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Adjustable-frequency ac winderdrive system

Michael L. Eikenberry and K e n n e t h D. Brink

The first US. installation of an ac-driven unwind backstand has operated

reliably since startup in late 1989.

In Nov ember 1988, the m ill division of Green BayPackaging ordered a fully digital ac sectional drive forthe rebuild of a paper m achine an d winder scheduled for

late 1989. The winder rebuild was the first U.S. instal-lation of an ac-driven unw ind b ackstand.

The ac driv e was selected because

The ac-drive option w as the most ad vanced drive-controltechnology available.

Totally enclosed fan cooled (TE FC ) and totally enclosedair over (TE AO) ac induction m otors offered a ruggedand reliable alternative to th e dc mo tors used previouslyfor machine drive applications. These types of ac motorsdo not require d uctwork for cooling air.

Most troubleshooting and tune-up work is carried outin the drive-control room through computer interfacedevices.

The ac drive has additional advantages. For example,the ac braking generator does not have the commutationproblems that plague dc drive motors (brush sparking ina w eak field). When stall tension is held at zero speed ina dc drive, the comm utator isheated un evenly, which couldlead to perma nent da mag e (raised commutator bars). Theac motor does not have this problem. In the dc drive, fieldflux control mu st be g enerated in the control system. Thisis not necessary in the ac drive, since field flux controlis already in the motor algorithm. Another importantadva ntage of the a c drives is improved power utilization.The ac system typically operates near unity power factorand 95% efficiency. Figure 1 is a photograph of the acmotors used to drive the winder.

Theory of ac-drive motors

A term frequently used in ac-drive technology is “slip.”Slip is defined as the difference in speed between thesynchronous speed an d th e actual rotor speed (1).This is

~ ~~ ~

Eikenberry is ma nag er of eng ineerin g in the Mill Divi-sion of Green Bay Packaging Inc., P. 0. Box 19017,Green Bay, Wis. 54307-9017. Brink is a project engineerwith Allen-Bradley/Stromberg, P. 0. Box 760, Mequon,Wis. 53092-0760.

-1. Totally enclos ed ac motors in winder drive system

commonly expressed as the di f fe rence per uni t ofsynchronous speed.

s = (nsyn- n)/n,,

where

s = slip

ns, = synchronous speed

n = rotor speed

In a squirrel-cage machine, when the slip is positive itacts as a motor while at negative slip it acts asa generator.The waveforms for the stator field and th e rotor field a reshown in Fig. 2. If the rotor field is lagging the statorfield, then the machine ismo torin g. Conversely, if the rotor

field is leading the stator field, then the machine isgenerating.

In ter ms of rotation and the mag netic field, if the rotoris rotating m ore slowly tha n th e magn etic field, the rotoris being pulled, and the machine is motoring. If the rotor

April 1991 TappiJournal

183



’.Field and torque waveforms for ac motor

Positive

is going faster tha n the m agnetic field, the rotor is beingheld back, and the machine is generating. Thus, for theac machine to go from motoring to generat ing, thefrequency of the stator must be reduced. No cu r r en treversal or dead band is involved, as is the case in a d cmachine. This is why ac motors provide such smoothoperation in unwind applications. When the rotor is at

synchronous speed , the mach ine draw s a m i n i m u mcurren t . As the slip increases in either direction, thecurrent begins to increase rapidly but smoothly withoutany discontinuity.

The torque is proportional to the m agnetic flux density

in the machine a ir gap. The torqu e of a n adjustable-speedac motor is controlled by controlling the flux density overthe operat ing frequency range. Flux densi ty can becontrolled with vector control. The a c cu rre nt (fundamen-tal) is a vector quantity w ith two components, a reactivecomponent and a real component. The reactive componentis used to produce the flux, while the real componentproduces the torque or power required to overcome theload. A digital tachometer is required for speed controlas well as computation of slip. Flux in our previous dcdrive was controlled by sepa rate shunt-field excitation.

Because the machine is a highly inductive load, theimpedance increases directly with applied frequency. Ifthe machine is to be operated in a constant-torque mode,the voltage mus t be incre ased in proportion to the increasein frequency to maintain a constant volts per hertz. T hisconstant-torque mode is similar to a dc motor operatingbelow base speed (variable a rm atu re voltage).

