883017aLCI

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Topic 17: Wound-Field Synchronous

Machine Drives

Spring 2004

ECE 8830 - Electric Drives

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Introduction

For high power (multi-MW) applications,the high efficiency of synchronous motors

makes them more appealing than inductionmotors. Indeed, most of today¶s electricalpower generators are 3* synchronousgenerators.

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Brushless dc Excitation

Wound-field synchronous motors require dccurrent excitation in the rotor winding. Thisexcitation is traditionally done through theuse of slip rings and brushes. However,these have several disadvantages such asrequiring maintenance, arcing (whichmeans they cannot be used in hazardous

environments), etc. An alternative approachis to use brushless excitation which isillustrated on the next slide.

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Brushless dc Excitation (cont¶d)

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Brushless dc Excitation (cont¶d)

A wound-rotor induction motor (WRIM) ismounted on the same shaft as the wound-field synchronous motor. This is acting as a

rotating transformer with the rotor as theprimary and the stator as the secondary.The stator of the WRIM is fed by a 60Hzsupply and the rotor of the WRIM rotates at

a speed set by the supply frequency. Theslip voltage in the rotor winding of theWRIM is rectified to provide the currentfeed to the rotor windings of the

synchronous motor.

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Load Commutated Current-Fed

Inverters

Thyristor current-fed, load commutatedinverters (LCI¶s) are very popular forhigh power (multi-MW) wound-field

synchronous motor drives.

We will briefly review current-fedthyristor inverters and then discuss

load commutated inverters in somedetail. We will then see how to applythem to wound-field synchronous motordrives.

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Review of Current-Fed Thyristor

Inverter

Let us first briefly review the operation of the current-fed thyristor inverter.

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Inverter (cont¶d)

Initially, ignore commutation considerations.

Induction motor load is modeled by back emf

generator and leakage inductance in eachphase of the winding.

The constant dc current Id is switched

through the thyristors to create a 3* 6-stepsymmetrical line current waves as shown onthe next slide.

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Inverter (cont¶d)

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Inverter (cont¶d)

The load or line current may be expressedby a Fourier series as:

where the peak value of the fundamental

component is given . Each thyristorconducts for radians. At any instantone upper thyristor and one lower thyristorconduct.

2 3 1 1cos cos 5 cos 7 ...

5 7a d

i I t t t [ [ [T

« »! ¬ ¼- ½

2 3 /d I T

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Inverter (cont¶d)

The dc link is considered harmonic-freeand the commutation effect between

thyristors is ignored.At steady state the voltage output fromthe rectifier block = input voltage of inverter.

For a variable speed drive the invertercan be operated at variable frequencyand variable dc current Id.

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Inverter (cont¶d)

If thyristor firing angle E > 0, inverterbehavior.

If thyristor firing angle E=0, rectifierbehavior.

Max. power transfer occurs when E=T.

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Inverter Operation Modes

Two inverter operation modes areestablished depending on the thyristorfiring angle:

1) Load-commutated inverter

Applies when T /2<E<T.

2) Force-commutated inverter

Applies when T<E<3T /2.

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Load-Commutated Inverter Mode

Consider E=3T /4. In this case vca < 0 => thyristor Q5 is turned off by the load. Thisrequires load to operate at leading power

factor => motoring mode of a synchronousmachine operating in over-excitation.

Vd=-Vd0cosE

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Load Commutated Inverters

Let us initially consider a single-phaseinverter before discussing the 3* case. Asingle-phase, current-fed, parallel

resonant inverter with load commutationis shown below:

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Load Commutated Inverters (cont¶d)

A phase-controlled rectifier provides the dcinput and a large capacitor C provides theload commutation of the thyristors. Assuming

perfect filtering of harmonics by the capacitorand the dc link inductor, the inverter loadvoltage and current waves are shown below:

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The thyristor pairs Q1Q2 and Q3Q4 areswitched alternately for T angle toproduce the square wave output. The

fundamental of the current wave leadsthe sinusoidal voltage wave by Fr. Thus,when Q1Q2 turn on, the Q3Q4 pair has anegative voltage for duration Fr which

provides the load commutation. Since F=[tq, the time tq must be sufficientlylong for the thyristors to turn off.

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The equations for the inverter circuit are:

where Rd is the resistance of the inductor Ld.

d iv i R L

d vi C d t

1 2 L L C i i i i i! !

1 2d i i i!

d d L d d d

div v i L

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These equations can be expressed in state-variable form and solved to model thesteady state and dynamic performance of

the inverter.

We will now consider an approximatesteady state analysis assuming that Ld is of

infinite size and is lossless. We will alsoassume that the load is highly inductive,i.e. [L>>R.

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Consider the series R-L load to be resolvedinto parallel R1 and L1 components in whichreal current IP flows through R1 and

reactive current IQ flows through L1. Theload impedance ZL can be written as:

R j L Z R j L

1 1 1 1

2 2 2 2 2 2

1 1 1 1

R L R L j

R j L R j L

[ [[ [

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If the load is highly inductive (as we hadassumed) R1>>[L1 and

and L } L1.

