d.c generator

Generator and its types

Generator is a machine which is used to produce electricity; it requires magnetic flux, conductors and mechanical torque. A machine which converts the mechanical energy is called a generator. A generator work on the principal of electromagnetic induction presented by Michael Faraday.

(A). Principal:- Whenever a conductor cuts magnetic lines of force an e.m.f. is induced in it

(B). Types:- 

(1) On the basic of prime mover-

(a) Water turbine type

(b) Steam turbine type

(c) Diesel engine type

(2). On the basic of excitation method used-

(a) Permanent magnet type

(b) Separately excited type

(c) Self excited type

1. On the Basic Of Prime Mover:- The driving system used to rotate the armature of a generator is called the Prime-mover

(a) Water Turbine Type- The conductors of armature coil are placed on the shaft of the generator. the shaft can be rotated with the power produced by a flowing water. water is stored at a high level by making a dam on a river etc. now the potential wheel. when the water-stream falls from a high level on the vanes of turbine-wheel, then the wheel begins to rotate, which in turn rotates the shaft of generator, as the generator produces the e.m.f.

Water Turbine

(b) Steam turbine type- In this generator, its shaft is rotated by the pressure of steam, discharged through a high pressure of steam and in turn it rotates the shaft of generator as in a steam-engine.

(c) Diesel engine type- In this generator, its shaft is rotated by an auto-engine which is operated by diesel or petrol.

2. On The Basic Of Excitation Method Used

(a). Permanent Magnet type- In this generator, a permanent magnet is used to produce the magnetic flux. these are small generators which are used in bicycle, telephone, megger etc.and are known as dynamos, as 

Permanent Magnet Type Dynamo

(b). Separately Excited type- In this generator, electromagnets are used to produce the gametic flux. the poles of field are excited from entirely separate source of supply such as battery or d.c. main. a.c. producing machine are known as A.C. generators or alternators.

(c). Self Excited Type- In this generator, the poles of field are excited from the current induced by the generator it self. for initial production of e.m.f. the residual magnetism of the poles is used. same flux is always present in the poles and when the armature is rotated, an e.m.f. is induced in it which to turn is supplied to the field coils and it strengthens the pole flux.

Separately Excited                                   Self Excited

Working Of An Alternator- According to Faraday's laws, if a conductor is moved in a magnetic field and it cut the magnetic lines of force, then an e.m.f. is induced in it. the magnitude of induced e.m.f. depends on the rate of change of flux or the velocity at which the conductor is rotated in the magnetic field. a simple generator consisting of a single rectangular loop abcd  connected to two metallic rings x and y. two carbon brushes make a sliding contact which are connected to the load.

A Single Alternator

the conductor loop is rotating in an anti-clockwise direction in the magnetic field such that conductor AB rises upwards while CD goes downwards. the direction of e.m.f. in each conductor is determined by 'Fleming's right hand rule', which is from b to a and d to c. when the loop is perpendicular  to the field, then maximum e.m.f. is induced in it. and when the loop is parallel to the field, then maximum e.m.f. is induced in it.

Different Position of loop

the different positions of loop are then the conductor ab is near to north pole, then the direction of current in this conductor is from b to a; but when it is near to the south pole, then the direction of current is from a to b in this way the current produced is of alternating nature.

Angles Between The Conductor and The Flux

Fleming's Right Hand Rule- if the thumb, fore-finger and middle finger of the right hand are stretched perpendicular to each other, and the thumb points the direction of motion of the conductor, fore-finger points the direction of the flux then the middle finger will point the direction of e.m.f. inductor. 

Fleming's Right Hand Rule

Working Of Slip-ring and Commutator

(a) Slip Ring- two copper rings, fitted on the shaft of a generator in such a way that these remain insulated from the shaft, are known as slip-rings. the conductors (or coils) are connected to these rings.two carbon brushes which make a sliding contact with the rings are connected to the external circuit to give A.C. supply

(b) Commutator- The commutator of copper segments which are assembled into a cylindrical shape. the segment and the whole commutator is kept insulated from the shaft with help of mica or micanite sheets. the ends of conductors (or coil) are soldered to the whole commutator is kept the segment and the shaft with whole commutator is kept insulated from the shaft with help of mica or micanite sheets.

Commutator

the ends of conductors are soldered the segment. the commutator supplies D.C. to the external circuit. because, for a half cycle the rotation of conductor, the direction of current remains the same. one conductor remains positive for half cycle and it supplies the e.m.f. to positive brush. in the other half cycle, second conductor comes near the positive brush and it supplies (+) e.m.f. to in this way, the e.m.f. supplied to the external circuit is D.C.

