DC machine consists of two main parts. One of them is armature and other is field system. The field system is stationary part of the dc machine and armature is rotating part. In this article we will discuss about the construction of DC machines
It is a outer body of the machine. It is cylindrical in shape. It serves the following purpose in machine:
- It supports inner part of the machines e.g. poles are fixed on it.
- It provides mechanical protection to the inner parts of the machine.
- It provides low reluctance part for the magnetic flux.
Cast Iron yokes are made for small machines and for large machine. It is made up of fabricated steel.
Pole consists of two main parts.
- Pole Core
- Pole shoe
The Pole Core is made of thin cast steel or wrought iron laminations which are riveted together. The pole core is circular in section and filed coil is wound over it.
Pole Shoes are also made of cast steel or wrought iron laminations and it screwed to the pole face. Pole shoes have larger cross section area. The poles sever following purposes.
- It supports the filed coils.
- It spreads out the magnetic flux in the air gap.
FIELD OR EXCITING COILS
Fields coils are made of enameled copper. The coils are wound on the former then placed around the pole core as the field coils of all the poles are connected in series in, such as way that when current flows through them, the adjacent poles attain opposite polarity. When direct current passed through the field coils, the required magnetic flux produced.
The armature is the rotating part of the dc machine. The conversion of energy takes place in the armature. The armature core is of laminated silicon steel. Laminations are used to reduce the eddy current losses and silicon steel is used to reduced the hysteresis losses. Armature is cylindrical in shape and keyed to the rotating shaft. At the outer periphery slots are cut, which accommodate the armature winding.
Enameled copper is used for the construction of armature winding. The armature winding is housed in armature slots, which is suitably connected. The armature winding is heart of the dc machine.
On the basis of connections, there are two types of armature winding.
- Lap Winding
- Wave Winding
It is the most important part of the DC machine. It is just a reversing switch. Commutator connects the rotating armature conductors to the stationary external circuit through brushes.
In generating action, it converts alternating voltage into direct voltage and in motoring action it converts unidirectional torque into alternating torque.
The commutator is a form of rotating switch placed between armature and external circuit and so arranged it will reverse the connections with the external circuit at the instant of each reversal of current in the armature.
The commutator is of cylindrical shape and is made up of wedge shape hard drawn copper segments. The segments are insulated from each other by a thin sheet of mica.
The Brushes are pressed upon the commutator and make the connecting link between the armature winding and external circuit. Carbon is used for the construction of Brushes because it is conducting material and good lubricating material.
It holds the spindles of the brush holders. It is fitted on to the stationary frame of the machine with nut and bolts.
End housings are attached to the ends of the main frame and support bearings. The front housing supports the bearings and the brush assemblies whereas the rear housing usually support the bearing only.
The function of Bearings is to reduce friction between the rotating and stationary parts of the machine. Usually ball or roller bearing are used.
The Shaft is made of mild steel. The shaft is used to transfer mechanical poser from or to the machine.