Synchronous Motors for Driving Steel RollingMills

BY W. T. BERKSHIRE' and H. A. WINNE1Associate, A. I. E. E. Associate, A. I. E. E.

Synopsis.-It is the purpose of this paper briefly to discuss, their advantages and disadvantages are; where they should, and wherefrom a practical standpoint, the application and design of synchro- they should not, be used; and what special precautions must be takennous motors for steel-mill main-roll drives, in an effort to show what in the design of motors for this service.

DRIVING main rolls of steel mills is universally In addition to being able to start the mill, and carryrecognized as very heavy duty. The loads are heavy and sudden overloads, the drive must usually behigh, and are applied and relieved very suddenly. capable of withstanding "plugging" in order to bring

Consider the case of a motor driving a single stand, the mill quickly to a stop in case of a "cobble" or otherFig. 1. Between passes it will run with only 5 per cent mishap. Any piece of metal which fails to go throughto 10 per cent load, due simply to mill friction. As themetal strikes the rolls the load jumps almost instantly topossibly 100 per cent or 150 per cent of normal, and isas suddenly reduced when the metal leaves. This j 'on

happens several times a minute. If such a drive Mtor t

has been properly selected, several passes on each bloomor billet may require 150 per cent to 175 per cent normalload on the motor. The load is intermittent in char- the mill properly is termed a "cobble." As soon asacter, so that the motor is selected with the idea of the operator sees that the steel is not going through aspermitting some of the passes to come up to these it should, he "plugs" the motor by disconnecting itlimits, so long as the r. m. s. value of the load is within from the line and then applying power with reversedthe normal rating of the motor. If the heavier passes phase rotation. After the mill stops, if the metal isare of not more than three or four seconds duration a not clear of all the stands, the portions between standsflywheel may be utilized to reduce the peak loads on are cut out and then the motor must start the mill inthe motor and power system. the reverse direction to back out the pieces in the rolls.

Considering these conditions which a main roll drivemust meet, it is not remarkable that for nearly all

l =0 constant-speed electric drives induction motors of thet5 % wound-rotor type have been used. This type of motor

PinionoStandPollStand has excellent starting characteristics, will carry heavyFIG. 1-ROLLING MILL, SINGLE STAND overloads, and withstands much abuse. In comnmon

with all induction motors, however, its power factor isWith a mill having a train of several stands as in lagging, and very much so in low-speed machines.

Fig. 2 or with a continuous mill as in Fig. 3, the drive is Now, the main rolls and lay-shafts on heavy mills donot subjected to quite as severe shocks as with a single not run at high speeds, and it is often desirable tostand, for it is apparent that as a piece of metal enters direct-connect the motor, so that there are now inthe mill the stands are filled in succession until all are service many low-speed motors, operating at low powerfull. The load increases to the maximum value in anumber of steps, and is similarly reduced.The torque required to start a mill from rest is often

quite high in comparison to the capacity of the drivingmotor. This is especially true in cold weather, asvery heavy grease is used on the roll necks and pinions, Ql Li vI.and this becomes very hard at low temperatures. MillsRoltnsi gused for cold rolling thin sheets, which operate with FIG. 3-CONTINUOUS MILLvery high pressure between the rolls and consequentlyon the bearings, may require as much as 200 per cent factors. As a matter of fact, one reason for the use ofof normal motor torque to break them loose. 25-cycle power in numbers of steel plants is that low-

1. othof eneal lecricCo. Scenetad, N ~.speed 25-cycle motors have better power factor thanPresented at the Regional Meeting of District No. 5 of the the corresponding 60-cycle machines. The use of

A. I. E. E., Chicago, IllI, Nov. 28-30, 1927. higher speed motors driving through reduction gears237

238 BERKSHIRE AND WINNE Transaetions A. I. E. E.

has helped the situation somewhat, but has still left power system can stand without disturbance, whilemuch to be desired in the way of power factor starting the motor; and last, but not least, to the charac-improvement. teristics that can be obtained in the motor, to determine

