Discussion on “electric drive for reversing rolling mills” (Wilfred Sykes and David Hall), Cleveland, Ohio, June 27, 1916. (see proceedings for June, 1916)

1474 ELECTRICALLY DRIVEN REVERSING MILL [June 27

DISCUSSION ON "ELECTRIC DRIVE FOR REVERSING ROLLING M I L L S " (WILFRED SYKES AND DAVID HALL), CLEVELAND, OHIO, JUNE 27, 1916. (SEE PROCEEDINGS FOR JUNE, 1916.)

(Subject to final revision for the Transactions.)

K. A. Pauly: There are several points in connection with this paper by Messrs. Sykes and Hall to which I wish to refer in discussion. One of the first points which is worthy of attention is that of the rating given these motors: namely, a horse power corresponding to the maximum torque which they will carry without any mention of the time during which they will carry the load. All will agree that a rating on this basis is not only contrary to the recommendations of the A. I. E. E., but is very unsafe for the purchaser on which to base the relative capacities of competitive equipments for any class of service. That the idea of so rating these motors is a new one will become apparent to any one examining the publications of the company with which the authors are connected. The early publications snowed, for example, 3000 h.p. as the capacity for the motors of the Steel Company of Canada now rated at 10,000 h.p. On this same basis, any one of the 6000 h.p. motors at the Gary Works of the Illinois Steel Company might be rated at 20,000 h.p.

As to the relative merits of the electrically-driven reversing mill and its steam competitor, it is difficult to discuss a question oï this magnitude in generalities, because there are so many factors affecting the problem, which are more or less important, depending upon the special conditions obtaining in individual cases. However, the characteristics of motors are in most of the essential details especially adapted to rolling mill conditions. They permit of the centralization of the steam plant and the generation of power in high speed, highly efficient units. The authors have made the mistake frequently found in comparing estimates of the relative operating costs of steam versus other methods of drive. Why stop with the steam consumption, when one of the greatest savings results from the increased boiler efficiency due to the more uniform demand for steam made possible by the use of the fly-wheel motor-generator set equalizing the load in the electrically-driven mill. Tests would indicate that boiler efficiencies may be raised twenty per cent or more by relieving the boiler of these excessive demands for steam required by reversing engines.

My experience indicates that a comparison on the proper basis including all the items of first cost, chargeable against each method of drive, would show little if anything in general in favor of steam equipment in first cost, and will always show the electrically-driven mill to be lower in operating costs. The speed and torque characteristics of the motors are ideal, there are no tendencies toward excessive speeds when the piece leaves the rolls and the motor is always in a position to exert its maximum effort when the maximum power is required.

The figures and curves of power consumed in rolling given by

1916] DISCUSSION AT CLEVELAND 1475

the authors, check within reasonable margin of the experience of the writer.

In considering the requirements of reversing rolling mill motors, it is interesting .to note the similarity between the factors affecting their electrical design and those for a mine hoist. Both are required to accelerate rapidly, and to carry very heavy dead loads, during acceleration. In fact, in these respects the requirements of a mine hoist motor are frequently more severe than for the reversing mill motor. The roll motor is required to accelerate but little in addition to its own armature, and usually reversal and acceleration to partial speed are accomplished without any dead load, although the equipment should have sufficient capacity to accelerate rapidly with the piece in the rolls. On the other hand, the hoist motor must accelerate in addition to its armature the drums, ropes, cages and load which may be equivalent to several times the armature, and must always handle full load throughout the acceleration. It is true that the time allowed for accelerating a mine hoist motor is more than that allowed for the reversing mill motor, but the increased time is not proportional to the increased load to be accelerated, so that frequently from the standpoint of peak loads, the requirements for the mine hoist motor are more severe than those for the reversing mill motor.

The shocks to which a reversing mill motor is subjected are, of course, more severe, and very much more so than those to which a mine hoist motor is subjected, but are the same as those to which the ordinary non-reversing steel mill motor is called upon to meet, so that the reversing mill motor becomes essentially a direct-current motor capable of withstanding the peak loads met with in mine hoist service, constructed mechanically to withstand the shock and vibrations incident to rolling with the windings held firmly to prevent injury from the mechanical and electrical shocks.

