D.C generator

 D.C. Generator is a machine which converts the mechanical power into electrical power. It works on the principle of Faraday's Law of Electromagnetic induction.

According to this law the conductor (Armature) are rotated in the magnetic field system and e.m.f. is induced in these conductors which is collected from the commutator fitted on the shaft of armature.

The D.C. Generator can work as D.C. Motor if the electrical power is fed to this machine. Then this machine will give mechanical power .

MAIN PART OF D.C GENERATOR : 

1. Yoke (body)

2. Pole

3. Armature

4. Commutator

5. Brush

6. Brush Holder

 7. Brush Lead

8. Brush Rocker

9. End cover

10. Cooling Fan 

11. Bearing

12. Eye Bolt

13. Bed Plate

14. Terminal box

15. Shaft and Pulley

1. Yoke (body ) :It is made of either cast iron, cast steel for big machines, rolled steel. It supports the field winding and it is outer part of the machine .it has two function first hold all the part of machine and second is provided Magnetic path .

2. Poles : The poles are either cast with frame or bolted to it. At the end of poles there are pole shoes bolted to it. The poles are surrounded by field coils supplied with current usually from the machine itself

for the excitation. it is the inner part of the machine and fitted with the body with the help of screw .

3. Armature. It consists of (i) A laminated core of magnetic material, (ii) Conductors in form of winding, (iii) Commutator at one end of armature core .the slots are cut in the core in which the Conductors are placed in the from of coil .the purpose of armatur is to rotate the Conductor in the magnetic field .

4. Commutator : It consists of copper segments insulated from each other and from the shaft by mica pieces. The ends of the is wound on the armature are soldered on the segments of commutator .

The purpose of the commutator is to take in and give out the currents from the armature. It keeps the -ve and + ve terminal constant in the circuit. Hence, commutator converts A.C. to D.C. 

this is a copper ring type but is the combination of small pieces. The one piece is called segment. Many segments insulated from each other are combined together tightly and fitted on the insulated shaft of the armature. This is called commutator. The terminals of the winding coils are soldered on the segments. Carbon brushes rest on this commutator to collect the supply from the armature. Mind it that commutator rotates and brushes are fixed. The work of commutator is to change the A.C. voltage produced in the

armature conductors to unidirectional or D.C

5.brush : It consists of two or more sets of stationary carbon brushes held in box-type brush holder. The brushes make contact on the commutator surface with a certain amount of pressure supplied by a spring in brush-holder. The brush- holders are clamped on the spindles which are insulated from the shaft and mounted on the brush rocker.The rocker is capable of movement so as to obtain best position for the brushes on the commutator. 

6. Brush Holder: It is hollow brass rod its holds the brush like lamp holder and spring is fixed with brush holder to provide spring pressure for brush. 

7. Brush Lead: it's the  small piece of thick wire which connected with in brush. Its work is to connect with the brush circuit. 

8. Brush Rocker: It is a round plate fitted with brush holders and brushes. Its function is to adjust the position of brushes by rotating it .

9. end cover : the end cover is made on cast iron . it is fitted both side of the machine .

10. Cooling Fan: the cooling fan is made on cast iron,aluminium and it is connect  with armature shaft opposite to commutator 

11. Bearing: Its connect with side cover and its function is to minimise friction effect.

12. Eye-Bolt: the upper most part of the machine, fitted  top side of the yoke. 

armatur winding : Usually two types of windings can be done on the armature - one is called lap winding and second is called wave winding. Difference between the two is as follows : 

lap winding : 

1. As many parallel paths as the no. of poles.

2. Full current is distributed between

these paths. 3. As many brushes set as the paths

 4. It is used in the machines which are for low voltage, high current.

wave winding : 

1. Only two parallel paths irrespective of no. of poles.

2. Full current is distributed between two paths only.

3. Only two brush sets are required.

4. It is used in the machines used for low current and high voltage.

E.M.F Equation of a D . C Generator : 

  Φ   = Flux per pole in wb (webers). 

Z= Total conductors on the armature.

P = No. of poles. 

A = Parallel paths on the armature.

N = Armature speed r.p.m. 

E=E.M.F. induced.

