in alternators winding is open i.e., two ends of each set of the winding are 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 emf in each set is called induced emf per phase denoted as Eph.All the coils used for one phase must be connected in such a way that their emf.s help each other. And overall design should be in such a way that the waveform of an induced emf is almost sinusoidal in nature.
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1) Conductor: The part of the wire, which is under the influence of the magnetic field and responsible for the induced emf 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 the below figure(a).
3) Coil: As there are a number of turns, for simplicity the number of turns are grouped together to form a coil. Such a coil is called a multi-turn coil. A coil may consist of single turn called single turn coil. Figure(b) shows a multi-turn coil.
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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 above figure(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 360° electrical of emf., 4 poles are responsible for 720° electrical of emf and so on. so 1 pole is responsible for 180° electrical of induced emf.
Key Point: So 180° electrical is also called one pole pitch.
Practically how many slots are under one pole which is responsible for 180° electrical, are measured to specify the pole pitch.
For example let us consider 2 poles, 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 for producing a phase difference of 180° between the emfs induced in different conductors.
This number of slots/pole is denoted as ‘n’.
Pole pitch = 180° electrical
= slots per.pole (no. of slots/P) = n
6) Slot angle (β): The phase difference contributed by one slot in degrees electrical is called slot angle As slots per pole contributes 180° electrical which is denoted as ‘n’, we can write,
1 slot angle = 180°/n
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 in between them.While if emf induced in the conductors which are placed in slots which are ‘n’ slots distance away, there will exist a phase difference of 180° in between them.
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Types of Armature Windings in Alternator:
The different types of armature windings in alternators are,
1) Single layer and double layer winding
2) Full pitch and short pitch winding
3) Concentrated and distributed winding
Let us see the details of each classification.
1)Single Layer and Double Layer Winding :
If a slot consists of only one coil side, winding is said to be a single layer. This is shown in figure(a). While there are two coil sides per slot, one, at the bottom and one at the top the winding is called double layer as shown in figure(b).A lot of space gets wasted in single layer hence in practice generally double layer winding is preferred.
2)Full Pitch and Short Pitch Winding:
As seen earlier, one pole pitch is 180° electrical. The value of ‘n’, slots per pole indicates how many slots are contributing 180° electrical phase difference. So if coil side in one slot is connected to a coil side in another slot which is one pole pitch distance away from the first slot, the winding is said to be full pitch winding and coil is called full pitch coil. For example, in 2 poles, 18 slots alternator, the pole pitch is n = 18/2 = 9 slots. So if coil side in slot No. 1 is connected to coil side in slot No. 10 such that two slots No. 1 and No. 10 are one pole pitch or n slots or 180° electrical apart, the coil is called full pitch coil. Here we can define one more term related to a coil called coil span.
Coil Span:
It is the distance on the periphery of the armature, between two coil sides of a coil. It is usually expressed in terms of number of slots or degrees electrical. So if coil span is ‘n’ slots or 180° electrical the coil is called 180° full pitch coil. This is shown in the figure to left. As against this if coils are used in such a way that coil span is slightly less than a pole pitch i.e. less than 180° electrical, the coils are called, short pitched coils or fractional pitched coils.Generally, coils are shorted by one or two slots.
So in 18 slots, 2 pole alternator instead of connecting a coil side in slot No 1 to slot No.10, it is connected to a coil side in slot No.9 or slot No. 8, the coil is said to be short pitched coil and winding are called short pitch winding.This is shown in the below figure.
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| Short pitch coils |
Advantages of Short Pitch Coils:
In actual practice, short pitch coils are used as it has following advantages,
1) The length required for the end connections of coils is less i.e. the inactive length of winding is less. So less copper is required. Hence economical.
2) Short pitching eliminates high frequency harmonics which distort the sinusoidal nature of e.m.f. Hence waveform of an induced e.m.f. is more sinusoidal due to short pitching.
3) As high frequency harmonics get eliminated, eddy current and hysteresis losses which depend on frequency also get minimised. This increases the efficiency.
3)Concentrated and distributed winding:
In three phase alternators, we have seen that there are three different sets of windings, each for a phase. So depending upon the total number of slots and number of poles, we have certain slots per phase available under each pole. This is denoted as ‘m’.
m = Slots per pole per phase = n/number of phases
= n/3 (generally no. of phases is 3)
For example in 18 slots, 2 pole alternator we have, 8
n = 18/2 = 9
and m = 9/3
So we have 3 slots per pole per phase available. Now let ‘x’ number of conductors per phase are to be placed under one pole. And we have 3 slots per pole per phase available. But if all ‘x’ conductors per phase are placed in one slot keeping remaining 2 slots per pole per phase empty then the winding is called concentrated winding.
Key Point: So in a concentrated winding, all conductors or coils belonging to a phase are placed in one slot under every pole.
