dc Generator Ppt

Electric motors and generators

Jitendra kumar1103107

D.C. GENERATORS-CONSTRUCTION & OPERATION

DC Generators Principle of operation Action of Commutator Constructional details of DC MachineTypes of DC generatorsEMF Equation

DC Generator

DC motor

D.C. GENERATORS PRINCIPLE OF OPERATION

DC generator converts mechanical energy into electrical energy. when a conductor move in a magnetic field in such a way conductors cuts across a magnetic flux of lines and e.m.f. produces in a generator and it is defined by faradays law of electromagnetic induction e.m.f. causes current to flow if the conductor circuit is closed.

Faradays laws

First Law :  Whenever the magnetic flux linked with a circuit changes, an e.m.f. is always induced in it.                                         or  Whenever a conductor cuts magnetic flux, an e.m.f. is induced in that conductor. Second Law :  The magnitude of the induced e.m.f. is equal to the rate of change of flux linkages.

Faradays Law of Electromagnetic Induction

A changing magnetic flux through a  loop or loops of wire induces an electromotive force (voltage) in each loop.

Lenz’s Law

“The induced currents in a conductor are in such a direction as to oppose the change in magnetic field that produces them..”

“The direction of induced E.M.F in a coil (conductor) is such that it opposes the cause of producing it..”

Fleming's Right Hand Rule

• The Thumb represents the direction of Motion of the conductor. • The First finger (four finger) represents Field. • The Second finger (Middle finger) represents Current

Fleming's Right Hand Rule

Are the basic requirements to be satisfied for generation of E.M.F

1.A uniform Magnetic field

2.A System of conductors

3.Relative motion between the magnetic field and conductors

Simple loop generator

Simple loop generator with slip ring

Generators

Basic operation of the generatorAs the loop rotates, the magnetic flux through it changes with timeThis induces an e.m.f and a current in the external circuitThe ends of the loop are connected to slip rings that rotate with the loopConnections to the external circuit are made by stationary brushes in contact with the slip rings

Simple loop generator with split ring

Working Principle of D.C Generator

Schematic diagram of a simple DC Generator1st half cycle(00 to 1800 ) Path of current ABR1B1MLR2B2CD

2st half cycle(1800 to 3600) Path of current DCR2B1MLB2R1BA

1)Yoke

- Acts as frame of the machine - Mechanical support - low reluctance for magnetic flux - High Permeability -- For Small machines -- Cast iron—low cost -- For Large Machines -- Cast Steel (Rolledsteel)

Large DC machine

Small DC machine

2)pole cores and pole shoes

Constructional Details Of DC Machine

Yoke:Rotor: Stator: Field electromagnets: Pole core and pole shoe:Brushes:Shaft: Armature: Coil: Commutator:Bearings:

Construction details of DC generator

Cross section view of dc machine

Practical Dc Machine

2)pole cores and pole shoes a) Pole core (Pole body) :- --Carry the field coils --Rectangle Cross sections -- Laminated to reduce heat losses --Fitted to yoke through bolts b) Pole shoe:- Acts as support to field poles and spreads out flux Pole core & Pole shoe are laminated of annealed steel (Of thickness of 1mm to 0.25 mm)

4)commutator:--Hard drawn copper bars segments insulated from each other by mica segments (insulation) -- Between armature & External circuit -- Split-Rings (acts like Rectifier AC to DC )

5&6 Bearings and Brushes 5)Brushes and brush gear:- Carbon, Carbon graphite, copper used to Collects current from commutation (in case of Generator)6)Shaft and bearings:- Shaft-- Mechanical link between prime over and armature Bearings– For free rotation

DC Machine Construction

Armature Winding

Armature Winding is classified into two types:

Lap winding

Wave windings

Armature windings

Lap Winding:

are used in machines designed for low voltage and high current

armatures are constructed with large wire because of high current

Eg: - are used is in the starter motor of almost all automobiles

The windings of a lap wound armature are connected in parallel. This

permits the current capacity of each winding to be added and provides a

higher operating current.

No of parallel path, A=P ; P = no. of poles

Wave winding:

are used in machines designed for high voltage and low current

their windings connected in series

When the windings are connected in series, the voltage of each winding adds, but the current capacity remains the same

are used is in the small generator.

No of parallel path, A=2,

Commutation process in D.C Generator

Commutation is the positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction.

The total losses in a dc machine

1.Cu losses2.Iron losses3.Mechanical losses

Cupper losses are mainly due to the current passing through the winding. 1.Armature cu losses (30 to 40% of full load losses)

Cu losses 2.Shunt field cu losses(20 to30% of full load losses)

3.Series field cu losses

1.Cu losses

Armature cu losse s= Ia2 Ra

Ra=Armature resistance , Ia= Armature current--Losses due to brush contact resistance is usually include in armature cu losses

Shunt field cu losses = Ish2Rsh

Rsh=Shunt field resistance, Ish=Shunt field current

Series field cu losses = Ise2Rse

Rse=Series field resistance , Ise=Series field current

2.Ironlosses (Magnetic losses) (20 to 30% of full load losses)

1)Hysteresis losses 2)Eddy current losses

1)Hysteresis losses (Wh)

The losses is due to the reversal of magnetisation of the armature coreEvery portion of the rating core passes under N and S poles alternately. There by attaining S and N polarity respectively. The core undergoes one complete cycle of magnetic reversal after passing under one pair of poles. P=No. of poles N= Armature speed in rpm frequency of magnetic reversals f=NP 120 The losses depends upon the volume and B max and frequency of reversals.Hysteresis losses is given by steinmetz formula Wh=η B1.6

maxf V wats V=Volume of the core in m3

η= Steinmetz hysteresis coefficient

2)Eddy current losses:-(We)

when the armature core rotates, it cuts the magenetic flux hence an e.m.f induced in in the body of the core according to faradays law of electro magnetic induction. This e. m.f through small sets up large current in the body of the core due to its mall resistance. This current is known as “Eddy Current”

We=k B2 maxf2t2v2 watts

Bmax=maximum flux densities f=Freequency of the magenetic reversals v=volume of the armaturecore t=Thick ness of lamination

Efficiency of D.C Generator

Efficiency of generator is defined as the ratio of output power to input powerEfficiency (η) =output ×100 input input=output+ losses (or) output=input-losses For D.C generator input mechanical & output electrical

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