2. D.C. GENERATORS-CONSTRUCTION & OPERATION
DC Generators
Principle of operation
Action of Commutator
Constructional details of DC Machine
Types of DC generators
EMF Equation
5. 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.
6. 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.
7. Faradays Law of Electromagnetic Induction
A changing magnetic flux through a loop or loops of wire induces an electromotive
force (voltage) in each loop.
8. 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..”
9. 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
11. 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
14. Generators
Basic operation of the generator
As the loop rotates, the magnetic flux through
it changes with time
This induces an e.m.f and a current in the
external circuit
The ends of the loop are connected to slip
rings that rotate with the loop
Connections to the external circuit are made by
stationary brushes in contact with the slip rings
16. Working Principle of D.C Generator
Schematic diagram of a simple DC
Generator
1st half cycle(00 to 1800 ) Path of current
ABR1B1MLR2B2CD
2st half cycle(1800 to 3600) Path of current
DCR2B1MLB2R1BA
17. 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
22. 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)
23. 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 )
24. 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
28. 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
29.
30. 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,
31.
32. 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.
33. The total losses in a dc machine
1.Cu losses
2.Iron losses
3.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
34. 1.Cu losses
Armature cu losse s= Ia
2 Ra
Ra=Armature resistance , Ia= Armature current
--Losses due to brush contact resistance is usually include in armature cu losses
Shunt field cu losses = Ish
2Rsh
Rsh=Shunt field resistance, Ish=Shunt field current
Series field cu losses = Ise
2Rse
Rse=Series field resistance , Ise=Series field current
36. 1)Hysteresis losses (Wh)
The losses is due to the reversal of magnetisation of the armature core
Every 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
37. 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
38. Efficiency of D.C Generator
Efficiency of generator is defined as the ratio of output power to input power
Efficiency (η) =output 100
input
input=output+ losses (or) output=input-losses
For D.C generator input mechanical & output electrical
39.
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