MESIN ELEKTRIK EBOOK

Topic 3 ac Generator

Topic 3 ac Generator

U.S. NRC image of a modern steam turbine generator

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Topic 3 ac Generator

Topic 3 ac Generator

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Introduction

Large alternators powered by engines, steam turbines, and water turbines
produce electricity on a commercial scale. Alternators and DC generators
make use of the same principle but are built differently and used for different
purposes. Alternators are sometimes called AC generators because they
generate alternating current (AC).

A.C generator operates as a d.c generator based on the principle of
electromagnetic induction. Where both the generators has armature coil and
magnetic fields. Known in d.c generator armature rotating and stationary
magnetic fields instead of a.c generator mostly the rotating magnetic fields
and stationary armature. Additional excitation from outside needed for a.c
generator because the generator can’t excited itself.

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Introduction

A generator consists of some magnets and a wire (usually a very long one
that's wrapped to form several coils and known as an armature). A steam
engine or some other outside source of motion moves the wire or armature
through the magnetic field created by the magnets.

An alternating current generator, or AC generator, produces an alternating
current, which means the voltage produced alternately reverses from positive
to negative polarity, producing a corresponding change in the direction of
current flow.

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Figure 3.1

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Introduction

Much like a DC generator, an AC generator requires a coil to cut across the
force lines of a magnetic field.

This coil is attached to two slip rings, which deliver the current to and from
the load destination, thus completing the circuit.

During the first half turn, the coil cuts across the field near the magnet's
north pole.

Electrons travel up the wire, and the lower slip ring becomes positively
charged.

When the coil cuts near the South Pole of the wire during the second half
turn, the lower slip ring becomes negatively charged, and electrons move
down the wire.

The faster the coil turns, the faster the electrons move, increasing the
frequency (in Hertz) of the current produced by the generator.

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Introduction

• In this instant, there is no voltage induced in the loop.
• As the loop rotates, sides will cut the magnetic lines of force inducing

voltage in the loop.
• When the loop is in the horizontal position, maximum voltage is induced.
• The rotation of the coil through 360 degrees results in an a.c sine wave

output.

Figure 3.2

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Basic types of ac generators :
i. Permanent

ii. standby
The source of backup power ultimately choose will be determined by
many factors, including your power requirements

iii. portable.
Portable are versatile, can use them for emergency electric power at
home, for power in remote locations where utility power is
unavailable, or for recreational purposes, like night market or boating
or camping.

The Type Of A.C. Generator
Ac generator is designed according the principles of the theory e.m.f.
generation. The types are:

• Rotating Armature and Stationary Field.
• Rotating Field and Stationary Armature.

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Figure 3.3:

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Components or main part of an a.c generator

Stator The stator of an AC generator is the part that is stationary. Like
the rotor, this component may be the armature or the field.
Depending on the type of generator.

Rotor The rotor of an AC generator is the rotating component of the
generator. The rotor is driven by the generator’s prime mover.
Depending on the type of generator, this component may be the
armature or the field.

Field The field in an AC generator consists of coils of conductors within
Winding the generator that receive a voltage from a source (called

excitation) and produce a magnetic flux.

Armature The armature is the part of an AC generator in which output
voltage is produced.

Slip Rings Slip rings are electrical connections that are used to transfer power
to and from the rotor of an AC generator.

Prime The prime mover is the component that is used to drive the AC
Mover generator. The prime mover may be any type of rotating machine,
such as a diesel engine, a steam turbine, or a motor and ect.

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

A generator consists of some magnets and a wire
(usually a very long one that's wrapped to form
several coils and known as an armature).

One advantage that ac has over dc is that it can
easily be "stepped up" or "stepped down" with a
transformer. In other words, a transformer can
take a low-voltage current and make it a high-
voltage current, and vice versa.

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

there are two types of construction ac generators:

1. The stationary field, rotating armature;
-Small ac generators

2. the rotating field, stationary armature.

“Note: Armature - The power-producing
component of an alternator or generator. The
armature can be on either the rotor or the stator,
depending on the type of generator. The rotor will
be the armature if the voltage output is generated
there and the stator will be the armature if the
voltage output is generated there.”