The all important question is production, and in my experience the steam advocate has made a greater point of the speed of the steam mill as compared with the electric mill than he has on the questions of economy or first cost, although many have claimed these advantages also, and the present electrically-driven mills are referred to by him as being slow due to the time required to accelerate. In this extremely important detail, the characteristics of the reversing rolling mill motor differ from those of the mine hoist motor apart from the effect of the time of reversal on the peak current taken by the motor. It is well known that time is required to build up and discharge a magnetic field: the rate being dependent upon the time-constant of the field winding.

The importance of this is suggested by the author, but apparently in the equipment thus far installed every reasonable means has not been resorted to, to bring about a rapid change of field and resulting rapid acceleration and retardation. The accumulative compound-windings on the motors tend to increase the time of rolling by lowering the rate of acceleration and decreasing the


time of retardation. This compounding is not necessary to protect the mill any more than it is necessary to protect a non-reversing mill driven by direct-current or alternating-current motor, many of which are driven by motors without this provision or its equivalent. The compounding, of course, relieves the motor and generator from electrical shocks sufficiently to permit the use of slightly smaller equipment, but this saving is made at the expense of output from the mill. In the equipments we are now building for this work, we will not only take advantage of the shorter time required for accelerating and retarding the shunt motor to reduce to a minimum the time required for rolling, but are using in addition a special system of control, which will be described later in a paper. This takes advantage of the actual characteristics of the rise and fall of the magnetic field to produce a maximum rate of acceleration and retardation of the roll motor, thereby obtaining a faster operating reversing mill than any thus far built.

One of the most important details in the construction of a reversing mill equipment is the motor shaft, and in many instances this detail has been rather slighted. It is true that these shafts are protected by breaking spindles, but it must be borne in mind that the breaking of a spindle must in no way endanger the shaft. A moment's consideration will convince anyonethatthe motor shaft should be at least one-third larger in diameter to even protect it against injury from a strain sufficiently great to break the breaking spindle; even then there will be little or no factor of safety in the shaft, in spite of the difference in material used for shafting and spindles. In addition to this, the shaft is frequently subjected to extremely severe bending moments in the event of the breaking of a spindle on the diagonal. It has been our practise to recommend shafts much stronger than those installed, and I cannot but feel that eventually serious delays are bound to result from unnecessary weaknesses in this feature.

The question of motor voltages is too elementary to warrant discussion, although considerable importance seems to have been attached to it by the authors. Practically all of the reversing mill motors installed, including the first one, have been designed for voltages ranging from 550to 750 volts, and wheremore than one motor has been used, they have been connected in series, resulting in a voltage of the combination from 1000 to 1500 volts. Here as in most problems, involving larger powers, it is essential to use as high voltage as is consistent with reliable operation and reasonable first cost, and the writer feels safe in predicting higher voltages for this work when more experience has been gained with high-voltage direct-current units of large capacity.

The question of windings, compensated and commutating, is also ancient history, the equipments for this service always having been provided with these special means for improving commutation.

E. S. Jefferies: The question brought out in Sykes' and Hall's paper regarding engine builders' claims is very interesting, and


I would like to point out a few results obtained on the Steel Company of Canada's mill referred to in their paper.

This mill has been operating since the early part of 1913 with very good results, and the experience gained brings out some very interesting answers to the five general questions raised by the steam men in favor of steam rather than electric drive. In the tables herewith, are complete costs covering this installation, and operating costs for the three years 1913 to 1915, and an average for the same period.

As the Hamilton Mill purchases power, I cannot give any comment on the relative costs of steam and electrical drive. However, in Table I is given a comparison showing the operating costs, interest on investment, depreciation and miscellaneous charges covering this particular installation. This shows an average total cost for the three years of only 43.1 cents per ton. This figure includes a depreciation charge which considers the installation as valueless at the end of a 20-year period, and a miscellaneous charge which includes all power for lighting, tables, crane, conveyor, pumps, motors, etc., used in this mill. The largest item, power cost, is exact, as it is metered, and the other items are charges made direct with no estimating, the result being that the total is an exact cost without any estimation whatever, in arriving at the results. These are the actual book figures.

TABLE No. 1.

Year

Operating Tonnage Kw-hr. per ton, Power cost, Repairs and maintenance. . .