If the a c machine operates w ith a constant voltage whilethe frequency is increased, the volts per her tz ratio is nolonger constant. In this case, the a ir-g ap flux decreases,as does the torque. This mode of operation is known asthe weak-field mode, and the machine can produceconstant horsepower in this range . A word of caution isin order concerning the breakdown torqu e in the constant-horsepower range . As the flux is reduced, the breakdowntorque is reduced inversely proportional to the flux

184 April 1991 Tappi Journal

Winder specifications

Web speed 7500 ft/min

Machine balance speed 8000 ft/min

Web tension 3-10 pli

Web width 164 in.

Drum face168 in.

Product Corru gating medium

Drum diameter 30 in.

Max. rewound roll diameter 60 in.

Max. unwind roll diameter 110 in.

Normal accelidecel time 90 s

Max. unwind E-stop time 35 s

Sitters 6 max.

Braking generator motor 350 hp

Drum motors 250 hp (each)

Rider roll motors

Basis weight 26-40 lb/1000 ft2

5 hp (each)

squared (speed). This is usually of little significance forbrakin g genera tors, since the required overload is greatlyreduced a t smaller diameters with low inertia.

Computers have simplified the task of d esigning acmotors to meet the needs of variable-speed drives. Withthe advent of high-speed digital microprocessor control,the performance of an ac vector-control led drive is

equivalent to or b et ter tha n the dc drive, with a steady-state speed accuracy of & 0.01%of top spee d.

Scalar-controlled ac drives a re comm only used in “standalone”-type applications or for applications requ iring lessaccuracy. In sca lar control, motor speed is controlled by

adjusting the supply frequency. The a ctual speed of themotor depends on the load, since the slip varies with load.Most modern ac motors are designed to have a low slip,

and as a result ther e is not much speed cha nge with loadvariation (typical slip values ar e 1-3%).The performanceof a drive with scalar control can be improved througha variety of techniques, such as output voltage boost atlow frequency, slip compensation (usually above 15%ofsynchronous spee d), torqu e con trol (above 10%of synchro-nous speed), an d speed control with tach ometer.

Scalar-controlled drives are the ac equivalent of avoltage-regulated d e d rive, while vector-controlled drivesare analogous to the accurate speed-regulated digital dcdrive.

Winder specification

The winder selected for this installation was th e BelwindSF manufactured by the Beloit Corp. The winder wasconstructed to meet the specifications listed in Table I.Table I1 l is ts the winder drive data, including rol ldiameters, RD C (recommended drive capacity) power,motor and roll rpm , and other electrical motor data.



II. Winder drive dataIRoll

diam.,Section in.

Unwind* 110118

Rear drum 30

Front drum 30

Rider roll rear 12.50

Rider roll front 12.50

Slitters (6m a . ) ...

lndrive

rPmat 7500ftlmin

265/706

955

955

2293

2293

...

ROC,

hP375

250

250

5

5

Reducer

Ratio 7vpe

2.26 New

... Dir

... Dir

1 Belt

1 Belt

... ...

ac motor

Powec

hP

350

250

250

5

5

0.90

GearrPm Frame

600/1597 HXUR1009H4

955 HXUR 808K3

955 HXUR 808K3

2293

2293

HXA 132SMA 4 83 E

HXA 132SMA 4 8 3 E

... ...

Enclo-sure

TEAD

TEAD

TEAD

TEFC

TEFC

...

Voltage,V

575

575

575

460

460

460

Full-load

amps

540

370

370

13

13

-...

"Maximum roll diameter is 110 in. Diameter of starting core is 18 in. The p aired values for indrive rpm and gear rpm correspond to these maximum andminimum rolldiameters.

Configurationof ac drive system

F i g u r e 3 is a one-line dia gr am of the w inder d rive system.The basic drive system comprises three m ajor compo nents.

