Synchronous Machines and SC

7/26/2019 Synchronous Machines and SC

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Synchronous motors and Synchronous condenser

Abstract — This paper provides a detailed discussion of

the basic theory of operation and control of thesynchronous motor. It describes typical synchronous motor

starting and provides information regarding control and

optimization of the motor through the use of power factor

control or VAr operation that can be helpful in reducing

VAr penalties and improving voltage stability of the plant.

Typical protection is also discussed involving motor pullout

and under and overexcitation concerns

Index Terms —

I. I NTRODUCTION

A synchronous machine is a machine whose speed alternatingcurrent under steady state conditions is proportional to the re!uency

o the current in his armor. The magnetic ield created "y armature

current rotates at the same speed that creates the ield current in therotor #rotating at the synchronous speed$% and a stationary tor!ue

occurs.

As the name suggests Synchronous motors are capa"le o

running at constant speed irrespecti&e o the load acting on

them. Unli'e induction motors where speed o the motor

depends upon the tor!ue acting on them% synchronous motors

ha&e got constant speed(tor!ue characteristics.

Synchronous motors ha&e got higher eiciency #electrical to

mechanical power con&ersion ratio$ than its counterparts. Its

eiciency ranges rom )* + ),-.

A. The Working Principle: RMF – Constant

Magnetic field interaction

The constant speed characteristic is achie&ed "y interaction

"etween a constant and rotating magnetic ield. Rotor o

synchronous motor produces a constant magnetic ield and

Stator produces a Rotating magnetic ield.

1) Stator: Revolving Magnetic Field

The ield coil o stator is ecited "y a / phase AC supply. This

will produce a re&ol&ing magnetic ield # RMF $% which rotates

at synchronous speed. The way R01 is produced with / phase

AC ecitation is eplained in a separate article. R01

produced in a synchronous motor and its direction is mar'ed

in 1ig.2

2) Rotor: Constant Magnetic field

Rotor is ecited "y a D.C power supply% magnetic ield

produced around the rotor coil "y DC ecitation is shown

"elow. It is clear that the rotor acts li'e a permanent magnet

due to such magnetic ield. Alternati&ely rotor can also "e

made o permanent magnet.

Interaction o Rotor and R01 is interesting. Assume you

are gi&ing an initial rotation to the rotor% with same direction

o R01. 3ou can see that opposite poles o R01 and Rotor will attract each other and they will get loc'ed magnetically.

This means that rotor will rotate at the same speed o R01% or

rotor will rotate at synchronous speed.

. Synchronous Speed

Speed at which R01 rotates or Synchronous speed can easily

"e deri&ed as ollows.

Starting methods of induction motors #Decem"er

,*24$Colcha I&5n ! "### % National 6olytechnic School

2




It is clear rom the relationship that speed o synchronous

motor%Ns#rpm$ is directly proportional to re!uency o the

electricity% f #78$.6 represents num"er o poles o the rotor. This

means that i one has got control o&er re!uency o the

electricity% speed o synchronous motor can "e &ery accurately

controlled. This is the reason why they are suita"le or

C. hy Synchronous motors are not self starting !

$t if the rotor has got no initial rotation! sit$ation is %$ite

different. &orth Pole of the Rotor 'ill o(io$sl* get attracted

(* +o$th Pole of RMF! and 'ill start to ,oe in the sa,e

-. Ma"ing Synchronous Motor Self Start # $se of

%amper &inding

To ma'e synchronous motor sel start% a s!uirrel cage

arrangement is cle&erly itted through pole tips. They are also

called as damper windings.

At the starting rotor ield coils are not energi8ed. So with

re&ol&ing magnetic ield% electricity is induced in s!uirrel cage "ars and rotor starts rotating 9ust li'e an induction motor starts.

:hen the rotor has achie&ed its maimum speed% rotor ield

coils are energi8ed. So as discussed earlier poles o rotor gets

loc'ed with poles o R01 and will start rotating at

synchronous speed. :hen rotor rotates at synchronous speed%

relati&e motion "etween s!uirrel cage and R01 is 8ero. This

means 8ero current and orce on s!uirrel cage "ars% thus it will

not aect synchroni8ed operation o the motor.

#. Synchronous motor out of Synchronism

Synchronous motors will produce constant speed

irrespecti&e o motor load only i the load is within thecapa"ility o motor. I eternal tor!ue load is more than tor!ue

produced "y the motor% it will slip out o synchronism and will

come to rest. ;ow supply &oltage and ecitation &oltage are

other reasons o going out o synchronism. It is interesting to

note that synchronous motor has got the same constructional

eatures o an alternator.

