Chapter4 Synchronous Machines

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Contents

Synchronous Generator Construction

Principle of Operation

Equivalent Circuit

Power Flow Synchronous Generator Operating Alone

Parallel Operation of Synchronous Generator

Synchronous Motor Equivalent Circuit

Torque Speed Characteristic Effect of Load and Field Current Changes

Synchronous Motor and Power Factor Correction

Starting Methods of Synchronous Motor


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Introduction

Synchronous machines areAC machines that have a field circuit supplied by

an external DC source. In a synchronous generator, a DC current is applied

to the rotor winding, which produces a rotor magnetic field.

The rotor of the generator is then turned by a prime mover (mechanical

torque which forces the rotor to turn), producing a rotating magnetic field withinthe machine. This rotating magnetic field induces a voltage within the stator

windings of the generator.

Synchronous motors reverse this process. The essential feature that makes

synchronous machines different from other electrical machines is that its

synchronous link between stator and rotor magnetic fields. Because of thatthere is a fixed relationship between rotor speed and the frequency of induced

EMF in the stator.


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Introduction

Anotheradvantage that makes synchronous machines different from other

machines is that varying its field excitation can vary its power factor of

operation.

This property makes it to be useful for the Industry, which is always operating

at low lagging power factor (motor inductive load). So part of the load is handled

by synchronous machine whose field is adjusted such that it is operating at

leading power factor to improve the overall power factor to nearly unity.


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Introduction

Synchronous Machines:

Synchronous Generators:A primary source of electrical energy largest

(energy converter).

Synchronous Motors: Used as motors as well as power factor compensators

(synchronous condensers).

Asynchronous(Induction) Machines:

Induction Motors: Most widely used electrical motors in bothdomestic and industrial applications.

Induction Generators: Due to lack of a separate field excitation,these machines are rarely used as generators


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Introduction

There are numerous reasons for such an inside-out construction of asynchronous generator, some of which are listed below.

1. Most synchronous generators are built in much larger sizes than their dc

counterparts. An increase in power capacity of a generator requires thicker

conductors in its armature winding to carry high currents and to minimize

copper losses.2. Since the output of a synchronous generator is of the alternating type, the

armature conductors in the stator can be directly connected to the

transmission line. This eliminates the need for slip rings for ac power

output.

3. Since most of the heat is produced by the armature winding, an outer

stationary member can be cooled more efficiently than an inner rotatingmember.

4. Since the induced emf in the armature winding is quite high, it is easier to

insulate it when it is wound inside the stationary member rather than the

rotating member.


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Introduction

Two terms commonly used to describe the windings on a machine aref ield w inding and armature winding s. In general, the term f ield w inding

applies to the windings that produce the main magnetic field in a machine

and the term armature wind ingapplies to the windings where the main

voltage is induced.

The magnetic poles on the rotorcan be of eithersalient ornonsalientconstruction. The term sal ientmeans protruding orstickingout and a

sal ient po leis a magnetic pole that sticks out from the surface.

Synchronous machines are AC machines that have a field circuit

supplied by an external DC source. In a synchronous generator, a DC

current is applied to the rotor winding, which produces a rotor magnetic

field.


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Introduction

As generators they can be quite large, rated a few hundred MV A, and almostall power generation is through these machines. Large synchronous motors

are not very common, but can be an attractive alternative to induction

machines. Small synchronous motors with permanent magnets in the rotor,

rather than coils with DC, are rapidly replacing induction motors in

automotive, industrial and residential applications. since they are more

efficient and lighter.

Synchronous generators are built with two types of rotors;

Salient-Pole RotorDriven by low-speed hydraulic turbines (btw 50 and 300

rpm). always possess a large diameter to provide necessary space for the

poles.

Cylindrical Rotor (non-salient) Driven by high speed steam turbines (3600 rpm)

are smaller and more efficient than low-speed turbines.


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Motor Construction

Round Rotor Machine (non-salient pole)

The stator is a ring shaped

laminated iron-core with slots.

Three phase windings are placedin the slots.

Round solid iron rotor with slots.

A single winding is placed in the

slots. DC current is suppliedthrough slip rings.


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Motor Construction

Round Rotor Machine (non-salient pole)


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Motor Construction

Salient Rotor Machine (salient pole)

The stator has a laminated iron-core

with slots and three phase windings

placed in the slots.

The rotor has salient poles excited by

dc current.