1. Line-supply equipm ent, which converts the three-phase

2 . Three-p hase inv erter, which converts the de voltage to

3. Motor.

ac power into fixed-voltage de po wer

variable-frequency ac voltage

The first component, the line-supply equipment, can bee i ther a three-phase diode br idge or a three-phase

regenerat ive SC R (silicon controlled rectifier) bridgecom posed of 1 2 SCRs. The SCR bridge, also called athyristor bra kin g unit (TBU), is used in the winder drivesystem, to accommodate the transfer of power back intothe power system th at occurs at certain times d urin g themachine cycle. The diode bridge is used in applicationswhere the power flow is always from the ac line into thedrive system. This winder system has one SCR bridge andone diode bridge. The line-supply equipment feeds a debus. Included in this front-end equipment a re an inductorand a capacitor bank to provide filtering on the d c bus.

The second component, the three-ph ase inv erter, utilizeseither GTOs (gate turn-off thyristors) o r GTRs (gianttransistors) to supply a variable voltage and frequency to

the motor. The motor algorithm, speed/torque regulation,and motor data reside a t the inverter level. Pulse-width-modulation inverters are used on this winder. The br akin ggenerator and both drum drives are vector controlled,while the ride r rolls and slitter drive are scalar controlled.Winder master functions, operator interface (control andmonitoring), and diagnostics a re done through the digitalreference con troller (DRC).

The drives a re three-phase ac squirrel-cage inductionmotors . Al l of the vec tor-cont rol led dr ives havetachometers.

Selection of line-supply equipment

Th e line-supply equ ipm ent is selected on the basis of th etotal drive requirements. The drive power requirementswere calculated using the TA PP I T IS 403-11 calculationsheets ( 2 ) .Table 111 lists the power requirements. F romthese values, the worst-case motoring load would be 430hp (321 kW), and regenerative load would be -1018 hp(-760 kW). The -1018 hp is fo r a 35-s emergency-stop (E-

stop) time from 7500 ft/min, a full reel at the unwind, an da ful l set at the dru ms.

With today's high-speed winders, there is concern aboutstopping the dr um s at a high rat e of deceleration. If the

deceleration ra te is too high, it is possible to cause slippagebetween the d rum s and the set on the dru ms . The resultantvibration can move the set up again st the raised loweringtable and da mag e the roll.

On this ac drive, it is possible to set a power limit, afeature that is lacking on most de drives. Both ac and dcdrives have torq ue limits. By using a combination of po werand torque limits, a two-rate ra mp can be obtained to stopthe drives a t an acceptable rate. Drives with these limitswi l l f i rs t dece le ra te in the power l imi t (power i sproportional to speed) and then in the torque limit. Thepower limit is determined by the line-supply equipment,and the torque limit is determined by the inverter. Bothlimits are set at the inverter. In the even t of an E-stop,

the condition of the web is not imp ortan t, since the w indermust be stopped as soon as possible. The only noticeableeffect of the two-rate deceleration is that-under the worst-case situation (m axim um speed, full rolls of paper)-thedrums will take a little longer to stop than the brak inggenerator. T his perm its the use of a sma ller power supply(in this case, less than the calculated 760-kW supply).

Th e line supply is selected on the ba sis of th e -760-kWdecelerating load rather than the 321-kW acceleratingload. The in pu t power factor of the ac-drive system isapprox imately 0.98 over the full range of the wind er speed

April 1991Tappi Journal 185



3. Winder drive system

iH

CBU = capacitor bank unitConverter8 Rectifier &capacitor unit capacitor unit DA C = digital ac controller575v 460V

DGC = digital graph ic controller

DR C = digital referenc e controller

INV = inverter

LSU = line supply unit

TBU thyristor braking unit5.75% z13.8kV primary 460/3/60primary

111. Drive power requirements

IRun power (sheet tension)

Braking generatorDrums (both)

Total

Accel power

Total

Decel power

Total

E-stop power




TotalI

-354 hp372 hp

18 hp

-159 hp589hp430 hp

-548 hp

-362hp186 hp

-500 hp-518 hp

-1018 hp

I

and load. Using this power factor, the -760-kW loadrequ ires an a c input of -776 kVA. With a motor efficiencyof 95%(acting as a generator) an d a n inverter efficiencyof 95%,the actual supply requirement drops to -700 kVA.During actual operation, the line supply equipm ent willhandle less than -760 kW of pow er, as this value wasconservatively calculated using a full parent roll as wellas a full set on the dru ms. Because this is a regenerative

186 April 1991 Tappi Journal

4. Braking generator ac motor

application, a 540-kVA TBU was selected. This is 16 0 kV Aless than calculated for the worst-case E-stop condition.The selection was based on the use of a sta nda rd T BU .

