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AMRITA SAI INSTITUTE OF SCIENCE AND TECHNOLOGY
AMRITA SAI INSTITUTE OF SCIENCE AND TECHNOLOGY
Approved by AICTE, New Delhi; Affiliated To JNTU, Kakinada
PARITALA(P), KANCHIKACHERLA(M), KRISHNA (D)–521 180(A. P.)
DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING
UNIT – VI
ALTERNATORS
An alternator operates on the same principle of electromagnetic induction as a d.c. generator i.e. when
the flux linking in the conductor changes, an e.m.f. is induced in the conductor. An alternator has an armature
winding and a field winding. But there is a one important difference in between the d.c. generator and
alternator. In a d.c. generator, the armature winding is placed on the rotor and field poles are placed on the
stationary part of the machine. But no commutator is required in alternator, it is usually more convenient to
place the field winding on the rotating part and armature winding on the stationary part as shown in the figure.
ADVANTAGES OF STATIONARY ARMATURE:
1) The output current can be led directly from fixed terminals on the stator (or armature windings) to the load
circuit, without having to pass it through brush contacts.
2) It is easier to insulate stationary armature winding for high AC voltage, which may have as high a value as
30KV or more.
3) The sliding contacts i.e. slip rings are transferred to the low voltage, low power DC field current which can,
therefore be easily insulated.
4) The armature winding can be more easily braced to prevent any deformation, which could be produced by
the mechanical stresses set up as a result of short circuit current & the high centrifugal forces brought in to
play.
5) As everywhere a.c. is used, the generation level of a.c. voltage may be higher as 11 KV to 33 KV. This gets
induced in the armature. For stationary armature large space can be provided to accommodate large number
of conductors and the insulations.
6) It is always better to protect high voltage winding from the centrifugal forces caused due to the rotation. So
high voltage armature is generally kept stationary. This avoids the interaction of mechanical and electrical
stresses.
7) It is easier to collect larger currents at very high voltage from a stationary member than from the slip ring
and brush assembly. The voltage required to be supplied to the field is very low (110 V to 220 V d.c.) and
hence can be easily supplied with the help of slip ring and brush assembly by keeping it rotating.
8) The problem of sparking at the slip rings can be avoided by keeping field rotating which is low voltage
circuit and high voltage armature as stationary.
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PREPARED BY CHINNA VEERAIAH CHINTA
AMRITA SAI INSTITUTE OF SCIENCE AND TECHNOLOGY
9) Due to low voltage level on the field side, the insulation required is less and hence field system has very low
inertia. It is always better to rotate low inertia system than high inertia, as efforts required to rotate low inertia
system are always less.
10) Rotating field makes the overall construction very simple. With simple, robust mechanical construction
and low inertia of rotor, it can be driven at high speeds. So greater output can obtained from an alternator
of given size.
11) If field is rotating, to excite it be external d.c. supply two slip rings are enough. Once each for positive and
negative terminals. As against this, in three phase rotating armature the minimum number of slip rings
required are three and can not be easily insulated due to high voltage levels.
12)The ventilation arrangement for high voltage side can be improved if it is kept stationary.
Due to all these reasons the most of the alternators in practice use rotating field type of arrangement. For
small voltage rating alternators rotating armature arrangement may be used.
CONSTRUCTION OF ALTERNATOR:
An alternator consists of:
1) Stator
2) Rotor
STATOR:
 Stator is the stationary part of the alternator and contains 3-phase armature windings. Stator core is
built up of silicon steel laminations to reduce eddy current losses. The laminations are provided with
slots on its inner periphery and are packed tightly together by cast iron frame.
 Open slots are used allowing easy installation of stator coils and easy removal in case of repair. Coils
are insulated before inserting in the slots and are further protected by fiber.
 The three phase windings are placed in these slots and serves as the armature windings of the alternator.
The armature windings are always connected in star and the neutral is connected to ground.
ROTOR:
 The rotor is rotating part of the alternator. It carries a field winding which is supplied with dc current
through two slip rings by a separate dc source. This dc source (exciter) is generally a small dc generator
mounted on the shaft of the alternator.