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Figure 3.4: stator & rotor

stator
i. Part that is stationary - Stator Windings or Armature Windings
ii. Will be the armature because the voltage output is generated there

Rotor
i. Rotating component of the AC generator - Rotor Windings or Field

Windings
ii. Will be the field because the field excitation is applied

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Figure 3.5: stator & rotor

http://www.electricaleasy.com/2014/02/AC-generator-
alternator-construction-working.html

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Figure 3.6: stator & rotor

3.1 CONSTRUCTION & PRINSIPLE AC GENERATOR(Understand)

Figure 3.7: stator & rotor

3.1.1 CONSTRUCTION & PRINSIPLE - STATOR AC GENERATOR(Understand)

The stator consists of a
i. cast iron frame which supports the armature core

- made of cast iron or mild steel plate.
- only acting as holder of the stator core

ii. The armature core has slots on its inner periphery for housing the
armature conductors.
- made of steel or alloy layer.
- have holes beside the groove.
- groove holes are placed in the armature windings.

Figure 3.8: stator

3.1.2 ADVANTAGES STATIONARY ARMATURE AC GENERATOR(Understand)

Rotor experience varying magnetic fields, therefore is constructed of
thin laminations to reduce eddy current losses.

To supply the rotor winding while it is rotating, special arrangement
employed to connect its terminal to dc supply
i. Supply dc power from an external dc source to rotor by means of

slip ring
ii. Supply dc power from a special dc power source mounted on

shaft of rotor

slip ring: are metal rings encircling shaft and are insulated from it
i. One end of rotor winding is connected to each of the 2 slip rings
ii. A stationary brush mounted on the machine casing ride on each

slip ring
mounted on shaft of rotor (brush): a block of graphite like carbon
compound that conducts and has low friction
Same dc voltage is applied to field winding during rotation

3.1.2 ADVANTAGES STATIONARY ARMATURE AC GENERATOR(Understand)

Some problems with slip rings and brushes increase the
required maintenance
- brushes should be examined for wear regularly.
- Brush voltage drop results in significant power losses if

field current is high.
Small synchronous machines(Rotating armature) – use slip
rings and brushes.

Larger machines(Static armature)
– brushless exciters are used to supply the dc field current.

3.1.2 ADVANTAGES STATIONARY ARMATURE AC GENERATOR(Understand)

Rotating armature and stationary field

• The rotating-armature alternator is similar in construction to
the d.c. generator in that the armature rotates in a stationary
magnetic field. In the alternator, the generated a.c is brought to the
load unchanged by means of slip rings. The rotating armature is found
only in alternators of low power rating

• Can generate only a low power rating and generally is not
used to supply electric power in large quantities., because it was
impossible to increase the strength of flux, if they want to increase the
number of generated voltage also be added as well as conductor size
brushes, slip rings and machine frame. This requires great expense
and difficult.

• Often these machines are not closed at the end of the back to
facilitate the observation and handling.

• 3.1 JENIS PENJANA A.U
• Penjana a.u direka mengikut teori prinsip penjanaan d.g.e. Tebahagi kepada 2

jenis iaitu :
• Jenis angker berputar medan pegun dan
• Jenis medan berputar angker pegun.
• 3.1.1 ANGKER BERPUTAR MEDAN PEGUN
• Dapat menjana voltan yang rendah sahaja, sebab sukar untuk menambah

kekuatan
• fluks, jika hendak meningkatkan juga voltan janaan maka bilangan pengalir

mestilah
• ditambah begitu juga saiz berus, gelang gelincir dan rangka alat serta mesin.