I an alternator is withdrawn the prime mo&er and the stator is

powered "y a three phase AC system it is generated in the

stator a rotating magnetic ield% whose speed is 'now that N <=* > p. I in such circumstances% with the rotor stopped

winding thereo is supplied with a ied rotor DC magnetic

ield% which passes the ront stator magnetic ield is produced.

The rotor poles are now undergoing attraction and repulsion%in short periods o time% "y the stator poles% "ut the rotor ails

to rotate% at most it will &i"rate. ?ut i we "ring the rotor to

synchronous speed% causing it to rotate "y an auiliary engine%

aced opposite sign poles o a magnetic latch that re!uiresthem to continue to rotate together% "eing a"le to remo&e the

auiliary engine is esta"lished.

Two electric circuits% one in the other in the rotor and stator :inding or ecitation winding or inductor@ One o the

windings% to "e tra&ersed "y an electric current produces a

magnetomoti&e orce that creates a magnetic lu.

Induced@ The other winding% in which an em is induced thatresults in a tor!ue.

Three coils in the stator% 2,* out o phase with each stay.

Bach o the coils is connected to one phase o a three phase

system and gi&e rise to a rotating magnetic ieldThe speed o the rotating magnetic ield is called the

synchronous speed # s$ and depends on the re!uency o the

electrical networ' to which is connected the motor.

Rotor@ 1rom a construction point o &iew there are two typicalorms o rotor@

S!uirrel(cage rotor@ It consists o copper "ars or aluminum and

9oined at their ends to two rings o the same material.

Rotor winding or slipring@ The rotor is made up o threewindings o copper wire connected at a common point. The

ends may "e connected to three copper rings integrally

rotating with the shat #slip rings$. Contacting the three rings

are some "rushes or connecting to these windings resistorsthat can regulate the speed o motor rotation. They are more

epensi&e and re!uire more maintenance.

1) TR## +/&CR0&0+

MAC"&#+ The open circuit and the short circuit test are used to o"tain

the unsaturated synchronous impedance and an approimate

&alue o saturated synchronous impedance. In the case o aconstant &oltage source ha&ing constant impedance% the

impedance can "e ound "y di&iding the open circuit terminal

&oltage "y the short circuit current. 7owe&er when the

impedance is a unction o the open circuit &oltage% as it iswhen the machine is saturated% the open circuit characteristics

or the magneti8ation magneti8ation cur&e in addition to the

short circuit characteristic re!uired.

2) 0P#& C"RC"T CARACT#R"+T"C+

,




To o"tain the open circuit characteristics the machine is dri&en

at rated speed without the load. Readings o the line(to(line&oltage are ta'en or &arious &alues o ield current. The

&oltage% ecept in &ery low &oltage machines% is stepped down

"y the means o a potential transormer.

The open circuit characteristics are shown "elow.

I not or the magnetic saturation o the iron% the open circuitcharacteristics would "e linear as represented "y the air gap

line.

T7B S7ORT CIRCUIT C7ARACTBRISTICS

The three terminals o the armature are short circuited each

through a current measuring circuit.

The machine is dri&en at approimately synchronous ratedspeed and measurements o armature short circuit currents are

made or &arious &alues o ield currents usually up to and

a"o&e rated armature current.

In con&entional synchronous machines the short circuitcharacteristics is practically linear "ecause the iron is

unsaturated up to rated armature current.

Connect the circuit or the open circuit test

Do not energi8e the circuit until the demonstrator chec's thecircuit and permission is gi&en to energi8e.

Start the D.C. motor couple to the alternator and ad9ust its

speed to correspond to the alternator. #The armature o thealternator is 'ept on open circuit.$

Increase the ield current o the alternator rom 8ero &alue and

note the no(load &oltage across the alternator or &arious ieldcurrents.

S7ORT CIRCUIT TBST

Using the circuit diagram connect the circuit or the short

circuit test.

Do not energi8e the circuit until the demonstrator chec's the

circuit and gi&es permission to energi8e the circuit.

;OAD TBST

In this test the machine is introduced to normal wor'ing

conditions. This test e&aluates how the machine operatesunder dierent wor'ing conditions. :e will compare the

results o the load test with the parameters de&eloped in the

open circuit and the short circuit test.

ethods of starting synchronous motor

?asically there are three methods that are used to start a

synchronous motor@

To reduce the speed o the rotating magnetic ield o the

stator to a low enough &alue that the rotor can easily accelerateand loc' in with it during one hal(cycle o the rotating

magnetic ieldEs rotation. This is done "y reducing the

re!uency o the applied electric power. This method is

usually ollowed in the case o in&erter(ed synchronous motor operating under &aria"le speed dri&e applications.