Total operating cost

($156,000) Depreciation (20 years) Total operating and fixed

Miscellaneous Total cost

1913

9 months 119,230

23.9 $.0160

0.077 0.004 0.014 0.185

0.078 0.065

0.328 0.126 0.454

1914

8 months 92,622

22.8 $0.153

0.009 0.005 0.016 0.183

0.101 0.084

0.368 0.133 0.501

1915

12 mo. 174,460

21.5 $.0144

0.004 0.003 0.013 0.164

0.054 0.045

0.263 0.115 0.378

Average or total

386,312 23.4

$0.157 0.006. 0.004 0.014 0.181

0.73 0.060

0.314 0.117 0.431

Per cent

36.40 1.40 0.90 3.25

16.90 13.95

27.20 100.00

The question of energy saved during reversal is a very interesting subject in connection with a paper of this kind. How many rollers on reversing mills in this country are paid straight time? How many are paid tonnage rates? The answer is that practically every mill is paying tonnage rates, with the result that speed is the sole question in the operators' mind. Furthermore this speed is obtained by using live steam to reverse the engine rapidly. In a motor-driven mill a certain per cent of the stored


energy in the rotating parts is saved by regeneration, and is stored in the flywheel for future use, regardless of operating conditions. The one great advantage in answer to question 4 of low power consumption with partial load is that the motor-generator set can be disconnected from the line when the mill is idle, thereby entailing no stand-by losses, such as are met with in steam installation.

The time taken for the Hamilton motor to reverse is still ahead of the handling of the metal on the tables, manipulation, screw-down, etc. That is, the motor is waiting on the mill. Since the figures given by Sykes and Hall on this mill were taken, we have developed a new governing relay which has given us considerably more positive protection and allowed more speed and, therefore, capacity. We have obtained a speed of 125 r.p.m. on our long passes which when rolling from sixteen to seventeen elongations, saves considerable time.

The advantage of an electric mill may be summed up as follows : 1. Low cost of power. 2. Low cost for repairs and maintenance. 3. Small time to get under way from complete shut-down to

rolling conditions. 4. Speed proportional to displacement of controller lever from

off position. 5. Part of rotative energy of mill parts recoverable for useful

work. 6. Stand-by losses nil. 7. Simplicity of control. 8. Few delays necessary. 9. Motor does not race when steel leaves rolls. 10. Constant turning moment. 11. Ideal load to add to any generating station. 12. Lends itself to centralization of power. 13. Simplified mill lay-out. 14. Mill breakages less. 15. Small area or ground space needed. The floor space necessary for the equipment described was

40 feet by 125 feet, which allows ample room between machines and wall and switchboard, no apparatus being cramped in any way. A 40-inch mill could easily be installed in this same area. In case of necessity the flywheel set need not be located in close proximity to the mill motor, so that in adapting a mill under extreme conditions where very little floor space was available, the flywheel set could easily be located some distance away where more area could be obtained. The real estate charges on some mills located in thickly settled communities must be considered, and in comparing this area with the area necessary for boilers, coal handling machinery, steam engines, pumps, etc. the result is good.

After the mill has been down for any reason, the time necessary for the attendants to have the entire equipment ready for


maximum rolling conditions is less than ten minutes. I t is doubtful whether a steam boiler equipment could be gotten under way from absolute standstill to running conditions in less than four hours. The simplicity of the control as compared to the levers, links and auxiliary cylinders necessary for the steam engine is very noticeable. The entire control wiring between pulpit and power house is contained in a one-inch conduit. All parts of the control are entirely accessible, and any part needing repairs can be changed in a very few minutes. Repairs have been exceptionally low, the largest item being the brush renewal. Delays in the last three years due to this equipment exclusive of the development period, have not amounted to twenty-four hours, and this period was taken up at various times more to be doubly sure that the equipment was in good order rather than take any chance.

When the mill is idle, the flywheel set can be disconnected from the line and allowed to rotate, which means that there is absolutely no loss as compared to steam equipment having to keep the boilers under steam, the steam-line condensation, small leaks, etc. When the steel leaves the roll, there is no racing, as would be the case in the steam engine run by the average operator, the motor maintaining uniform speed, corresponding to the displacement of the control lever from off position. Such complete control of the speed of the mill is ideal when steel is entering and leaving the rolls, as there is no change of speed unless the operator so wishes. The motor exerts a constant turning moment in all positions, whereas the steam engine has its maximums and zeros, every revolution. The saving due to the return of the rotative energy of the mill parts to the flywheel gives a means of saving power which is normally lost in steam-driven mills. If 60 per cent of the rotative energy of the mill motor is returned to the flywheel, 60 per cent of this, namely, 36 per cent of the whole, is available again on the mill shaft for active work.