DC current is supplied to the rotor

through slip-rings and Brushes


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ROTORSalient pole type:

used in low and medium speed

Has a number of projecting poles having cores bolted onto heavy magneticwheel of cast iron

Have large diameters and short axial lengths

Pole and pole shoes are laminated


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Motor Construction

Salient Rotor Machine (salient pole)


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Motor Construction

Operation conceptThe field winding is supplied with a DC

current -> excitation.

Rotor (field) winding is mechanically

turned (rotated) at synchronous speed(ns).

The RMF (rotating magnetic field)

produced by the field current induces

voltages in the outer stator (armature)

winding.


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DC Power Supply

DC current must be supplied to the filed winding

on the rotor. There are two common approaches to

supplying this DC power:

1. From external DC source use slip rings and

brushes (small synchronous machines)

2. From special DC power source mounted

directly on the shaft. (large synchronousmachines)


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Small Synchronous Machines

Slip rings and brushes create a few problems

when they are use d to supply DC power to the

field windings.

Brushes must be checked for wear regularly increase maintenance

Despite, slip rings and brushes are used on small

synchronous machines.


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Large Synchronous Machines

Brushless exciters are used to supply DC field current. Brushless exciters is a small AC generator with its field

circuit mounted on the stator and its armature circuit

mounted on the rotor shaft.

A 3 phase current is rectified and used to supply the

field circuit of the exciter (on stator). The output of the armature circuit of the exciter (on

rotor) is then rectified and used to supply the field

current of the main machines.

To make the generator completely independent, a small

exciter is included in the system.

A pilot exciter is a small AC generator with permanent

magnets mounted on the rotor shaft and a 3 phase

winding on the stator.


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Block diagram of a large

synchronous generator

Permanent

magnets

Pilot exciter

fieldExciter

armature

Pilot exciter

armature

Three-

phase

rectifier

Exciter field

Three-

phase

rectifier

Main field

Synchronous

Generator

ExciterPilot Exciter

Main

armature

Rotor

StatorRF

R

Y

B


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Speed of rotation of a synchronous

generator

Operation concept The rate of rotation of the magnetic fields in the machine is related to the

stator electrical frequency

Where fe= electrical frequency, in Hz

nm= mechanical speed of magnetic field, in r/min (equal speed of rotor

for synchronous machines)

P= number of poles

Typical rotor speeds are 3600 rpm for 2-pole, 1800 rpm for 4 pole and 450

rpm for 16 poles.

120

Pnf

me


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The Internal Generated voltage of a

Synchronous Generator

fNEA 2

The magnitude of the voltage induced in a given stator is;

Where EA= induced voltage/generated voltage

OR

The rms. value of the induced voltages is:EA= 4.44N BA f , (BA = )

where:

N = number of turns,

B= flux density,

A = cross sectional area of the magnetic circuit,

f= frequency,= flux per pole

This voltage depends on the flux in the machine, the frequency or speed of

rotation and the machine construction. The simpler form is;KEA

AE


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Equivalent Circuit of A Synchronous

Generator

The voltage EAis the induced voltage produced in one phase of a

synchronous generator. EAis not usually the voltage that appears at the

terminals of the generator. The only time EAis the same as the output voltage V

of the phase when there is no armature current flowing in the machine (during no

load).

There are many factors that cause the difference between EAand V including

the resistance of the armature coils, the self inductance of the armature coils,

and the distortion of the air-gap magnetic field by the current flowing in the stator,

called armature reaction.

With two voltages present in the stator windings, the total voltage in a perphase circuit is just the sum of the induced voltage EAand the armature reaction

voltage EX.


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Generator

A simple circuit

AAAsA IRIjXEV

We realize that the three phases of a synchronous generator are identicalexcept for phase angle. It is very important to know that the three phases

have the same voltages and currents only when the loads attached to them

are balanced. If the machiness loads are not balanced, more complicated

techniques of analysis are required.


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Generator

The full equivalent circuit of a three-phase synchronousgenerator

You observe the DC powersource supplying the rotor field

circuit. The figure also shows that

each phase has an induced voltage

with a seriesXSand a series RA.

The voltages and currents of the

three phases are identical but 120apart in angle.

The three phases can be eitherY

or . If they areY connected, then

the terminal voltage VTis related to

the phase voltage by

VVT 3

If connected

VVT


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Phasor Diagram

Voltages in a synchronous generator are expressed as phasors becausethey are AC voltages. Since we have magnitude and angle, the relationship

between voltage and current must be expressed by a two-dimensional plot.