Working Of D.C. Generator- A single conductor loop abcd is placed between two field-poles N and S. the conductor ab is connected to x and c,d to y segment of the commutator as if the loop is rotated in an anti-clockwise direction, then the direction of induced e.m.f. in conductor ab is from b to a.

Simple Generator

the induced current flows towards B to A through the brush A. while in the same time the direction of induced e.m.f. in conductor cd is from d to c. there fore. the potential of brush A is position higher then that of brush B or the brush A is positive and the brush B is negative. After half cycle, conductor cd comes near field-pole and the direction of induced e.m.f. in it is from c to d, while in conductor ab, it is from a to b. in this position, commutator segment x comes in contact with brush A and the segment y with brush B. even now the direction of current in the external circuit is from B' to A; therefore, the induced e.m.f. is unidirectional, as two coils are used, then 4 commutator segment will be required. the brush will get positive e.m.f.4 time in each cycle of rotation, and thus the output available at the brushes A and B will be of the shape, as in this way if more then two coils are used then the resulting e.m.f. will be of nearly straight line shape.

Construction Of D.C. Generator- A D.C. generator consist of following parts:

1. Yoke and plates                                                         2. Field 

3. Armature                                                                   4. Commutator

5. brush and rocker                                                        6. Bearing

1. Yoke and Plates- it is made of cast iron or cast steel. it give mechanical support to the pole cores and also serves as a part of magnetic circuit. generally, machine can be lifted and conveyed to any other place. easily. two side disc plates are also fitted to cover the machine add to give support to the the bearings and shaft. 

Parts of D.C. Generator

2. Field- field coils are placed around the pole cores. these coils produce the magnetic flux for the machine. the pole face, also known as pole as pole shoe, is made larger then the main body, as in small machine the pole are made of cast iron or cast steel, but in large machine these are made of sheet-steel lamination's which are insulated from each other and riveted together. cast iron or cast steel provides an easy path to the magnetic lines of force. 

3. Armature- It is a cylindrical drum and the armature coils are fitted in it. it has two parts

1. Armature Core
2. Armature Winding

1. Armature Core- they are made of sheet-steel lamination's which are insulated form each other by insulated-varnish coating. laminated cores have a low degree of eddy current loss. the thickness of a lamination varies from 0.33 mm to 0.5 mm each lamination is slotted, as all the laminations are bolted together to form a drum and the armature coils are fitted in the slots.

Armature                        Armature Core

2. Armature Winding- Armature winding are wound to the correct shape and size of the armature cores of formers with enamelled copper wire. these are following two types of Windings:

(a) Lap Winding.

(b) Wave Winding.

(a) Lap Winding- In this method, windings are overlapped on each other, as in it is also known as parallel winding because the number of parallel path is kept equal to the member of poles. this type of winding is suitable for large current and comparatively low voltage machine.

Lap Winding                             Winding Connection

(b) Wave Winding- In this method the armature coils are divided in two parallel paths irrespective of the number of poles. the whole winding is done in the shape of progressive wave, as two sets of brushes are enough for this winding. this type of winding is suitable for high voltage and low current machine.

Wave Winding                          Winding Connection

4. Commutator- it is used to provide a D.C, output from the alternating e.m.f. induced in the armature. it as made of hard drawn copper, which is a good conductor and has a low specific resistance. Along with it, copper is suitable for soldering the winding wires on it.

Commutator Segment

 it consists of number of segment insulated from each other and also from from the shaft with mica sheets, as these is a groove of V- shape on one side of the segment. A V- shaped mica sheet is placed between the segment and the steel ring.

Armature Assembly

5. Brush and Rocker- A rocker is fitted on the front end plate it is made of backeline  and it has a round brush are used to make a sliding electrical contact between the armature and the external circuit.

Rocker and Brush Assembly

6. Bearing- They are used to support the shaft and are fitted on the front and near end plates. they enable the shaft to rotate smoothly in the field alongwith the armature. following types of bearings are used - (1) Gun metal bush-bearing -these are used in small size machine.

(2) ball or roller-bearing- these are used in medium size machine

(3) Babbit metal bush- these are used in large size machine and are made by joining two half pieces.

Leakage flux- the magnetic flux leaked out from the poles is called the leakage flux or flux-loss. it is of no use and it causes a power loss. the magnet flux, passing through the pole and pole-shoes, is the only useful flux. when the armature coils cut the magnetic flux produced by pole-shoes, then an e.m.f. is induced in then.

• Its resistance decreases with a rise in temperature.
• It does not oxides.
• It can work satisfactory even on high temperatures.
• It can easily be moulded into required shape.
• It has a current carrying capacity of 4 to 6 amperes per square cm, its width is kept 3 to 4 time greater then that of a commutator segment.

Assembly of a D.C. Machine