The important parts of a DC machine are as follows
1. Yoke
2. Field winding
3. Poles
4. Armature
5. Commutator, brushes and gear
6. Bearings
Yoke
The important function of the yoke is that
it is an outer cover of a DC machine. Sometimes, the yoke is also called a
frame. It is used to protect other parts of the machine from moisture, dust,
etc.
The yoke is an iron body and it provides
the path for the flux. This is essential to complete the magnetic circuit. And
it also provides mechanical support for the poles.
Generally, low reluctance materials used
for yoke like; cast iron, silicon steel, rolled steel, cast steel, etc.
Poles, Pole Shoe and Pole Core
A pole of a DC machine is an electromagnet.
The pole shoes provide housing to the field winding. When the field winding is
excited, it is used to produce the magnetic flux.
The poles extend at the end is known as
pole shoes. It enlarges the area of the pole by its typical shape. Due to this more
area, more flux can pass through the air gap to the armature.
For the construction of a pole or pole shoe,
a low reluctance magnetic material such as cast steel or cast iron is used.
To reduce the power loss due to eddy
currents, the construction of poles is done using the laminations of a particular
shape.
Field Winding (F1 -F2)
The coils wound around the pole are known
as field coils. To form the field winding, field coils are connected in series
with each other.
To magnetize the poles and pole shoes, a current
is passed through the field winding in a specific direction. The magnetic flux
is produced in the air gap between pole shoes and armature.
The field winding is also known as exciting
winding. The copper is used for the field winding.
Alternate N and S poles are produced, due
to the current flowing through the field winding. A right-hand thumb rule is
used to know which pole is produced at a particular core.
Armature Core
The armature core is a cylindrical drum. It
is mounted on the shaft. It is provided with a large number of slots all over
its periphery. All these slots are parallel along with the shaft axis.
Armature conductors are placed in these
slots. It provides a low reluctance path to the flux produced by the field
winding.
For the armature core, Low reluctance, high
permeability materials like; cast steel or cast iron are used. The air holes
are provided for the air circulation which helps in the armature core. To minimize
the eddy current losses armature core is constructed using laminations.
Armature Winding
The armature conductors are made of copper.
And it placed in the armature slots present on the periphery of the armature
core.
To form the armature winding, these
armature conductors are interconnected. Using a prime mover, the armature
winding is rotated and it cuts the magnetic flux lines. Due to this, the voltage
gets induced in it.
Through the commutator and brushes, armature
winding is connected to the external circuit.
The armature winding is supposed to carry
the entire load current. Hence it should be made up of a conducting material
such as copper.
Commutator
A commutator is a cylindrical drum and it
is mounted on the shaft along with the armature core. It is made of several
wedge-shaped segments of hard-drawn copper.
The segments are insulated from each other
by thin layers of mica. The armature winding is tapped at various points and these
tapings are successively connected to various segments of the commutator.
The commutator is used to convert the
alternating EMF generated internally into a DC voltage in the case of the DC
generator. Hence, it works like a rectifier. It collects the current from the
armature conductors. And passes the current to the external load via brushes.
It helps to produce a unidirectional torque
for DC motors.
The commutator segments are made of copper
and the insulating material between the segments is mica.
Brushes
The commutator is a rotating part of the machine.
Therefore, it is not possible to connect the load directly to it.
By the carbon brushes, a current is
conducted from the armature, and that current is held against the surface of the
commutator by springs.
Brushes wear with time. Hence, they should
be inspected regularly and replaced occasionally.
The brushes are used to collect current from the commutator and apply it to the external load. Brushes are made of carbon. They are rectangular.
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