To any plant, whether purchasing power from central station or receiving power from their own power house, the ligner system adds an ideal load due to the fact that it is a fairly constant load. If the mill is run to capacity, the power variations will be very slight. The central station load applies in the same way and lends itself, where power is being purchased on a peak basis, to a very low rate. For a large power plant, the increased load does not amount to very much. Taking as an example of this the Hamilton Mill: A 1200-kw. generator capacity would easily take care of the load. Where mills are located at various points in the plant, which, from a steam power point of view is inefficient, the linger system eliminates such inefficiency by centralization.

The exceptionally low cost of power is probably the most striking feature of this system, the figures shown being actual figures in no way having been adjusted for cost-keeping purposes. The simplicity of the mill lay-out is another feature which must be considered.


D. M. Petty: I have in mind one or two points I think it would be well to consider in comparing electrical and steam reversing mills. One of the most important points is the flexibility of lay-out. Steam engines, in order to reduce condensation losses, must be near the boiler plant, and while it is possible to locate reversing drives at any position that may be necessary, the fact that mills in recent years are being much more frequently laid out with the idea of reducing the distance that the steel has actually to travel from the soaking pit to the finished product, whatever that may be, is of considerable importance.

The commutators of reversing mill motors I think are probably the most important of electrical problems, because with the reversing mill d-c. drive the commutator is naturally the place where most trouble will be experienced, and most trouble has been experienced. This trouble is not only electrical but mechanical in a good many respects. The number of brushes on the commutator is a direct item in the maintenance charges, but the size of the commutator has a great deal to do with the mechanical troubles which may arise.

So far as the speeds of the motor itself are concerned, they are governed very largely by the speed at which it is desired to run the mill, but the speed of the generators attached to the motor-generator set is not so limited, and I think it is pretty safe to say in regard to d-c. steel-mill motors and generators that the lower the speed, within reasonable limits, the more satisfactory the operation and the lower the operating charges.

The speed of reversal has been emphasized. This should be taken into consideration, but I feel sure that electrical engineers will have no trouble meeting the requirements. It is far more important to insure reliability of operation than to obtain rapid reversal. Size of bearings, method of lubrication, size of shaft and holding of field coils and armature coils in place against heavy shocks are points that stand near the head of the list in importance, in order to make a mill drive reliable.

R. Tschentscher: I would like to correct, first, the statement made by Mr. Hall that the first reversing set was put in at Gary. The first reversing set was in operation, in December, 1905, at Chicago. It was a very small set, about 75 h.p., and the next set of a much larger size, was put in at the same place.

I think there is too much stress being laid on the question of the electrical characteristics of these equipments. As the result of my observation and my experience in the last ten years in operating one of these sets, it is my opinion that the designers should lay more emphasis on the physical arrangement, than on the time of reversal; for example, on the stiffness of the shaft, to prevent oscillations, etc., and on the size of the bearings, to put off as far as possible the time when the bearings must be renewed.

When the first set was purchased in~this country, the time of reversal was a subject which was given very careful consideration.


I think that the time of reversal was specified as three seconds. As a matter of fact, there are practically no mills,—there might be one or two sawmills—that can use a reversal of three seconds that is from full speed in one direction to full speed in the other.

Much has been made out of the rapidity of reversal of the steam engine—true, it does reverse very rapidly, but any one who has watched a blooming mill operate will be impressed with the fact that the reversal is fast, but the time consumed from the moment the steel enters the rolls and the rolls grip the piece, and the engine is started up again, is from one to five seconds, and it is a fallacy to put too much stress on the question of the time of reversal when such rapid reversals cannot be used. The piece has to be manipulated, the screw-down operated, etc. The time required for such operations determines the time required for motor reversal.

Any one about to purchase an outfit always has the question of the relative merits of the reversing drive versus the continuous mill, to decide. Efficiencies that may be guaranteed at full load, to me are more or less valueless. What may be called the capacity factor, the average yearly input to the capacity of the outfit, is very low, indeed. I think in continuous mills it will be found that this capacity factor will vary from 15 to 40 per cent and in the reversing mills from 30 to 70 per cent. Figures I have taken show, in a reversing mill operating for twenty-four hours on ,the basis of the input to the motor of the motor-generator set, divided by the kw. ratio of that motor, that 60 per cent is extremely rare. On that basis, it appears to me that we ought not to spend too much time in attempting to get the last bit of efficiency, but rather get reliability physically, so that when the demands for the mill are increased in times of high pressure and high prices, the outfit will then respond.

