Construction of an alternator salient pole .ppt

Alternator

1. Synchronous Machines

2. CONTENTS
Introduction
Types of Synchronous Machine
Construction
Applications

3. TYPES OF SYNCHRONOUS MACHINES
Synchronous machines are principally used as AC machines that have a field
circuit supplied by an external DC source.
Synchronous machine is an important electromechanical energy converter
Based on the output they are called as
Synchronous Generator (or) Alternator
Synchronous motor
Synchronous compensators – used to control of reactive power (P = VI sin )
in power supply networks.
Speed – upto 3000 rpm
Rating upto 100MVAR

4. Major parts of synchronous machine
Stationary part – Stator
Rotating field system – Rotor
Field windings - windings producing the main magnetic field (rotor
windings)
Armature windings - windings where the main voltage is induced
(stator windings)

5. Types of synchronous generators
There are basically two types of alternator
Single phase Alternator
Three phase Alternator
According to the arrangement of armature and field system,
Rotating field type
Rotating armature type

6. Advantages of rotating field system
 Easily supplied - 110 V to 220 V d.c.
 Size of the slip ring will be very small.
 Sparking

7. Number of slip rings - two slip rings.
Increase in number of conductors
Bracing & Deformation

8. Thus the conductors in the overhang must be braced i.e.
their mechanical strength be raised.

9. Less Mechanical stress
Easier to collect larger currents
Low Inertia - field system has very low inertia.
Increase in capacity
Cooling arrangements

10. Based on the type of the prime movers employed the synchronous generators
are classified as
Hydro generators - which uses hydraulic turbines as its prime
mover (used in Hydro power stations)
Rating – upto 750 MW, Speed – 100 to 1000 rpm
Hydraulic turbines are of different types
1. Pelton wheel turbine – high water level 400 m and above
(P=4)
2. Francis turbine - water high below 400 m and above 100 m
(Vertical shaft)
3. Kaplan turbine – water high is below 80 m

11. Classification cont..
Turbo generators
Rating – 1000 MW
Speed – upto 3000 rpm
Engine driven Generators
Rating – 20 MW
Speed – upto 1500 rpm

12. Cross-section of a large turbo generator.

13. Large hydro generator rotor with view of the
vertical poles.
Stator of a large salient pole hydro generator; inset
shows the insulated conductors and spacers.

14. INTRODUCTION TO TYPES OF
ALTERNATORS
1.Salient pole Machines:
Salient pole or projecting poles
Concentrated field windings.
Hydraulic turbines or Diesel engines.
2. Non salient pole or Cylindrical rotor or Round rotor Machines:
Cylindrical smooth rotor
Distributed field winding in slots.
Steam turbines

15. Construction of Hydro-generators
low speed machines (125 rpm – 500 rpm)
low speed demands large number of poles.
To withstand the centrifugal force and stress

16. CONSTRUCTION OF HYDRO-GENERATORS
Stator
Stator is the outer stationary part of the machine, which consists of
Frame
Terminal box
Eye bolt or lifting eye
Feet or base
Stator core
Three phase stator windings
End shield (2 numbers)

17. Frame
The outer cylindrical frame is called as yoke.
In D.C. machines it is used to carry the magnetic flux.
But here it is used for holding the armature stampings and windings in position.
In small and medium capacity machines , the frame have a single piece of short,
hollow cylindrical cast iron castings
In large capacity machines, where the diameter of the machine is large the frame
is made of welded sheet steel.
Frame for large capacity machines are fabricated in four or more arc shaped steel
pieces and are jointed together to form a cylindrical shape.
Ventilation – is provided with help of holes cast in the frame itself.
An Eye Bolt is fitted on the top of the frame for transit purpose.

18. The section through the top of a sectional stator

19. Stator core
The magnetic path, which comprises a set of slotted steel
laminations called stator core pressed into the cylindrical
space inside the frame.
The stator core provides the space (slots) for housing the
three phase stator windings.
Any part of the stator core subjected to alternate changes in
polarity of the magnetic field - causes hysteresis and eddy
current losses.
The magnetic path is laminated to reduce eddy currents,
reducing losses and heating.

20. CRGO laminations of 0.5 mm thickness are used to reduce
the iron losses.
The laminations are either pasted with paper on one side or
insulated from each other by thin layer of varnish.
To provide ventilation laminations are stacked in packets of
about 10cm thickness and a spacing of about 1 cm is
provided between adjacent packets.
The number of slots (s) provided by integer multiples of 3
times the number of poles
S = x(3P) where x = 1,2,3…

21. Different shapes of armature slots are provided
(a) Wide open slot
Advantages:
Permitting easy installation of form – wound coils (coils are wound
before they are inserted into the slot) and their easy removal in case of
repair.
Disadvantages:
Distribution of air gap flux into the bunches that produces ripples in the
wave of the generated emf.
(b) Semi – closed slot
(c) Closed slot

22. Closed slot
Advantages:
They do not disturb the air gap flux.
Disadvantages:
(a) They tend to increase the inductance of the windings
(b) The armature conductors have to be threaded through,
thereby increasing initial labour and cost of winding.
(c) Present a complicated problem of end connections.
(d) They are rarely used.
Semi – closed slot
These slots are better than the above two but they do not allow the use of form-
wound coils.

