The Drive of Tandem Rolling MillsBY A. F. KENYON*

Associate, A. I. E. E.

Synopsis.-The paper briefly discusses the development of the for such mills, and presents mnethods of calculation of powertandem type of rolling mill and the requirements of electrical drives requirements.

CHANGING conditions in the steel rolling mills of d-c. adjustable speed motors for the individual drive ofthis country have caused the development and each of the three tandem finishing stands. Thewidespread application of what are quite com- arrangement of the stands is shown by Fig. 1. The

monly termed "tandem" rolling mills. The tandem distance from stand 14 to 15 was 12 ft. and from standtype of mill has the advantages of great flexibility, high 15 to 16 was 15 ft. The three finishing stands weretonnage output, low labor costs, compactness of layout, driven by direct-coupled 575-volt d-c. motors of 600and uniform quality of product. This type has been hp., 150/250 rev. per min.; 800 hp., 210/315 rev.applied successfully to the production of thin flat stock per min.; and 800 hp., 260/390 rev. per min., capacityup to 50 in. wide, rod, bar, merchant, and structural respectively. At the top speed of 390 rev. per min. onshapes. In this type of mill, the several finishing stand 16 motor, the delivery speed was approximately

1325 ft. per min. The mill has a range of capacity fromIWOH2RePperMin. 18 gage strip up to about 4 in. wide to 14 gage material

Induction Motor up to about 10 in. wide. In its arrangement of thefinishing stands the mill varies but little from more

3 b b d b beurecent designs.With the more recent installations the trend has

been toward the elimination of stands in train, withfinal development of the present mills in which the

16 15 ] )

1500 Hp.1800 1500 705 Rev. per Min

800 Hp. 800 Hp. 600 Hp. 1350 25/25\260-390 210-315 150-250180 r i - i50

Rev per Min Rev, per Min. Rev per Min 180065biMotors 16,350 2515 evpe5M

1v8ftXl ft.- 18f 46 f 1Vt- 1 5/0i ftFIG. 1-14-IN. STRIP MILL AT CUYAHOGA WORKS OF AMERICAN J lb, ErJ EbD E1

STEEL AND WIRE COMPANY 13 12 1 213 4 95 68

Illustrating first application of d-c. ad;ustable-speed motors to tan- Shear 15 ft2t 28 1.-3rI 251,381ftdem finishing stands. Installed 1908-9 Heating Heating 10 an.1/2 in.

Furnace Furnace

stands, and in some recent installations the roughing 34.5 Rev. per MnMm. .500 H80 Rev peM.stands,andinsome Rev,perMm~~~~~~~~~~~~~~~~~~~~~~5.Rvprin.0Hp \\729Re, e Mnstands also, are closely spaced in a continuous line and 50.4 lee. per Mm. 705 Rev. per Min. _95 Rev. per Min.are individually driven by separate motors. The FIG. 2-20-IN.-6-IN. SEMI-CONTINUOUS STRIP MILL AT WESTlength of the billets and the spacing between stands LEECHBURG STEEL COMPANYare such that a single piece of steel may be in several The four finishing stands are individually driven by two 1500-hp. and

stands at one time, and for periods of several seconds. two 1800-hp. d-c. motorsIt is therefore essential to the successful operation ofsuch a tandem mill that the speeds of the several stands are arranged in one continuous line. As andriving motors be susceptible of easy and accurate intermediate step in the development, Fig. 2 is includedadjustment, in order that stretching or looping of the to show the layout of the semi-continuous mill of thesteel between stands may be kept within limits. West Leechburg Steel Company. The equipment and

DEVELOPMENT OF TANDEM OPERATION operation of this mill were described in a paper readDuring 1908-9, the American Steel and Wire Com- before the April 1924 Convention at Birmingham by

pany installed at its Cuyahoga Works a 14-in. mill Jones and Wilson.which is of considerable interest in that it was the first The Acme Steel Company has a mill very similarelectrically driven mill for the rolling of wide strip in equipment to the West Leechburg mill, but in whichmaterial, and that it was the first application of separate the stands are arranged in a single line, eliminating the- ~~~~~~~~~~~~~broadsidetransfer. Also the two intermediate stands*General Engineer, Westinghouse Elee. & Mfg. Co., East

