electricalandelectronics.org » Construction of D.C. Machines

Important terms regarding armature winding

arjun — Wed, 29 Apr 2009 17:23:32 +0000

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

Armature Shaft Bearings

arjun — Tue, 16 Sep 2008 09:51:50 +0000

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. Share and Enjoy: read more

Brush Holders & Brush Rockers

arjun — Tue, 16 Sep 2008 09:24:33 +0000

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

Brushes

arjun — Tue, 16 Sep 2008 08:50:40 +0000

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

Brush Gear

arjun — Tue, 16 Sep 2008 07:20:23 +0000

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 Share and Enjoy: read more

Commutator

arjun — Tue, 16 Sep 2008 06:51:38 +0000

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. Share and Enjoy: read more

Armature Windings

arjun — Tue, 16 Sep 2008 06:33:49 +0000

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

Armature

arjun — Mon, 15 Sep 2008 17:39:23 +0000

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

Commutating poles /Interpoles

arjun — Sat, 13 Sep 2008 09:48:54 +0000

Interpoles or commutating poles are similar to that of main poles and has got a core which terminates with in a poleshoe.it is constructed in various shapes and the coils are mounted on the core.It is usually spaced in between two two main poles and bolted to yoke.Interpoles are usually made of solid steel but for the machines with varying loads sheet steel is used for construction.Interpoles are used to reduce spark while commutation. Share and Enjoy: read more

Field Poles

arjun — Sat, 13 Sep 2008 09:31:32 +0000

In early stages or say in the case of small machines the poles were cast integral to the yoke.But nowadays we use completely laminated pole or solid steel poles with laminated pole shoes. Why we use laminated field poles? We use laminated field poles because the surface of armature is not uniform since it has got notches or slots.And so when it rotates through the field produced by the pole shoes there may be pulsations in the field and this varying field could produce an eddy current inside the field pole.So to avoid this we use laminated field poles. How laminated field poles reduce eddy current ? The laminated field poles allows only the eddy current to pass through the length of lamination not through the entire body of pole ie from lamination to another laminations.These lamination read more