11 Synchronous Machine (Week 11)

7/31/2019 11 Synchronous Machine (Week 11)

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Synchronous Machine

Electric Machinery :


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I. Synchronous Machine Fundamentals

A. Function in electric power system

What is a synchronous machine

A synchronous machine is an ac machine whose speed under steady-state

conditions is proportional to the frequency of the current in its armature.

The rotor, along with the magnetic field created by the dc field current on the

rotor, rotates at the same speed as, or in synchronism with, the rotating

magnetic field produced by the armature currents, and a steady torque

results.


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B. Major components

S

N

Field winding

Rotor

Stator

Armature winding

Slip rings & brushes


Number of pole

- Pole number of stator depends on number of coil group per-phase

- Pole number of rotor is equals to pole number of stator


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1. Salient pole- Pole faces project out from the rotor

- Normally used for rotors with four or more poles

Type of synchronous machine is determined by the design of the rotor pole

For synchronous machine there is two type of rotor design.

- Utilisation: low speed synchronous machine

- Application: hydroturbine as prime mover

C. Types of synchronous machine



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2. Non-salient pole

- Cylindrical rotor

- Normally used for rotors with two and four poles

- Utitilisation: high speed synchronous machine

- Application: steam and gas turbines as prime mover




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D. Physical Phenomena inSynchronous Machine Operation

Generator

Rotating magneticfield passes the

armature windingof stator

Strong magneticfield is producedaround the field

winding

The field windingof rotor is

supplied with DCpower supply

DCsupply

If

IL

Load

Rotor is rotatedby using prime

mover

S

N

AC voltage isinduced in the

armature winding

ACvoltage


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II. Equivalent Circuit

A. Generator

Field winding Armature winding

I

F

RF

LF

VF (DC)

+

-

+

+

+

-

-

-

IA1

IA

2

IA3

RAj XS

j XS

j XS

RA

RA

EA1

EA2

EA3

V 1

V 2

V 3

+

+

+

-

-

-

+

-EA1

+

-

EA2

+

-EA3

RA

j XS

j XS

RA

RA

j XS

V

ILIA

VL

+

-

+

-EA1

+ -EA2

+

-EA3

RA

RA

RA

j XS

j XS

j XS

IL

+

-

VL

V

+

-

IA


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A. Generator

I

F

RF

LF

VF (DC)

+

-

+

-

IA

RAj XS

EA V

+

-

1 phase equivalent circuit

As XXX

AAASA IRIjXEV (4-2)

(4-3)(4-1))( FFFF jXRIV

VVL

In Delta connection

(4-4)

AL II 3 (4-4a)

VVL 3

In Wye connection

(4-5)

AL II (4-5a)

EA : internal generated voltagein one phase

V : output voltage of onephase

XS : synchronous reactanceRA : stator resistance

IA : stator (armature) current

IF : excitation (field) currentRF : excitation (field) resistanceX : reactance corresponding

armature reactionXA: reactance corresponding

stator self-inductance

VL : terminal voltageVF : excitation (field) voltage

IL : line current


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Generator Phasor Diagram

.a a a sV E jI X


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Equivalent circuit of synchronous generato

Xar = Armature reactanceXL = Leakage reactanceRa = Armature Resistance

Because of the voltages in thesynchronous generator are AC voltages ,there are usually expressed by phasordiagram.


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III. Performances

Losses

Voltage Regulation of generator (alternator)

(4-9)mechcucoreoutin PPPPP

AAcu RIP 2

3 (4-10)

%100

fl

flnl

V

VVVR (4-11)

+

-

IA

RAj XS

EA V

+

-

Using 1 phase equivalent circuit of alternator

Anl EV

sAAAfl jXRIEVV

(4-12a)

(4-12b)

because 0

AI


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IV. Characteristics

A. Open circuit and short circuit tests of alternator

Open circuit test (considered stator windings are connected in delta)

- Objective

Observing the relationship between field current IF and internal generated voltage EA

