What are the important parts of DC machine?
Construction wise DC machine has four main parts namely field magnet, armature, commutator and brush. Actually a DC machine means a DC motor or a DC generator constructionally both are same. If rotor of a DC machine is rotated through means of a prime mover. It produces electricity at armature on the other hand electricity if supplied to the armature, the rotor of a DC machine starts rotating. In first case where dc machine produces electricity due to rotating rotor it acts as dc generator. In the second case where dc machine produces rotation of rotor due to applied electricity. It acts as a dc motor, hence theoretically same dc machine can be used as generator or motor. From this point of view essentially the constructional features of both dc motor and generator are same.
Main parts of DC system:
1) Field system:
The objective of the field system is to create a uniform magnetic field in which the armature rotates. Permanent magnet or electromagnet can create this magnetic field. Field magnet system consists of four main parts, yoke or frame, pole cores, pole shoes and magnetizing coils. The yoke is generally of cylindrical shape, the yoke acts as covered frame of the machine. It carries the magnetic flux produced through the poles. Since the field is at rest, no need of using laminated yoke for the normal machine. In small machines cast and yokes are used because of cheapness, but the yoke of a large machine is made of fabricated steel due to its high permeability.
Lifting eye, feet and terminal box are welded on the outer surface of the frame. In small machine the yoke may be of one piece. But for large machines, it is usual to employ a split yoke the two of sewing bolted together. Pole core of DC machine is of round section and is used to carry the field current, pole cores are usually not laminated and generally made of cast steel. In some cases these pole cores are laminated and composed of electrical grade steel sheets of about 1 mm thick and insulated with each other. The pole shoes acts as support to the field coils and spread the flux over armature more uniformly. It also being reduces the reluctance of magnetic path. The pole shoes are always laminated to avoid heating and eddy current losses caused through the fluctuations, in the flux distribution on the pole face due to moment of armature slots and teeth .
2) Construction of armature:
The purpose of armature is to rotate the conductors in the uniform magnetic field. It contains coils of wounded(insulated) wires around an iron and electric currents are induced to wires when armature is rotating in a magnetic field. In addition to providing support to the armature coils, it also provides low reluctance path to the magnetic flux. The armature core is made of a thin lamination of silicon steel, each lamination is insulated through from other thin paper or a thin coating of varnish. The armature is made of laminated grain oriented silicon steel because in DC machine armature induces alternating current.
Hence the effect of eddy current and hysteresis should be avoided. The slots are either dicot are punched on the outer periphery of the circular core. Stamping is the key bay is located on the inner diameter. The slots are normally open type usually placed parallel to the axis of the armature better also sometimes cute at a small angle to avoid vibration of teeth. The lamination stamping of the armature is usually punched with holes near the shaft to give longitudinal ventilating ducts, the air is drawn in through these ducts to provide defecting ventilation. For larger machines, it is not economical to punch out a complete circular stamping from one sheet of silicon steel. Armature core stamping, in this case, are essentially build up of 4 to 8 segments of lamination. A dovetail joint means is attached to the spider.
3) Commutator :
The commutator is a rotating switch, which is placed at the armature and the external circuit and it will inverse the connections of the other circuit at the time of each reverse of current in the armature. It is essentially of cylindrical structure and it is build up of high conductivity hot drawn copper. These layers (segments) are covered from each other thin layer of mica. the segments are help together two V-shaped rings that fit into the v glows cut into the segments.
The brushes are used to collect the current from the commutator and supplied to external load circuit of Dc generator. In case of dc motor brushes supply current to the commutator, the brushes are rectangular in shape and rest on the commutator.
Also Read- Construction of DC machines