IJnquestionably the desire for a better operating whether it can meet the requirements.power factor has been the chief factor in bringing the STARTING CHARACTERISTICSsynchronous motor into consideration in steel mill Practically the only reason synchronous motors haveservice. It possesses, however, advantages other than not been widely used on mill drives in the past is because

their starting characteristics are not so desirable asthose of the wound-rotor induction motor. For 100per cent kv-a. input the induction motor developsapproximately 100 per cent rated torque at starting,whereas the synchronous motor will give from 30 percent to 60 per cent starting torque with the same kv-a.input at a much lower power factor. However, thetorque obtainable from a synchronous motor is ampleto start most types of mills, and its other advantagesmake it the logical choice in many cases.By proper design, good starting torque character-

istics, as shown in Figs. 5 and 6 can be obtained in millFia. 4-9000-H.P., 107-RFv. PER MIN., 6600-VOLT SYNC1iRON- type synchronous motors with a single squirrel-cage

OUS M(OTOR DRIVING A CONTINUOUS SIIEET BAR MILL winding. The double squirrel-cage has, at times, beenThis motor has a greater continuous horsepower rating than any other considered, but in each case it has been found that by

motor in industrial service, the proper choice and distribution of materials in thebars and rings, the proper spacing of the bars with

its good power factor, as well as some disadvantages, respect to the stator slot pitch and the depth and widthand these will be brought out in the following detailed of the slots in the pole face over the bars, the torquecomparison of the characteristics of the two types ofmachines.FIELD OF APPLICATTON OF SYNCHRONOUS MOTOR 340 TEX _The field of application of the synchronous motor

300in main-roll service is limited to strictly constant-speeddrives. This eliminates it from consideration on 260 _ treversing mills,-mills requiring flywheels, and mills _

needing adjustable speed. 24 -

It is not as a rule advisable to attempt to apply it toany type of mill which may have to be started with __

,I1Ometal in the rolls, such as a cold strip mill, nor to cold .e16'__ _sheet mills, which have excessively high friction. Such -1.mills may require at starting considerably more torque E300 110than is needed to carry their full load at full speed, and 4 _unless the motor is sufficiently small in comparison to -200 3-the power system so that it can be started at full voltage, ------

difficulty may be experienced in getting started and -o 40synchronized. 20C

In connection with constant-speed continuous mills 0 0 0i 0 010i10 20 30 40 10 r 7 80 9 10of the type shown in Fig. 3, looping mills as illustrated Poe,nt SyCchmono.s Speed

in Fig. 2, and in fact, almost any constant-speed hot FIG. 5-STARTING CIIARACTERISTICS OF 6500-H. P., 0.8-metal mill, the synchronous motor deserves very Careful POw7ER FACTOR, 187-REV. PER MIN., 6600-VOLT, 25-CYcLzconsideration. Every individual case must be studied SYNCHRONOUS MOTOR, FROM TEST DATAvery thoroughly to make certain that no misapplica-tions are made. Careful thought must be given not requirements have been amply met with a single squirrel-only to the full-load rating required, but also to the cage. In practically all cases it has been found possiblemaximum torque that may be necessary to break the to obtain more than sufficient torque to start and bringmill from rest under the most adverse conditions; to the mill to synchronous speed, or to even back out athe maximum torque needed at pull-in; to the torque cobble, with from 70 per cent to 100 per cent normalrequired to back out cobbles; to the maximum peak load kv-a. input. Unlike that of the squirrel-cage inductionthat may be encountered; to the kv-a. demand that the motor the squirrel-cage of the synchronous motor can

Nov. 1927 SYNCHRONOUS MOTORS FOR DRIVING STEEL ROLLING MILLS 239

be changed in design at will, with a corresponding the starting current and decreases the starting torquechange in torque characteristics, without affecting the to some degree, it also increases the pull-in torque.efficiency of the synchronous motor during its normal The amount and capacity of this resistance to give theoperation under load. best torque characteristics can be determined byThe curves shown give the torque and kv-a. values calculation.

with full voltage applied to the motor. In normal A synchronous motor may be plugged for a quickoperation of course, these large motors are started at stop, by first opening the "forward" breaker andreduced voltage obtained from a suitable auto-trans- removing field, then closing the "reverse" breaker andformer. For example, the 6500-hp., 187-rev. per min. connecting the motor to the starting tap of the auto-motor for which starting torque and kv-a. curves are transformer. The current drawn when plugging is

approximately 15 per cent more than the starting160 current, and the torque developed about 75 per cent of

1450 the torque at starting.