H. D. James: I think that we all appreciate that the development of the electrically-driven reversing mill is another opportunity to "do it electrically." We all of us believe in electric power, many of us believe in central station power. The use of the electric rolling mill has enabled us to occupy another field with our motor applications.

We have learned a great deal in the past ten years about this reversing drive, and I want to emphasize Mr. Tschentscher's remarks that the-time of reversal is not the main essential. It was my privilege to assist in developing the drive of which Mr. Tschentscher spoke, and I wish to add that, he himself, did a great deal to make that drive a success. We started out with the idea that time was one of the most important points, and we ended with other ideas. The tendency now seems to be towards a little more time, and a little more substantial mechanical construction.

F, G. Liljenroth: Having been very closely connected with the European practise of reversing rolling mills during the past ten years, it is with the greatest interest that I have read the


above paper. As Messrs. Sykes and Hall point out, there is quite a difference between the practise in Europe and in the United States, the most noticeable differences being as follows:

REVERSIBLE MOTOR It seems to be the general practise in the United States· to

divide the motor, especially the larger sizes, into two units on the same shaft, while the European practise, at least as far as the leading electrical manufacturing companies are concerned, is to build the motor as a single unit, even for the very largest capacities. So, for example, there are to be found in Europe several reversible motors of the same size, that is, with the same torque as the Bethlehem steel motors, which are built in single units, and as far as I know, there is at least one single unit motor built which has a maximum torque of 240 meter-ton—1,750,000 ft-lb.

The advantages of using single-unit motors are evident : lower first cost and better efficiency, besides the advantages of having to deal with only one commutator, which part is always the weakest in a direct-current machine.. Such a single commutator need not be larger than each of the commutators of a two-unit motor, inasmuch as the voltage which is used is as high as from 1000 to 1500 volts and consequently the current is the same as at 2 X 600 volts. There seems to be no disadvantage in using a single unit motor instead of two. It is true that on account of the higher operating voltage the potential between the brush edges (or rather the voltage which would appear between the brush edges on account of the armature reaction, if this wasnot compensated for by the commutating poles) is higher than if the machine were, divided into two parts, but by correctly determining the ratio of the tangential width of the brush to the commutator pitch, etc., it is possible to limit said potential at maximum peak load to less than 2\ per cent of the operating potential, that is, at 1200 volts to less than 30 volts which is allowed even according to American practise, as stated by Mr. Lamme in his very excellent A. I. E. E. paper dealing with the Commutation of Direct-Current Machines. There are, of course, just as here in the United States other provisions made in order to completely compensate for the potential between the brush edges under all conditions of load. For example, these motors are always equipped with compensating winding, as well as commutating poles, the winding of the latter being placed as close as possible to the armature in order to reduce the leakage field of the commutating pole to the lowest possible value. Furthermore, the commutating poles are tapered and have the same axial length as the armature. In order to avoid a time lag at rapid load fluctuations between the armature current and the commutating field, it has been found advisable to make the commutating poles of laminated iron. The air gap of these poles is made very large, at least one inch, while that of the main poles


is usually about 3/16 in. The armature winding is furthermore provided with numerous equalizing connections, at least one for each slot.

With the above design it has been found that the motors will operate absolutely sparkless, even for the highest peak loads and in watching the commutators of several such machines, they have operated so perfectly that it was impossible to determine whether they carried a load or not. This applied even to peak loads where thenon-compensated potential between the brush edges would have been approximately 40 volts. The average voltage between the two commutator bars, that is, the operating voltage divided by the number of commutator bars between two brush positions does not, for such machines, exceed 20 volts which must be considered a conservative value.