Ini
• memerlukan perbelanjaan yang besar dan menyukarkan.
• Biasanya mesin ini tidak ditutup di penghujung belakangnya bagi

memudahkan
• pemerhatian dan pengendalian

Gambarajah Angker Berputar Medan Pegun

• 3.1.2 MEDAN BERPUTAR ANGKER PEGUN Dapat menjana voltan yang tinggi, voltan yang
dijana diambil daripada medan kutubpegun bukan dari angker. Arus yang dikeluarkan besar.
Tidak ada bahagian pengalir yang bergerak. Oleh itu pengalir boleh ditambah tanpamenambah
saiz angker dan kekuatan penggerak.Izdihar AB8/9/20074.32pm2(The rotor will bethe
armature if thevoltage output isgenerated there.)

• E3106 – MESIN ELEKTRIK DAN KAWALAN JKE POLIMAS Mesin ini biasa ditutup sepenuhnya
dan disejukkan dengan cara hembusan angin. Mengeluarkan voltan 3 fasaGambarajah Medan
Berputar Angker Pegun3.2 DUA BAHAGIAN UTAMA PENJANA A.UStator (Pemegun) Rotor
(Pemutar)Bahagian yang tidak bergerak danmenempatkan lilitan medan.Bahagian yang
berputar danmenempatkan gelung angker.Mengandungi lilitan utama dan
lilitanpermulaan.Lilitannya tidak sisambung kepadabahagian lain motor.Terdiri dari kerangka
dan teras pegun. Jenis yang selalu digunakan:i. Rotor sangkar tupaiii. Rotor gelang pisah
Kerangka bertindak sebagai penyokongteras pegun dan dibuat dari besi tuang.3.2.1 STATOR
(PEMEGUN)Terdiri dari dua bahagian iaitu ;a) Bingkai diperbuat dari besi tuang atau plet keluli
lembut,Izdihar AB8/9/20074.32pm3

3.1.2 ADVANTAGES STATIONARY ARMATURE AC GENERATOR(Understand)

U Prime The Rotor will be
rotor Mover the Armature if the
Voltage output is
S generated there.

A.C Output

Field Excitation DC Supply

Figure 3.9 : Rotating Armature and Stationary Field

Basic Operating Principle

In the rotating armature AC generator as illustrated in figure 3.9. The stator
provides a stationary electromagnetic field. The rotor, acting as the armature,
rotates in the field, cutting the lines of force and producing the desired output
voltage. The output voltage is taken from the rotor by the slip ring and brushes.
One slip ring is attached to each end of the rotating loop. The brushes make
sliding electrical contact with the slip rings. The generator’s AC output voltage
can be transferred from the slip ring through the brushes to an external circuit.

3.1.2 ADVANTAGES STATIONARY ARMATURE AC GENERATOR(Understand)

Rotational field and stationary armature

1. Rotating field is comparatively light and can run with high speed.

2. High voltage can be generate due to high speed and there is very
little difficulty in providing high voltage on a stationary part than a
moving part.

3. It is easier to insulate armature coils for high pressure usually
generated. Since the stator is outside the rotor, so more space is
available for greater insulation required for armature winding.

4. Very little difficulty is experience in supplying the field magnet
current as it is very low in comparison with the armature current.

3.1.2 ADVANTAGES STATIONARY ARMATURE AC GENERATOR(Understand)

U

rotor Prime The Stator will be the
Mover Armature if the Voltage
output is generated
Field Excitation DC S there.
Supply A.C Output

Figure 3.10 : Rotating field and Stationary Armature

Basic Operating Principle

The rotating field AC generator as illustrated in figure 3.10. is by far the most
widely used generator. In this type of generator, direct current from a separate
source is passed through windings on the rotor by means of slip rings and
brushes.

3.1.2 ADVANTAGES STATIONARY ARMATURE AC GENERATOR(Understand)

This maintains a rotating electromagnetic field of fixed polarity (similar to a
rotating bar magnet). The rotating magnetic field of the rotor extends outward
and cuts through the armature winding embedded in the surrounding stator.

As the rotor turns. alternating voltages are induced in the winding because
magnetic fields of first one polarity and then the other cut through them.
Because the output power is taken from stationary windings. The output may be
connected through fixed terminals. The advantage in this types of construction is
that larger amounts of currents can be handled because there are no sliding
contacts and the whole output circuit is continuously insulated.