Pittsburgh, Pa. are much closer together and are driven by adjustable-Presented at the Regional Meeting of the A. I. E. E., St. Louis, speed direct-current motors. This arrangement per-

Mo., March 7-9, 1928. mits of additional flexibility and permits the rolling764

28-43

July 1928 KENYON: THE DRIVE OF TANDEM ROLLING MILLS 765

of finished strips up to 600 ft. or longer. Fig. 3 shows as the steel enters the rolls, and if the loaded speeds ofthe arrangement of the mill and driving motors. each of the several stands are in the correct relation

Latest designs show a tendency to apply individual for the reductions being made, there will be no stretch-motor drives to the roughing as well as the intermediate ing or looping. From this standpoint successfuland finishing stands. A mill for the production of operation can and is being obtained with motors havingstrip up to 50 in. wide, construction of which will soon as much as 8-10 per cent speed regulation on thebe completed, consists of eleven roll stands, the first finishing stands. However, with motors of flatterfour driven by wound-rotor induction motors, and the speed characteristics, the speeds of the several motorslast seven driven by d-c. motors. The total capacity may be more accurately set at no-load, and speedof the eleven motors is 21,800 hp. A similar eleven changes due to load variations caused by changesstand mill will be driven by one constant-speed motorand nine adjustable-speed d-c. motors. lo 9 8 E 7 6 E 5 4321 E

On the tandem stands of the mills so far discussed inthis paper, d-c. adjustable-speed motors have been i 3applied. The question may logically be asked: Whynot use some type of alternating-current adjustable-

speed drives? To answer this question we shoul FIG. 4-10-IN. STRIP MILL OF LACLEDE STEEL COMPANYspee.dries?Toaswei q n we shoud ILLUSTRATING TIlE USE OF A MULTI-STAND ROUGHING MILL IN

consider first the suitability of such machines for TANDEM WITH INDIVIDUAL MOTOR DRIVEN FINISHING STANDS

tandem mill operation, and second the relative cost ascompared to other drives.

Pass Diam. Rev, per min. Motor hp.It is a requirement of any continuous mill, in which Diam_ Rev. per mi.Mto_p

steel is in two or more stands simultaneously, that the Edger .. 275/825 1001 12 31/632 12 45/903 12 61/122

14131211 10 98 76 5 4 3 21 4 12 90/1815 11 123/249

I $,rL,M M Edger .. 400/1600 35EUliFLFLE nM X , .610 198/400 1500

7 10 185/470 600Edger .. 400/1600 35

FIG. 3-20 IN.-16 IN. STRIP MILL AT ACME STEEL COMPANY 8 10 270/635 6009 10 395/825 720

The vertical edgers and the intermediate and finishing stands are driven 1010 480/1000720by d-c. motors

Pass Diam. Rev. Per. Min D. to next pass Motor hp, 'in draft, temperature of steel, etc., are kept at a mini-______ ________ _________ _________ _______ mum. This makes a comparatively flat speed charac-

1 20"t 35 15'-6" teristic in the motors very desirable. Most operators2 20" 35 16'-7"3 20" 350 33'-1" therefore insist on motors of flat speed characteristics.4 20" 95 30'-7" 1500 There is a type of mill, such as the 10-in. strip mill of5 20" 79 376 .S. .. 8--0 100 the Laclede Steel Company, Alton, Illinois, Figs. 4 and 5,7 20" 9.5 66'-0" where motors of good speed regulation are quite neces-

S ....S-O", 1009 16" 76-152 18'-0" 1500 sary. In this mill, the six roughing stands are driven10 16" 100-200 77'-0" 1800 by a 1500-hp. 198,/400-rev. per min. motor, and theT1 ~~16"T 125-250 1S'-O"W 1800

12 16" 125-250 18'-0" 1800 four finishing stands are driven by individual motors of13 16" 185-370 18'-()" 1800 600 hp. and 720 hp. capacity. The billets are of such

1_4 16" -185-3701800 length that one piece may be in all stands of the millsimultaneously.