/




To use an eternal prime mo&er to accelerate the rotor o

synchronous motor near to its synchronous speed and then

supply the rotor as well as stator. O course care should "eta'en to ensure that the direction o rotation o the rotor as

well as that o the rotating magnetic ield o the stator are the

same. This method is usually ollowed in the la"oratory( the

synchronous machine is started as a generator and is thenconnected to the supply mains "y ollowing the

synchroni8ation or paralleling procedure. Then the powersupply to the prime mo&er is disconnected so that the

synchronous machine will continue to operate as a motor. To use damper windings or amortisseur windings i these are

pro&ided in the machine. The damper windings or amortisseur

windings are pro&ided in most o the large synchronous

motors in order to nulliy the oscillations o the rotorwhene&er the synchronous machine is su"9ected to a

periodically &arying load. Bach o these methods o starting a

synchronous motor are descri"ed "elow in detail.

otor Starting by !educing the supply "re#uency

I the rotating magnetic ield o the stator in a synchronous

motor rotates at a low enough speed% there will "e no pro"lem

or the rotor to accelerate and to loc' in with the statorEsmagnetic ield. The speed o the stator magnetic ield can then

"e increased to its rated operating speed "y gradually

increasing the supply re!uency up to its normal 4*( or =*(

78 &alue. This approach to starting o synchronous motorsma'es a lot o sense% "ut there is a "ig pro"lem@ :here rom

can we get the &aria"le re!uency supplyF The usual power

supply systems generally regulate the re!uency to "e 4* or =*

78 as the case may "e. 7owe&er% &aria"le(re!uency &oltagesource can "e o"tained rom a dedicated generator only in the

olden days and such a situation was o"&iously impractical

ecept or &ery unusual or special dri&e applications. ?ut the

present day solid state power con&erters oer an easy solution

to this. :e now ha&e the rectiier( in&erter andcyclocon&erters% which can "e used to con&ert a constant

re!uency AC supply to a &aria"le re!uency AC supply. :ith

the de&elopment o such modern solid(state &aria"le(re!uency dri&e pac'ages% it is thus possi"le to continuously

control the re!uency o the supply connected to the

synchronous motor all the way rom a raction o a hert8 up to

and e&en a"o&e the normal rated re!uency. I such a &aria"le(re!uency dri&e unit is included in a motor(control circuit to

achie&e speed control% then starting the synchronous motor is

&ery easy(simply ad9ust the re!uency to a &ery low &alue or

starting% and then raise it up to the desired operating re!uencyor normal running. :hen a synchronous motor is operated at

a speed lower than the rated speed% its internal generated&oltage #usually called the counter B01$ BA < GH will "e

smaller than normal. As such the terminal &oltage applied tothe motor must "e reduced proportionally with the re!uency

in order to 'eep the stator current within the rated &alue.

Jenerally% the &oltage in any &aria"le(re!uency power supply

&aries roughly linearly with the output re!uency.

otor Starting with an $xternal otor

The second method o starting a synchronous motor is to

attach an eternal starting motor #pony motor$ to it and "ringthe synchronous machine to near a"out its rated speed #"ut not

eactly e!ual to it% as the synchroni8ation process may ail to

indicate the point o closure o the main switch connecting the

synchronous machine to the supply system$ with the ponymotor. Then the output o the synchronous machine can "e

synchronised or paralleled with its power supply system as a

generator% and the pony motor can "e detached rom the shat

o the machine or the supply to the pony motor can "edisconnected. Once the pony motor is turned O11% the shat o

the machine slows down% the speed o the rotor magnetic ield?R alls "ehind ?net% momentarily and the synchronous

machine continues to operate as a motor. As soon as it "eginsto operates as a motor the synchronous motor can "e loaded in

the usual manner 9ust li'e any motor. This whole procedure is

not as cum"ersome as it sounds% since many synchronous

motors are parts o motor(generator sets% and the synchronousmachine in the motor(generator set may "e started with the

other machine ser&ing as the starting motor. 0ore o&er% the

starting motor is re!uired to o&ercome only the mechanical

inertia o the synchronous machine without any mechanicalload # load is attached only ater the synchronous machine is

paralleled to the power supply system$. Since only the motorEs

inertia must "e o&ercome% the starting motor can ha&e a much

smaller rating than the synchronous motor it is going to start.Jenerally most o the large synchronous motors ha&e