The only way to limit the amou nt of power return ed tothe dc bus is to set limits at each inverter. For thisapplication, the braking-gen erator power limits were seta t 160%,while the winder drums were set at 140%fo rmotoring and less than 100%for regeneration. The torque



5. Unwind torquekpeed requirements at roll for maximum sheettension

8000

’380i- - - - -

-x\\

\

\

\

\

\

\

\

\

\

\ .\......

‘ x

0 26 5 892 1000

SPEED, rpm

limits (both directions) a re set for 160% on the bra kin g

generator and 145% for the drums. When the winder isE-stopped, all the drives initially regenerate within theirpower limits. The limits cause the braking generator to

stop as soon as possible (linear ra m p to zero speed), whilethe drums begin to decelerate more slowly (in the powerlimit). These limits were chosen to meet the windermanufacturer’s requirements.

For the two rem aining drives, a small 180-kVA diodebridg e was selected for the supply equipment. This ratin gwas chosen because of convenience rather than loadrequirements, as it is much larg er than required.

Drive inverters

The calculated braking-generator require men ts for runpower, acceleration pow er, deceleration power, an d E-stoppower were -354 hp, -159 hp, -548 hp, and -500 hp,respectively, as seen in Table 111. The largest value was-548 hp (408 kW). Using a 0.7 power factor, a motorefficiency of 95%, and an inver ter efficiency of 98%, th e..actual.. supply require men t is -544 kVA .

Using appl ica t ion guide l ines involving the motorcharacteristics, a n 870-kVA inverter was selected for theunwind drive. The next size smaller, 540 kVA, was toosmall to supply the peak cu rren ts for this large machine.

The largest power requirem ent for any drum motor wascalculated to be 394 hp (294 kW) for fro nt-d rum acceler-

ation. Using the same selection criterion as used for thebra kin g generator, a 540-kVA inverter was selected. Bothdru ms ar e identical.

Based on an economic analysis, 460-V GTR inverterswere selected for the rider-roll motors and slitters.

Drive motors

Braking generator

The build-down ratio for this winder is about 6 to 1,andso the brak ing generator h as a large speed range. Figure

4 is a photograph of the bra kin g generator. Dur ing normalwinder operation, the operator slows down the winder atthe end of th e p are nt reel to avoid “tailing out” at full webspeed. This mode of operation can he lp to reduce the sp eedrange fo r the brakin g generator.

Core speed was limited a t 892 rpm . The length of paperthat unwinds during a normal stop was calculated for asheet of average c aliper. This length was used to determinean unwind diameter, and the limit for core rpm wasdetermined from this diameter. The core torque require-ments were used as the limit for core rpm . A speed reducer

is needed to increase the motor frequ ency a t slow speeds.The motor designer was given the information in Fig.

5 and asked to recommend the number of poles, motorspeed range, and a gear ratio (standard ratios wererecommended ). The increased power needed du rin g accelfdecel periods could be obtained by allowing short-termoverloads f o r t he b ra k i ng ge ne ra t o r . The de s i gne rrecommend ed a 3 50-hp, 588-2016-rpm, 39-Hz, 8-pole, 575-V machine with the capability to withstand 160%overloadfor 120 s. A ge ar ratio of 2.26 was selected so that the 588-rpm motor would correspond to a roll speed of 265 rp m .

All of th e ac motors used on this win der ar e totallyenclosed (no separ ate ai r is blown through the motor). Thebraking generator is cooled by a separate motor blowing

air over the outside of the machine. The blower motor ismounted axially on the nondrive end. A tachome ter is alsomounted on the nondrive end. A totally enclosed motorcooled by air blown over the outside of the machine isknown as a TE A 0 motor.

The braking generator was designed for a class Ftemp erature rise, as well as normal features required fora paper machine environment . These basic featuresinclude cast-iron construction, special insulation, andt rea tment to resist corrosion. Resistance temperaturedevices (RTD) were also installed on this m otor. On larg emotors like this, an insulated bea rin g on the nondrive endand an insulated tachometer coupling are standardfeatures as a precaution ag ainst possible bearin g currents.