MAXIMUM TORQUE120 __ _ _ _ _ _ _ _ _ _ _ _ t _ _A synchronous motor can be designed for fully as high

_10_ ___ ___ V__maximum or pull-out torque as an induction motor andV1C ____ _ ________ ___ \ _ _ for steel mill service this pull-out torque varies from

90_ _ _ _ ___ ______ _ _ X _225 per cent to 300 per cent of normal full load running

400E' eo80 2CtAi 4 WS torque. The synchronous motor has the advantage,0_________ ______ _that for any reduction in applied line voltage the pull-

>°00 LO SS p S p X 3out torque decreases only in direct ratio to the voltage,so ___ _ _ ___ ____ s _ _whereas the torque of an induction motor decreases as

z°200 40 the square of the voltage. Furthermore because of itso) 30 ___ _ ___ _ ____ _ _ _ _better power factor, the synchronous motor helps to10ioo 20 maintain the voltage at its terminals; consequently the

20 ___ _ __ ___I ___ _drop in line voltage due to a given load is not likely to00 1 20 30 40 50D 7

0 be so great as if an induction motor were used.Pe cent Synchronous Speed

FIG. 6-STARTING CHARACTERISTICS OF 5000-H.P., 0.85- POWER FACTORPOWER FACTOR, 240-REV. PER MIN., 2200-VOLT, 60-CYCLE One of the most desirable features of the synchronousSYNCHRONOUS MOTOR, FROM TEST DATA motor is its ability to improve the power factor of the

system on which it operates. ft is usually designed toshown is regularly started on 32 per cent voltage. give a leading power factor at normal load, and will thenSince the starting torque and kv-a. input varies as thesquare of the voltage, it is apparent that under thiscondition the motor demands only 60 per cent ofnormal ky-a, and gives about 35 per cent of its normal 50T ttl il T Itorque. This has proved ample to start the mill under 40

all conditions. Similarly, a 9000-hp., 107-rev. permin. motor is always started at 32 per cent voltage, a0giving 27 per cent of normal torque with 70 per cent N S 010120 130040 050

normal kv-a.~~~~~~~~~~~~~~~~~~~~~~~~~~~~0 output inPercent Norrnal ShaftHo,sepoweri JJnormal kv-a. 07)06 5 COIS +The "pull-in" torque, or the torque available at

approximately 95 per cent synchronous speed before the 40application of field, must of course be in excess of the FIG. 7-APPROXIMATE REACTIVE KV-A. AVAILABLE FOR POWERPmill friction at this speed, but can be considerably less FACTOR CORRECTION, WITH FIELD EXCITATION CONSTANT ATthan the starting torque, as the latter must overcome NORMAL LOAD VALUEthe "dead" friction of the mill, with the bearingspractically dry. furnish a considerable amount of corrective ky-a, at allThe fields of these motors are usually wound for loads up to a considerable overload, as shown in

250-volt excitation, and if the field were left open- Fig. 7. The low power factor of low-speed inductioncircuited at starting the induced voltage across the motors, particularly 60-cycle machines, has necessi-rings, with 33 per cent normal voltage applied to the tated the use of reduction gears in some cases where forstator, would be from 5000 to 10,000 volts. In order to other reasons a direct drive would have been preferable.protect the operators from the induced field voltage it The use of synchronous motors permits direct driveis the practise when starting, to close the field circuit with low-speed machines, operating at unity or leadingthrough a discharge resistance. While this increases power factor.