product of delivery speed and cross sectional area of For illustration, assume that the speed regulation ofsteel after the pass must be the same for each pass of the the 1500-hp. motor is 5 per cent and that the speed-mill, else the steel will be stretched or a loop formed load curve is a straight line. Further assume that thebetween stands. Speed adjustments must therefore friction load is 100 hp. and that the rolling load of eachaccompany any changes in the drafts made in the vari- stand is 300 hp. making a total load of 1900 hp. whenous passes. Mill operators, in purchasing tandem mill metal is in all stands. In order to avoid stretching ofdrive equipment, have commonly specified that the the steel between stands 6 and 7, it is necessary to havemotors should have a speed regulation of not more the delivery speed of stand 6 equal to or greater thanthan 2 per cent. Analysis shows, however, that the the take-up speed of stand 7. That is, the speed of theactual speeds while steel is in the mill, rather than the 1500-hp. motor when carrying 1900 hp. load mustdifference between the friction and rolling load speeds, give a delivery speed to stand 6 at least equal todetermine the amount of looping or stretching which the take-up speed of stand 7. But, as the end of thewill take place. Regardless of the friction-load speeds, strip leaves stands 1, 2, 3, 4, and 5, in succession the loadeach motor will quickly assume its loaded speed as soon on the 1500 hp.-motor is reduced, the speed increases,

766 KENYON: THE DRIVE OF TANDEM ROLLING MILLS Transaetions A. I. E. E.

and a loop is formed. Table I shows the data for a sive looping of steel, until adjustments were made toparticular rolling schedule, and indicates that nearly reduce the speed regulation to below 2 per cent. Quitefive ft. more material will be delivered from stand 6 satisfactory operation is now obtained under this lat-than is taken up by stand 7, during the interval between ter condition.the time when the tail end of a piece leaves stand 1 If we analyze the case of a mill consisting of two or

more multi-stand sections each driven by a motor ofpoor speed regulation, the loop formed between sectionsmay be much longer than in the case just considered.Referring to Fig. 6, stands 1, 2, 3, and 4 are driven byone motor while stands 5, 6, 7, and 8 are driven by asecond motor. Assume again that billets are of suchlength that one piece may be in all stands simultane-ously. Now in order to avoid stretching of the steel, itis necessary that the delivery speed of stand 4, withsteel in stands 1, 2, 3, and 4, be equal to or greater thanthe take-up speed of stand 5, when steel is in onlystand 5 of the second section. But, as the steel entersstands 6, 7, and 8, the speed of the second motor sreduced, and the take-up speed of stand 5 is lower than

FIG. 5-Two 720-HP., AND 'lwo 600-HP. D-c. MOTORS 1 2 3 4

DRIVING THE FINISHING STANDS OF A 10-IN. TANDEM HOT STRIPMILL AT LACLEDE STEEL COMPANY LL { _

The series exciter set for indirect compounding of the two 720-hp.motors is shown between these motors FIG. 6-EXAMPLE OF MILL CONSISTING OF Two MULTI-

STAND SECTIONS IN TANDEM, TO ILLUSTRATE LOOPING OF STEELand the time it leaves stand 6. Remember, too, that BETWEEN SECTIONS CAUSED BY SPEED REGULATION OFDRIVING

this is on the basis that the delivery speed of stand 6, MOTORSwhen steel is in all the roughing stands, is exactly equalto the take-up speed of stand 7, and that there is no the delivery speed of the preceding stand. A loop isloop formed until the tail end of the piece leaves stand 1. thus formed. Also as the tail end of the piece leavesIf, as is quite often the case, in order to avoid all pos- stands 1, 2, and 3, the speed of the first motor increases,sibility of stretching the normal delivery speed of stand and the delivery speed of stand 4 exceeds the take-up6 with the roughing mill full is slightly in excess of the speed of stand 5 by a still greater amount, so that thetake-up speed of stand 7, there will be an initial loop rate of loop growth is accelerated.between stands 6 and 7 before the steel leaves stand 1. It is to be noted that in the case of the mill shown byThen by the time the steel leaves stand 6, the loop may Fig. 4, the total length of excess material caused by thehave become so large as to be unmanageable. When increased speed of the roughing mill due to decreasedthis motor was first installed, the speed regulation was load as the steel leaves the early passes, is independentof the order of 7-8 per cent, due to a misadjustment. of the length of billets being rolled, and depends onlyConsiderable trouble was experienced due to the exces- on the speed regulation of the motor and on the layout

