Excitation and Armature coils

Excitation Coils:

Each pole core has one or more field coils (windings) placed over it to produce a magnetic field. The enamelled copper(insulated copper wire) wire is used for the construction of field or exciting coils. The coils are wound on the former and then placed around the pole core.

When direct current passes through the field winding, it magnetizes the poles, which in turns produces the flux. The field coils of all the poles are connected in series in such a way that when current flows through them, the adjacent poles attain opposite polarity.

Armature Core:

The armature core of DC Generator is cylindrical in shape and keyed to the rotating shaft. At the outer periphery of the armature has grooves or slots which accommodate the armature windings.

The armature core of a DC generator or machine serves the following purposes.

1. It houses the conductors in the slots.

2. It provides an easy path for the magnetic flux.

As the armature is a rotating part of the DC Generator or machine, the reversal of flux takes place in the core, hence hysteresis losses are produced. The silicon steel material is used for the construction of the core to reduce the hysteresis losses.

The rotating armature cuts the magnetic field, due to which an emf is induced in it. This emf circulates the eddy current which results in Eddy Current loss. Thus to reduce the loss the armature core is laminated with a stamping of about 0.3 to 0.5 mm thickness. Each lamination is insulated from the other by a coating of varnish.

Types Of Armature Winding:

1.Lap Winding.
2.Wave Winding.

Lap Winding:

Lap winding is the winding in which successive coils overlap each other. It is named “Lap” winding because it doubles or laps back with its succeeding coils.

In this winding the finishing end of one coil is connected to one commutator segment and the starting end of the next coil situated under the same pole and connected with same commutator segment.
Here we can see in picture, the finishing end of coil – 1 and starting end of coil – 2 are both connected to the commutator segment – 2 and both coils are under the same magnetic pole that is N pole here.
Lap winding are of two types –

1.Simplex lap winding.

2. Duplex Lap Winding.

Simplex Lap Winding

A winding in which the number of parallel path between the brushes is equal to the number of poles is called simplex lap winding.

Duplex Winding

A winding in which the number ofparallel path between the brushes is twice the number of poles is called duplex lap winding.

In lap winding, the number of brushes is equal to the number of parallel paths. Out of which half the brushes are positive and the remaining half are negative.

Advantages of Lap Winding

1. This winding is necessarily required for large current application because it has more parallel paths.

inding is suitable for low voltage and high current generators.

Disadvantages of Lap 

 1.  It gives less emf compared to wave winding. This winding requires more no. of conductors for giving the same emf, it results high winding cost.

2. It has less efficient utilization of space in the armature slots.

Wave Winding:

In wave winding, we connect the end of one coil to the starting of another coil of the same polarity as that of the first coil. In this type of winding the coil side (A – B) progresses forward around the armature to another coil side and goes on successively passing through N and S pole till it returns to a conductor (A₁-B₁) lying under the starting pole.

This winding forms a wave with its coil, that’s why we call it as wave winding. Since we connect the coils in series here, we also call it series winding.

Types of Wave winding:

1.If after one round of the armature the coil falls in a slot right to its starting slot the winging is called Progressive wave winding.

2.If after one round of the armature the coil falls in a slot left to its starting slot the winging is called Retrogressive wave winding.

Wave Winding Advantages

1.In this winding, only two brushes are required but more parallel brushes can be added to make it equal to the no. of poles. If one or more brushes set poor contacts with the commutator, satisfactory operation is still possible.

2.This winding gives sparkles commutation. The reason behind that it has two parallel paths irrespective of no of poles of the machine. The conductors in each of the two parallel path distributed around the armature in the entire circumference.

Wave Winding Disdvantages

1.Wave winding cannot be used in the machines having higher current rating because it has only two parallel paths.

Let's discuss the working of the D.C generator and it's classifications in the forth comming posts..

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