Construction Of D.C. Generator

A d.c. Generator Consists Of Following Parts:
⇒ Yoke And Side Plates


⇒Brush and Rocker


1). Yoke and Side Plates
It is made of cast iron or cast steel. it gives mechanical support to the pole cores and also serves as a part of the magnetic circuit. Generally, an eye bolt is fitted at the top of the Yoke so that the machine can be lifted and conveyed to any other place easily. Two side disc plates are also fitted to cover the machine add to give support to the bearings and shaft. see figure 12.

2). Field
Field coils are placed around the poles course. These coils produced the magnetic flux for the machine. The pole face, also known as pole shoe, is made larger than the main body, as shown in figure 12. In small machines, the poles are made of cast iron or cast steel, but in large machines, these are made of sheet- steel limitations which are insulated from each other and riveted together. cast iron or cast steel provides an easy path to the magnetic lines of force.
Field coils are wound on insulating formers and then these are put on the poles. the shoe portion of the poles increases the magnetic field strength.
Interpoles or commutating poles are placed Midway between the main poles, as shown in figure 12. These are used to reduce the sparking produced at the commutator. if the armature is rotating from N-pole to S-pole, then the polarity of the Interpole placed after N-pole will be S-pole.
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3). Armature
It is a cylindrical drum and the armature coils are fitted in it.

It has two type-

→ Armature  Core 

They are made of sheet-steel laminations which are insulated from each other by insulating-Varnish coasting. Laminated cores have a low degree of eddy current loss. The thickness of a lamination varies from 0.35 mm to 0.5mm. Each lamination is slotted, as shown in figure 13. All the laminations are bolted together to form a drum and the armature coils are fitted in the slots. ducts are also made in the cores so as to reduce their mass and to keep them cool by allowing the fresh air to pass through them. These cores provide a path of low reluctance to the magnetic flux.

→ Armature Winding

armature windings are wound to the correct shape and size of the armature cores or formers with enameled copper wire.

There are the following two types of windings:

〉 Lap Winding

In this method, windings are overlapped on each other, as shown in figure 14. It is also known as parallel winding because the number of parallel paths is kept equal to the number of poles. this type of winding is suitable for large currents and comparatively low voltage machines.

〉 Wave Winding

In this method, the armature coil is divided into two parallel paths irrespective of the number of poles. the whole winding is done in the shape of the progressive wave, as shown in figure 15. two sets of brushes are enough for this winding. this type of winding is suitable for high voltage and low current machines.

4). Commutator
it is used to provide a D.C. output from the alternative EMF induced in the armature. it is made of hard drawn copper, which is a good conductor and has a lower specific resistance. along with it, copper is suitable for soldering the winding wires on it. it consists of a number of segments insulated from each other and also from the shaft with mica sheets, as shown in figure 16.

There is a groove of V-shape on one side of the segments. a V-shaped mica sheet is placed between the segment and the steel Ring. steel Ring tights the segment on the shaft. there is a projected portion called riser for soldering the winding wires on it, as shown in figure 16 (a). The Assembly of the armature is shown in figure 17.

5). Brush and Rocker

A rocker is fitted on the front end plate. It is made of bakelite and it has a round shape. brush holders, brushes, adjusting springs and a pigtail wire (flexible copper wire) are fitted On The Rocker. brushes act as a medium for the collection of current from the armature. a brush is made of carbon or a mixture of copper and graphite particles.

Advantages of using a carbon brush are as follows.
* Its resistance decreases with rising in temperature.
* It does not oxides.
* It can work satisfactorily even on high temperatures.
* It can easily be molded into required shape.
* It has a current carrying capacity of 4 to 6 amperes per square cm. Its width is kept 3 to 4 times greater than that of a commutator segment.

For more current, copper-graphite brushes are used. a brush can be fitted into a box type, Morgan type or a constant pressure type brush holder under pressure of a spring. See figure 18.

The pressure of the spring is kept in the range of 75 to 150 grams per square cm.
Brushes are used to make a sliding electrical contact between the armature and the external circuit.
6). Bearing

They are used to support the shaft and are fitted on the front end near end plates. they enable the shaft to rotate smoothly in the field along with the armature.

Three Types Of Bearing are used in generator-

*  Gunmetal bush-bearing- these are used in small size machines.
* Ball or roller-bearing- these are used in medium size machines.
* Babbit metal bush- These are used in large size machines and are made by joining two half pieces.
Note- Given that in figure 19, an assembly of a d.c. the machine, the direction of the flux of the poles and the magnetic circuit.

  • Leakage Flux
The magnetic flux leak out from the poles is called the leakage flux or flux-loss. it is of no use and it causes a power loss.
The Magnetic flux passing through the poles and poles-shoes is the only useful flux. when the armature coils cut the magnetic flux, produced by pole-shoes, then an EMF induced in them. the induced EMF is collected by the brushes and it is supplied to the external circuit, white interpoles reduce the sparking at the commutator.

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