Electrical machines

1. DC Generator –
Construction & Working
Principle

2. MODULE 2 DC GENERATORS
• Construction
• Windings
• Principle of operation
• Types
• Characteristics
• Armature reaction
• Commutation
• Parallel operation
• Applications

3. D.C. GENERATORS PRINCIPLE OF
OPERATION
 Converts mechanical energy into electrical energy
 Working Principle - Faradays law of electromagnetic
induction

4. Constructional Details Of DC Machine
 Yoke
 Rotor
 Stator
 Field electromagnets
 Pole core and pole shoe
 Brushes
 Shaft
 Armature
 Coil
 Commutator
 Bearings

5. Cross section view of dc machine
N
S
shaft

6. Practical Dc Machine

7. 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 (Rolled steel)
Large DC machine Small DC machine

8. 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)

9. c) Field coils (Magnetizing coils):-
-- Provide excitation (exciting coils) i.e field flux
--Number of poles depends speed of armature on
and the output for which the machine designed
--Frame to used for design for exciting coils
Different types of fields
i) Separately Exciting
ii) Self Exciting
Pole cores and Pole shoes

10. Armature core
a) Armature core (Armature):-
-- To support armature windings
--To rotate conductors in a magnetic field
-- it is cylindrical or drum shaped
--Made of high permeability silicon steel stampings
(of 0.5 mm thick)

11. -- Each stamping is separated from its neighboring one by
thin varnish as insulation
--Laminated to reduce eddy current losses
-- A small air gap between pole pieces and armature so that
no rubbing between them
-- High grade silicon steel used to reduce
i) Hysteresis loss
ii) Eddy current loss
-- Ventilating ducts are provided to dissipate heat to
dissipate heat generated by above losses
b) Armature Winding:-
Main flux cuts armature and hence E.M.F is induced
--winding made of Copper (or) Aluminum
--windings are insulated each other
Conductor system

12. 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 )

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

14. DC Machine Construction

15. DC Machine Construction

16. DC Machine Construction

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

18. 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
E.M.F

19. 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
Magnetic field :-
Permanent Magnet
(or)
Electro Magnet (practical)
Conductor :-
Copper (or) Aluminum bars placed in
slots cut around the periphery of
cylindrical rotor
Relative motion:-
By Prime Mover – Turbine,
I.C Engine (Internal combustion)

20. Simple loop generator

21. Simple loop generator with slip ring

22. Simple loop generator with split ring

23. Simple loop generator with split ring

24. Armature Winding
 Lap winding
 Wave windings

25. Lap Winding
• Used in machines designed for low voltage and high
current
• Armatures are constructed with large wire because of
high current
• Windings 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

26. Wave winding
Used in machines designed for high voltage and low
current
windings connected in series
When the windings are connected in series, the
voltage of each winding adds, but the current capacity
remains the same
Used is in the small generator.
No of parallel path, A=2

30. EMF Equation of a generator
Let  = flux/pole in Weber
Z =Total number of armature conductors
=No. of slot × No. of conductors/slot
P= No. of generator poles
A =No. of parallel paths in armature
N= Armature rotation in revolutions per minute (r. p. m)
E= e.m.f induced in any parallel path in armature
Generated e.m.f Eg= e.m.f generated in any one of the
parallel paths i.e E
Average e.m.f generated/conductor = d  volt
dt
Now, flux cut/conductor in one revolution d  = P wb

31. No. of revolutions/sec=N/ 60
Time for one revolution , dt= 60 /N sec
According to Faraday’s Law of electro magnetic induction
E.M.F generated/conductor = d= PN volts
dt 60
No. of conductors (in series) in one parallel path= Z / A
E.M.F generated/path=  PN × Z Volts
60 A
Generate E.M.F, Eg= Z N × P Volts
60 A
For
i) Wave winding A = 2
ii) Lap winding A = P