- Procedure Terminal (output) of generator are disconnected from load

Field current is set to zero Generator is turned in rated speed

The current field is gradually increased while measuring the terminal voltage VT

- Condition

0AI

LA VE

(4-13)

(4-14)

VL

IF

airgap line

+

-

IA

RAj XS

EA V

+

-

I

F

RF

LF

VF (DC)

+

-


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IV. Characteristics

Short circuit test (considered stator windings are connected in delta)- Objective

Observing the relationship between field current IF and armature(load) current IA

- Procedure Terminal (output) of generator are short circuited through ammeters

Field current is set to zero

Generator is turned in rated speed The current field is graduaaly increased while measuring the armature current IA

- Condition

IA

IF

0 VVL

sAAA jXRIE

(4-15)

(4-16)

sA

A

A jXRI

E (4-17)


+

-

IA

RAj XSEA

I

F

RF

LF

VF (DC)

+

-


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IV. Characteristics

Supposed :

Where :

Ratio of the field current required for obtaining the rated voltage at open circuit test tothe field current required for getting the rated armature current at short circuit test

AR

A

AS

I

EX

(4-18)

IA = armature current obtained in short circuit test

EA = terminal (output) voltage obtained in open circuit test


- Synchronous reactance

- Short circuit ratio


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IV. Characteristics

B. Operating Characteristics of Alternator

Speedactive power characteristic

Prime movers speed will decrease in nonlinear when the power

drawn from it (or the load of alternator) increasesGovernor mechanism is included to the prime mover to make the

decrease of the speed linear

Speed Drop of prime mover

%100fl

flnl

n

nnSD (4-19)

ns

nnl

nfl

PflPout0


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IV. Characteristics


(4-20)

f

fnl

fsys

PflPout0

120

pnf s

Based on (4-8a), it is obtained :

Frequencyactive power characteristic

sysnlout ffSPP

Note :

SP : slope of curve (W/Hz)

fnl : no load frequency of generator

fsys : operating system frequency

Output power of alternator

(4-21)


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IV. Characteristics


Terminal voltagereactive power characteristic

VL

V T nl

V L fl

Qfl

Q out

0

- QKVAR suppliedKVAR consumed


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IV. Characteristics


Operating alone

Operation condition

- The governor set points of prime mover of alternator will control the operatingfrequency of system (the frequency generated by the alternator)

- The real and reactive power supplied by alternator will be equal tothe amount demanded by attached load

loadoutPP

loadoutQQ

- The field current of alternator control the terminal voltage of powersystem ( the output voltage of the alternator)

fL IV


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IV. Characteristics


Operating alone (contd) The effect of load on output voltage

- Lagging load (+Q): when load is increased (more lagging),phase voltage V and line voltage VLdecrease significantly

- Unity power factor : when load is increased, phasevoltage V and line voltage VLdecrease slightly

- Leading load (-Q) : when load is increased(more leading), phase voltage V and linevoltage VL willrise

V V

jXsIA

EAjXsIA

EA

IAIA

+

-

IA

RAj XS

EA V

+

-

V V

jXsIA

EA jXsIA

EAIAIA

V V

jXsIA

EA

jXsIA

EA

IAIA


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IV. Characteristics


Operating alone (contd)

Keeping V or VL constant in load changes

By decreasing or increasing field current (IF ) of alternator

KEA

When IF EA V

AAASA IRIjXEV (4-2)

(4-22)

When IF EA V

fI (4-23)


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IV. Characteristics


Parallel with large power system Characteristics of large power system (infinite bus)

Voltage and frequency do not vary regardless of how muchactive and reactive power (Q and P) is drawn from andsupplied to the infinite bus

V L

Q, kVAR- Q

suppliedconsumed

f

P, kW- P

suppliedconsumed


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IV. Characteristics


Parallel with large power system (contd) House diagram

f

PG, kWPGPinf bus, kW

Pload

f nl

Frequency-powercharacteristic of generator

Pinf bus

Frequency-powercharacteristic of infinite bus

fsys


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IV. Characteristics


Parallel with large power system (contd)

