3 - LCI Fundamentals

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GEH-6373

Innovation™ Series

AC Drives

Load Commutated Inverter

User’s Manual



Innovation™ Series

AC Drives

Publication: GEH-6373

Issue date: 1998-03-31

Load Commutated InverterUser’s Manual



© 1998 by General Electric Company, USA.All rights reserved.

Printed in the United States of America.

These instructions do not purport to cover all details or variations in equipment, nor to

provide every possible contingency to be met during installation, operation, and

maintenance. If further information is desired or if particular problems arise that are not

covered sufficiently for the purchaser’s purpose, the matter should be referred to GE

Industrial Control Systems.

This document contains proprietary information of General Electric Company, USA and

is furnished to its customer solely to assist that customer in the installation, testing,

operation, and/or maintenance of the equipment described. This document shall not be

reproduced in whole or in part nor shall its contents be disclosed to any third party

without the written approval of GE Industrial Control Systems.

GeniusTM

, Field ControlTM

, LogicMasterTM

, and Series 90TM

are trademarks

of GE Fanuc Automation North America, Inc.

InnovationTM is a trademark of General Electric Company.

TeflonTM

is a trademark of E.I. DuPont de Nemours and Co., Inc.

Windows® is a registered trademark of Microsoft Corporation.



Load Commutated Inverter, User’s Manual GEH-6373

Safety Symbol Legend • a

Safety Symbol Legend

Indicates a procedure, practice, condition, or statement that, if not strictly

observed, could result in personal injury or death.

Indicates a procedure, practice, condition, or statement that, if not strictly

observed, could result in damage to or destruction of equipment.

Note

Indicates an essential or important procedure, practice, condition, or statement.

CAUTION

WARNING



GEH-6373 Innovation Series AC Drives

b • Safety Symbol Legend

This equipment contains a potential hazard of electric shock or burn. Only

personnel who are adequately trained and thoroughly familiar with the

equipment and the instructions should install, operate, or maintain this

equipment.

Isolation of test equipment from the equipment under test presents

potential electrical hazards. If the test equipment cannot be grounded to the

equipment under test, the test equipment’s case must be shielded to prevent

contact by personnel.

To minimize hazard of electrical shock or burn, approved grounding

practices and procedures must be strictly followed.

To prevent personal injury or equipment damage caused by equipmentmalfunction, only adequately trained personnel should modify any

programmable machine.

WARNING

WARNING




Section 2, Functional Description • 5

2 Functional Description

Section 2 of this manual is a functional description of

the LCI. Its purpose is to provide a basic technical

overview of the operating theory, to help the user bet-ter understand how to run and maintain the drive.

This description requires the reader to be already fa-

miliar with the principles of power conversion and

microprocessor-based control. This section is organ-

ized as follows:

Section Heading Page

2-1. LCI System Basics ........................................... 5

2-1.1. Power Conversion......................................... 5

2-1.2. Excitation Voltage Controller....................... 6

2-1.3. Microprocessor-Based Control ..................... 6

2-2. Power Converter Operation .............................6

2-3. Control Operation ............................................9

2-3.1. Synchronization ............................................9

2-3.2. Commutation................................................. 9

2-3.3. Force Commutated Operation/Modes......... 10

2-3.4. Load Commutated Mode ............................ 10

2-3.5. Torque Control............................................ 13

2-3.6. Phase-Locked Loop..................................... 15

2-4. Dual-Channel, 12-Pulse Configuration.......... 15

2-4.1. Shutdown Operations.................................. 16

2-5. Series 12-Pulse Configuration ....................... 15

2-1. LCI System Basics

The LCI is a static, adjustable frequency drive system.

It uses application-specific, microprocessor-based

software to control the speed of a synchronous ma-

chine (motor or generator). The basic LCI is a 6-pulse

configuration that consists of two functional sections:

a power converter assembly and a control assembly

Figure 2-1 is a simplified one-line diagram of a single

channel LCI. Sections 2-1.1 through 2-1.3 describe the

function of the components shown. Sections 2-5 and

2-6 cover 12-pulse configurations.

2-1.1. Power Conversion

The LCI power converter is made up of a rectifier

that feeds an inverter through a dc link reactor. A

transformer isolates the LCI from the ac system bus

and provides the correct voltage at the rectifier termi-nals. The transformer’s internal impedance limits the

magnitude of any downstream bus faults.