It is noticed that, for a given phase voltage and armature current, a larger

induced voltage EAis required for lagging loads than leading loads.

Phasor diagram of a synchronous generator at unity power factor (Resistive Load).


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Phasor Diagram

Phasor diagram of a synchronous generator at leading factor (Capacitive Load).

Phasor diagram of a synchronous generator at lagging factor (Inductive Load).


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Power Relationships

Not all the mechanical power going into a synchronous generator becomeselectrical power out of the machine. The difference between input power and

output power represents the losses of the machine. The input mechanical power

is the shaft power in the generator.

Pin (Motor)

Rotational

losses (Pr)

Pconverted(Pm)

Pout

Stray losses

(Pst)

Core losses

(Pc)Copper losses

(Pcu)

cos3 LTIV

AARI

2

3

mindconvP

msinP

strc PPP


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Power Relationships

The power converted from mechanical to electrical is given by;

cos3 AAIE

mindconvP

; Where is the angle between EA and IA.

If the armature resistance RA is ignored (XS >> RA), Therefore;

S

A

A

X

EI

sincos

S

A

X

EVP

sin3

; Substituting this equation into Pout, gives;.

The induced torque can be express as;.

Sm

A

indX

EV

sin3

; Where is the angle between EA and VT.


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Power Angle Characteristics

The P() curve shows that theincrease of power increases the

angle between the induced voltage

and the terminal voltage.

The power is maximum when

=90o

The further increase of input

power forces the generator out of

synchronism. This generates large

current and mechanical forces.

The maximum power is the static

stability limit of the system.

Safe operation requires a 15-20%

power reverse.S

A

X

EVP

3

max


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Efficiency

100 %out

in

in out losses

P

P

P P P


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Voltage regulation

As the load on the generator increases, the terminal voltage drops. But,the

terminal voltage, must be maintained constant, and hence the excitation on the

machine is varied, or input power to the generator is varied. That means, EG has

to be adjusted to keep the terminal voltage VT constant.

Voltage Regulation, V.R; %100

FL

FLNL

V

VV


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Example

A 240 V, 50 Hz, 4-pole, Y-connected synchronous generator has a per-

phase reactance of 0.2 (ignore armature resistance). At full-load, the

armature current is 50 A at 0.83 lagging power factor. Also at full-load,

the friction and windage loss is 1.2 kW, and core loss is 1.1 kW. The

field current is initially adjusted so that the terminal voltage is 240 V at

no load, after which it is kept constant. [Assume phase voltage = VS/0].

i. What is the speed of rotation of the generator?

ii. What is the terminal/generated voltage of the generator if it is operated

at full-load rated current at 0.83 lagging power factor?

iii. What is the efficiency of the generator when it is operating at full-loadrated current at 0.83 lagging power factor?

iv. What is the voltage regulation?


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Parallel Operation of AC Generators

The generation of electric power, its transmission and its distribution must beconducted in an efficient and reliable way at a reasonable cost with the least

number of interruptions.

As the demand for electric energy can fluctuate from a light load to a heavy

load and vice versa several times during the day, it is almost impossible to

operate a single alternator at its maximum efficiency at all times.

A single alternator cannot ensure such a reliable operation owing to thepossibility of its failure or a deliberate shut-off for periodic inspection. Therefore,

a single alternator supplying a variable load cannot be very efficient, cost-

effective and reliable.

To overcome this problem, it becomes necessary to generate electric power

at a central location where several alternators can be connected in parallel to

meet the power demand.When the demand is light, some of the alternators can be taken offline while

the other alternators are operating at their maximum efficiencies.

As the demand increases, another alternator can be put on line without

causing any service interruption.


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Parallel Operation of AC Generators

The following requirements have to be satisfied prior to connecting analternator to the infinite bus (connection line).

1. The line voltage of the (incoming) alternator must be equal to the constant

voltage of the of the infinite bus.

2. The frequency of the incoming alternator must be exactly equal to that of the

infinite bus.3. The phase sequence of the incoming alternator must be identical to the

phase sequence of the infinite bus.


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Power System Operation

In a network several hundred synchronous generators operate in parallel.

Each generator operates with the same speed.

The load increase is achieved by increasing the input power, that

increases the power angle . The speed remain constant.

The power angle must be less than 90 degrees. The load should be 30-

20% less than the maximum power (= 90o).

Chapter4 Synchronous Machines

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