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 key.
The core(except small size is divided into number of packets by radial ventilation spacers.The spacers are usually ‘I’ sections welded to thick steel laminations and arranged to pass centrally down each tooth.
Armature Lamination for small machine
For small machines the punching is made on the same piece which is built directly on the shaft and ventilation holes are provided to pass air into ventilating ducts.
Armature lamination for medium size machine
Medium size machines are machines having more than four poles,these machines are built on a spider.The spider may be fabricated and the lamination about 100cm are punched in one piece and keyed directly to spider.
Armature lamination for large machine
For large type of machines segmental lamination is employed since thin lamination may get distorted or become wavy when assembled together.Hence instead of being cut into one piece it is cut into a number of segments or sections which forms a complete ring.A complete circular ring could be made of 4,6 or even 8 segmental sections or laminations.Usually two key ways are notched in each segments and are dove tailed or wedge shaped to make the laminations self locking in position.