construction of Dc generator:
Previously we talked about working principle of dc generator, advantages and disadvantages of Dc generator, and we give a short note about the construction of Dc generator which will be our interest today. Let’s start.
construction of Dc generator:
Now we will recognize in detail construction of Dc generator
We will deal with a 4 pole Dc generator.
and we had explained that it consists of a stator and a rotor.
The stator of Dc generator:
It’s the main fixed part of the generator, the stator responsible for supplying the magnetic fields during coil rotation, and it mainly consists of:
dc generator Yoke (magnetic frame):
The yoke is the outer frame of the machine used to support and protect the internal parts, it’s made of permeability material processing sufficient mechanical strength and it also carries the magnetic field flux produced by field winding.
In small generators.
We take care to make yoke of cheap and heavier cast iron.
but in a large generator, we fabricate yoke of light cast steel or rolled steel.
Pole cores and pole shoes:
Pole cores are the most important of the field magnet, they have this great importance as they help:
Establishing the required magnetic flux how! when the current pass through the field winding placed on this pole, the core becomes electromagnetic hence, it establishes the magnetic flux and this flux can be varied by varying the current through field winding.
We fabricate them of cast steel or iron laminations to reduce the eddy current losses.
We connect this poles to yoke with bolts or welding.
In the other hand, the pole shoes is very important to
- Reduce the reluctance of the magnetic path by Increasing the cross-sectional area of the magnetic circuit.
- Support the field winding.
dc generator Field windings:
Generally, we fabricate field winding of low resistive materials copper or aluminum, we can connect this field windings to the armature in series or shunt.
When we connect the windings in series, we use less numbered turns with large cross-section conductors, instead when they are shunted they would be in large turns with a less cross-section to withstand the supply voltage.
When the current pass through the winding coils the pole core become electromagnetism then produce the necessary magnetic flux.we take care to connect the field coils in a way that the poles are made of positive polarity.
Brushes and brush holders:
Brushes are important to us because we use them to ensure electrical connections between the rotating commutator and external load circuit because they collect current from commutator segments.
They have a rectangular block shaped usually made of carbon or graphite which are conducting materials.
The brushes are housed in the brush holders which are secured to the front end housing with clamps.
Brushes also gain added from its cheap coast, ability to minimize the sparking and facilitate the collection of current from rotating commutator to stationary terminals.
But Unfortunately they require high maintenance, and they reduce the terminal voltage due to brush contact drop.
End covers (housing):
We fabricate end covers from cast iron or cast steel attached to the ends of the mainframe.
The rotor :
Of course; it has no different than other machines but here in dc generator differ according to the type permanent, separately excited or self-excited which lead us to look more in types of dc generator, but in all, it consists of:
dc generator Armature core:
Armature core has slots to accommodate the armature winding, and it is important as:
- it helps to house The conductors in the slots.
- It provides low reactance path to the magnetic flux.
It’s also a cylindrical rotating part made of high permeability silicon steel stamping or lamination to reduce eddy current losses and minimize the hysteresis loss.
The armature windings are the heart of the generator in which the conversion of mechanical power to electrical power takes place.
usually, We make Armature winding of aluminum to reduce the cost of the machine.
We can connect in one of the following:
In lap winding, we connect the conductors in a way that the number of parallel paths equals the number of poles to make each path in final have (armature conductors/pole) connected in series.
Here we use a number of brushes equal the number of parallel paths, half of this brushes are positive and the others are negative.
In wave winding, we connect the conductors in a way that we have two parallel paths irrespective of the number of poles.
And finally, there will be two parallel paths (armature conductors/2) conductors in series.
And here the number of brushes used equal two (number of parallel paths).
of course, there are more and more about the lap and wave winding but it isn’t our interest now.
In future, we will give them the space they should occupy.
fairly we consider it the rotor, and it is the most important in dc generator as:
- It connects between the rotating armature conductors and stationary external circuit by the brushes.
- It converts the alternating current produced in armature conductors to a unidirectional current in the external load circuit.
- Sequentially It also converts the alternating torque to continuous torque in the armature in motor action.
And you should know that commutator is a mechanical rectifier made of wedge-shaped high conductivity hard drawn copper bars rotates with the armature.
we also use the commutator to transfers the current from the wire coil to the brushes and it keeps the current at the brush positive.
commonly, we use the shaft to transfer mechanical power to the generator and turns the coil through the magnetic field.
And it is mild steel, cast iron or cast steel with a maximum bearing strength. And we support this shaft between two bearings. the armature core, commutator, and cooling fans are housed on the shaft.
When we take previously about motors we give a Wide background about bearings but today we will add that: we use ball bearings in small machine and roller bearing in heavy dc generator, and both are fitted in the end housing.
We also take care to use high carbon steel in bearing construction because the carbon is a very hard material.
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