"rushless ecitation systems mounted on their shats. It is then

possi"le to use these eciters as the starting motors. 1or many

medium(si8e to large synchronous motors% an eternal startingmotor or starting "y using the eciter may "e the only possi"le

solution% "ecause the power systems they are tied to may not

"e a"le to handle the starting currents needed to use the

damper #amortisseur$ winding approach descri"ed net.

otor Starting by %sing damper &Amortisseur' (inding

As already mentioned earlier most o the large synchronous

motors are pro&ided with damper windings% in order to nulliy

the oscillations o the rotor whene&er the synchronousmachine is su"9ected to a periodically &arying load. Damper

windings are special "ars laid into slots cut in the pole ace o

a synchronous machine and then shorted out on each end "y alarge shorting ring% similar to the s!uirrel cage rotor "ars. A

pole ace with a set o damper windings is shown in 1igure..

:hen the stator o such a synchronous machine is connected

to the /(6hase AC supply% the machine starts as a /(6haseinduction machine due to the presence o the damper "ars% 9ust

li'e a s!uirrel cage induction motor. Kust as in the case o a /(

6hase s!uirrel cage induction motor% the applied &oltage must

"e suita"ly reduced so as to limit the starting current to thesae rated &alue. Once the motor pic's up to a speed near

a"out its synchronous speed% the DC supply to its ieldwinding is connected and the synchronous motor pulls into

step i.e. it continues to operate as a Synchronous motorrunning at its synchronous speed.

)ehavior of a synchronous motor

The "eha&ior o a synchronous motor can "e predicted "yconsidering its e!ui&alent circuit on similar lines to that o a

synchronous generator as descri"ed "elow.

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Synchronous*motor power e#uationBcept or &ery small machines% the armature resistance o asynchronous motor is relati&ely insigniicant compared to its

synchronous reactance% so that e!uation to "e approimated to

II. S3NC7RONOUS CONDBNSBR

A synchronous condenser #sometimes called synchronouscompensator$ is a synchronous generator that is not attached to

a prime mo&er. There is typically a small pony motor attached

to the synchronous machine input shat% "ut it is used only to

accelerate it to synchronous speed. Once the synchronous

machine is on(line% the pony motor is de(energi8ed and spinsreely. The synchronous machine ield current is controlled "y

a &oltage regulator as needed to control the system &oltage or

to urnish>a"sor" a speciied amount o reacti&e power.Increasing the de&iceMs ield ecitation increases its reacti&e

power #'AR$ output. The synchronous condenser is thus a

continuous &aria"le source>sin' o reacti&e power. A

synchronous condenser perorms se&eral 'ey unctions in awind(diesel hy"rid power system. :hen in running in parallel

with one or more diesel generators% it shares the reacti&e

power load with the diesel generator#s$% which impro&es the

system &oltage regulation and oten allows or the net load to "e met with a smaller genset than would otherwise "e

re!uired. Also% the inertia o the rotating machine resists rapidchanges in speed and there"y assists the diesel generator#s$

with re!uency regulation. This impro&ed &oltage andre!uency regulation counteracts the desta"ili8ing inluence o

the wind tur"ines% which can "e signiicant in gusty winds%

when wind power luctuations can "e rapid and large. In high

penetration wind(diesel systems% which re!uently aredesigned to support diesel(o operation #such that the wind

generators carry 2**- o the load$% the synchronous

condenser is sometimes used to regulate the grid &oltage% a

tas' normally perormed "y the diesel generators. B&en wherethere is a "attery>in&erter or lywheel>in&erter system that

regulates grid &oltage% a synchronous condenser is oten

necessary to ensure an ade!uate supply o ault clearing

current% which in&erters oten cannot pro&ide unassisted.

Synchronous motors can also "e used to impro&e o&erall

power actor o the system. :hen the sole purpose o

application is power actor impro&ement synchronous motorsare reerred as synchronous condenser. In such situation shat

o the motor is not connected to any mechanical load and itspins reely.

Synchronous motors load the power line with a leading poweractor. This is oten useull in cancelling out the more

commonly encountered lagging power actor caused "y

induction motors and other inducti&e loads. Originally% large

industrial synchronous motors came into wide use "ecause othis a"ility to correct the lagging power actor o induction

motors.

This leading power actor can "e eaggerated "y remo&ing the

mechanical load and o&er eciting the ield o the synchronousmotor. Such a de&ice is 'nown as a synchronous condenser.