The rectifier is a line commutated, phase-controlled

thyristor bridge that functions as a source converter.

Its microprocessor-controlled gating produces a vari-

able dc voltage output to the dc link reactor. The re-

actor smoothes the current and keeps it continuous

over the system’s operating range.

The reactor output is fed to the inverter, which is a

load commutated thyristor bridge. The inverter is also

microprocessor-controlled and functions as a loadconverter (see section 2-2.1.1). The inverter produces

a variable frequency ac output to a synchronous mo-

tor’s stator terminals.

ACLINE

ISOLATIONTRANSFORMER

RECTIFIER(SOURCE)

DC LINKREACTOR

INVERTER(LOAD)

SYNCHRONOUS MACHINE

FIELD

MICRO-PROCESSOR –

BASED CONTROL

EXCITATION

VOLTAGE

CONTROLLER

Figure 2-1. Simplified LCI System One-Line Diagram




6 • Section 2, Functional Description

2-1.1.1. Power Flow Reversal . The LCI’s rectifier

and inverter bridges use the same power hardware and

are both controlled by microprocessor-based elec-

tronics. Their functions can be reversed by reversing

their power flow. In this case, the synchronous motor

is braked by pumping its energy back into the ac line.

Because of this reversing capability, the line sidebridge (labeled rectifier in Figure 2-1), can also be

called the source converter ; the machine side bridge

(labeled inverter ) can be called the load converter.

2-1.2. Excitation Voltage Controller

The synchronous motor field is usually excited by a

brushless exciter coupled to the motor shaft. The

brushless exciter is a wound rotor induction motor. Its

rotor voltage is rectified to supply field current to the

synchronous motor.

The LCI control cabinet includes a static excitationvoltage controller to supply stator voltage for the

brushless exciter. The LCI’s electronic control (see

section 2-1.3) gates/controls this voltage controller.

This controls the excitation to produce the required

machine flux. It also provides field overcurrent and

undercurrent protection.

For applications with slip-ring excitation, the LCI

supports control of a dc bridge exciter from the same

control hardware as the brushless exciter uses.

For applications using an external ac or dc exciter,

the control fully supports any external excitation that

is compatible with a simple set of control signaling.

2-1.3. Microprocessor-Based Control

(Refer to Figure 2-3.) The LCI’s microprocessor-

based electronics control firing of both the source and

load bridges. It processes input signals for this func-

tion as follows:

• Attenuated source and load bus voltage signals:

– To synchronize source and load thyristor firing

– For voltage feedback

– For overvoltage and undervoltage detection

• Attenuated source and load current signals from

current transducers:

– For regulator current feedback

– Electronic overcurrent detection

– Software-implemented fault detection

• Speed reference signal

• Process commands, such as stop and start

The LCI control evaluates the process commands and

internal status signals to determine whether the LCI

should be in a stopped, started, alarmed, or faulted

condition.

If in a started condition, the control provides gate sig-

nals to the thyristor bridges and the excitation voltage

controller. These signals are low-level and are condi-

tioned in the power bridge circuitry to provide the

necessary isolation and power level.

2-2. Power Converter Operation

Note

The system elementary diagrams contain de-tails on the source and load bridge configu-

ration for each customer requisition.

The power bridges are 6-pulse, 2-way types. (See sec-

tions 2-4 and 2-5 for 12-pulse bridge configurations.)

The elementary diagram shows the physical arrange-

ment of the thyristors (SCRs) in the power bridges.

The bridge legs fire in the order that they are num-

bered (see Figure 2-2). The synchronous machine sta-

tor voltages transfer the source converter current from

one leg to the next.

1

CONDUCTS

3

CONDUCTS

5

CONDUCTS

FIRE 1 FIRE 3 FIRE 5 FIRE 1

CONDUCTS

2 4

CONDUCTS

6

CONDUCTS

6

FIRE 2 FIRE 4 FIRE 6 FIRE 2

120

1

CONDUCTS

CONDUCTS

Figure 2-2. Bridge Leg Conduction Sequence





Figure 2-3. LCI Control Block Diagram

Microcomputer

(DSPC Board)

High Speed I/O(ADMA/DDTB Board)

Innovation Series Controller(UCVA Board)(Option)

Genius Bus

Series 90 Protocol(SNPA Board)

(Option)

Series 90-30 PLC(Option)