GENERATORS It is the general European practise to use two generators of

from 500 to 750 volts (usually 600 volts) connected in series and consequently the same practise as in the United States. For the newest installations the speed is, however, considerably higher than was the case some years ago, and which still seems to be maintained in the United States. But this, only refers to the modern installations which were completed shortly before and after the beginning of the war. Sufficient time has elapsed since these motor-generator sets were installed and the results obtained have clearly demonstrated that such speeds are entirely satisfactory and safe, and, as far as Europe is concerned, they have caused a revolution in the design of such machines. The first cost is considerably lower, while the space required is also much lower and the efficiency higher. Comparing such a flywheel motor-generator set with the older designs, one is immediately astonished by its small dimensions. The Bethlehem steel set could have had a speed of at least 514 instead of 375 revolutions, that is, at 60 cycles the induction motor should have had 14 poles.

With the higher speed the motor generator can be built much smaller and this particularly refers to the flywheel. For a certain WR2 it is evident that the mass of the wheel and consequently also its weight can be the same, independent of what speed is chosen, if only the peripheral speed is maintained constant. In most cases, however, the diameter of the flywheel is limited by shipping facilities to about 4 meters—13 ft. and the peripheral speed by the permissible stresses at maximum runaway speed, this usually being taken as 25 per cent above normal speed. The stresses should in no part in the wheel at its runaway speed be permitted to exceed 10kg. per sq. mm. or about 14,000 lb. per sq. in. which values at normal speed would correspond to about 9000 lb. These values are, as seen, very low and conservative, and it is not necessary to use any especially expensive material in the wheel, but only ordinary


cast steel with an elasticity of 40,000 and an ultimate strength of approximately 70,000. Regardless of these low stresses, it is possible, by using a suitable design of flywheel, to go to a peripheral speed of as high as 430 feet per second and still not exceed a stress of 14,000 lb. per sq. in. at runaway speed. From the above it follows that the lowest speed which should ever be used for such a flywheel should be :

430 X 0.8 X 60 κππ 0 w = 500 rev. per. mm.

l o X 7Ã

The largest generator which can be built at this speed has a maximum load, that is, peak load, of about 4300 kw., this being under the assumption that the peripheral speed of the armature is not to exceed 150 feet per second and the average voltage between the two commutator bars not to exceed 20 volts and that at peak load, amperes X armature conductors per cm. circumference is about 800.

The Bethlehem motor requires approximately 8500 kw. and it follows, therefore, that its motor generator could readily have been built for 500 revolutions with two series connected generators each for 600 volts. The weight of the flywheel could

( 375 V -£7γτ ) or to approximately one-half and the flywheel effect would still have been retained. This is under assumption, that the diameter of the Bethlehem flywheel is about 13 feet. It is possible, however, that there are other reasons for using the low speed and it would be interesting to obtain information in regard to :

1. The largest permissible diameter of the flywheel limited by transportation facilities.

2. The percentage of runaway speeds which are usually specified.

3. The corresponding permissible stresses in the flywheel as well as the properties and the material which is used.

4. The corresponding peripheral speed whose relation to the stresses is dependent on the design of the flywheel.

EXCITATION As far as the excitation is concerned, the European practise

comprises what is known as the "indirect" regulation, that is, the rheostats or controllers are not inserted in the generator or motor fields, but in the fields of the exciters. The exciters are used, one for the generators and one for the motor, the voltage of the exciters being regulated. The exciters are, therefore, in turn magnetized from a third exciter which consequently can be very small, working normally as a shunt generator with constant potential. These three machines are driven by a common small induction motor.

The advantages of this scheme are: smaller equipment,


lower first cost and especially greater reliability as far as the rheostats and controllers are concerned, in that the currents which have to be handled are only from one to two per cent of what would be the case if the "direct" excitation were used. For rolling mill service it is evident that the control equipments have to withstand very severe service both electrically as well as mechanically on account of the rapid breaking of comparatively large magnetic energies and also due to the fact that they are in almost continuous service. The advantages of smaller equipments which are obtained by the smaller apparatus used with indirect excitation are, therefore, obvious. The excitation will be just as rapid as by the direct method, on account of the fact that the magnetic energy stored in the small exciter fields is negligible compared to the large machines.

The compounding is usually made in the way that the main poles of the generators have an opposing series winding which therefore does not have to be reversed. Against this practise it may be argued that an assisting compound winding on the motor would be better, inasmuch as the torque will be increased by strengthening the field. This, however, is more or less an imaginary advantage, as for machines of this kind the saturation curve at normal speed is so flat that a large increase in the field current creates a very small or almost no strengthening of the field.

Concerning the safety devices and the arrangement of the automatic load regulation, etc., the European practise involves several quite interesting departures from the practise in the United States, but space does not permit of further discussion of this subject.

