TYPES OF D.C GENERATOR

The power station of modern design generate practically only three phase alternating current.A large part of this power is used in the form of alternating current in industry,for lighting and domestic needs.When industrial needs make it necessary too or when it is of greater advantage to use direct current .It is generally obtained by converting A.C to D.C with help of converters of ionic or machines type.

As primary sources of power,D.C generators are mainly used in self contained plants such as automobiles and airplanes,for electric arc welding,train car lighting,in submarines etc,.

Types of D.C generator :

sending current through the field winding for production of magnetic flux is called Excitation.Depending on the method of excitation D.C generator are classified as

1.separately excited D.C generators

2.self excited D.C generators.

1.Separately excited D.C generators

These generators whose field magnets are energised from independent external source of D.C current.

 In separately excited D.C generators,the exciting field current is supplied by a separate source as shown in above.

2.Self excited D.C generators :

These generators whose field magnets are energised by the current produced by the generators themselves.Due to residual magnetism there is always present some flux in the poles .When the armature is rotated,some emf and hence some induced current produced which is partly or fully passed through the field coils there by strengthening the pole flux.

self excited generators can be divided in to accordance with how field winding is connected into generators as follows,

(i) D.C shunt generators

(ii) D.C series generators

(iii)D.C compound generators

(i)D.C shunt generators:

In shunt generators the field winding is connected in parallel with the armature winding terminals and have the full voltage of the generator across them.

some important relations are

The shunt generator field winding has many turns of fine wire having high resistance.In DC shunt generator the armature current (Ia) will be the sum of load current (I) and the field current(Ish).

Electric shock and safety measures

INTRODUCTION ELECTRIC SHOCK:

Our human body is a conductor.When the live wire is touches the body,current completes its path through body and earth.Hence the muscular function body are paralysed due to the currents action on the nervous system and causing breathing to stop,severe burns and resulting death.

This is the symbol of electric shock is

Prevention and precaution against electric shock:

To prevent electric shock and danger the following precaution should be observed before using the electric equipment's.

1.Always be careful.

2.All electrical equipment's are connected with good quality wires.

3.Before replacing  a blown out fuse always put the main switch off.

4.Always keep the earth connection in good condition.

5.Before using portable electrical things like table fan,iron box heaters etc,see that these are well earthed.

6.Line wires should always be connected through the switch.

7.Replace immediately broken or damaged switches and plug etc.

8.A plug point should never disconnected by pulling the flexible cable.

9.Plug point connection should be made only by plug tops not by bare wires.

10.Check proper working devices.

11.Keep proper condition of electrical hand tools.

The purpose of earthing the electrical equipment's is to bring it to zero potential and thereby to avoid the shock to the operator. 

Due to failure of insulation of a live conductor it may be in contact with the frame of the machine.If the frame is not earthed when somebody is touches the frames of the equipment's they will get shock.

But if the frame of the equipment is earthed the voltage of parted with respect to the earth doesn't rise to dangerously higher value.Thus severe shock is prevented. 

Corona effect in Electrical energy

INTRODUCTION :
This type of effect can be noticed that near the overhead lines there exists a hissing noise and sometimes a faint violet glow.
CORONA EFFECT:

When normal alternating voltages is applied across two conductors which conductors are enough spacing in between,then there is no change in the atmospheric conditions surrounding conductors.but if the applied voltage exceeds a particular limits value then the air surrounding the conductors gets ionized due to which hissing noise or faint violet glow appears.

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona.

If the conductors are polished and smooth, the corona glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter. With d.c. voltage, there is difference in the appearance of the two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow.

Theory of corona formation in transmission lines :

The corona effect takes place when applied voltage exceeds the particular value which is called critical disruptive voltage.