Alternator,Uses of Alternator ,Working principle ,Basic structure ,Types of Rotor ,Pitch Factor,Distribution Factor,Speed of alternator ,Unity Power Factor ,Zero Power Factor Lagging,Zero Power Factor Leading ,Alternator on Load

1. WELCOME TO
OUR
PRESENTATION

3. Alternator
An alternator is an electrical generator that converts mechanical energy to
electrical energy in the form of alternating current. For reasons of cost and
simplicity, most alternators use a rotating magnetic field with a stationary
armature. Nikola Tesla invented it in 1830.
* It’s capable to generate AC power at specific frequency.
* It’s also called synchronize generator.
* It’s based on electromagnetic induction.
* Used in the load shedding and power cast.

4. Uses of Alternator :
Alternators are used in modern automobiles to charge the battery and to power the electrical
system when its engine is running. Until the 1960s, automobiles used DC dynamo generators
with commutators. With the availability of affordable silicon diode rectifiers, alternators were
used instead.
It is used in :
Nuclear Power Station Thermal Power Station Hydro
Eclectic Power Station

5. Working principle :
Flemings right hand rule
1.The thumb is pointed in the direction of motion of the conductor.
2.The first finger is pointed in the direction of the magnetic field. (north to
south)
3.Then the second finger represents the direction of the induced or
generated current

6. Basic structure

7. Rotor :
The rotor is a moving
component of an
electromagnetic system in the
electric
motor , electric generator ,
or alternator. it’s rotation is
due to the interaction
between the windings and
magnetic fields which
produces a torque around the
Rotor’s axis.

8. Types of Rotor :
Salient Type rotor Turbine driven
rotor

9. (c) Armature Reaction:
1. Armature reaction is the effect of armature flux on the main field flux.
2. The power factor of the load has a considerable effect on the armature
reaction.
We will consider three case:
1. When load of p.f. unity ;
2. When p.f. is zero lagging ;
3. When p.f. is zero leading .

10. Construction of Stator:
Stator is identical to the induction motor Laminated low silicon steel rings joined
together Slots insulated with Mylar Example of 36 slot stator with 3 coil conductors
per slot, 12 slots per phase. Slot insulator inserted by hand Coils inserted by hand
Coils can be placed in single or double layers.
Stator Frame:
Holding the armature stampings Holding the armature windings in
position Maintaining ventilation with the help of holes cast in the
frame. Instead of castings, frames are fabricated from mild steel
plates welded together. Having a box type section.

11. Pitch Factor:
The ratio of vector sum of the induced e.m.f s per coil and arithmetic sum
of the induced e.m.f per coil is called pitch factor .Which is denoted by kp.
Kp=Vs/As
Here,
Vs=vector sum of e.m.f per coil
As= Arithmetic sum of e.m.f per coil
It is always less than unity.
Ø/2 Ø/2
Es
Es

12. Let Es be the induced e.m.f in each side of the coil. If
the coil were full pitch i.e if its two sides were one
pole – pitch apart then the total induced e.m.f in the
coil
E=Es+Es
=2Es
And it is short - pitch by Ø then the vector sum of the
e.m.f s is Vs .
Vs=Es cos Ø/2+Es cos Ø/2
=2Es cos Ø/2
So, pitch factor Kp =Vs/As
= (2Es cos Ø/2)/2Es

13. Distribution Factor:
The ratio of vector sum of coils e.m.f and arithmetic sum of coils e.m.f
is called distribution factor which is denoted by Kd .
Kd =V.S.D.E/A.S.D.E
Here,
V.S.D.E =vector sum of distributed e.m.f
A.S.D.E =Arithmetic sum of distributed e.m.f
Ø
Ø
Ø
Es
Es
Es
Es

14. Let Ø be the value of angular displacement between slots . Its value
is :
Ø=180 /n [n=(number of slots)/pole]
Let, m=number of slots /pole/phase
mØ=phase spread angle
Then the voltage induced in one polar group = mEs
So, the vector sum of E.M.F = Es x Sin(mØ/2)
Arithmetic sum of E.M.F = m x Sin(Ø/2)
Distribution factor of Kd = { Es x Sin(mØ/2)} / {m x Sin(Ø/2) }
= Sin(mØ/2) / mSin(Ø/2)
,

15. From Factor :
Kf = r.m.s value / average value
Here,
r.m.s value = r/ √2
average value= (2/ π)
So, we have Kf = (I/ √2) / (2I/π)
= 1.11
Which is always fixed.

