Alternator Windings:
ere we get detailed information about alternator(Synchronous Machines) windings types.Armature winding in an alternator may be either closed type open type. Closed winding forms star connection in armature winding of alternator.but here basic DC windings also included for your knowledge.Windings in Alternators:
DC Windings:-
(i) Lap winding,
(ii) wave winding and
(iii) mush winding.
Based on pitch of the coil
(i) full pitched
(ii) short pitched windings
Based on number of layers
(i) Single layer
(ii) Double layer
Lap winding:-
In a lap winding, the number of parallel paths, a, is always equal to the number of poles, p, and also to the number of brushes.When two adjacent commutator bars make contact with a brush, one coil is shorted by the brush in the lap winding.
Wave winding:-
Coils are laid out in a wave pattern and cross all the poles.In wave windings, the number of parallel paths, a,is always two (2), and there may be two or more brush positions.When two adjacent commutator bars make contact with a brush, p/2 coils are shorted by the brush in the wave winding.Because wave windings have only two parallel paths, this winding is more suited for high voltage, low-current dc machines.
Based on number of layers:-
Single layer winding: Only one coil side placed in one slot.
Double layer winding: Two coil sides are placed in a single slot.
1) This winding is very commonly used for small induction motors having circular conductors.
2)This is a single layer winding where all the coils have same span (unlike the concentric winding where coils have different spans).
3)Each coil is wound on a former, making one coil side shorter than the other.
4)The winding is put on the core by dropping the conductors, one by one into previously
insulated slots.
5)The short coil sides are placed first and then the long coil sides. The long and short coil sides occupy alternate slots.
6)It will be also observed that the ends of coil situated in adjacent slots cross each other
i.e. proceed to left and right alternatively.
That is why sometimes it is known as a basket winding.
AC Windings:-
Generally open windings except when delta connected
Classification:
Based on :
Supply: 1Φ & 3 Φ
Placing: Concentrated and Distributed
No. of layers: Single and Double layer
End connection: Lap, wave and concentric
The pitch: Full pitched and short pitched
Terms Used in AC windings:
Balanced winding: If the number of coils per pole per phase is same and a whole number then the winding
is balanced winding.
Unbalanced winding: If the number of coils per pole per phase is not a whole number then the winding is
balanced winding.
Slot angle: Slot angle = 180/ pole pitch (electrical degrees)
Full pitched winding: The pitch of the coil is equal to full pitch or equal to 180(degrees) then the coil is called the full pitched winding.
Short pitched winding or short chorded winding: If the pitch of the coil is less than 180(degrees) or less than the full pitch then the coil is called short pitched coil.
Coil span: Coil span is the distance between two coil sides measured in terms of slots.
Coil span = winding pitch/slot angle; For full pitched winding 180/slot angle
Short chorded winding or Fractional slot winding:
• Coil pitch in poly phase machines is usually less than pole-pitch and such a winding arrangement is called short pitch or chorded or fractional slot winding.
• Usually the coil pitch varies from 2/3 pole pitch to full pole pitch.
• A coil span less than 2/3 pole pitch is not used in practice. Because a chording more than 1/3 pole pitch would noticeably reduce the phase emf.
• Advantages of short pitched,( chorded, fractional slot) windings are:-
– The amount of copper used in the overhang (end winding) reduced and hence a saving on copper,
– The magnitude of certain harmonics in the emf and also mmf is suppressed.
Note :-
• In integral full pitch winding, a slot contains coil sides of the same phase.
• In integral chorded pitch winding, some slots contain coil sides pertaining to different phases.
• Interconnection between the phase belts of chorded three phase winding is done in a similar manner to that explained earlier for full pitch winding.
These are the most used winding connections in generators.
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