The fundamental component of the current

is given by:

2 2 L d

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The real and reactive components of theload current are given by:

where . Through somealgebraic manipulation we get:

R F! !

'sin L

X F! !

R V T!

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From the above equations we cancalculate the load voltage, currents, andcommutation angle F.

Example:

Single-phase synchronous motor;

Vd=200V, f=60Hz, R=0.2;, L=1.2mH,Id=240A, C=150QF. Find F.

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There are basically two control variablesfor the load commutated inverter - thedc link current and the frequency. For a

variable load, a variable capacitance canbe used to provide desired margin of commutation angle F. However, a betterway to operate is to use a PLL to control

the inverter frequency to just above theload resonance frequency.

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The single-phase inverter concepts can beextended to 3* LCI¶s. The figure belowshows a three-phase LCI with lagging

power factor load. Here load commutationis achieved by using a leading VAR loadconnected at the load terminal.

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In the case of a variable load, a fixedcapacitor bank is connected at the

terminals and the inverter frequencyadjusted so that the effective inverterload has a leading PF so thatcommutation occurs at a fixed angle F.

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As mentioned earlier, thyristor current-fed,load commutated inverters (LCI¶s) are verypopular for high power (multi-MW) wound-field synchronous motor drives where it iseasy to maintain the required leading PFangle by adjusting the field excitation.

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The fundamental frequency phasor diagram fora salient pole synchronous machine undermotoring condition is shown below:

Note: The winding resistance and the

commutation overlap effect have been neglected.

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A flux linkage has been included in thephasor diagram where ]f = field flux linkage,]a=armature reaction flux linkage and

]S=resultant stator flux. We can write thede and qe components of ]a as follows:

For a salient pole machine, Ld{Lq the phasors]

and Is

are not in phase.

2 sin( )ds ds ds ds s

L I L I ] H J ! !

2 sin( )q s q s q s q s s

L I L I ] H J ! !

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The motor phase voltage and current wavesare shown below including the commutationoverlap effect:

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The load commutated inverter with anover-excited synchronous machine loaddepends on sufficient back emf which is

not available at low speeds. The criticalspeed required for load commutation towork is about 5% of base speed. A forcedcommutation approach is required below

these speeds and to start the motor. (seeBose text pp. 284-285 for details).

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Load-Commutated Inverter Drive

Having seen how a current-fed, thyristorinverter can be load commutated with awound-field synchronous motor by

operating the machine at a leading powerfactor, we can now consider how to designa self-controlled drive system for a wound-field synchronous motor based on a load-

commutated inverter drive. As mentionedearlier, this type of drive is popular for highpower (multi-MW) drives for compressors,pumps, ship propulsion, etc.

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(cont¶d)

A block diagram of a self-controlled load-commutated, current-fed inverter drive for awound-field synchronous motor is shown below:

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(cont¶d)

The phasor diagram for the LCI inmotoring mode driving a synchronousmotor is shown below:

Note: The saliency and stator resistance

have been neglected.

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(cont¶d)

The field flux ]f is established by the fieldcurrent If and depends on the rotor position.The armature flux ]

is determined bythe stator current and stator windinginductance. The delay angle command Ed

sets the position of ]a relative to ]f since ]a

leads ]f by J¶ given by:

where H= torque angle.

T J T E H J ¨ ¸! ! © ¹

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(cont¶d)

Thyristors require a minimum turn-off time toff for successful commutation. Thiscorresponds to a turn-off angle K=[toff .

For reliable operation of a LCI drive andminimum reactive current loading to thesynchronous motor, turning off thethyristors at a fixed time every cycle is a

good approach. A complete speed controlsystem for a LCI synchronous motor driveincorporating constant turn-off anglecontrol is shown on the next slide.

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(cont¶d)

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(cont¶d)

This drive operates in the constant torqueregion in motoring mode with stator flux]s maintained constant (open loop). There

are four control elements: Speed and dc link current control

Field flux /field current control

Generation of ]f

* andQ

* commandsignals (where Q is the commutationoverlap angle)

Delay angle control.

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(cont¶d)

Speed and dc link current control:

[r compared to [r* and error goes

through P-I controller and absolute value

circuit -> Id*. Id and Id* compared and

controls thyristor firing angle E in rectifierto control dc link current.

The generated motor torque w Id (seeBose text pg. 499 for derivation).

