The field winding produces a magnetic flux in the air gap between the armature and field windings and the armature is placed in this magnetic field.
The construction of a DC machine is shown in Figure.
The main parts used in the construction of a DC motor are the yoke, poles, field winding, commutator, carbon brushes bearings etc. A brief description of the various parts is as follows:
The yoke acts as the outer cover of a DC motor and it is also known as the frame. The yoke is an iron body, made up of low reluctance magnetic material such as cast iron, silicon steel, rolled steel etc.
Yoke serve two purposes, firstly it provides mechanical protection to the outer parts of the machine secondly it provides low reluctance path for the magnetic flux.
Poles and Pole Shoe
The pole and pole shoe are fixed on the yoke by bolts. These are made of thin cast steel or wrought iron laminations which are riveted together. Poles produce the magnetic flux when the field winding is excited.
Pole shoe is an extended part of a pole. Due to its shape, the pole area is enlarged and more flux can pass through the air gap to the armature.
The coils around the poles are known as field (or exciting) coils and are connected in series to form the field winding. Copper wire is used for the construction of field coils. When the DC current is passed through the field windings, it magnetizes poles which produce magnetic flux.
It is a cylindrical drum and keyed to the rotating shaft. A large number of slots are made all over its periphery, which accommodates the armature winding. Low reluctance, high permeability material such as cast iron and cast steel are used for armature core.
The laminated construction is used to produce the armature core to minimize the eddy current losses. The air holes are also provided on the armature core for the air circulation which helps in cooling the motor.
The armature winding plays very important role in the construction of a DC motor because the conversion of power takes place in armature winding. On the basis of connections, there are two types of armature windings named:
- Lap winding
- Wave Winding
In lap winding the armature conductors are divided into P groups.
All the conductors in a group are connected in series and all such groups are connected in parallel. For a lap winding the number of parallel paths (A) is equal to the number of poles (P).
Due to the existence of a large number of parallel paths, the lap wound armature is capable of supplying larger load currents, therefore, lap winding is used for low voltage high current DC motors.
In wave winding, all the conductors are connected in series to form a single closed circuit. This closed circuit is tapped at various points. The two carbon brushes touch two diametrically opposite tappings.
For wave winding the number of parallel paths is equal to two irrespective of the number of poles. The wave winding is useful for high voltage low current motors.
It is a mounted on the shaft. It is made up of a large number of wedge-shaped segments of hard drawn copper, insulated from each other by a thin layer of mica.
The commutator connects the rotating armature conductor to the stationary external circuit through carbon brushes. It converts alternating torque into unidirectional torque produced in the armature.
The current is conducted from voltage source to armature by the carbon brushes which are held against the surface of commutator by springs. They are made of high-grade carbon steel and are rectangular in shape.
The ball or roller bearings are fitted in the end housings. The friction between stationary and rotating parts of the motor is reduced by bearing. Mostly high carbon steel is used for making the bearings as it is very hard material.