TABLE ISHOWING LOOP FORMED BETWEEN STANDS 6 AND 7 OF MILL SHOWN IN FIGURE 4, WHEN 1500-HP. ROUGHING MILL MOTOR

HAS SPEED REGULATION OF 5 PER CENT. CALCULATION BASED ON ASSUMPTION THAT DELIVERY SPEED OF STAND6 WITH STEEL IN ALL ROUGHING STANDS IS EQUAL TO TAKE-UP SPEED OF STAND 7

1. Passnumber ............................... 1 2 3 4 5 62. Roll diameter, in .......................... 12 12 12 12 11 103. Roll rev. per min. at friction load ........... 42 6081 120 165 2654. Crosssectionofsteel after pass, sq. in. 7.50. 4.45 3.13 2.32 1.57 1.24 .855. Distance to next stand, in ........ .......... 40.25 44.50 40.25 345.00 304.50 130.006. Volume of steel to next stand, cu. in......... 179 139 93 541 378 1107. Motor load with preceding stands empty, hp.. . 1900 1600 1300 1000 700 400 1008. Delivery speed of stand 6 with preceding

stands empty, in. per se .130.2 131.6 133.0 134.4 135.8 137.2 138.69. Volume deUvered from stand 6 with preceding

stands empty, cu. in. per sec .110.7 111.9 113.1 114.3 115.5 116.7 117.910. Interval from time piece leaves preceding

stand until it leaves this stand, sec 1.60 1.23 .81 4.69 3.2511. Rate of overfeed from stand 6 during interval

(10), in. per se . .1.4 2.8 4.2 5.6 7.012. Amount of overfeed, in . .2.24 3.44 3.40 26.20 22.7513. Total accumulated excess strip between stands

6 and 7,in . .2.24 5.68 9.08 35.28 58.0314. Take-up speed of stand 7, in. per sec ........ 1 110.7 110.7 110.7 110.7 1O0.7 110.7

July 1928 KENYON: THE DRIVE OF TANDEM ROLLING MILLS 767

of the mill. However, in the case of the mill shown by TABLE IIFig. 6, there is a continuous overfeed as soon as the steei SPEED REGULATION OF 2500 HIP., 160/320 Rt. P. M., 600 VOLT,enters the second section of the mill. The loop steadily 3360 AMPERE. INDIRECTLY COMPOUNDED, DIRECT-increases and the total length of excess material is CURRENT MOTORdirectly proportional to the length of billets being - 1 2 3 4rolled. Armature Bus SftFrom the foregoing, it is seen that the speed regula- amperes volts amperes R. P. M.

tion of motors on individual stands need not be exces- - __60sively close. On motors driving several stands, good 168o 600 35.8 160speed regulation is of relatively more importance. 3360 600 35.8 160

However, such drives are usually applied on the rough- 6720 600 35.8 159ing stands where the length of piece is comparatively 120 300 19.2 210short and rolling speeds low so that loops are more easily 1680 600 19.2. 210taken care of. On most mills, the author believes that 3360 600 19.2 210

* * ~~~~ ~ ~~~~~~~~~5040fi00 19.2 210motors with as much as much as 4 or 5 per cent speed 6720 600 19.2 210regulation will operate successfully, although certain 140 No 14.0 265types of mills may require motors of 2 per cent or less 1680 600 14.0 265regulation. 3360 600 14.0 265

5040 600 14.0 264D-c. motors are ideally suited to the drive of such 6720 600 14.0 264

tandem mills. Speed ranges of 2:1 or greater can besecured by shunt field adjustment. The number of 1680 600 10.8 320speed points is limited only by the design of the field 3360 600 10.8 320rheostats, and by the use of coarse and fine adjustment 6720 600 10.8 319rheostats, each having 100 or 120 points, the totalnumber of settings may be 1000 or more. The inherent directly compounded equipment was placed in servicespeed regulation of shunt or lightly compounded motors in November 1926, and there are now installed or build-can usually be kept within 3 or 4 per cent, over a speed ing 46 motors totaling over 60,000 hp., to be used onrange of 2:1, and by the addition of simple auxiliary eleven different mills.equipment the regulation can be maintained as low as Alternating current adjustable speed motors may be1 per cent. made to have a speed regulation sufficiently low for theCompensating pole face windings are used to mini- successful operation of some tandem mills, and there is

