An electrical generator is a machine that converts mechanical energy into electrical energy. This conversion is based on the principle of the production of dynamically induced emf .whenever a conductor cuts magnetic flux, dynamically induced emf is produced in it according to Faraday’s law of electromagnetic induction. This emf causes current to flow if the conductor circuit is closed.
The construction of the DC generator is shown in above fig.
The DC generator has the following important parts :
- yoke or frame:
the yoke of the dc generator has two functions
(i)It provides mechanical support for the machine and acts as a cover for the machine.
(ii)It formats a portion of the magnetic circuit. The yoke is made up of cast iron for smaller machines and for larger machines, it is always made up of fabricated steel either in one or more pieces.
2. Interpol :
An Interpol is also called a commutating pole. These poles are arranged strictly midway between the main poles and are bolted to the yoke. These poles are usually made up of solid steel, but machines operating on sharply varying loads are made up of sheet steel.
A commutator converts the alternating voltage into direct voltage. A commutator cylindrical structure built up of segments made of hard-drawn coppers.
4. Armature :
The Armature core is keyed to the machine shaft and it rotates between the field poles. It consists of slotted steel lamination. The laminations are insulated from each other by a thin coating of varnish. The purpose of laminating the core is to reduce the eddy current loss. The laminations are slotted to accommodate the armature winding.
Armature winding is wound in two ways
(i) Lab winding: The finish of each coil is connected to the start of the next coil so that the winding or commutator pitch is unity. It is suitable for comparatively low voltages but high current generators.
(ii) Wave winding: The finish of the coil is connected to the start of another coil well away from the first coil. It is suitable for high voltages and low currents.
5. Field coils :
field coils are usually wound with enameled copper wire. sometimes cotton insulation is used. The coils are dried in a vacuum and then impregnated with an insulating compound. The magnetic field strength depends upon the current flowing through the coil.
6. Dummy coils :
These coils are also called idle coils. These are used to provide mechanical balance for armature because having some slots without winding’s would be out of balance mechanically.
E.m.f equation of generator:
Emf equation of a generator is
Eg = generated emf per parallel path in the armature
p = number of poles
= flux/pole in wb
Z = total no of conductors
N = rotational speed of the armature
a = no of the parallel path in the armature.
Types of D.c generator :
sending current through the field winding for the production of magnetic flux is called excitation. depending on the method of excitation D.C generators are classified
According to methods of excitation D.c Generator
classified as follows as
- separately excited generators
2. self-excited generators
the self-excited generator can be classified as dividend in accordance with how the field winding is connected into generators as follows,
- shunt-wound generators
2. series wound generators
3. compound wound generators
(i) short shunt
(ii) long shunt
Applications of dc generators :
It is mainly used in self-contained plants such as automobiles and airplanes. for electric arc welding, train car lighting, submarines, etc.