But in practice, an attempt is always made to use all the ‘m’ slots per pole per phase available for distribution of the winding. So if ‘x’ conductors per phase are distributed amongst the 3 slots per phase available under every pole, the winding is called distributed winding. So in distributed type of winding all the coils belonging to a phase are well distributed over the ‘m’ slots per phase, under every pole.Distributed winding makes the waveform of the induced e.m.f. more sinusoidal in nature.Also in concentrated winding due to a large number of conductors per slot, heat dissipation is poor.
Key Point: So in practice, double layer, short pitched and distributed type of armature winding is preferred for the alternators.
Full pitch coils are to be used so if phase 1 says R is started in slot 1, it is to be connected to a coil in slot 7. so that coil span will be 6 slots i.e. ‘n’ slots i.e. 1 pole pitch.As distributed winding is to be used, both the slots per pole per phase (m = 2) available are to be used to place the coils. And all coils for one phase are to be connected in series.
So from slot No.7 we have to connect it to coil slot No.2 and slot No.2 second end to slot No.8 and so on.After finishing all slots per phase available under the first pair of pole, we will connect the coil to slot No.13 under next pole and winding will be repeated in a similar fashion. The starting end Rs and final end Rf winding for R-phase are taken out finally. Connections for R-phase only are shown in the below figure.
Now, we want to have a phase difference of 120° between ‘R’ and ‘Y’. Each slot contributes 30° as β = 30°.So start of ‘Y’ phase should be 120° apart from the start of ‘R’ i.e. 4 slots away from the start of R. So start of ‘Y’ will be in slot 5 and will get connected to slot No.11 to have full pitch coil. Similarly, the start of ‘B’ will be further 120° apart from ‘Y’ i.e. 4 slots apart start of ‘Y’ i.e. will be in slot No.9 and will continue similar to ‘R’.
Finally, all six terminals of three sets will be brought out which are connected either in star or delta to get three ends R, Y and B outside to get three phase supply. The entire winding diagram with star connected windings is shown in the below figure.
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Integral Slot Winding:
The value of slots per pole per phase decides the class of the winding.
m = slots / pole / phase
Key Point: When the value of m is an integer, then the winding is called Integral slot winding.
Consider 2 pole, 12 slots alternator hence,
n = slots / pole = 12/2 = 6
Pole pitch = 180° = 6 slots
m = n/3 = 6/3 =2
As m is an integer, the type of winding is integral slot winding. This winding can be full pitch winding or short pitch winding.
Let, the winding is full pitch winding. For integral slot winding, coils of one coil group lying under one pole pair are connected in series. Thus the end of the first coil is connected to start of the next coil lying to the right of the first coil.The alternate coil groups must be reversely connected such that EMI is induced in them is additive in nature.Any slot contains the coil sides which belong to the same phase.Such a winding is shown in the below figure.
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| Double layer integral slot winding |
If the short pitch coils are used for integral slot winding then in each group of the slots per pole phase, the coil sides of different phases exist.
Fractional Slot Winding:
This is another type of winding which depends on the value of m.
Key Point: The winding in which slots per pole per phase (m) is a fractional number is called fractional slot winding.
In such a winding, the number of slots (S) must be divisible by 3. Thus slots per phase is an integer which is necessary to obtain symmetrical three phase winding.But slots per pole (n) and slots per pole per phase (m) both are fractional. As n is a fraction, the coils cannot be full pitch. Thus if there are 54 slots and 8 poles then the slots per pole n = 54/8 = 6.75 hence coil span can be 7 or 6. Generally, short pitch coils are used.Such a fractional slot winding can be easily achieved with double layer winding.
In a balanced three phase winding, a basic unit under a pole pair (N and S) is repeated for remaining pole pairs where m is an integer. In fractional slot winding, the m is reduced to an irreducible fraction by taking out highest common factor in number of slots and poles.
Let S = Number of slots
P = Number of poles
then for a 3 phase winding,
where k = Highest common factor in S and P
Sk/Pk = Characteristic ratio
The number k indicates the number of repeatable units and number of possible parallel paths. The characteristic ratio indicates that there are Sk coils per phase distributed among Pk poles.Thus the winding is to be considered only of Pk poles out of P poles and for other poles it is repeated.
Similarly, the winding arrangement is to be considered for Sk slots out of total S slots and for other slots it is repeated.In a double layer winding, only the arrangement of the top layer is to be considered. This gets repeated in the bottom layer in which the corresponding coil sides are located one coil span away.
Advantages of Fractional slot Windings:
The various advantages of fractional slot winding are,
1. Though appearing to be complicated, easy to manufacture.
2. The number of armature slots (S) need not be an integral multiple of number of poles (P).
3. The number of slots can be selected for which notching gear is available, which is economical.
4. There is saving in machine tools.
5. High frequency harmonics are considerably reduced.
6. The voltage waveform available is sinusoidal in nature.
Conclusion:
Now here we have learnt armature windings in alternators and types of armature windings.You can download this article as pdf, ppt.
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