Construction

CONSTRUCTION

An electric generator is a machine which converts mechanical energy into electrical energy. The conversion is based on the dynamically induced Emf. The principle of operation based on faraday`s laws of electromagnetic induction.

Construction:

The two basic essential parts of an electrical generator are

1. Magnetic field

2. A conductor to cut the magnetic field

The parts of generator are listed below

1. Magnetic frame (or) Yoke:

The purpose of yoke is

*To provide mechanical strength (or) support for the poles

*It carries the magnetic flux produced by the poles

Yoke is made up of cast iron for small machines and cast steel for large machines. The structure of yoke is different, it consists of rolled steel slab due to this the machine withstand from several distortions.

2. Pole cores and Pole shoes:

The field magnet consist of pole core (or) pole shoe, these serve two purposes

i) They give support to the field coils

ii) The flux produced by field coils are distributed in the air gap by reducing reluctance.

*The pole core is made up of the solid material named cast iron (or) cast steel. The lower part of the pole shoe is laminated and named as pole shoe.

*The pole core and Pole shoes are having thin lamination made up of annealed steel, the thickness varies from 1mm to 0.25mm.

3. Pole coils (or) Field coils:

The pole coils are also known as field coils. It consists of copper wire (or) strip this wire is correctly dimensional and place over the core. When supply is given the copper conductor produces the required flux.

4. Armature core:

The function of armature core is to allow the mechanism of faraday`s laws of electromagnetic induction. Armature core is made up of silicon steel with a thickness of each combination about 0.3mm to 0.5mm to reduce the eddy current losses.

Functions of Armature core:

i) It gives accommodation for armature winding

ii) It provides low reluctance magnetic flux path

iii) In DC machines open slots are prefferd.

5. Armature windings:

The single winding is formed by the two copper conductors. In machines when you remove the outer part you observer the bunch of windings. These are settled in certain place and placed in order manner. These windings are supported by Iron metal and laminated to reduce the eddy current losses.

Basically windings are of two types

1. Lap Winding

2. Wave Winding

1. Lap Winding:

Lap winding is a type of winding it is used where the machine bears high current and runs at low voltage.

The emf equation is Eg = φZNP/60A Volts

For lap winding A = P

The emf equation becomes Eg = φZN/60 Volts

2. Wave Winding:

Wave winding is a type of winding it is used where the machine require high voltage and Low current.

The emf equation is Eg = φZNP/60A Volts

For wave winding A = 2

The emf equation becomes Eg = φZNP/120 Volts.

6.Commutator:

Every electric student knows that current is of two types one is alternating current ( AC ) and another one is direct current (DC )

We are discussing about DC machines which works on DC. "Commutator is a device that collect the current from the armature and converts alternating current into direct current."

*It is a cylindrical structure and is built up of wedge shape segments of high conductivity hard drawn (or) drop forged coppers.

*Each and every segment is laminated itself by thin layer of Mica.

*Number of segments = Number of armature coils

7. Brushes and Bearings:

The brushes are made up of carbon whose function is to collect the current from the commutator. Graphite is also used for making brushes.

*The shape of brushes is rectangular blocks

*The brushes are housed in brush holders.

The Bearings are used for smooth rotating operation of a machine instead of getting heavy friction. The ball bearings are frequently employed for heavy duties and high reliability. The balls and rollers are generally packed in hard oil for quiet operation.