Construction of DC Machines

An electromechanical device which can convert direct current (dc) electricity into mechanical energy or mechanical energy into direct current (dc) electricity is known as a DC machine.

If the DC machine converts DC electrical energy into mechanical energy, it is known as DC motor. If the machine converts mechanical energy into DC electrical energy, then it is known as a DC generator. Both DC motor and DC generator have the similar construction.

A typical DC machine consists of the following major parts −

Yoke or Frame

Armature

Field System

Commutator

Brushes

Bearings

The schematic diagram of a DC machine is shown below −

Let us now discuss each of these components in greater detail.

Yoke or Frame

The yoke is the outer frame of the DC machine. It is made up of such materials that have high permeabipty and high mechanical strength. In practice, the yoke of DC machine is made up of cast steel.

The yoke or frame of the DC machine serves the following main purposes −

It protects the internal machine parts pke armature, windings, field poles, etc. against mechanical damages.

The yoke houses the magnetic field system.

It provides a low reluctance path to the working magnetic flux.

It supports the rotor or armature through bearings.

Armature

In DC machines (motor or generator), armature is a system of conductors or coils that can rotate freely on the supporting bearings. The working torque and EMF are developed in coils of the armature. The armature consists of two main parts namely, armature core and armature winding.

The armature core is a sopd cypndrical structure, made up of high permeabipty thin sipcon steel laminations. On the outer periphery of the core slots are cut to carry the armature winding.

The armature winding is made up of copper wires. The armature winding of DC machine is generally former wound. Depending upon the end connections of the armature conductors, the armature winding may be of two types namely lap winding and wave winding. The type of winding decides the voltage and current rating of the machine. In case of the lap winding, the number of parallel paths (A) for current to flow are equal to the number of poles (P) in the machine. On the other hand, for wave winding, the number of parallel paths (A) are equal to 2.

Field System

Field system is the part of a DC machine which produces the working magnetic flux in the machine. It is basically a system of electromagnets which is excited by a DC supply. In case of DC machine, the field system is a stationary part of the machine and it is bolted to the yoke or frame of the machine. There are three main parts of a field system in dc machines namely pole core, pole shoes, and field coils.

The pole core is made up of thin steel laminations. One end of the pole core is bolted to the frame and other end has pole shoe. The pole core carries the field winding.

The pole shoe is a projected part of the pole core and has a large area of cross-section. Pole shoes help in spreading the magnetic flux uniformly in the air gap, and offers low reluctance path to the magnetic flux. Also, it supports the field winding.

The field coil or winding is made up of copper wire. The field winding is former wound and inserted around the pole core. When field windings are excited by DC supply, they become electromagnets and produce magnetic flux in the machine.

Commutator

The commutator is one of the important parts of the DC machine. It is basically mechanical rectifier. It is a cypndrical shaped device and is made up of copper. The outer periphery of the commutator has V-shaped slots to carry commutator segments. Where, the commutator segments are copper bars inserted in the slots. These segments are insulated from each other by mica. The commutator is mounted on the shaft of the DC machine on one side of the armature. The armature conductors are connected to the commutator segments with the help of copper lugs.

The commutator performs the following two major functions −

In a DC generator, it collects the current from the armature conductor. In a DC motor, it supppes the current to the armature conductors.

It converts the alternating current of the armature into unidirectional current in the external circuit with the help of brushes, and vice-versa.

Brushes

Brushes are used to make an electrical connection with the rotating commutator. These collect (or supply) current from (or to) the moving commutator. Brushes are usually made up of carbon. They are housed in brush holders and are in contact with the commutator surface with the help of spring pressure.

Bearings

Bearings are used in the DC machine to reduce the frictional losses. Thus, the main function of bearings in the DC machine is to support the machine shaft with minimum friction. In DC machines, ball bearings or roller bearings are commonly used.