mize the effects of armature reaction and thus make the a number of mills so driven. However, most tandemspeed drop proportional to the load. The speed curveof a shunt wound machine is usually slightly rising withload, so that a light series field is necessary to produce aflat or slightly drooping characteristic. The amount ofseries excitation required at full field speed with maxi-mum shunt field excitation is different from that re-quired with weakened shunt field. Therefore, tomaintain good speed regulation over the entire speedrange, it is necessary to vary the series excitation. FIG. 7-DESK FOR THE CONTROL OF EIGHT MOTORS AND THREEOn some machines this was accomplished by means of GENERATORS ON 10 IN. STRIP MILLseries field shunts and one or more switches to cut mills require several drives, and if d-c. motors are used

More recently,or outh sercuies ectio ifuisd power can be supplied from a few large motor generator

by a "series exciter," which is excited by the motor sets or rotary converters, while with a-c. drives aseparate regulating machine is required for each motor.armature current, thus giving the same effect as a sernes Under such conditions, the total cost of a d-c. installa-winding on the motor itself. A rheostat in this excita- tion may be as low as that with a-c. drives. This, withtion circuit is mechanically coupled to the main shunt the undisputed advantages of simplicity, flexibility,field rheostat. Both rheostats are thus operated to- and easy operation, has been responsible for the largegether and resistance values are so proportioned that proportion of tandem mills driven by d-c. motors.the proper compounding is secured for each speedsetting. Table II shows the almost negligible speed POWER REQUIREMENTSchanges from no-load to double load of a 2500-hp. Tests have been made on many of the mills now in160/320-rev. per min., 600-volt motor. Six of these operation, so that the energy consumption and capacitymotors are used to drive the intermediate and finishing of driving motors for any proposed new mill may bestands of a modern continuous mill recently placed in estimated with considerable accuracy.service in the Chicago district. The first such in- Table III shows a power calculation for a thirteen

768 KENYON: THE DRIVE OF TANDEM ROLLING MILLS Transactions A. 1. E. E.

stand mill, making three edging and ten flatting passes,l -> -i C'C| for the reduction of 16'2in. by 3 in. by 72 in. billets to

¢___________ _________________________ 16 in. by 0.062 in. strip, delivering the strip at 1200 ft.4D ° ° ° per min. Columns 1 to 10 inclusive show the details

- 0 C . of the rolling schedule, while columns 11 to 14, and 15to 19, show the horse power for each pass by two dif-

cc x s X n : s °: ~°: < >° X ° n ° < ferent methods of calculation. The last column showsthe averages of the results of the two methods ofcalculation.

m m c = t 0 i It c t cc There are two general methods of calculation. One,

1: - F \ A .007 n

Co L--

t o s: OcO4uC3OXOO<OOOOO.: > t o i.s.4.<..

0 ~ _<:t ~ ~~¢nt:eQCQt~ ° O _ \ I _ L-o; High C Steel_

EH | :w: ti o;: ~~ c0 ~~ u: 0t Ct e cr,0 C". cc Cs 50 ainnn 1 X t

; C,, Peren6 0 Pret C,__________ I_I_______I__________________________o___ 5_ 0!__0-_.7005MnL-

H~~~~ ~~~ IO

0oo 0t) =, tOsN o' a,+d 0.1 Pecn C 6 Pe 1Cn:- xlii = = a ~ ~ nt QS 0 20 40 60 80 1)0

E- Sof 0. H. SteeLLQnNOOO> I.TADMMLX~~~~~~~~~~~~~~. to I_ It I II- 'I 'I 0I I -IeetC