The generator must have frequency slightly higher than the frequency of infinite busin order to be able to supply an active power

Condition must be fulfilled by alternator before paralleling

Condition after paralleling

- The frequency and terminal voltage of alternator are controlled bythe infinite bus to which it is connected

- The governor set points of alternator controlthe active power supplied to the infinite bus

- The field current of alternator controls the reactive powersupplied to the infinite bus

If constant

s constant

EAconstant

EA

EA

EA

V

IA

IAIA

PG

PG

PG

P constant

s constant

EA EA EA

V

IA

IA

IA

P

QGQG

P


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IV. Characteristics


Parallel with other alternator


- The total active power supplied by the two alternators togethermust equal the amount consumed by the load

loadGG PPP 21

- To adjust active power sharing between the alternators withoutchanging fsys, simultaneously increase the governor set pointsof one alternator while decreasing the governor set points ofthe other

- To adjust fsys,without changing the active power sharing,simultaneously increase or decrease both of alternatorsgovernor set point

Alternator 1 Alternator 2

PG1

PG2

P

f

fsys

PG1 PG2

'

2

'

121 GGGGloadPPPPP

fsys

PG1 PG2

fsys

f

P PG1 PG2


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IV. Characteristics


Parallel with other alternator (contd)


- To adjust reactive power sharing between the alternators,without changing the terminal voltage, simultaneouslyincrease the field current of one alternator while decreasing

the field current of the other

- To adjust VL , without changing the reactive power sharing,simultaneously increase or decrease both of alternatorsfield current

Alternator 1 Alternator 2

QG1

QG2

Q

VL

VL sys

QG1 QG2

QG1 QG2

VL

sys

VL

Q

VL sys

QG1 QG2


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IV. Characteristics

C. Paralleling an alternator with infinitebus and other alternators

Required conditions

The rms line voltage must be equal

The phase sequence must be similar

The phase angles of them must be equal

The frequency of the on coming alternator must same with thefrequency of the running system

Procedure of paralleling

1. The field current of oncoming alternator should be adjusted until its line voltageis equal to the line voltage of the running system

2. The phase sequence of the oncoming alternator must be compared to thephase sequence of the running system

3. The frequency of the oncoming alternator is adjusted to be slightly higher thanthe frequency of the running system

4. Switch on the connection when the phase angles of the incoming alternator andthe running system are equal.


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I. Synchronous Machine (Motor)Fundamentals

D. Physical Phenomenon inSynchronous Machine Operation

Motor

If

IL

S

N

DCsupply

Field winding ofrotor is suppliedwith DC power

supply

A steady-statemagnetic field BR is

produced

A uniform rotatingmagnetic field BS

is produced

Rotor field BR willtend to line up with

the stator field BS

Armaturewinding of statoris supplied with

AC powersupply

ACvoltage

There is torque in rotorwinding, motor starts rotating

S h M t


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Synchronous Motor

C. Starting of a motor

Synchronous motor Field Condition 3 phase synchronous motor has two rotor pole NR and SR.

The rotor will be wound by two pole Ns and Ss.

The motor has direct voltage applied to the rotor winding and a 3 phasesupply applied to the stator winding

Stator winding produce rotating field Ns

So this create rotating field for stator armature and stationary field for rotor

S h M t


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Synchronous Motor

Initial condition rotor repel each other due to same polarity causing the rotorrotate anti-clockwise

After period of cycle (1/2 1/100 second, polarities of the stator reversed but

the polarity of the rotor same. Different poles attract each other causing tomove clockwise

Stator change polarity rapidly and high inertia of rotor, motor failed to start.

S h M t


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C. Starting of a motor

Making synchronous motor self starting

Synchronous motor cannot start by itself To make the motor self starting damper winding is provided on rotor

Damper winding copper bars embedded on the poles faces of the salientpoles and shorted at the end to create squirrel cage winding.

Damper winding serves to start the sync motor. Initially motor starts asinduction motor

As motor reach Ns , rotor excited with DC. Resulting poles facing oppositepolarities making rotor poles lock with poles rotating flux.