1urthermore% the leading power actor can "e ad9usted "y

&arying the ield ecitation. This ma'es it possi"le to nearly

cancel an ar"itrary lagging power actor to unity "y parallelingthe lagging load with a synchronous motor. A synchronous

condenser is operated in a "orderline condition "etween a

motor and a generator with no mechanical load to ulill this

unction. It can compensate either a leading or lagging poweractor% "y a"sor"ing or supplying reacti&e power to the line.

This enhances power line &oltage regulation.

Since a synchronous condenser does not supply a tor!ue% the

output shat may "e dispensed with and the unit easilyenclosed in a gas tight shell. The synchronous condenser may

then "e illed with hydrogen to aid cooling and reduce

windage losses. Since the density o hydrogen is - o that o

air% the windage loss or a hydrogen illed unit is - o that

encountered in air. 1urthermore% the thermal conducti&ity ohydrogen is ten times that o air. Thus% heat remo&al is ten

times more eicient. As a result% a hydrogen illed

synchronous condenser can "e dri&en harder than an air cooledunit% or it may "e physically smaller or a gi&en capacity.

There is no eplosion ha8ard as long as the hydrogen

concentration is maintained a"o&e *-% typically a"o&e )2-.

The eiciency o long power transmission lines may "eincreased "y placing synchronous condensers along the line to

compensate lagging currents caused "y line inductance. 0ore

real power may "e transmitted through a ied si8e line i the

power actor is "rought closer to unity "y synchronouscondensers a"sor"ing reacti&e power.

The a"ility o synchronous condensers to a"sor" or producereacti&e power on a transient "asis sta"ili8es the power grid

against short circuits and other transient ault conditions.Transient sags and dips o milliseconds duration are sta"ili8ed.

This supplements longer response times o !uic' acting

&oltage regulation and ecitation o generating e!uipment. The

synchronous condenser aids &oltage regulation "y drawingleading current when the line &oltage sags% which increases

generator ecitation there"y restoring line &oltage.

#1igure "elow$ A capacitor "an' does not ha&e this a"ility.

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Synchronous condenser impro&es power line &oltage

regulation.

The capacity o a synchronous condenser can "e increased "yreplacing the copper wound iron ield rotor with an ironless

rotor o high temperature superconducting wire% which must

"e cooled to the li!uid nitrogen "oiling point o oG #(

2)=oC$. The superconducting wire carries 2=* times thecurrent o compara"le copper wire% while producing a lu

density o / Teslas or higher. An iron core would saturate at ,

Teslas in the rotor air gap. Thus% an iron core% approimatePr<2***% is o no more use than air% or any other material with

a relati&e permea"ility Pr<2% in the rotor. Such a machine issaid to ha&e considera"le additional transient a"ility to supply

reacti&e power to trou"lesome loads li'e metal melting arcurnaces. The manuacturer descri"es it as "eing a Qreacti&e

power shoc' a"sor"er. Such a synchronous condenser has a

higher power density #smaller physically$ than a switched

capacitor "an'. The a"ility to a"sor" or produce reacti&e power on a transient "asis sta"ili8es the o&erall power grid

against ault conditions.

Synchronous condensers or reacti&e power compensationA??Es synchronous condensers ensure eicient and relia"le

operation o power grids "y pro&iding reacti&e power

compensation and additional short circuit power capacity.A?? can tailor synchronous condenser modules to matchsystem perormance re!uirements and site conditions% and

deli&er optimum cost(eiciency.

To ensure enduring and relia"le operation% A?? synchronous

condensers are designed or high relia"ility% dura"ility and thecapa"ility to operate or long periods o time "etween

recommended ser&ice inter&als.

III. CONC;USION

:ith the completion o this trial we can say that a

synchronous motor is an electromechanical de&ice capa"le o

liting a high load range at a gi&en speed% called synchronousspeed% drawing the necessary energy rom a networ'.

The synchronous motor is used in those cases where it is

desired constant speed

The synchronous motor% uses the concept o a rotatingmagnetic ield produced "y the stator% "ut now the rotor

consists o permanent magnets which rotate synchronouslywith the stator ield.

Ultimately this trial "een o great help as my 'nowledgeincreased and clearing my dou"ts a"out a synchronous motor.

R B1BRBNCBS

2 K. Chapman.% 05!uinas Blctricas.% 0D.1.% Cuarta BdiciVn. .

, A. B. 1it8geral% 05!uinas Blctrica% W

Iv+n .,olcha- was "orn in Rio"am"a% in 2))*. 7e studied al 6linio

Ro"alino School. 7is secondary school ternimed al the Technical

Saint Koshep. It is in the race o Blectrical Bngineering at the

National 6olytechnic School.

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Synchronous Machines and SC