Zn C)L7 0IC OxmCoO700CO Maximum_

m~~~~~~~~~~~~~- r- r- !- °- cq I--4 200 - 11E5

4172 1 0PecentnH| NCoe C0C-COO 0 2 4 6 8 10Mn C /| C

~00 Ic., A 0 20n X b <) C; °° t: ° C I c40 60, 0 10

~~~~~~~~~~~~c r- Ic ,) 50 1- r- 00 0.5Pecn C, Maxmu |0Ct~~~~~~~~~~~~~l UI I 6 I~ o I I .4 i~ Ie I_i

00t q 0 0 C t' )C 0 10 20 30 40 50

FIG. 8-ENERGY -CONSUMPTION PER UNIT DISPLACEMENTo OON3~0C0CoO I CURVES FOR ROLLING3 IN. THICKBILLETS TO STRIP 0N20-16Z; -I 4.. Id C9 0 0 C) co 't m cli cli NIN. TANDEM MILL

-4 o 4

LO -1C-C- 0-4 _ -N

-~~~ ~~~tOCOtOCO~~~~~~~~CONCO 21K~~~~~~~ High Carbon Steel

00 ~~c m ~~0 10 00 41. t- LO___ ___ 0.60-0.75 Percent C, .

¢O -070 Mn 1J - O Mn

Z 5 a,,,, 0 a, a, CO .0 o 1 03 Nn~oto no C-r

z o - C I O -0~~~~~~~~~~~~~~~~~~~

I~~~~~~~~~~~LID¢ 10 I Cmt- x C C ioo nn~

o t LoC- X C I 0O O

I I 0 ,,, S I ,<, <, > ~~~~~~~~~~~~~~~~~~~~~~oo b1-ecmsoredenseCulgrol,Motaximurvsmr

N I w w ~cd _X -X-XX-X4X-X-X XXXX

0 10 20 30 '40 500 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ELONGATION

aI Nt LO X 0 O 'I L FIG. 9-NET ENERGY CONSUMPTION-ELONGATION CURVES0 ;X C -- FOR ROLLING 3 IN. THICKnBILLETS TO STRIP ON A 20-16 IN.

_TANDEM ILL

o 2 m- 0 U~ C -4 t knownas the displacement method has been described

in detail in a paper before the Institute by Mr. WilfredSykes in 1912. By this method it is assumed that

C~0~ 00 t N -during a pass there is "displaced" a volume of metalU) ~~~~~~~~equalto the product of the length before the pass andthe difference in area before and after the pass, and that

July 1928 KENYON: THE DRIVE OF' TANDEM ROLLING MILLS 769

mill, rolling 3 in. thick billets to strip varying from Discussion0.049 in. to 0.240 in. thick. Results shown in column Fred Butterfield: We have a thing that was not mentioned14 were calculated using unit energy consumption values in the paper, namely, a split bus which helps us out on overloadsread from the lower curve on Fig. 8. on one end of the mill or the other, using two generators.The other method of calculation is based on the We need close regulation,-5 per cent is not satisfactory.

energy required to elongate a unit volume or weight of We have had the mill up to 2300 ft. per min., delivering a piece atthe initial elongation before starting to the finish that was sixty times as long as when it entered the

steel, assumingThe curves shown in Fig. 9areplotted furnace, and when you consider more than 2 per cent of 2300roll as one. The curves shown in Fig. 9 are plotted ft., you will see what it will do with only 16 ft. between the lastfrom the same test data as Fig. 8, and results in column two stands.19 are calculated from values read from the lower curve. Our problems are fully as much operating as they are engineer-The results by the two methods of calculation check ing, and I do not believe this is the place to discuss them.

closely, and such calculations enable the designer L. P. Staubitz: We happen to be one of the mills that requireto quite accurately select motor drives for a similar 2 per cent regulation. I agree with Mr. Kenyon that there are

manv applications where close regulation might not be so essen-proposed mill, although the data must be used with tial as it is in our particular application, but I feel that if theycare, and due regard given to the effect of variable could produce a motor which would give absolutely flat regulation,conditions, such as mill layout, rolling speed, tempera- even in mills where it is not essential, it would be better for theture and analysis of steel to be rolled, percentage operating men.reduction per pass, etc., any or all of which may Mr. Kenyon mentioned the difficulty we had, at first, with