Field I/O

Fiber-Optic Gate & Status

Current Feedback

Voltage Feedback


Current Feedback

Voltage Feedback


Current Feedback

Voltage Feedback


Current Feedback

Voltage Feedback

Gate, Current, & Voltage Signals

Source A Bridge Control(FCGD Board)

Source B Bridge Control(FCGD Board)(Option)

Load A Bridge Control(FCGD Board)

Internal (AC/DC) ExciterControl(FCGE Board) (Option)

Load B Bridge Control(FCGD Board)

(Option)





Figure 2-4 shows the process of switching motor/

inverter current from one leg to the next. The princi-

ples apply to both the rectifier bridge and inverter

bridge. This phase-controlled switching is based on

the following two thyristor characteristics:

• When the voltage across the thyristor is positive, it

can be triggered into conduction.

• It does not permit current flow in the reverse di-

rection. Thus, in an alternating voltage circuit, thy-

ristor conduction stops and reverse voltage begins

to appear when the current becomes zero.

Figure 2-4. Load Converter Voltage and Current





Current transfer must be completed before voltage

crossover with a positive margin angle. This angle

must be long enough to allow the previously conduct-

ing leg thyristors to recover to their blocking state be-

fore forward voltage is applied.

This is why the fundamental component of current

must lead the voltage for the inverter/motor, but lagthe voltage for the rectifier/source. For successful

commutation, angle α = 180° − β = 180° − µ − γ must

always be less than 180°, with a practical limit at

155°. A practical minimum value for β for the inverter

bridge is 25°. Therefore, the motor power-factor angle

is always greater than zero.

The LCI control system must conform to the charac-

teristics of the synchronous machine operating at

leading power factor. Figure 2-5 shows a phasor dia-

gram for a synchronous machine operating at leading

power factor.

Figure 2-5. LCI-Driven Synchronous Motor Diagram; Leading Power Factor

With a fixed amount of field excitation, the machine

voltage characteristic is mainly a function of the rotor

field-excitation, E f 1, and the de-magnetizing action of

direct-axis current. This produces the I D X AD2 voltage

in opposition to the voltage produced by field excita-

tion.

As shown, an increase in stator current I S results in

higher direct-axis current, which increases I D X AD3.

This, in turn, decreases the motor voltage E G4 avail-

able for commutation, thereby increasing displacement

angle θ . This increase of angle θ further increases

stator current, and so on, until equilibrium is reached

at a new operating point.

In actual practice, the motor field excitation is fixed in

the speed range of approximately 0 – 10%. It is con-

trolled to produce a desired profile of motor flux at

higher speeds.

At speeds greater than 10%, the LCI operates in a

flux-regulated mode. It adjusts the output of its static

exciter voltage controller (EVC) to maintain motorflux at the desired level.

2-3. Control Operation

2-3.1. Synchronization

When operating in any mode (see sections 2-3.3 and

2-3.4), the electronic control must synchronize firing

of both the source and the load converters. It synchro-

nizes these to the ac line and synchronous machine

bus voltages, respectively, using attenuated bus-to-

ground signals as its primary feedback. The controlcombines these inputs to produce line-to-line analog

voltages for both converters. It then integrates these

voltages to obtain flux signals.

The flux signals’ zero-crossings are then used in syn-

chronizing the phase-locked loop for firing control of

both converters. At low speed, before the phase-

locked loop is effective on the load side and if tach

position mode is not enabled, the zero-crossing marks

are used as a timing reference for firing in force com-

mutated operation.

2-3.2. Commutation

The LCI’s source side converter always operates line

commutated. Therefore, the ac line voltage transfers

conduction from one thyristor to the next. The load

side converter may operate either force commutated

(see section 2-3.3) or load commutated (see section

2-3.4), depending on motor speed and flux level.

As the synchronous machine’s rotor (field) rotates, the

near-sinusoidal shaped field flux cuts the stator wind-

ings. This produces a set of three sinusoidal voltages

in the stator. These sinusoidal voltages are angularlydisplaced by 120 electrical degrees. The magnitude of

this counter-electromotive force (cemf) is proportional

to speed and field strength.

At low speeds, the induced emf is insufficient to

commutate the thyristors in the load side converter.

Therefore, in this mode, the load converter must oper-

ate force commutated.