There are two types of rotors:
1) Salient pole type rotor
2) Non-salient pole type rotor.
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PREPARED BY CHINNA VEERAIAH CHINTA
AMRITA SAI INSTITUTE OF SCIENCE AND TECHNOLOGY
SALIENT POLE TYPE ALTERNATOR
 Salient means sticking out or projected out. A salient pole is a magnetic pole that is projected out of the
rotor surface.
 The salient pole alternators are slow-speed machines, speed varying from 150 to 600 rpm. These
alternators are driven by hydraulic turbines. They are also called water-wheel generators or hydrogenerators. Pelton wheel, Francis turbine and Kaplan turbine are types of hydraulic turbines used with
these alternators:
 Salient type rotor has large diameter, small length and low speed. Diameter is usually between 3-15 m.
 Salient type rotor has non-uniform air-gap and two or four poles.
 Low and medium speed alternators (120 – 400 rpm) driven by diesel engines or water turbines have
salient pole type rotors due to following reasons:
 The salient field poles would cause an excessive windage loss if driven at high speed and would tend to
produce noise.
 Salient-pole construction can not be made strong enough to withstand the mechanical stress at higher
speeds.
NON-SALIENT POLE TYPE ALTERNATOR





Non-salient pole is non-projecting surface type.
Non-salient type rotor has small diameter and large length.
Non-salient type rotor is used for high speed and has uniform air-gap.
Non-salient pole rotors have four or more poles.
High speed alternators (1500 – 3000 rpm) are driven by steam turbines and use non-salient type rotors
due to following reasons:
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PREPARED BY CHINNA VEERAIAH CHINTA
AMRITA SAI INSTITUTE OF SCIENCE AND TECHNOLOGY
 Gives noiseless operation at high speeds.
 Flux is uniformly distributed along the periphery, so proper sine wave is obtained which gives better
emf.
ALTERNATOR OPERATION:
The rotor winding is energised from the d.c. exciter and alternate N and S poles are developed on the
rotor. When the rotor is rotated in anticlockwise direction by a prime mover, the stator or armature conductors
are cut by the megnetic flux or rotor poles. Consequently, e.m.f. is induced in the armature conductors due to
electromagnetic induction. The induced e.m.f. is alternating since N and S poles of rotor alternately pass the
armature conductors. The direction of induced e.m.f. can be found by Flemings right hand rule and frequency
is given by;
𝑓=
𝑁𝑃
120
Where
N = Speed of rotor in r.p.m.
P = Number of rotor poles
The magnitude of the voltage induced in each phase depends upon the rotor flux, the number and
position of the conductors in the phase and the speed of the rotor.
WINDING FACTORS:
The armature winding of an alternator is distributed over the entire armature. The distributed winding
produces nearly a sine waveform and the heating is more uniform. Likewise, the coils of armature windings are
not full-pitched i.e., the two sides of a coil are not at corresponding points under adjacent poles. The fractional
pitched armature winding requires less copper per coil and at the same time waveform of output voltage is
improved. The distribution and pitching of the coils affect the voltages induced in the coils.
There are two winding factors:
(1) Distribution factor [Kd]
(2) Pitch factor or coil span factor [Kp]
DISTRIBUTION FACTOR [KD]:
If all the coil side of any one phase under one pole are bunched in one slot, the winding obtained is
known as concentrated winding and the total emf induced is equal to arithmetic sum of the emfs induced in all
the coils of one phase under one pole. But in practical cases, for obtaining smooth sinusoidal voltage wave
form, armature winding of alternator is not concentrated but distributed among the different slots to form polar
groups under each pole. In distributed winding, coil sides per phase are displaced from each other by an angle
equal to the angular displacement of the adjacent slots. Hence, the induced emf per coil side are not an angle
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PREPARED BY CHINNA VEERAIAH CHINTA
AMRITA SAI INSTITUTE OF SCIENCE AND TECHNOLOGY
equal to the angular displacement of the slots. So, the resultant emf of the winding is the phasor sum of the
induced emf per coil side. As it is phasor sum, must be less than arithmetic sum of these induced emfs.