Brent Wiley: It would be well to consider the blooming mill equipment from the standpoint of the entire mill drive, as practically every steel plant is composed of finishing or semi-finishing mills, as well as blooming mills.

During the last eleven years more than three hundred large motor units, totalling over 500,000 h.p., have been installed to drive the main rolls of steel mills.

The following list gives the electrically-driven reversing blooming mills installed, and on order in the United States and Canada. The majority of the equipments have been purchased during the last few years.

Installed — Blooming Mill—Algoma Steel Co. Sault Ste. Marie, Can. 1911 34 in. " . " —Steel Co. of Canada, Hamilton, Ont. 1913 34 in. « " —Central Steel Co., Massillon, Ohio. 1914 35 in. " " —Bethlehem Steel Co. Bethlehem, Pa., 1915 35 in. " " —United Steel Co., Canton, Ohio, On order. 40 in. " " —Inland Steel Co., Indiana Harbor, " " 32 in. " " —Inland Steel Co., Indiana Harbor, " " 40 in. " " —Illinios Steel Co., Gary, Ind. 40 in. " " —National Tube Co., Lorain, Ohio " " 34 in. " « —Chattanooga Steel Co., Chattanooga " " 35 in. " " —Mark Mfg. Co., Indiana Harbor, 34 in. " " —Ashland I. & M. Co., Ashland, Ky. " 34 in. " " —Keystone Steel & Wire, Peoria, 111. * u


REVERSING MOTOR EQUIPMENTS—FOR OTHER TYPES OP MILLS.

Installed 30 in. Universal Plate Mill 111. Steel Co., S. Chicago, 1908

Plate Mill—Am. Sheet & T.P. Co., Gary, Ind. . 1910 « — ditto 1910

28 in. Structural Mill—Inland Steel Co., Indiana Harbor, On Order 27 in. Universal Plate Mill Mark Mfg. Co., Indiana Harbor

The principal object of the earlier installations was to utilize cheap power and the incidental advantages possible were not taken into account. Experience has demonstrated that motor drive has many favorable features which are of economic value in not only the every day operation of the mill, but assist most materially in the development of the method of operation.

Motor drive gives the greatest latitude regarding the arrangement and design of mill and of the entire plant. Motors can be designed with either high, intermediate or low speed, and with a wide variation in maximum or pull-out torque. Adjustable speed motors give a wide range of operating speeds, with very economical operation of mill, either for constant torque or constant horse power requirements. The regulation of the alternating-current motor is very close, even under a wide range of load, varying only about two and one-half per cent from light to full load. It is capable of standing heavy overloads frequently for comparatively long periods, without undue strain or deterioration. The ease with which power readings can be made instantaneously and for any desired period is of great value in compiling records to ascertain the effect and value of any change that is inaugurated in the development of the mill design.

The general experience in the operation of a new mill is that the results obtained, after a few years, are radically different from the ideas of the possibilities and expectations of what could be accomplished when the design was first conceived. In the majority of cases it is necessary to make a number of assumptions regarding the possibilities of each particular portion of the mill, including capacity of heating furnaces required, permissible reductions per pass, speed of rolling, size and shape of product, most desirable from the standpoint of trade demands. The mill is developed as more information is obtained regarding these factors, and the incidental advantages of motor drive have played an important part in accomplishing the most satisfactory results in reversing blooming mills, as well as other types of rolling mills, as the general advantages are the same for all types of motor equipment.

FIRST COST It is assumed that the plant is designed with motor drive for

the blooming mill, finishing mills, and for the auxiliaries. The average load on the electric power station to drive a re

versing blooming mill with maximum peaks of 15,000 to 20,000 h.p. is approximately 3000 to 3500 h.p., with variations of not more than 15 per cent during the active rolling period of the


mill. The station capacity will be divided approximately 80 per cent for motors of auxiliary machines and finishing mills and 20 per cent for the reversing mill equipment. In other words, it will require a comparatively small increase of power plant to provide for the blooming mill work which is quite a contrast with the requirements in an addition to a boiler plant for steam engine-operated blooming mill.

Furthermore, the addition of a very uniform load assists in; equalizing the total plant load, and thereby increases the efficiency of operation.

