Armature Windings The mean­ing of the following terms used in connection with armature winding should be clearly kept in mind. Pole-pitch It may be variously defined as : i) The periphery of the armature divided by the number of poles of the generator i.e. the distance between two adjacent poles. ii) It is equal to the number of armature conductors (or armature slots) per pole. If there are 48 conductors and 4 poles, the pole pitch is 48/4 = 12. Conductor The length of a wire lying in the magnetic field and in which an e.m.f. is induced, is called a conductor (or inductor) as. for example, length AB or CO in Fig. 26.21 Coil and Winding Element With reference lo Fig. 26.21. the two conductors AB and CD along with their end connections constitute one coil of the armature winding. The read more

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For small machines roller bearings are used at both ends. For large machines roller bearings are used at driving end and ball bearings at non driving or commutative end.Sometimes pedastal bearings are also used for large machines.The bearings are housed in the endshield.The figure below shows a pedestal bearing. read more

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Usually we use box type brush holders in all D.C. machines.At the outer end of arm a brush box is provided which is open at top and the bottom is attached.The brush is pressed to the commutator by means of a clock spring and this pressure could be adjusted by a level arrangement in the spring.The brush is connected to a flexible conductor called pigtail.The flexible conductor may be attached to the brush by a screw or may be soldered. Brush boxes are generally made of bronze casting or sheet brass.For small machines working on low voltage commutation conditions are easy and galvanized steel boxes are employed.There are individual and multiple brush holders available on the market.In multiple brush holders a number of single brush holders are built into one long assembly. Brush read more

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The brushes used for machines are divided into five classes according to the material with which it is made they are Metal Graphite brush Carbon Graphite brush Graphite brush Electro – Graphite brush Copper brush The maximum current density at brush contact varies from 0.5A/sq.cm to 23A/sq.cm for copper.For large current machines working at low voltage copper brushes are employed.Lubrication should be done properly else it may cut the commutator rapidly leading to high wear and tear.Graphite and carbon graphite brushes are self lubricating and therefore it is used very widely. Even for soft brushes there is wear but it takes place gradually.Though the mica segments between the commutator doesn’t wear easily but it reduces the effective contact with the segments and it may read more

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We use brush gear to collect or feed current from a rotating commutator.A brush gear consists of following parts. Brushes Brush Holders Brush studs or Brush holder arm Brush Rocker Current collecting busbars read more

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A commutator converts alternating voltage into direct voltage.A commutator is a cylindrical structure built up of segments made of hard drawn copper.These segments separated from each other and from frame by means of mica strips.These segments are connected to the winding by means of risers .The risers have air spaces between one another or that the air is drawn across the commutator there by keeping the commutator cool.The components of a commutator is shown below The general appearance of commutator when completed and commutator and armature assembly is shown below. read more

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The armature windings are placed or housed in the slots grooved on the surface of armature and and are so spaced such that one coils side or say a conductor of coil comes under north pole and the other coilside or second conductor comes under south pole. Generally in D.C. machines two layer windings with diamond shaped coil is used.The coils are usually former wound. How the coils are kept in position? In small machines armature coils are held i position by means of band of steel wire which is wound under tension along core length.In large machine wedges of fiber or wood is used to keep the coils in position ie in the slots and use the wire band to overhang the coil. Factors affecting armature winding Armature winding plays an important role in the conversion of energy fro one form to read more

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The armature consists of a core and winding.Due to the good magnetic properties of iron it is used as the armature core.Iron is also a good conductor of electricity so the rotation of iron core in the magnetic field could produce a current in the core and this current called eddy current cause the wastage of energy as heat.To reduce eddy current the core is made of thin laminations.These laminations are made up of low loss silicon steel of of 0.4 to 0.5 mm thick and insulated with varnish. In small machines armature laminations are fitted to the shaft and clamped tightly between the flanges.These flanges also acts as a support for the armature winding.One end of flange rests against a shoulder on the shaft,the laminations are fitted and then end is pressed on the shaft and retained by a read more

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