240 BERKSHIRE AND WINNE Transactions A. 1. E. E.

EFFICIENCY The following table shows the relative base dimen-Two curves are shown, illustrating the very high sions of the two types of motors, for several different

efficiency obtained from large synchronous motors, ratings:both 25 and 60 cycles. The fact that approximately Floor Space75 per cent efficiency is obtained at 5 per cent of normal Rating Synchronous Motor Induction Motorload is quite noteworthy. Rev.The full-load efficiency of synchronous motors for H. P. per min. Cycles Ft. In. Ft. In. Sq. ft. Ft. In. Ft. In. Sq.ft.

steel mill service varies from 0.5 per cent to 2 per cent 9000 107 25 18 0 24 8 - 445 20 7 x 25 0 - 515more than that of the corresponding induction motors. 6500 187 25 17 5 x 15 7 - 272 16 7 x 19 3 - 319This better efficiency of course means some saving in 500 24°0 60 13 2 x 21 11 195 4 0 x 1236 0 = 224power cost. 5000 83 25 14 3 x 19 9 -282 15 0 x 20 0 = 300

1500 300 25 10 10 x 11 8 - 123 12 3 x 11 9 - 143

OPERATING VOLTAGESynchronous machines can very readily be built for -0°°

any operating voltage up to and including 13,200. 90 IWhile a very few induction motors are operating at '8013,200 volts, it is better practise not to exceed 6600 volts 7

on an induction motor, as the design becomes difficult 60and the machine expensive.

30

2090 10

--

10>70 I0 10 20 30 40 50 60 10 60 90 100

Pe, cwt Load

o'60at _ _ T T T _ _ _ g FIG. 9-EFFICIENCY OF 5000-H.P., 0.85-POWER FACTOR, 240-SO r __ T I T _ _ _ m REV. PER MIN., 2200-VOLT, 60-CYCLE SYNCHRONOUS MOTORu40 E W + i t X FROM TEST DATA

30L20

10

0 0 IC' 20 30 40 50 60 10 60 90 100Pe, cent Load

FIG. 8-EFFICIENCY OF 9000-HP., 1.0-POWER FACTOR, [107-REcv. PER MIN., 6600-VOLT, 25-CYCLE SYNCHRONOUS MOTOR,

FROM TFST DATA

EXCITATIONOne disadvantage of the synchronous machine is that

it requires a separate source of excitation, while theinduction motor does not. On an important drive itis wise to employ an individual exciter, either directconnected or driven by a separate motor. The excita-tion voltage is always 250, so that as an emergencysource the 250-volt d-c. power circuit which exists in FIG. 10-STATOR FRAME FABRICATED FROM STEEL PLATESall steel mills can be used. AND BAR8

FLOOR SPACE SPEED CONTROLThe amount of floor space required by a synchronous Control, or rather adjustment, of the speed of a syn-

motor is almost invariably less than that needed for an chronous motor in mill service is of course impractical,induction motor of the same rating. One reason for and its use must, therefore, be confined strictly tothis is that it is the usual practise to make the motor constant-speed mills. This fact also eliminates itbase long enough so that the stator can be moved along from consideration on any so-called constant-speedthe shaft a sufficient distance to make both rotor and mill on which a flywheel is necessary, for to get anystator windings accessible for cleaning or repairs. beneficial effect from the wheel the speed must varyThe rotor of an induction motor is inherently somewhat inversely with the load.longer than that of a synchronous motor, because of the The fact that the motor runs at truly constant speed,space required for the end conneetions of the coils on except for what variation in frequency occurs on thethe former, and this necessitates a greater space for system, is an advantage on some types of mills. Formovement of the stator. example, if the product from a continuous mill of the

Nov. 1927 SYNCHRONOUS MOTORS FOR DRIVING STEEL ROLLING MILLS 241

type shown in Fig. 3 is cut into lengths by a flying The stator frames of the earlier motors of this typeshear, as it leaves the mill, the lengths will be more are of cast iron. Those built within the past year and auniform if the mill speed is absolutely constant than if half, however, are fabricated of steel plates securelyit varies slightly. welded together and braced to form an exceedingly

COST strong and rigid structure. To the inner periphery ofThe cost of a synchronous motor, of the capacity used the frame are welded steel dovetailed keys. The coreThe cost ofasynchronousriscmoltor othe capcityrundclaminations are held on these keys and clamped be-for main roll drives, complete with exciter and control, tween heavy welded steel finger flanges. Air ducts are

is usually less than that of a similarly rated induction provided in the core and complete ventilation is furtheraccomplished by the use of air-slide wedges.