16.  Speed Of Alternator:
Let,
P= Total number of magnetic poles,
N= Rotative speed of the rotor in r.p.m,
F= Frequency of generated e.m.f in Hz.
Here,
Number of cycles/revolution=P/2
Number of revolutions/second=N/60
So, frequency,(f)=(P/2) x (N/60)
f=PN/120
Here,N is the synchronous speed, because it is the speed at
which an alternator must run,in order to generate an e.m.f of the
required frequency.

17. ARMATURE REACTION :
In D.C. generators, armature reaction is the effect of armature flux on the main field
flux. In the case of alternators ,the power factor of the load has a considerable effect on
the armature reaction . In different condition of power factor the armature reaction is
different . Here , we will consider three cases :
 When load of power factor is unity ;
 When power factor is zero lagging and
 When power factor is zero leading.

18. Unity Power Factor :
In this case the armature flux is cross- magnetising . The result is that the
flux at the leading tips of the poles is reduced while it is increased at the trailing
tips. However, these two effects nearly offset each other leaving the average field
strength constant. In other words, armature reaction for unity p.f. is distortional.
Zero Power Factor Lagging:
here the armature flux is in direct opposition to the main flux. The main flux
is decreased. Therefore, it is found that armature reaction, in this case, is wholly
damagnetinsing with the result, that due to weakening of the main flux, less e.m.f. is
generated. To keep the value of generated e.m.f. the same, field excitation will have
to be increased to compensate for this weakening.

19. Zero Power Factor Leading :
In this case armature flux wave has moved forward by 90 so that it is in
phase with the main flux wave. The result in added main flux. Hence in this case
armature reaction is wholly magnetizing, which results in greater induced e.m.f.
To keep the value of generated e.m.f. the same, field excitation will have to be
reduced somewhat.

20. Alternator on Load :
When the synchronous generator is connected to load , current flows from armature winding to the load . As
the load on an alternator is varied , its terminal voltage is also found to vary as in D.C generator . This
variation in terminal voltage V is due to the following reasons :-
1. Voltage drop due to armature resistance Ra ;
2. Voltage drop due to armature leakage reactance XL ;
3. Voltage drop due to armature reaction.
(a)Armature Resistance :
1. Causing voltage drop
2. Phasing with the armature current
3. This voltage drop is practically negligible
(

21. (b)Armature Leakage Reactance :
1. Current flowing through the armature conductor ;
2. Fluxes are set up ;
3. Do not cross the air-gap and taking different paths ;
4. Fluxes are known as leakage fluxes ;
5. The leakage flux is practically independent of saturation .
6. Depend on I and its phase angle with terminal voltage(V).
7. Leakage flux setting up an emf of self-inductance known as reactance
emf
8. Being ahead of I by 90
9. E =V+ I(R+ j XL)

22. Stator core:
Supporting armature core. Building up of laminations of
magnetic iron. Laminating to minimize loss due to eddy currents.
Laminations are stamped out in complete rings. Permitting easy
installation of form wound coils. Easy removal in case of repair.

23. Equation of induced E.M.F :
Average e.m.f induced per conductor = dØ/ dt
= Øp/(60/N)
=Øp NP/60
Let, Z = No.of conductors or coil sides in series/phase = 2T
p = No. of poles
f = frequency of induced e.m.f
Ø = flux/pole in weber
Kd = Distribution Factor
Kp=pitch or coil span factor
kf = from factor=1.11
N = rotor r.p.m

24. In one revolution of the rotor each rotor conductor is cut by a flux of ØP webers.
dØ = ØP and d = 60/N second
Average e.m.f induced per conductor =(dØ/dt)
Now we know that f= PN/120
substituting this value of N above, we get Average e.m.f per conductor = (ØP/60) x (120f/p)
= 2fØ volt
If there are Z conductors in series/phase,then Average e.m.f/phase = 2fØZ volt
= 4fØT volt
R.M.S value of e.m.f /phase= 1.11 x 4fØT volt
This would have been the actual value of the induced voltage of all the coils in a phase were
(i) full- pitched and (ii) concentrated or bunched in one shot. But this nor being so, actually
available voltage is reduced in the ratio of these two factors.
Actually available voltage/phase = 4.44 Kc Kd Kf fØT volt