27-08-2013, 03:05 PM
DC MACHINE CONSTRUCTIONAL DETAILS
It serves two purposes
Protecting cover to the whole machine
A number of salient poles bolted to the inside of circular frame (generally called yoke)
Allow the flux to pass through
Materials used
For small machine – cast iron
For large machine – cast steel
POLES
Poles consists of pole cores, pole coils & pole shoes. The Pole shoes serve two purposes .
1. They spread out the flux in the air gap
2. They support the exciting coils
When current is passed through field coils, they electromagnetise the poles which produce the necessary flux.
A field system to produce the magnetic field, which is the stationary element.
Field coils(pole coils) are mounted on the poles and carry the d.c.exciting current. The field coils are connected in such a way that adjacent poles have opposite polarity.
The m.m.f. developed by the field coils produces a magnetic flux that passes through the pole pieces, the air gap, the armature and the frame.
Poles are made up of
-> For small machine – cast iron
-> For large machine – cast steel
ARMATURE CORE
The armature core is keyed to the machine shaft and rotates between the field poles. It consists of slotted soft-iron laminations (about 0.4 to 0.6 mm thick)
The purpose of using laminations is to reduce the eddy current loss.
ARMATURE WINDING
The slots of the armature core hold insulated conductors that are connected in a suitable manner. This is known as armature winding
This is the winding in which “working” e.m.f. is induced. The armature conductors are connected in series-parallel; the conductors being connected in series so as to increase the
voltage and in parallel paths so as to increase the current.
COMMUTATOR
Commutator is a mechanical rectifier, which converts the alternating voltage generated in the armature winding into direct voltage across the brush.
It is made of copper segments insulated from each other by mica and mounted on the shaft of the machine. The armature windings are connected to the commutator segments.