the speed. Now, that is particularly true in the finishing stands,materially influeiice the power requirements. For because of the short distances we have between them.instance, the effect of the carbon content of the steel Mr. Butterfield mentioned the split bus, and Mr. Kenyonis shown by the curves, the lower curve on each of the spoke of the speed change between stands 6 and 7. We found thatfigures showing energy consumption for "soft open we were able to overcome much of this difficultyat timeswhenwehearth" steel, with carbon up to about 0.15 percent and could tie the busses together. Whenever possible we attempt tothe upper curve showing energy consumption for rolling carry the two busses in parallel. The motor operates from nosteel with about 0.60-75 per cent carbon, and 0.50-70 per load to full load in parallel, which was a point somewhat doubtfulsteelnwithmanabout0.67pher centcarbon, and 0.50-70aper in our minds at the time when we were first considering thecent manganese. The high carbon and manganese installation.steel requires over 30 per cent more total energy to The motors of the Laclede steel plant are smaller than thosereduce to 16 gage strip than does the soft steel. of West Leechburg. That is largely due to the fact that in the

Laclede plant the main mills have roller bearings, while theCONCLUSIONS other mills have ordinary bearings.

The paper indicates that the tandem type of mill with We found that this mill when shut off, when it was originallyindividual stands separately driven byadjustable speed driven with a-c. motors, would coast, due to these roller bearings,

motors has great flexibility and iscasome 12 to 15 min. The old mills ran only about 6 or 7 min. andstopped. In steel-mill operation we often have to stop, and one

large tonnages, with very low labor operating costs. of the advantages of using d-c. equipment is that, throughA-c. equipment may be used for drives, but d-c. motors the use of regeneration in stopping, we are able to bring this millare in general more simple and have better characteris- to a complete standstill in 45 see.tics, and where several drives are installed the cost may W. M. Ballenger: Modern tandem or continuous mills, asbe as low or lower than for the a-c. drives. Test data designed by the mill builders, are such that a wide variety of

products must be rolled. This means that the finishing standsfrmeitn ilsaeaalbeandpowemust have reasonably good speed regulation and must operate

ments of any proposed mill may be quite accurately through a comparatively wide speed range.estimated and correct applications of driving equip- Since power is practically always brought to the motor roomment made. in the form of alternating current at some convenient voltage as

2200 or 6600, it is advisable to use a-c. motors wherever possibleBibliography in order to get arouind the losses in conversion apparatus.

1. A New 20-16 in. Hot Strip Mill, by Noble Jones and G. P. If the roughing and intermediate stands can be grouped intoWilson, A. I. E. E. JOURNAL, August 1924, pp. 710-715. trains, large a-c. motors of the induction or synchronous types

2. "The Electrical Control for a Hot Strip Mill," by M. J. can be used to advantage. Synchronous motors are becomingWohlgemuth, Electric Journal, September 1923, pp. 322-325. more and more popualar, wvherever applicable, because of their

3. "The Application of Direct Current Motors to Main Roll ability to improve the inherent low power factor of steel mills.Drives," by H. A. Winne, Iron and Steel Engineer, April 1927, On the finishing stands, because of the diversity of products topp. 194-202. be rolled, adjustable-speed motors are practically always required.

4. Seed egultionof te Man Rol Drves, by . A.Where extremely close speed regulation is unnecessary, the speedUmansky, Iron and Steel Engineer, MaJy 1927. pp. 207-218. range iS not too great, and the motors required are of compara-' ' ~~~~~~~~~~~~~tivelylarge size, induction motors employilng either the Seberbius

5. "Adjustable Speed Drives for Rolling Mills," by L. A. or Kraemer sy,stems of speed conltrol may be advantageouslyUmansky, Iron and Steel Engineer, September 1924, pp. 515-532. applied. W9here extremnely close speed regulation and a very

6. "Electric Drive for Ten-Inch Strip Mill at the Laclede wide range of speed are necessary, d-e. motors are almost inva-Steel Company," by A. F. Kenyon, Electric Journal, June 1927, riably used. In instances where the adjustable-speed motors arepp. 312-317. to be relatively small it is usually better to use d-c. machines.

7. Power Requirements of Rolling Mills, by Wilfred Sykes, A. F. Kenyon: Mr. Butterfield has mentioned the use, atA. I. E. E. TRANS., 1912, Vol. XXXI, Part II, pp. 2051-2066. the Laclede mill, of a split bus. In this case, the supply for the