DIRECT AXIS

QUADRATUREAXIS

IS

ID

EG

EFIQ

IQXAQ

ID XAD

θ





2-3.3. Force Commutated Operation/Modes

Force commutated operation is used:

• When starting the synchronous motor from zero

• During low speed until the motor cemf is sufficient

for load commutation

(See Figure 2-6.) In force commutated operation, con-duction of the load converter is stopped by phasing the

source converter to inversion limit until the dc link

(reactor) current is zero. Thus, the dc link current is

chopped into 60°-wide segments of motor frequency

(angle).

There are several modes of force commutated opera-

tion. These can be separated into two types:

• Modes where the digital pulse tach is used to track

rotor position

• Modes where the tach is not used

During initial startup (commissioning) of the LCI, the

tach is deselected until initial operating checks are

completed.

2-3.3.1. Starting Without Tach. In this mode, the

starting current must be large enough to accelerate the

motor to about .5 Hz in one or two inverter firings.

This is approximately the minimum frequency at

which the LCI can reliably sense motor flux and begin

to control torque and speed.

When a start from standstill is initiated, the LCI ap-plies a fixed current level at a fixed frequency to the

motor’s stator. The frequency is set by tuneup

STFREQ and the starting current level is set by tuneup

CRSTART (see section 5).

When the LCI control senses that flux has reached

sufficient magnitude, it transitions into the segment

firing mode of forced commutated operation. In this

mode:

• Inverter firing is synchronized to crossovers of the

motor flux

• The motor is operated near unity power factor to

obtain maximum torque

• Inverter firing is adjustable in 30° steps or seg-

ments

• The speed regulator becomes active

At approximately 5% of motor speed, the load phase-

locked loop can lock. Inverter firing resolution in-

creases to 0.35°, ending segment firing mode.

Force commutated operation continues until the syn-

chronous motor reaches a frequency with enough emf

to commutate the load side converter. At this point,

the control changes to load commutated operation.2-3.3.2. Starting With Tach . For LCIs using a pulse

tach for applications with high starting torque, the tach

pulses are counted to keep track of the rotor position.

Starting from standstill does not depend on zero-

crossings of motor flux.

The LCI ramps up stator current until it detects shaft

rotation. At that point, it freezes current and fires the

thyristors based on the rotor position determined from

the tach count. This continues for several firings to

ensure that the motor is rotating. Then the speed

regulator is enabled.

The speed regulator then controls stator current to

produce the correct torque to accelerate the motor as

required. Force commutated operation continues until

there is enough motor cemf to commutate the load

side converter.

2-3.4. Load Commutated Mode

Load commutated operation (mode) requires that the

motor be operated at a leading power factor. This

ensures commutation of the load converter.

The LCI control keeps the motor power factor, and

therefore torque-per-ampere, as high as possible. It

does this by firing the load converter as close to the

inversion limit as possible, while maintaining suffi-

cient margin for successful commutation of current

from one thyristor to the next.

For successful commutation, the volts-seconds re-

quired are proportional to product of the load current

and motor inductance. The LCI controls commutation

and firing time by processing the following three val-

ues:• Motor (load) current

• Motor commutating inductance (a constant stored

in the microprocessor system memory)

• Available volt-seconds from the integrated line-to-

line motor voltages





Using the current and inductance, the control

calculates the amount of commutation volt-seconds

required. The control then uses this value of volt-

seconds and the latest calculation of available volt-

seconds to determine the latest possible time to fire.

This gives a specified margin after commutation com-

pletes.

0IA

+

-

IB

0

+

-

-

0IC

+

0ILINK

-

+

34

45 6

5 61

12

23

LOAD CONVERTERLEGS CONDUCTING

FIRING TO ESTABLISH

PHASE-ON OCCURS

WITH FIRING OF A NEW

THYRISTOR LEG PAIR.

FIRING LEVELTO MAINTAINLINK AND MOTOR

INVERTING TOSHUTOFF

0

+

-

LINK VOLTAGEAT SOURCE

THYRISTOR BRIDGEOUTPUT

0

+

-

LINKCURRENT

OUTLINED AREA ABOVESHOWN IN DETAILWITH SOURCECONTROLLING ACTION

Figure 2-6. Forced Commutation Firing Mode





Figure 2-7 shows the relationship of the system volt-

ages, currents, and flux waves. The commutating

“notch” identified in the A-C line-to-line voltage is

equal in amplitude to the simultaneous commutating

“bump” on the B-C voltage. The corresponding notch

in the A-B voltage is twice this amplitude (A and B

are the two lines commutating together at this instant);

the notch area is twice the commutating inductanceper phase times the current. The voltage at the com-

mutating point, where the lines are temporarily con-

nected by the thyristor legs, is practically zero during

commutation; the line-to-line voltage is only the for-

ward voltage drops of the conducting thyristor legs.