3.1.2 ADVANTAGES STATIONARY ARMATURE AC GENERATOR(Understand)

Advantage of alternator

Num. ROTATING ARMATURE STATIONARY ARMATURE

1 Least used because of limited Permits higher output because it is
output. connected directly to the stator and not
routed through brushes/slip rings.

2 Can be connected delta or wye. can be connected delta or wye.

3 A.C output connected to slip A.C Output connected to stator.
rings (loops of wire).

Power is carried to the outside Three sets of windings 120 degrees

4 circuit via brushes riding apart.

against the sliprings.

Two types of rotor designs used are:

5 - i) Salient Pole and

ii) Cylindrical

6 low-voltage generation High-voltage generation, High output
current

7 - easy to insulating armature windings,
while operating (high voltage)

3.1.2 ADVANTAGES STATIONARY ARMATURE AC GENERATOR(Understand)

Advantage of alternator

Num. Rotating Armature STATIONARY ARMATURE
more complex structure of the armature
Construction same, but slip windings
8 rings connected armature and
add conductor but armature size still
enter the polar field is Vdc same
To increase the emf, total
number of conductor(Z), size
9 of brushes, slip rings and
frame size should be
increased, thus the added cost

3.1.3 CONSTRUCTION & PRINSIPLE - ROTOR AC GENERATOR(Understand)

The rotor is like a flywheel having alternate North (N) and South
(S) poles fixed to its outer rim.

The types of rotors are used in alternators are a
i. salient pole
ii. non salient pole (smooth cylindrical type).

Figure 3.11:

3.1.3 CONSTRUCTION & PRINSIPLE - ROTOR AC GENERATOR(Understand)

Figure 3.12: Non Salient Pole & Salient Pole

Non Salient Pole/smooth cylindrical: Salient Pole:
i. 2 or 4 pole rotor i. more 4 pole rotor
ii. High speed ii. Low and Medium speed
iii. rotors do not need damper iii. usually contain damper

windings windings to prevent rotor

oscillations during operation

3.1.3 CONSTRUCTION & PRINSIPLE - STATOR AC GENERATOR(Understand)

Figure 3.13: stator

3.1.3 CONSTRUCTION & PRINSIPLE - ROTOR AC GENERATOR(Understand)

Salient Pole

Form like a “flywheel”, used for low and medium-speed generator

Salient pole rotors are used in application with speeds from 100 to
1500rpm. They are alternative known as "projected pole" type of
rotors.

The poles mounted on the rotor are made of laminations made of
steel. The poles are connected to the rotor shaft by means of dovetail
joints. Each pole has a pole shoe around which the winding is wound.

The salient pole rotor is generally used in applications where the
prime mover is a hydro turbine or a combustion engine which have
low or medium speeds.

Salient pole rotors usually contain damper windings to prevent rotor
oscillations during operation.

3.1.3 CONSTRUCTION & PRINSIPLE - ROTOR AC GENERATOR(Understand)

Advantage rotor-salient pole types.
i. Is used in low-speed alternators
ii. The salient-pole rotor would have a greater diameter with cylinder.
iii. Easy to maintenance
iv. Low maintenance cost
v. This type of rotor lighter compared to cylindrical polar types.

Figure 3.14: Salient Pole

3.1.3 CONSTRUCTION & PRINSIPLE - ROTOR AC GENERATOR(Understand)

Non Salient Pole / smooth cylindrical

Non-salient pole rotors are generally used in application which
operate at higher speeds, 1500rpm and above. The prime movers in
these applications are generally gas or steam turbines.
These are sometimes known as "drum rotors".
The rotor is a cylinder made of solid forged steel. The slots on which
the windings are fixed are milled on the rotor. The number of poles is
usually 2 or 4 in number.
Since these rotors are cylindrical, the wind age loss is reduced.
The noise produced is also less. These rotors have higher axial length.
These rotors do not need damper windings.

3.1.3 CONSTRUCTION & PRINSIPLE - ROTOR AC GENERATOR(Understand)

Advantage rotor – Non Salient Pole / smooth cylindrical
i. are favored for large, high-speed generators.
ii. This type of rotor has a good balance.
iii. Quieter running.
iv. Have minimal wind loss.

