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# Armature Windings of Synchronous Generators and Alternators

## Armature Windings

The armature Windings of DC Machines are usually closed Circuit windings but alternator may be closed giving - Connections or open giving star - Connections.

 ARMATURE WINDING

The general Principles governing windings for DC Machines also good for alternator windings e.g.,

1) The span of each coil must be equal to Pole pitch i.e., the two sides of any Coil must be under adjacent poles.

2) The coils must be so connected that emfs induced in them help each other.

3) The winding can be eighter single layer or double layer.

The single layer winding will have one coil side per slot, while double layer winding will have two coil sides/slot.

4) In addition to above, winding must be so designed that it may approximately give Induced EMF of Sinusoidal form.

### Such winding give maximum Induced emfs for a given number of Conductors but the waveform of Induced emf is not exactly of Sinusoidal form.

#### Distributed winding :-  If the Conductors are placed in several slots under one pole, the winding is known as Distributed winding.

The winding may be partially or completely distributed accordingly as the slots are spread over only a portion of it or Over the entire armature surface.

Through it reduces the Induced emf it yet it is most commonly employed due to its manifold advantages, such as improved waveform of Induced EMF, diminished armature Reaction and of copper over the surface of the Armature resulting in even Distribution of copper loss and better utilization of copper due to even spacing of a number of slots.

• Coil Group :- It is the product of number of phases bad number of poles i.e., coil - groups number of poles × number of phases.

• Balanced winding :- If the number of coils per group is a whole number, the Winding is know as the balance winding.

Each pole contains equal number of coils of difference phases in such a case. It is important to not that there must be equal number of coils in each phase eighter the winding is balanced or unbalanced.

• Full pitch and short pitch windings :- When the two coil sides forming a complete coil of a winding are 180 Electrical space degree apart, the winding is known as the full pitch winding.

i.e., the two coil sides forming a complete coil are less than 180 Electrical space degree apart, the Winding are known as fractional pitch Winding.

• The Short pitch or chorded coil winding have the advantage of

1) Improved waveform of Induced emf due to reduction of distorting harmonics,

2) Reduced winding Inductance,

3) Reduced tooth ripples due to use of fractional number of slots per pole.

4) Saving in copper Requirement owing to less span and

5) Increased Mechanical strength of the coil.