Classification of D . C Generator : 

1. Separately Excited. 

2. Self-excited. Self-excited generators are further subdivided into : 

1. Series wound generator.

2. Shunt wound generator.

3. Compound wound generator. Compound wound generators are again subdivided into :

1. Short shunt compound. 

2. Long shunt compound

1. Separately excited generator : 

Such generator has its winding like shunt wound generator . thin wire many turns. The winding is excited through a battery and armature is driven by prime mover. The voltage is induced at brushes and can be loaded . Such generators are used to excite the big Alternators

The principle of D.C. Generator is according to the Faraday's Law of the Electromagnetic induction, i.e., whenever a conductor is moved in a magnetic field, e.m.f. is induced in the conductor.

Shunt-wound Generator : 

 In this generators the field windings are connected in parallel with the armature conductors and are made with many turns of fine wire. Shunt generators with regulators are used for lighting and power supply purposes. These are also used for charging batteries because these generators supply the suitable charging voltage according to the condition of the battery .

Series-wound Generator : 

The field coils are wound with few turns of thick wire of large cross-section and are connected in series with the armature. Before the machine excites, the external circuit must be closed. Series generators are used for specific purposes . boosters. In this type the voltage increases as the load increases

Compound Generator : 

There are several types of compound generators, the most common being the short-shunt cumulative generators. This has the shunt field 75 connected across the armature, and the current flow in the shunt field is in the same direction as in the series field. This generator can also be connected long shunt

Differentially Compound Generator: 

 It is used only for winding to its characteristics. At starting, more voltage in available which is necessary to produce the spark. Then due to load, voltage falls down to the limit necessary to maintain the flame. Now due to wavering of hand, if the distance between the electrode and job increases i.e. load decreases thereby increasing the voltage of the Generator which maintains the flame in the increased gap. If the goes to the same distance then the load increases thereby decreasing the voltage and maintaining a constant flame. Thus a constant smooth flame is obtained from the Generator in welding. A choke coil is also connected in series off load to get more smooth flame.

Series Generators are used for lighting arc lamps. These can also used as constant current generators. These generators are used as boosters for which these are connected in series with the load instead of connecting them in parallel.

Armature reaction. 

When the generator is loaded, the armature conductor carries current the current carrying conductors produce a magnetic flux of its own which effects the flux due to main poles. This effect of m.m.f. set up by the armature currents on the distribution of flux under the main poles is called armature reaction .

 this armature reaction is there only when the armature conductors carry current and this effects increases as the armature carries more current .

Before understanding the armature reaction let us understand two definitions: 

G.N.P. Full name is Geometrical Natural Plane. It is the line which bisects

the angle between the two centres of the adjacent poles.

M.N.P. Full name is Magnetic Neutral Plane. It is the line where no em is induced in the armature conductors. Meeting and separating point of two e.m.f. occur at this line, and the brushes are placed at these points.

Now, to understand armature reaction  It shows the armature of a 2-pole generator rotated in a clockwise direction, the e.m.f. induce or the direction of current in the conductors is represented by dots and crosse according to Fleming's Rights Hand rule. The brushes are placed along the G.N.P. which is also an M.N.P.

the two fluxes. One flux is due to the field winding in the air gap and armature core. It is seen that the main flux is parallel to the axis of the poles. Second, flux is due to the armature conductors carrying current. Its field direction is according to the right hand cork screw rule.

These two m.m.fs. act at 90° to each other at the same time . These two m.m.fs. produce a resultant m.m.f. which is twisted in the direction of rotation . Thus, the magnetic axis is no longer magnetic neutral axis. Thus, magnetic neutral axis will be shifted along the M.N.P. Hence, the sparking will be there. So, to have sparkles commutation the brushes must also be shifted and placed alo neutral plane.

Interpole : 

 It is a better method of providing the commutating field. These are small auxiliary poles placed in the G.N.P., that is midway between the main poles. This polarity must, in case of generator, be that of the next main pole further ahead . These  are connected in series with the armature so that the commutating field should be proportional to the armature current. The modern machines will operate between no load and 20 or 25% overload with fixed brush position without appreciable sparking.