23. Construction of Hydro-generators
In case small machines the laminations are stamped out in a complete rings.
In case of generators where the diameter is too large stator lamination can not be
stamped out in a complete rings.
In such cases the laminations are punched in segments.
A number of segments are assembled together to form one circular laminations.
The stator core sheets are held together
with end –plates and insulated bolts
The figure shows the stator stampings

24. STATOR core

25. Construction of Hydro-generators
Stator winding - A set of insulated electrical windings are placed inside
the slots of the laminated stator after placing the insulation sheet (mica)
The cross sectional area of these windings must be large enough for the
power rating of the machine.
For a 3- phase generator, 3 sets of windings are required, one for each
phase connected either in star or delta.
The total number of coils C required for all the three phases is given by
C = ½(uS) where u = number of coil sides.

26. STATOR CORE

27. Stator after winding and insulation
(small diameter)

28. Stator of a salient pole alternator
(large diameter)

29. ROTOR CONSTRUCTION
Rotor of water wheel generator consists of salient poles (or) projecting poles
 Poles of identical dimensions are assembled outside by stacking thin silicon steel
laminations of 0.5mm to 0.8 mm thickness to required length – to reduce eddy
current losses.
The laminations are clamped by heavy end plates and secured by studs or rivets.
 For low speed rotors poles have the bolted on construction for the machines with
little higher peripheral speed poles have dove tailed construction as shown in Figs.

31. Generally rectangular or round pole constructions are used for
such type of alternators.

32. ROTOR CORE CONSTRUCTION
However the round poles have the advantages over rectangular poles.
Generators driven by water wheel turbines are of either horizontal or vertical
shaft type.
Generators with fairly higher speeds are built with horizontal shaft and the
generators with higher power ratings and low speeds are built with vertical shaft
design.
Vertical shaft generators are of two types of designs
Umbrella type where in the bearing is mounted below the rotor.
Suspended type where in the bearing is mounted above the rotor.

33. ROTOR

34. ROTOR PARTS
A synchronous rotor with 8 salient poles
Salient pole with field windings
Salient pole without field
windings – observe
laminations

35.  

Rotor Salient Pole Generator 12 MW 16 Poles

36. FIELD WINDING
The field coils are formed from rectangular aluminum or flat
copper strips wound edgewise with inner – turn insulation.
The field windings and the overhangs of the field windings are
secured in place by steel retaining rings to protect against
high centrifugal forces.
The coils may have a smooth or may have some of the turns
made of a wider copper strap so that they protect outside and
act as cooling fins.
The cooling fins increase the external bar copper cooling
area thereby lowering temperature rise.

37. The pole phases are so shaped that the radial air – gap length increases
from the pole centre to the pole tips so that the flux distribution in the
air gap is sinusoidal in shape, which will help the machine to generate
sinusoidal emf.
After placing the field coils around the pole body, they are fitted to a
steel spider of good magnetic quality by T headed joint.

40. All the field coils are connected in series and the resultant
two leads are connected to two slip – rings mounted on the
shaft.
The corresponding brush leads are taken to the terminal box
of the machine and the field winding terminals are marked
as X & XX.
When external DC current is passed through the field
winding, magnetic fields with alternate North and South
poles are set up around the pole faces and get sinusoidally
distributed in the air gap.

41. Damper windings
Damper windings are nothing but the copper or aluminum bars
housed in the semi – closed slots of the pole faces.
The ends of the damper bars are short circuited at the ends by short
circuiting rings
FUNCTION:
providing mechanical balance;
provide damping effect,
reduce the effect of over voltages and damp out hunting in case of
alternators,
In case of synchronous motors they act as rotor bars and help in self
starting of the motor.

42. Damper
bars

43. Assembling of rotor and stator

44. Slip rings & brushes
Slip rings are the rings completely encircling the shaft of a machine made of steel
and are shrunk over cast iron sleeve with micanite between the two.
The brushes are made of Graphite or carbon and are in shape of rectangular
block.
The brushes are connected to DC terminals that ride on each slip ring, supplying
DC voltage to field windings regardless the position or speed of the rotor.
Slip rings
Brush

45. Bearings
For horizontal shaft hydro generators – conventional
type (ball or roller)
For vertical shaft – special features have to be
incorporated in the bearings – because of the
requirements of the rotor and the turbine runner and
the hydraulic thrust – which may be twice the dead
weight of rotating masses.
Oil is supplied to the bearings by pumps and cooled
externally.

46. Field excitation
The field windings of an alternator are excited by direct
current supply which may be obtained in any of the
following ways:
(a)From a dc generator called exciter, mounted on
the shaft extension of the alternator
For moderately rated alternators, exciters are dc shunt
generators.