Because of the size and weight of the stator coils inthese large motors they are insulated very carefully toprotect them from mechanical injury. After theirassembly in the stator the end projections are securelylaced to insulated steel bracing rings which are sup-ported from the stator frame. The larger machinesare supplied with resistance temperature detectors.The stator coils are liberally designed to safely take careof sudden overloads or the condition where the motormay be required to develop its maximum torque as an

induction motor.The rotor spiders of the machines of small diameter

FIG. 11-PARTLY WOUND STATOR, SHOWING METHOD OF are built up of laminations punehed from heavy steelBRACING THE COILS plates, those of larger diameter being of cast steel.

The laminated pole pieces are either dovetailed intomotor. For machines of medium capacities, speeds, and the punched rotor or secured to the cast rotor byvoltages, the differential is not great, but for large means of bolts screwed into steel keys imbedded in thelow-speed units, the synchronous machine is consider- pole pieces.ably less expensive. The field windings are usually of edgewise-wound

RELIABILITY AND EASE OF REPAIR copper strip. Here again great care is given to theF'rom the standpoint of reliability it can hardly be insulation between the turns of the winding, and of the

said that either type of motor has the advantager A coils as a whole from the pole pieces and rotor spider.sai tha eihe type of.....moo.asteadatg. A One of the recent improvements in design consists inmachine any more reliable than the well bUilt mill-type Oeo h eetipoeet ndsg osssi

minecanymo* r rea e than th bul mill-ye the addition of fins to the ends of the field coils whichinduction motor has proved itself to be, would bedifficult to find, but there is no reason why the syn-chronous motor should not have an equally good recordin the years to come.As far as ease of repairing is concerned, the stators of

the two machines are practically on a par. The coilsof the synchronous motor are somewhat larger andheavier as a rule, but there are fewer of them. Therotor of a synchronous motor could probably be re-paired more quickly than that of an induction machine.The fact that the synchronous motor has a fairly largeair-gap helps to facilitate the moving over of the stator ;~for cleaning or repairs.

CONSTRUCTION FIG. 12-ROTOR WITH CAST STEEL SPIDERObviously the details of design and construction

described in the following paragraphs apply to motors are made by simply projecting every second or thirdbuilt by the company with which the writers are turn during winding. These fins provide an increasedassociated. The practise of other manufacturers may area of radiating surface on the ends of the coilsdiffer in some respects. and have proved very effective in reducing 'the fieldThe mechanical and electrical construction of the temperature.

mill-type synchronous motor is fully as sturdy and Since, at times, these motors may be required toreliable as that of the mill-type induction motor. The develop their maximum torque as induction motors,quantity and kind of the materials used are such that considerable attention is given to the heat storageall stresses are kept within a conservative minimum. capacity of the amortisseur winding, the materials

242 BERKSSHIRE AN1) WIN-NE Transaetions A. 1. E. E.

used being such that their strength will be retained at We will assume that the motor is at rest and that thehigh temperatures. The bars are silver soldered into operator throws the handle of the master switch to thethe end-ring segments. The end-ring segments have "forward" position. Oil circuit breaker A closes atbolted joints between poles so that each individual once, establishing the neutral connection of the auto-pole may be readily removed from the rotor without transformer,-F follows immediately, connecting thedisturbing the others. line end of the auto-transformer to the line andThe specially designed mill-type pedestals are thereby applying the first step of reduced voltage to

securely bolted to both the base and foundation. They the motor. With this voltage the motor should startare equipped with babbitted thrust collars when this and gradually increase its speed.feature is desired. These pedestals are insulated from When it reaches a predetermined speed, usually from

50 per cent to 75 per cent of synchronism, as indicatedby the frequency obtained from a small pilot generatoron the main motor, a relay operated in response to thefrequency causes breaker A to open, and immediatelythereafter breaker B closes. B connects the motorto the line through the reactor. The reactor is soproport-oned that the voltage drop across it at the timeit is connected in the circuit is sufficient to reduce thevoltage at the motor to a value between the line voltageand that given by the auto-transformer tap. As themotor speed increases, the current will drop and thevoltage at the motor terminals will rise.

