In this article, we will discuss the construction of DC machines (DC Motor and DC Generator). But before that let us first learn a bit about DC machines.
Introduction to DC Machine
A device
that converts direct current electricity into mechanical energy or mechanical
energy into direct current electricity is known as a DC Machine.
Therefore, a DC machine is an electromechanical energy conversion device.
When the DC
machine converts mechanical energy into dc electrical energy, it is called a dc
generator. When the dc machine converts dc electrical energy into
mechanical energy, it is called a dc motor.
Actually,
the basic construction of a dc motor and dc generator is similar. Therefore, a
dc machine can be made to operate as a dc motor and a dc generator.
However, the principle of operation of the dc motor and dc generator is different.
Construction of DC Machines
A typical
dc machine consists of the following major parts:
- Yoke or Frame
- Magnetic Field System
- Armature
- Commutator
- Brushes
- Bearings
The overall
constructional view of a dc machine is shown in figure-1 below.
Now, let us discuss each part of the dc machine in detail.
(1). Yoke or Frame:
The yoke is the
outer frame of the dc machine. The yoke or frame of the dc machine serves the
following three purposes:
- The yoke protects the internal parts like the windings of the dc machine from mechanical damage.
- It provides housing for the magnetic field system.
- The yoke carries the working magnetic flux in the machine.
The yoke of
a dc machine is made of a material having high permeability and high mechanical
strength. Usually, the yoke of the dc machine is made up of cast steel.
(2). Magnetic Field System:
The magnetic
field system in a dc machine is a system of electromagnets that are excited by a dc
supply. It produces the required working magnetic flux in the machine. In dc
machines, the field system is a stationary part and is fixed with bolts to the
yoke or frame of the machine.
In large dc
machines, the magnetic field system has the following parts namely, pole core,
pole shoes, and field windings.
- Pole
Core: The pole core
houses the field windings. The Pole core is made up of thin steel laminations. One
end of the pole core is fixed to the machine frame with the help of bolts and
the other end has a pole shoe.
- Pole
Shoes: It is the
projected part of the field pole having a large cross-sectional area. The primary
function of the pole shoe is to spread the magnetic flux uniformly in the air
gap. It also supports field winding.
- Field
Windings: Field
windings are coils made up of copper wire. Field windings are former wounds, and are inserted around the pole core. When the field windings are excited by a dc
supply, they become electromagnets and produce the working magnetic flux in
the air gap of the machine.
(3). Armature:
In dc
machines, the armature is a rotating part (rotor). It has a system of conductors.
The EMF and torque are developed in the armature of the dc machine. The Armature of
a dc machine has two major parts: armature core and armature winding.
- Armature
Core: The armature
core is a solid cylindrical structure. The armature core usually comprises thin silicon steel laminations with high permeability. The Armature core has
slots on the outer periphery to carry the armature winding.
- Armature
Winding: Armature
winding of a dc machine is a coil of conductor wires such as copper wires. In
dc machines, depending upon the end connection of armature conductors, the
armature winding can be of two types namely, Lap Winding and Wave
Winding. The type of armature winding decides the voltage and current
ratings of the dc machine.
(4). Commutator:
In dc
machines, the commutator is an important part. The commutator is basically a
rotating rectifier switch. It performs the following two functions:
- It collects the current from the armature conductors or allows the current to flow into the armature conductors.
- It converts the alternating current of the armature winding into a unidirectional current in the external circuit and vice-versa.
The commutator of a DC machine is shown in the above figure. It is a cylindrically shaped component made up of copper. The V-shaped slots are provided on the outer periphery of the commutator to carry the commutator segments. Commutator segments are nothing but copper bars inserted in these V-shaped slots. Each commutator segment is insulated from the other by the mica material. The commutator is mounted on the rotor shaft of the dc machine on one side.
(5). Brushes:
In dc
machines, brushes are made up of carbon or graphite. Brushes are used to
collect current from the rotating commutator. The carbon brushes are housed in
brush holders and be in contact with the commutator surface with the help of
pressure created by springs.
(6). Bearings:
In dc
machines, bearings are used to reduce frictional losses. Bearings support
the machine shaft. In dc machines, two types of bearings are generally used
viz. ball bearing and roller bearing.
Hence, this
is all about the construction of a DC machine.