47. Exciters for large alternators may be separately excited type, whose
field windings are fed from another shunt generator called Pilot
exciter.
The pilot exciter is also mounted on the same shaft as that of the
alternator.

49. (b) Using a separate three – phase synchronous generator as
exciter , mounted on the same shaft as the main synchronous
generator.
The output of the exciter is rectified through a bank of rectifiers and
then fed to the windings of the main generator.

50. BRUSHLESS EXCITER
The brushless excitation system eliminates the usual commutator,
collector rings, and brushes.
One arrangement in which a permanent magnet pilot exciter, an ac
main exciter, and a rotating rectifier are mounted on the same shaft
as the field of the ac turbo generator is shown in Fig.
The permanent magnet pilot exciter has a stationary armature and a
rotating permanent magnetic field.
 It feeds 400 Hz, three-phase power to a regulator, which in
turn supplies regulated dc power to the stationary field of a rotating-
armature ac exciter

51. The output of the ac exciter is rectified by diodes and delivered
to the field of the turbo generator.
Brush less excitation systems have been also used extensively
in the much smaller generators employed in aircraft
applications where reduced atmospheric pressure
intensifies problems of brush deterioration.
Because of their mechanical simplicity, such systems lend
themselves to military and other applications that involve
moderate amounts of power.

53. TURBO - ALTERNATORS
STATOR – the stator core is built up of segmental laminations.
Stator winding of turbo alternator is a double layer winding
The overhang has to be highly reinforced in turbo-alternators – to make
the material structure stronger.
ROTOR - The cylindrical or non salient pole rotor is adopted for Turbo
alternator. (i.e. The poles does not project out from the surface of the
rotor)
The field winding being distributed in the slots, instead of being wound
as in salient poles.
The rotor is generally made up of chromium nickel – steel or
chromium molybdenum steel.

54. TURBO - ALTERNATORS
The rotor consists of core and shaft generally forged(to shape metal by
heating in the fire) in one piece except in vary large sizes.
Number of slots are milled out at the intervals along the outer periphery
of the field winding.
Rotors are distinguished as
(i) radial slot rotor
(ii) parallel slot rotor – because rectangular conductors are used.
 Normally two third of the rotor periphery is slotted to accommodate
the winding and the remaining one third unslotted portion acts as the
pole is called as tooth.

55. ROTOR OF TURBO ALTERNATOR
Polar Axis

56. ROTOR OF TURBO ALTERNATOR

57. ROTOR WINDING
To avoid excessive peripheral velocity they are constructed with small
diameter (V
a = Dn
s)
Concentric multi turn coils are accommodated in slots.
The field winding consists of copper strips laid flat in the slots and
insulated with moulded micanite or asbestos – thus obtaining a solid
winding which will not shrink under the effects of centrifugal
stresses and temperature rise.
A manganese bronze or steel wedge is driven into the mouth of each
slot for the purpose of keeping the winding in place.

58. Large machine rotor completely assembled

59. ROTOR WINDING
The end connectors (overhang) of the field winding must be
rigidly supported by end bells – to with stand large
centrifugal forces to which it is subjected under short
circuit conditions.
The end bells are made of an non-magnetic austenitic steel
in order to reduce leakage flux.

60. Rotor with conductors placed in the slots

61. Operation Principle

The rotor of the generator is driven by a prime-mover
A dc current is flowing in the rotor winding which
produces a rotating magnetic field within the machine
The rotating magnetic field induces a three-phase
voltage in the stator winding of the generator

62. APPLICATION OF
ALTERNATOR
CAR ALTERNATOR FLIGHT ALTERNATOR

THE TURBO-GENERATOR BEING ASSEMBLED THE BRITISH THOMSON HOUSTON ON THE
LOCOMOTIVE FRAME TURBO - ALTERNATOR

63. MAIN PARTS OF
ALTERNATOR
ROTOR
SLIP RINGS
END COVER
TERMINAL BOX
BALL
BEARINGS
BRUSHES
STATOR
SHAFT
TYPES
SALIENT POLE
ROTOR
SMOOTH
CYLINDRICAL
ROTOR
STATOR FRAME
STATOR CORE
STATOR WINDINGS
COUPLED TO
PRIME MOVER
MOUNTED ON
SHAFT
EXCITER
BUILD UP OF
SHEET STEEL
LAMINATIONS
SLOTS LIE ALONG THE
INNER PERIPHERY OF
THE CORE
HOLDING THE ARMATURE
STAMPINGS AND WINDINGS
HOLES CAST IN FRAME
PROVIDES VENTILATION
USED IN HYDRO
ALTERNATORS DRIVEN BY
WATER TURBINES
USED IN TURBO ALTERNATORS
DRIVEN BY STEAM OR GAS TURBINES
INSULATED FROM EACH
OTHER
MIND MAP FOR CONSTRUCTION OF ALTERNATOR