3 6cos '

2e s d d

P T I K I ] J

¨ ¸! !© ¹

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(cont¶d)

Field flux /field current control:

The command field flux ]f * is given by:

where ]s*= constant, ]a

*=LsIs=KaId* and

J*= K*+kQ*. To obtain J we need Q* whichcan either be measured or calculated usingthe expression:

* *2 *2 * * *2 cos

2 f s a s a

T] ] ] ] ] J ¨ ¸! © ¹ª º

*1 * *

2cos cos

L I Q K K

! ¬ ¼

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(cont¶d)

The command flux current If * is then

generated from the command flux ]f * by

through a function generator thatcorrects for saturation effects. A phase-controlled rectifier can then be used tocontrol the flux current as shown in thesystem block diagram.

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(cont¶d)

Generation of ]f *, Ed

*, H* and Q* commandsignals:

We have discussed how all of the

command signals can be obtained withthe exception of the H* angle. This isobtained from the equation:

* 1 *6 cossin s

L I J H T]

« »! ¬ ¼¬ ¼- ½

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(cont¶d)

Delay Angle Control:

For a six-step inverter we need six

discrete firing pulses at T /3 intervalsapart within a cycle. A block diagram

showing how this can be achieved isshown on the next slide.

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(cont¶d)

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(cont¶d)

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(cont¶d)

The corresponding alignment of referencesignal S1 and the waveforms for phase a inmotoring mode are shown below. Thesediagrams can be used to determine the

inverter firing angles.

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(cont¶d)

The absolute position sensor can beeliminated and the machine terminalvoltage signals can be used instead to

estimate the rotor position for inverterfiring angle determination. Details arepresented in the Bose textbook pp. 504-507.

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Cycloconverter Drive

High power, wound-field synchronousmotors can be operated at unity powerfactor when excited by phase-controlled,

line-commutated, thyristorcycloconverters. Drive control for suchdrives can be both scalar and vectorcontrol, similar to that of the voltage-fed

inverter drive.

The next slide shows a simple scalarcontrol method for a cycloconverter drivefor a wound-field synchronous motor.

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Cycloconverter Drive (cont¶d)

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There are three control variables in thiscontrol system:

The stator current amplitude,

Phase angle, J¶ (see phasor diagram below)

The field current, If .

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The torque generated by the motor isproportional to the in-phase stator current.The command stator current Is

* is generated

from the error in the speed control loop.The angle J¶ and the field current If can bedetermined from Is as

shown in the figure. Thus,

Is* is used to generate J¶ *

and If * using function

generators.

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The position sensor and encoder generatethe cos Ue and sin Ue signals and the speedsignal, [r. The 2-phase unit signals are

converted to 3-phase unit signals using thefollowing transformations:

cosa eU U!

2 1 3cos cos sin3 2 2

2 1 3cos cos sin

a e e e

TU U U

¨ ¸! ! © ¹

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Each of the 3* unit signals is thenmultiplied by Is

* and phase shifted byangle J¶ * to produce the three phase

current command signals as follows:*

* '* s

a ai I U J !

sb bi I U J !

i I U J ! $

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The performance of the cycloconverter drivecan be enhanced if vector control is usedrather than scalar control. In the constanttorque region, the field current must beincreased if we want to increase the developedtorque at the constant rated stator flux.However, the field current response is slowand this leads to sluggish motor response. In

vector control we inject a transientmagnetizing current in the direction of thestator flux to obtain a much faster responsethan with scalar control. This current is set to

zero in steady state to maintain unity PF.

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A vector control implementation is shown:

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A phasor diagram for the vector controlapproach is shown below:

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Notable points from phasor diagram:

The torque component of the stator current IT

is in phase with Vs and forms a triangle withIs and the injected magnetizing current IM.IM=0 at steady state and IT=Is.

The magnetizing current, Im, the field current

If , and the torque component of the statorcurrent IT form a right-angled triangle (whichis a scaled version of the flux triangle).

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There are three sets of d-q axes:

- de-qe in reference frame of rotor;

- ds-qs in reference frame of stator;

- de¶ -qe¶ with qe¶ aligned with Vs and de¶ aligned

with ]s.

At steady state, ]s and ]a are at quadrature.

Also, Is is in phase with Vs which leads ]s by90° => unity PF.

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From the phasor diagram, at steady state, wecan write:

This equation gives the control equation forIf

*. Under transient conditions, the commandinjected transient magnetizing current, IM

given by:

Under steady state conditions, IM=0 and theabove steady state equation is re-established.

cosm f I I H !

* *cos

M f I I I H !

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Control features of the vector control of awound-field synchronous motor drive:

Speed control error generates the torque

component of current through P-I controller. Command currents IT

* and IM* are compared

to feedback currents, IT and IM to generatecommand voltages vT

* and vM* through P-I

compensators. A vector rotator uses unit control signals

cosE and sinE to transform the vT* and vM

signals to phase command voltages va*, vb

and vc*.

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A transient change in the required torquecauses IM to be injected because of thesluggish response of If , thereby maintaining

a constant flux ]s. As If builds up, IM dropsdown to reach zero when If has reached itsnew steady state value.

The complete vector control feedbacksignal processing is shown on the nextslide.

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