Synchronous Motor

As rotor rotate with Ns, no cutting flux happened and no induced current

S h M t


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C. Starting of a motorSynchronous Motor

Synchronous Motor


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C. Starting of a motorSynchronous Motor

II Equivalent Circuit


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B. Motor

Field winding Armature winding

I

F

RF

LF

VF (DC)

+

-

+

+

+

-

-

-

IA1

IA2

IA3

RAj XS

j XS

j XS

RA

RA

EA1

EA2

EA3

V 1

V 2

V 3

+

+

+

-

-

-

+

-EA1

+

-

EA2

+

-EA3

RA

j XS

j XS

RA

RA

j XS

V

IL

IA

VL

+

-

+

-EA1

+ -EA2

+

-EA3

RA

RA

RA

j XS

j XS

j XS

IL

+

-

VL

V

+

-

IA

II Equivalent Circuit


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B. Motor

I

F

RF

LF

VF (DC)

+

-

+

-

IA

RAj XS

EA V

+

-

1 phase equivalent circuit

AAASA IRIjXEV

AAASA IRIjXVE

(4-6)

(4-7)

VVL In Delta connection

(4-4)

AL II 3 (4-4a)

VVL 3

In Wye connection

(4-5)

AL II (4-5a)

(4-1))( FFFF jXRIV

IV Characteristics


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IV. Characteristics

D. Operating Characteristics ofSynchronous Motor (in steady state)

Torquespeed characteristics

The rotation speed of the synchronous motor is locked tothe applied electrical frequency.

pullout

ns

ind

rated

nm

%100

fl

flnl

n

nnSDSR (4-22)

%0SR

smnn constant

flnlnn

sm

A

pullout X

EV

3

max (4-23)(N.m)

IV Characteristics


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IV. Characteristics


Effect of load changes

If load is attached to the shaft of a synchronous motor, the motor will developenough torque to keep the motor and its load turning at a synchronous speed

Magnitude of EA remains constant

IL, VL and PF angle ( ) change

V

P1

P2P3

P4

IA1IA2

IA4EA1

EA2

EA4

IA3

EA3

+

-

IA

RAj XS

EA V

+

-

IV Characteristics


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IV. Characteristics


Effect of field current changes

When the field current (If) is increased, the magnitude of EA is also increasedbut the active power supplied to the motor is still constant

Speed nm is not affected by the field current (if)

V and VL is kept constant by the power supply

IA and EA change

V

P = constantIA1

IA2

IA4

EA1 EA2 EA4

IA3

EA3P = constant

+

-

IA

RAj XS

EA V

+

-

IV Characteristics


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IV. Characteristics


Synchronous motor V curves

Relationship between IA and If corresponding to a type of power factoroperating system and a value of active power supplied to the motor.

The motor is

over excited

The motor is

under excited

PF = 1

Lagging PFLeading PF

P1

P2IA

IF

Minimum IA occurs at unity PF

When If less than If giving the minimumIA, the motor operates with lagging PF orconsumes Q.

When If greater than If giving the minimum IA,the motor operates with leading PF or supplies Qto the power supply. The motor operates assynchronous condenser.

IA

EA

V jXS IA

IV Characteristics


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IV. Characteristics

E. Starting Methods of Synchronous Motor

Reducing Electrical Frequency

Using external prime mover

Using armotisseur winding

Reduce the speed of the stator magnetic field so that the rotor can accelerateand lock in with the magnetic field during one half-cycle of the magnetic fieldsrotation

Prime mover is used to accelerate the rotor up to the synchronous speed (thespeed of the stator magnetic field). Then, the prime mover is disconnected

Additional windings that lay on the face of synchronous motors rotor. It helps

the rotor being bale to rotate within the synchronous speed

Three basic approaches used to safely start a synchronous motor

V. Nameplate


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V. Nameplate

- Voltage supply

- Frequency

- Speed

-Apparent power(kVA)

- Power factor

- Field current

- service factor

- Current supply

Nominal values of

- Field voltage


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11 Synchronous Machine (Week 11)

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