Figure 3.14: Non Salient Pole

3.2.1 PRINCIPLE OPERATION OF AC GENERATORS AC GENERATOR(Apply)

Alternating voltage may be generated by
i. rotating a coil in the magnetic field
ii. rotating a magnetic field within a stationary coil.

The value of the voltage generated depends on
i. the number of turns in the coil.
ii. strength of the field
iii. the speed at which the coil or magnetic field rotates

Figure 3.15:

3.2.1 PRINCIPLE OPERATION OF AC GENERATORS AC GENERATOR(Apply)

“Note: Armature - The power-producing component of an alternator
or generator. The armature can be on either the rotor or the stator,
depending on the type of generator. The rotor will be the armature if
the voltage output is generated there and the stator will be the
armature if the voltage output is generated there.”

The concept of synchronous generators A.C. here referring to is
located in the stator armature coil and loop field on the rotor or
which is said from a.c generator rotating field type stationary
armature.

3.2.2 SYNCHRONOUS CONCEPT AC GENERATOR(Apply)

drive speed

The turning of a coil in a magnetic field produces motional emf in
both sides of the coil which add.

Since the component of the velocity perpendicular to the magnetic
field changes sinusoidal with the rotation, the generated voltage is
sinusoidal or AC.

This process can be described in terms of Faraday's law that the
rotation of the coil continually changes the magnetic flux through the
coil and therefore generates a voltage.

3.2.2 SYNCHRONOUS CONCEPT AC GENERATOR(Apply)

The concept of synchronous a.c generators simply is
as follows.
i. When supply at given to the rotor, it will produce a

magnetic field in each of the poles of a magnet and
magnetizing the iron core.
ii. The shaft is rotated by the prime mover will cause
the rotor was rotates, cutting the magnetic field
produced by the armature then produces e.m.f. in
the armature coil.
iii. Next was armature rotating magnetic field,
according to the rotor field rotation at a certain
speed.
iv. This is known as the concept of synchronous a.c
generator.

3.2.2 SYNCHRONOUS CONCEPT AC GENERATOR(Apply)

i. definite relationship between
the rotational speed (N) of the
rotor, the frequency (f) of the
generated emf & the number
of poles(P), refer to the Figure
3.16.

Figure 3.16: Relationship ii. armature conductor (X)at the
between speed, frequency center of a N-pole rotating in
and pole clockwise direction

iii. The conductor at maximum flux
density will have maximum emf
induced in it.

3.2.2 SYNCHRONOUS CONCEPT AC GENERATOR(Apply)

Figure 3.16: Relationship iv. When the conductor is in the
between speed, frequency inter-polar gap as at ‘A’ (figure
and pole 3.16), it has minimum emf
induced in it, because flux
density is minimum there

iv. when it is at the center of a S-
pole, it has maximum emf
when conductor is over s N-
pole is opposite to that when
it is over a S-pole. Obviously,
one cycle of emf is induced in
a conductor when one pair of
poles passes over it.

3.2.2 SYNCHRONOUS CONCEPT - generated frequency AC GENERATOR(Apply)

Since one cycle of emf is produced when a pair of poles passes past a
conductor, number cycles of emf produced in one revolution of the
rotor is equal to the number of pairs of poles.

N is known as the synchronous speed because it is the speed at which
an alternator must run in order to generate an emf of the required
frequency.

Number of cycles generated emf in a full cycle movement of the rotor
is equal to the number of pairs of poles available on the generator.

Frequency emf generated for a full rotation of rotor;
P
Number of cycle/revelation, f = 2

Number of revelation/second = N
60
By the number of cycles (f) for the emf generated in one second is:
P N PN
f = 2 x 60 = 120 Hz

3.2.2 SYNCHRONOUS CONCEPT - generated frequency AC GENERATOR(Apply)

Note : (The induction generator is nothing more than an

induction motor driven above its synchronous speed by an

amount not exceeding the full load slip the unit would have as a

motor. Assuming a full load slip of 3%, a motor with a

synchronous speed of 1200 rpm would have a full load speed of

1164 rpm. This unit could also be driven by an external prime

mover at 1236 rpm {1200 +3%}, for use as an induction

generator.)