At high load on the motor, the apparent power factor

“seen” by the power source increases. This is because

the source converter firing angle advances (is reduced)

to obtain more current. The harmonics in the current

and the resultant harmonics in the voltage, caused by

commutation notching, decreases.

The fundamental control strategy is to increase motorcurrent in response to a load torque increase. The dc

link voltage on the source side is then increased, rais-

ing motor current so that it keeps motor speed con-

stant.

EXCESS VOLT-SECONDS AFTER COMMUTATION(COMMUTATION MARGIN)

POWER FACTOR ANGLE,BETWEEN CENTERS OFVOLTAGE & CURRENT WAVES

LOAD LINE-NEUTRAL VOLTAGES

CURRENT INTO LOAD,NUMBERS REPRESENTINVERTER LEGSCONDUCTING & COMMUTATING

ANGLE OF OVERLAP, µ(COMMUTATION ANGLE)MARGIN ANGLE, γ

LOAD LINE-LINE VOLTAGES

COMMUTATING VOLT-SECONDS=COMMUTATING INDUCTANCE X

STATOR CURRENT

PEAK VOLT-SECONDS AVAILABLE

(READ BY PROCESSOR TOCALCULATE LATEST TIME FOR NEXTFIRING)

LOAD "FLUX" WAVES

Figure 2-7. Load Voltage and Current in Load Commutated Mode



S ect i on2 ,F unct i onal Descr i pt i on•

1 3

F i g ur e2 - 8 .L C I S y s t em

R e

g ul a t or B l o c k Di a gr am





The torque command signal is applied to both the

source and load side control. Since motor torque is a

function of flux, current, and the angle between them,

torque can be controlled either of two ways:

• By adjusting stator current magnitude from the

source side at a fixed load firing angle

• By maintaining a constant current and varying the

displacement angle (firing delay angle) on the load

side

However, at any one time, only one of these means can

actively control the torque.

The torque command to the source side control is ap-

plied to a maximum and minimum current limiter. The

minimum current level is set to maintain continuous

current in the dc link. The minimum current is usually

set at 0.2 per unit (pu) of rated dc current.

The minimum current limit also affects the load firingangle (therefore, motor power factor) whenever the

torque command produced by the speed regulator is less

than the minimum current limit. In this case, the load

firing angle (and motor power factor) is varied as a

function of the torque command, while stator current is

held constant. Thus, torque is controlled by adjusting

motor power factor whenever the torque command is

lower than minimum current limit.

The minimum current limit may also be dynamically

increased by the action of the voltage limit regulator.

This regulator reduces stator voltage by simultaneouslyincreasing current and decreasing power factor. The

voltage limit regulator is used mostly in applications

where the field excitation is fixed.

When the torque command is greater than the mini-

mum current limit, the load firing angle functions as

follows:

• If motoring, the load angle is at its inversion limit.

• If regenerating (braking), the load angle is at its

rectifying limit.

When motoring, the load control adjusts the firingdelay angle to be as late as possible to maintain a

fixed commutation safety margin (usually 20°). This

fire-as-late-as-possible control adapts to changes in

stator current and voltage to maintain the margin angle

constant.

To regenerate the drive, the load side thyristors fire

full advance (point “X” in Figure 2-9). At this time,

the source side controls current by reversing the dc

voltage to match the rectified motor voltage.

VAN VBN VCNX

Y

Z

I N T E G R A T

E D

L I N E - T O - L I N E V O L T A G E

VCA VAB VBC

X =

CELL 1FULL

ADVANCE

Y =

CELL 1

FULL

RETARD

Z =

INVERSION

LIMIT L I N E - T O - N E U T R A L V O L T

A G E

Figure 2-9. Flux Wave Zero-Crossing

The drive current command is the greater of the ab-

solute value of the torque command (from the speed

regulator) and the minimum current limit. The current

command is compared with current feedback and the

error is applied to the current regulator.