Drum motors

The power and rpm requirements for the drums weregiven to the motor designer. Ahead of this winder is anac-driven paper machine. The Beloit E N P press required500-hp, 1495-rpm motors. In orde r to reduce the numb erof spare m otors, the designer was asked if a common motorcould be used for both applications.

A 500-hp, 1495-rpm, 75-Hz, 6-pole, 575-V motor wasdesigned to meet both app lications. These motors have allthe features th at w ere supplied on the b raking generator.

April 1991 Tappi Journal 187



1 *e Both motors are 5 hp , 4 pole, 460 V. These are TE FCmotors wi th shaf t -mo unted fans . The motors weredesigned for a class F tempe rature rise, as well as normalfeatures required for a paper-machine environment .Bimetallic temperature switches were installed on thesemotors instead of the RTD s used on the larg er machines.

Operator interface and master control

The drop-in control panel and color monitor provide theoperator with all the information needed to run the winder.The operator also can elect to run with roll density andtension control from th e Beloit control equip men t. All fiveinverters comm unicate with the DRC, which is the w indermaster. The drive maintenance system is used for drivediagnostics.

Winder installation

Design work fo r the installation of the drive wa s completed

by S.J. Baisch Associates Inc. du ri ng late spr in g of 1989.The winder drive system w as delivered to the mill in June1989. The me chanical an d electrical installation w as begunat th at time, and th e drive system was re ady for the Allen-Bradley/Stromberg s ta rtup team in September 1989.

Al l the dr ive motors associa ted wi th the winderinstal lat ion were set on the floor alongside the oldoperating winde r, and temporary connections were ma deso that all initial tune-up work could be completed beforethe shutdown was to take place. On October 11, the winder

drive system was deemed ready. The paper m achine wasshu t down two days later.

During the 25 days that followed, the old winder wasremoved, its foundation was demolished, and a newfoundation was constructed. The mechanical and electricalinstallation and preliminary checkout were completed,and on November 6 the winder was ready for tune-up.

Winder startu ps can be troublesome, so we decided earlyon to have some paper available to get the winde r run ningbefore the paper machine was ready. B ecause this was acomplete m achine reb uild, including a new reel, the paperru n on the old mach ine could not be used for testing, sincethe reel spools from th at mac hine were not compatible withthe new equipment. The backstand of the new winder w asmodified so tha t it could a ccept reel spools used in our m illin Mo rrilton, Ark . Thr ee 110-in.-diam. rolls of line r wereshipped to Green Bay, Wis. , for use in the s tartupoperation.

The sta rtu p of the new winder w as almost trouble free.All mechanical and electrical tune-up was completed usingtwo of the three 110-in. rolls. All of th e produ ct fr om these

first two rolls was salable. The third roll, which was notrequired for tuneup, was used for operator training.

On November 16, the rebuilt paper machine began its

initial startu p, and the w inder was fully operational andready for full production when the first paper came offthe machine.

Performance

The winder was subjected to several performance tests,with the results documented using actual brush re cordingsof various machine operating parameters. Performanced a t a a r e av a i l ab l e i n t h e P ro ceed i n g s o f t h e 1 9 9 0Engineering Conference (3) .

The paper machine ahead of the winder is running 20%faster than before the shutdown, and new productionrecords have been set. To date, the winder has had notrouble keeping up with the increased production.

Conclusion

A mill that produces corruga ting medium installed a fullyac winder drive system, including the first U S . installa-tion of an ac-driven unwind backstand. Winder perfor-mance has met or exceeded the mil l ’s expectat ions,indicating th at adjustable-speed ac drives can be appliedto production w inders.0

Literature cited1. Cochran, P. L., Polyphase Induction Motors, Marcel Dekker,

2. Winder power requirements, TIS 403-11, T A P P I PRESS,

3. Eikenberry, M. L., and Br ink, K. D., 1990 Engineering

N.Y., 1989, p. 62 .

Atlanta, 1982.

Conference Proceedings, TAPPI PRESS, Atlanta, p. 45.

Received for review June 15, 1990.

Accepted Nov. 7,1990.

Presented at the TAPPI 1990 Engineering Conference.

drive for pay of reel

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