The magnitude of the AC voltage generated is controlled by the
amount of DC exciting current supplied to the field.
The frequency of the voltage developed by the generator
depends on the speed of the rotor and the number of field
poles. For a 60Hz system,

frequency = speed(rpm) x pole pair /60.

3.2.2 SYNCHRONOUS CONCEPT - generated frequency AC GENERATOR(Apply)

Electrical power generated at 50 Hz so rotor must turn at fixed

speed depending on number of poles on machine

To generate 50Hz in 2 pole machine, rotor must turn at 3000
rpm.

Generator frequency the frequency f (in Hz) of the ac voltage is
a function of speed of the rotor n (in RPM):

N = 60f (RPM)

If the rotor contains a multiple number of poles (2, 4, 6 etc)

then 2

wrotor = 2π f (rad/sec)

Np = 120 RPM


Synchronous speed versus poles for a 60hZ machine:

3.2.3 DISTRIBUTION FACTOR (kd) AC GENERATOR(Apply)

From Figure 3.18 Full pitch
5/6 pitch
i. if the coil sides are placed in slots
5678
1 and 7, then it is full-pitched

ii. If the coil sides are placed in

slots 1 and 6, then it is short– 1800

pitched or friction pitched 234

because it is equal to 5/6 of a 24 1

pole-pitch

∝ = x number of slots by which coil ∝
are shot pitched or
∝ = 1800 – actual coil span of the Figure 3.18
coils

= 1800 , n= slots per pole
n

3.2.3 DISTRIBUTION FACTOR (kd) AC GENERATOR(Apply)

Short-pitched coil are deliberately used because of the following
advantages:

i. They save copper of end connection
ii. Improve the waveform of the generated emf, example the generated

emf can be made to approximate to a sine wave more easily and the
distorting harmonics can be reduced or totally eliminated
iii. Due to elimination of high frequency harmonics, eddy current and
hysteresis losses are reduced thereby increasing the efficiency

But the disadvantages of using short-pitched coil is that the total voltage
around the coil is somewhat reduced. Because the voltages induced in
the two sides of the short-pitched coil are slightly out of phase.

3.2.3 PITCH FACTOR (kp) AC GENERATOR(Apply)

The pitch factor or coil-span factor kp or kc is defined as

= vector sum of the induced emf per coil = E
arithmetic sum of the induced emf per coil Es

It is always less than unity C

Es Es E Es
2Es
A 300
Figure 3.19a Es B

Figure 3.19b

If the coil were full-pitched i.e if If it is short-pitched 300 (elect)
its two aides were one pole-pitch there resultant is E which is the
apart, then total induced emf in vector sum of two voltages 300
the coil wound have been = 2Es (elect) apart.

3.2.3 PITCH FACTOR (kp) AC GENERATOR(Apply)

E = 2Es cos 300
2

In general, if the coil span falls short of full pitch by an

∝
angle ∝ (electrical), then kc / kp = cos 2

The distribution factor is always less then ONE.

kd = sinβ2 (m)
m sinβ2

Example 3_1
In a 4 pole, 3 phase alternator, armature has 36 slots. It is using an armature
winding which is short pitched by one slot. Calculate is coil span/pitch
factor(kc/kp )

3.2.4 FORMULA EMF GENERATED AC GENERATOR(Apply)

Z : Number of conductors or coil sides in series or phase

P Z = 2T ; where T is number of coil or turn per phase.
F
  remember one turn or coil has two side.
kd
: Number of poles.
kc or kp
kf : Frequency of induced E.M.F in Hz.

N : Flux pole in Weber.

: Distribution factor. sin m   2 

k
d m sin  2

: Pitch or coil span factor

: Form factor
; if E.M.F is assumed sinusoidal

: Rotative speed of the rotor in rpm.