Which Brushes Are Not Insulated

March 31, 2024

37

DC Machine:

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F1 J.P Madhu 29.05.20 D2

The above figure shows constructional details of a simple 4-pole DC machine.

A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

Brushes:

The brush in a DC machine is a component that collects the current from the moving parts and feeds it to the stationary circuit
Brushes are usually made from carbon or graphite, but copper brushes are preferred in low voltage high current requirements.
Brush contact loss attributes to the resistance between the surface of the brush and the commutator
This loss is mostly enclosed in armature copper loss
As copper losses depend on load current, brush contact losses also depend on load current; These are directly proportional to the current

Yoke:

The outer frame of a dc machine is called a yoke.
It is made up of cast iron or steel.
It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.

Poles and pole shoes:

Poles are joined to the yoke with the help of bolts or welding.
They carry field winding and pole shoes are fastened to them.
Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in the air gap uniformly.

Field winding:

They are usually made of copper.
Field coils are former wounds and placed on each pole and are connected in series.
They are wound in such a way that, when energized, they form alternate North and South poles.

Armature core:

Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding.
The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
Armature core is made of silicon steel laminations which are insulated from each other by insulating varnish coating. These laminations are used to reduce eddy current losses.
It may be provided with air ducts for the axial air flow for cooling purposes.
Armature is keyed to the shaft.

Armature winding:

It is usually a former wound copper coil which rests in armature slots.
The armature conductors are insulated from each other and also from the armature core.
Armature winding can be wound by one of the two methods; lap winding or wave winding.
Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.

Commutator and brushes:

In the case of the DC generator, the commutator is used to convert generated AC in armature into DC.
In the case of DC motor, the commutator is used to convert DC to A.C
Due to the limitation of the commutator dc generators are not usually designed beyond 650 V
The physical connection to the armature winding is made through a commutator-brush arrangement.
The function of a commutator in a dc generator is to collect the current generated in armature conductors.
While in the case of a dc motor, the commutator helps in providing current to the armature conductors.
A commutator consists of a set of copper segments which are insulated from each other.
The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
Brushes are usually made from carbon or graphite, but copper brushes are preferred in low voltage high current requirements.
They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.