FIG. 13-ROTOR WITH LAMINATED SPIDER When the motor reaches approximately 95 per centsynchronous speed, as determined by a relay which

the base to eliminate the possibility of shaft currents. operates only when the difference between the lineThe spherical-seat, self-alining bearings may be frequency and the pilot generator frequency is 5 perTheipspherical-seperat, rerelf-aying Abearingslay bned cent or less, field excitation is applied, of sufficient valueequipped with temperature relays. A liberally designed to give approximately unity power factor. This pullsring oiling system insures ample lubriaationbut in the motor into step, and so reduces the current drawnaddition, provision is made so that flood lubrication by the motor through the reactor that the voltage at

The base is provided with rollers under the carrier the motor terminals increases almost to the line value.plates supporting the stator feet in order that the statorframe may be easily moved in a direction parallel to A-. Supplythe shaft. The carrier plates are keyed to the base in Illorder to maintain the alinement of the stator frame OilCi°e.it 6''kem

F R

during this movement. F itorstting.All breakers op4l.Air heaters may be installed in the lower halves of the 1BCloseA.Motor StArto.stator frames of these motors to prevent the possible A Ti- 1.ta-a.

accumulation of moisture on the windings in case the 4C.c.nt speed.mill is idle for any considerable length of time. 5-Appig r,a,', fla.

Traos$oroBr fr6.Close C.

CONTROL I^ fld.

A few main-roll synchronous motors are startedat full voltage, but most of them are so large thatsuch practise is not desirable because of the result-ing demand on the power system. Consequently, FIG. 14-ELEMENTARY DIAGRAM OF CO11BINATIONan auto-transformer is usually employed to give reduced KORNDORFER AND REACTOR METHOD OF STARTINGvoltage for starting. For some of the largest machines SYNCHRONOUS MOTORSit has proved desirable to employ two reduced voltagesteps in the starting operation, and to meet this con- After a short-time interval, breaker C closes, Bdition the combination Korndorfer and reactor method opens, and the field excitation is automatically increasedillustrated diagramatically in Fig. 14 has been de- to the full value.veloped. It will be noted that at no time during the sequenceThe sequence of operations for starting, stopping, or of starting operations is the motor entirely disconnected

plugging the motor is initiated by the simple movement from the line. Furthermore, owing to the use of theof the handle of a master switch placed near the mill, reactor, the transition from the second starting voltageand the operation is completed automatically under the to the line is made with extreme smoothness..control of relays on the control panel. Protection is provided against under-voltage, loss of

Nov. 1927 SYNCHRONOUS MOTORS FOR DRIVING STEEL ROLLING MILLS 243

excitation, and failure to synchronize within a definite they are sufficient and on those mills it can often beinterval after the master switch is operated. used to advantage.The control equipment for a synchronous motor It seems safe to predict the widely increasing use of

involves more oil circuit breakers than does that for the synchronous motor in mill service, and with thisan induction motor, but the latter requires a number prediction goes the hope that such motors will beof large contactors and resistors for its rotor circuit, applied, designed, and built only with a fullknowledge ofwith relays for controlling the same. Neither is the demands of the load and the limitations of theespecially complicated in installation or maintenance. motor.So far as the mill operator is concerned, he simplymoves the handle on one kind of master switch to start Discussioneither type of motor. H. V. Putman: Mr. Berkshire classifies the application of

INSTALLATIONS synchronous motors to cold strip rolling mills as undesirablebecause of the high starting torque (usually 200 per cent)

A considerable number of synchronous motors is now required. We already have one installation of a svnchronousinstalled or being built for main roll service. Those motor on a cold strip mill starting up under 200 per cent ofsupplied by one manufacturer include the following: normal torque and giving entire satisfaction.A 9000-hp., 107-rev, per mmn., 25-cycle, 6600-volt The principal problem in applications of this kind is to obtain

is0rivng coninuus*hee bar mll a60 lth the necessary starting torque with a reasonable inrush kv-a.