Resultant emf would be arithmetic sum, if the winding would have been a concentrated one.
As per definition, distribution factor, is measure of resultant emf of a distributed winding in
compared to a concentrated winding.
It is expressed as ratio of the phasor sum of the emfs induced in all the coils distributed in a number of
slots under one pole to the arithmetic sum of the emfs induced. Distribution factor is,
𝐾𝑑 =
𝑒. 𝑚. 𝑓. 𝑤𝑖𝑡ℎ 𝑑𝑖𝑠𝑡𝑟𝑖𝑏𝑢𝑡𝑒𝑑 𝑤𝑖𝑛𝑑𝑖𝑛𝑔
𝑒. 𝑚. 𝑓. 𝑤𝑖𝑡ℎ 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑒𝑑 𝑤𝑖𝑛𝑑𝑖𝑛𝑔
𝑛𝛼
sin( 2 )
𝐾𝑑 =
𝛼
𝑛𝑠𝑖𝑛( 2 )
Where 𝛼 = 𝑠𝑙𝑜𝑡 𝑎𝑛𝑔𝑙𝑒 =
1800
𝑁𝑜.𝑜𝑓 𝑠𝑙𝑜𝑡𝑠/𝑝𝑜𝑙𝑒
n = Slots per pole per phase
PITCH FACTOR [KP]:
In short pitched coil, the induced emf of two coil sides is vectorically added to get, resultant emf of the
coil. In short pitched coil, the phase angle between the emfs induced in two opposite coil sides is less than 180°
(electrical). But we known that, in full pitched coil, the phase angle between the emfs induced in two coil sides
is exactly 180° (electrical). Hence, the resultant emf of a full pitched coil is just arithmetic sum of the emfs
induced in both sides of the coil. We well know that, vector sum or phasor sum of two quantities, is always less
than their arithmetic sum. Pitch factor is the measure of resultant emf of short pitched coil in comparison with
resultant emf of full pitched coil.
Hence, it must be the ratio of phasor sum of induced emfs per coil to the arithmetic sum of induced
emfs per coil. Hence it must be less than unity.
Let’s a coil is short pitched by an angle α (electrical degree). Emf induced per coil side is E. The
arithmetic sum of induced emfs is 2E. That means, 2E, is the induced voltage across the coil terminals, if the
coil would have been full pitched.
𝐾𝑝 =
𝑒. 𝑚. 𝑓. 𝑖𝑛𝑑𝑢𝑐𝑒𝑑 𝑖𝑛 𝑠ℎ𝑜𝑟𝑡 − 𝑝𝑖𝑡𝑐ℎ 𝑐𝑜𝑖𝑙
𝑒. 𝑚. 𝑓. 𝑖𝑛𝑑𝑢𝑐𝑒𝑑 𝑖𝑛 𝑓𝑢𝑙𝑙 − 𝑝𝑖𝑡𝑐ℎ 𝑐𝑜𝑖𝑙
E.M.F. EQUATION OF AN ALTERNATOR:
Let
Z = No.of conductors or coil sides in series per phase
Ф = Flux per pole in webers
P = Number of poles
N = Rotor speed in r.p.m.
The e.m.f. induced in the alternator is depends on the rate of change of flux
𝐸=
𝑑∅
𝑑𝑡
In one revolution (i.e. 60/N second), each stator conductor is cut by P Ф webers i.e.,
𝑑∅ = 𝑃∅ ; 𝑑𝑡 =
60
𝑁
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PREPARED BY CHINNA VEERAIAH CHINTA
AMRITA SAI INSTITUTE OF SCIENCE AND TECHNOLOGY
Therefore average e.m.f. induced in one stator conductor =
𝑑∅
𝑑𝑡
=
𝑃∅
60⁄
𝑁
=
𝑃∅𝑁
60
volts
Since there are Z conductors in series per phase,
Therefore average e.m.f. per phase =
=
𝑃∅𝑁𝑍
[But speed N =
60
𝑃∅𝑍
60
∗
120𝐹
𝑃
]
120𝐹
𝑃
= 2F ФZ
The R.M.S. value of e.m.f. / phase = (Average value / phase) * form factor
= 2F ФZ * 1.11
= 2.22 F ФZ
If Kp and Kd are the pitch factor and distribution factor of the armature winding, then,
𝐸𝑟.𝑚.𝑠
⁄𝑝ℎ𝑎𝑠𝑒 = 2.22𝐾𝑃 𝐾𝑑 𝐹∅𝑍
VOLTAGE REGULATION:
The voltage regulation of an alternator is defined as the change in terminal voltage from no-load to full
load and it is expressed as fraction of full load voltage.