The current regulator controls the firing of the thyris-

tors in the source converter (rectifier). Thus, the

source control adjusts the dc link voltage as required

to produce the current and torque needed to drive the

load.

Note

The load firing control reverses the polarity

of dc link voltage if braking torque is re-

quired.





2-3.6. Phase-Locked Loop Operation

The source and load controls use a phase-locked loop

(PLL) to track bus voltage angle. This enables the LCI

to fire the thyristors at specific angular displacements

from the ac bus voltages.

The PLL uses the zero-crossings of reconstructed

3-phase flux waves as a timing reference. At eachflux wave crossing, it is possible to determine the an-

gular position within the present cycle of phase A-to-

neutral of the ac bus voltage.

The PLL uses the FCGD board (see Section 3) to

capture the time and polarity of each zero-crossing of

the 3-phase flux waves. The control maintains a run-

ning estimate of electrical degrees based on elapsed

time and rate of change of the electrical angle. As each

zero-crossing occurs, the corresponding electrical de-

grees and timing are compared to the control estimate.

From the comparison, an error in the estimated de-grees is determined and applied to the PLL regulator.

The regulator increases or decreases the rate of change

of the angle, to drive the error toward zero.

The control determines a firing angle for each thy-

ristor. Using the estimated electrical degrees, it calcu-

lates the time when the drive will be at the desired

firing angle. The control then places a firing command

and time to activate into FCGD board registers, which

completes the firing process.

2-4. Dual-Channel, 12-Pulse Configuration

(Refer to Figure 2-10.) A dual-channel, 12-pulse LCI

is configured as two identical, separate 6-pulse drives

operating from a common source. This allows two

motors to be combined into one frame, reducing both

the motor and installation costs.

Twelve-pulse operation best uses the motor and drive

capabilities. It minimizes the harmonics present in

each 6-pulse channel, canceling 5th and 7th harmon-

ics. Other higher-order harmonics also cancel, but

their amplitudes are much smaller.

Note

Refer to the system elementary diagrams for

the exact configuration and detail of each

customer’s system.

The transformers feeding the two drives (channels)

are identical, except their windings are 30° apart. This

design enables the drives to operate as follows:

• With the same current and firing angle

• The firing reference angles shifted by 30° between

the two channels

• Equal source-side converter voltage, but 30° apart

in the two channels

• Reduced harmonic distortion on the power system

and higher harmonic frequencies

Figure 2-10.

Dual-Channel,

12-Pulse LCI

Source Bridge

Load Bridge

Load Bridge

LoadSource Bridge

Exciter




The two motors use a common magnetic frame and a

common field. This causes the load side converter

voltage to be equal in amplitude and frequency be-

tween the two drive channels.

The load motor’s stator winding is separated into two

identical windings, but isolated and phase-shifted 30°.

This reduces the torque pulsation amplitude whileraising the torque pulsation frequency. The result is

smoother torque for equal current.

Inter-channel communication allows one channel to

be master and the other the follower (also called

slave). The follower takes its torque reference from

the master, enabling the two motor winding currents to

be balanced. Thus, the channels deliver equal power,

take equal current, and fire at the same relative firing

angle.

2-4.1. Shutdown Options Some dual-channel, 12-pulse systems allow one chan-

nel to be shut down for maintenance while the motor

continues to run on the other channel (with reduced

torque and usually reduced speed range). When the

out-of-service channel is ready for operation, it can be

returned to service without interrupting the LCI sys-

tem.

However, dual-channel, 12-pulse systems that use a

single control to operate both power converter chan-

nels cannot run with one channel shut down.

2-5. Series Twelve-Pulse Configuration

(Refer to Figure 2-11.) Some LCIs are configured for

series 12-pulse operation of the source converter. This

is done primarily to reduce the harmonic distortion

imposed by the drive on the power system. Twelve-

pulse operation eliminates half the harmonics pro-

duced by a 6-pulse system, starting with the 5th and 7th

harmonics.

Note

Refer to the system elementary diagrams for

the exact configuration and detail of each

customer’s system.

The 12-pulse converter consists of two identical SCR

bridges connected in series. Each bridge is operated at

approximately half the motor voltage. The ac sources

for the two bridges are supplied from delta and wyetransformer secondary windings and displaced in

phase by 30°.

12-PulseSource Bridges

Load

BridgeLoad

Figure 2-11. Series 12-Pulse LCI

3 - LCI Fundamentals

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