Recent developments in synchronous motors indicate that it isCleveland plant of the Corrigan-McKinney Steel possible to obtain these high values of starting torque withCompany. This motor has a higher continuous horse- little more inruslh kv-a,. than would be required bv a normalpower rating than any other motor in industrial service wound-rotor motor.in this country. As an example, we are now building a low-speed synehronous

To oron60p 18r,et motor for driving a ball mill in a rock-erushing plant. ThisTwo motors, one 6500-hp., 187-rev. per min. and the 'nmill requires approximately 200 per cent starting torque andother 4000 hp., 83 rev, per mmn., 25 cycles, 6600 volts, slightly over 100 per cent pull-in torque. This synchronousform part of the drive of a continuous skelp mill at the motor will deliver 200 per cent starting torque with aboutBethlehem Steel Company's Sparrows Point plant. 390 per cent inrush. In fact, the starting torque may be variedA 5000-hp., 240-rev. per min., 60-cycle, 2200-volt at will from about 160 per cent torque to 235 per cent, the inrush

synchronous motor is being installed to drive a tube being almost proportional to the torque. The calculated pull-synchonomill at the Standard Seamless Tube Company's in torque is 125 per cent.piercing mill at the Standard Seamless Tube Company s Now this is almost as good torque per ky-a. as w-ould be ob-plant at Economy, Pa. tained from an ordinary slip-ring motor. While the slip-ringThe Continental Steel Corporation, Kokomo, Indiana, motor will give 100 per cent torque with 100 per cent kv-a. to

has purchased a 5000-hp., 100-rev. per min., 60-cycle, get 200 per cent torque at starting it is necessary to reduce the2200-volt motor to be used in driving a continuous sheet resistance in the rotor circuit so that its power factor is much

bar mill.poorer. Also, it should be remembered that the normal kv-a.on which the per cent inrush is based, is figured on a power factor

The Copperweld Steel Company of Glassport, Pa. considerably less than unity, especially on low-speed machines.will use three 60-cycle, 2300-volt synchronous motors, The slip-ring motor would therefore give 200 per cent startingone 600-hp., 400-rev. per min., one 600-h. p., 514-rev. torque with an inrush of about 350 per cent based on unity-per mm. and one 600-h. p., 900-rev, per mi. to drive power-factor kv-a. This is to be compared with the 390 per cent

various merhatndodmilinrush for the special synchronous motor mentioned above.various merchant and rod mill stands. This leaves the slip-ring motor but little advantage for applica-Two 400-hp., 720-rev. per min., 60-cycle, 4600-volt tions requiring high starting torque.

motors have been purchased by the Higgins Brass and While some features of this new motor are still experimental,Manufacturing Company, Detroit, Michigan, to drive it may be said at this time that it is of the salient-pole construc-brass and copper mills. tion and has the high efficiency and low exciter capacity charac-Another manufacturer has built several synchronous teristic of machines of this type.m notorsefor samlesstue m ll *seve, som chofohe In view of these recent developments, I think it is reasonable

beings forpieringamllss andemillservisomef u olling ml to predict that in the near future synchronous motors will berecommended and used for any constant-speed, non-reversing

drives. drive, regardless of the starting duty.

CONCLUSION There are two other points I should like to comment on: the

The foregoing discussion, we believe, has made clear matter of power factor and the construction of damper windings.that the synchronous motor is a real competitor of the I think most users of synchronous motors have the idea that

wound-rotorinduction moor for some types of mai unity-power-factor motors are cheaper than 80 per cent power-factor machines. This is ordinarily true, but it iS not true ofSroll service. The number of installations which have motors having a pull-out torque of between 250 per cent andbeen made within a comparatively short space of time 300 per cent. An 80 per cent motor costs no more than a unity-certainly proves this contention. The synchronous power-factor motor when the pull-out torque is between thesemotor has certain definite advantages, such as better values. InvariaJbly the electrical manufacturer prefers to supply

the leading-power-factor machine because it has a lower mag-power factor, efficiencies, and cost, which make it very netizing current a,nd hence better starting characteristics.attractive. Its starting characteristics are not so good Leading-power-factor motors are therefore to be recommendedas those of the induction motor, but for many drives when high pull-out torque is required. When no corrective