𝑁𝑜 𝑙𝑜𝑎𝑑 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 − 𝑓𝑢𝑙𝑙 𝑙𝑜𝑎𝑑 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
% 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 =
∗ 100
𝑓𝑢𝑙𝑙 𝑙𝑜𝑎𝑑 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
𝐸0 − 𝑉
%𝑉𝑅 =
∗ 100
𝑉
DETERMINATION OF VOLTAGE REGULATION:
Generally there are several indirect methods of determining the voltage regulation of an alternator.
These methods are required only a small amount of power as compared to the direct loading method.
To calculate voltage regulation, the following data are required:
(1) Armature resistance
(2) Open circuit characteristic (O.C.C)
(3) Short circuit characteristic (S.C.C)
Open circuit characteristics:
Like the magnetization curve for a d.c. machine, the open – circuit characteristic of an alternator is the
curve between armature terminal voltage (phase value) on open circuit and the field current when the alternator
is running at rated speed.
The figure shows the circuit for determining the O.C.C. of an alternator. The alternator is run on no –
load at the rated speed. The filed current is gradually increased from zero by adjusting field rheostat until open
circuit voltage E0 reaches to the rated phase voltage.
The graph is drawn between the open circuit voltage values and the corresponding values of If as shown
in figure.
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PREPARED BY CHINNA VEERAIAH CHINTA
AMRITA SAI INSTITUTE OF SCIENCE AND TECHNOLOGY
+
L
A
F
D
220 V
D.C.
Supply
P
A
X1
S
T
A
AA
A
Isc
P
S
X2
_
E2 V
Z
M
_
T
ZZ
+
A
If
C
B
T
-
Short circuit characteristic (S.C.C.):
In a short circuit test, the alternator is run at rated speed and the armature terminals are short circuited
through ammeter. The field current is gradually increased from zero until the short circuit armature current is
reached to rated value. The graph between short circuit armature current and field current gives the short
circuit characteristic (S.C.C.) as shown in figure.
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PREPARED BY CHINNA VEERAIAH CHINTA
AMRITA SAI INSTITUTE OF SCIENCE AND TECHNOLOGY
There is no need to take more than one reading because S.C.C. is a straight line passing through the
origin.
SYNCHRONOUS IMPEDANCE METHOD:
In this method of finding the voltage regulation of an alternator, we find the synchronous impedance ZS
of the alternator from the O.C.C. and S.C.C. for this reason, it is called synchronous impedance method. The
method involves the following steps.
(1) Plot the O.C.C. and S.C.C. on the same field current as shown in figure.
(2) Consider a field current If the open circuit voltage corresponding to this field current is E1 the short
circuit armature current corresponding to field current If is I1.
𝐸1 (𝑂𝑝𝑒𝑛 − 𝑐𝑖𝑟𝑐𝑢𝑖𝑡)
𝑍𝑆 =
𝐼1 (𝑠ℎ𝑜𝑟𝑡 − 𝑐𝑖𝑟𝑐𝑢𝑖𝑡)
(3) Synchronous reactance, 𝑋𝑆 = √𝑍𝑆2 − 𝑅𝑎2
(4) 𝐸0 = √(𝑉𝐶𝑂𝑆∅ + 𝐼𝑎 𝑅𝑎 )2 + (𝑉𝑆𝐼𝑁∅ + 𝐼𝑎 𝑋𝑆 )2
% 𝑉𝑂𝐿𝑇𝐴𝐺𝐸 𝑅𝐸𝐺𝑈𝐿𝐴𝑇𝐼𝑂𝑁 =
𝐸0 − 𝑉
∗ 100
𝑉
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