The use of central station power is also a very material factor in reducing the first cost, and many steel plants are taking advantage of this point as well as of other favorable factors which purchased power affords. At present, there are more than fifty steel plants obtaining part or all of their electric power from central stations, and are using a total of approximatley 425,000,-000 kw-hr. per year, which is approximately 18 per cent of the total electric power required per year by the iron and steel industry.

ECONOMY OF OPERATION There is more economy of operation to be gained by the elec

trification of the reversing blooming mill than of any other type of mill. In the case of the averaging existing steam engine-driven mill, the steam consumption can be reduced fully fifty per cent by use of motor drive. This comparison is made on the basis of using electric power as furnished by steam turbines at approximately 5000-kw. capacity. Undoubtedly there are many cases where the gain would be even greater than this, but on account of the limited test data available regarding the steam requirements and general analysis of plant conditions, it has been difficult to establish these facts definitely.

H. S. Page: Analysis of the sequence of operations during rolling might serve to bring about closer unanimity of opinion in regard to the permissible time of reversal. The actual reversal occurs during the time the metal is out of the rolls between passes: is from a comparatively low speed in one direction to about the same, or lower speed in the opposite direction, and can easily be accomplished while the metal is being returned to the rolls. After the metal enters the rolls it is of the utmost importance to have a driving motor capable of accelerating the mill to the maximum speed of the pass just as quickly as possible. The retarding action of any device installed for the protection of the motor should be carefully considered ; as the function of most of these auxiliary devices is to prevent rapid acceleration and thus limit the output of the mill.

A few comments might be added on the subject of general design of reversing mill apparatus which is treated at some length in the paper. Generally speaking it is permissible to sacrifice efficiency to a slight degree in order to gain more rapid acceleration and for this reason it seems advisable to work all


magnetic and current carrying material in the motor armature at the highest possible densities in order to reduce the stored energy to a minimum, the voltage being determined by the most economical arrangement of armature conductors and commutator segments. A decided advantage is gained by designing the motor with a view to cooling by means of forced ventilation. Valuable protection of commutators and brushes from the sharp steel mill dust and grit, as well as insurance from burnouts caused by deposits of this same material can be obtained by blowing sufficient thoroughly washed air through the motor for the complete ventilation of the dynamo room. As mentioned, before, it would seem advisable to let the voltage of these equipments be fixed by conditions governing the design of the motor rather than choose a voltage best suited to the generating set. The speed/voltage curves submitted with the paper apply fairly well to present day design but as the tendency is toward higher speed generating apparatus it is questionable whether they will apply a few years hence.

The rapid fluctuating speed and load conditions required by the cycle of operation of this type of equipment makes the question of rating, especially of the driving motor a rather vexed one. A very convenient working basis for heating can be taken as the maximum continuous safe load capacity at full field on the motor and maximum continuous generator voltage. If some such rating is established and the heating properly calculated from the rolling cycle for each installation much of the trouble which has been met in the past with electrically driven rolling mills will be avoided. Of course in addition to this it is necessary to make proper allowances for the range in load and speed as well as to carefully consider the mechanical stresses which are brought about by the rapidly fluctuating load conditions.

Peter Lindemann: There is one point in my mind which the schematic diagram shown in Fig. 3 of the paper does not make entirely clear.

It may be that there are other controlling devices used which are not shown in this diagram, and which will prevent the starting of the direct-current motors while the generator field rheostat is in neutral position, for it seems to me that the shifting of the brush holders on one of the direct-current separately excited generators would cause voltage to be built up on the closed motor circuit.

It would be interesting to know what precaution, if any, has been provided against such an occurrence.

Alex. Gray: One of the previous speakers brought up the question of the speed of the flywheel motor-generator set, and stated that whereas we use a speed of 375 rev. per min., European engineers have gone up to 500 rev. per min. for the same horsepower output. To me this would indicate one thing only, namely, that Eurpoean engineers work much closer to the limit than we care to do on this side.

1916] . DISCUSSION AT CLEVELAND 1489

I once made the remark that there were limitations in the design of electrical machinery, and was told by a well known operating engineer that such limitations did not exist, and that as soon as the operating engineer demanded anything, the" designing engineer found a way to overcome his limits and supplied the demand.

Mr. Hall in his paper has drawn attention to the fact that for each speed there is an output rating that cannot be exceeded unless the engineer is looking for trouble. This may be explained as follows : Taking a machine of given diameter, there is a safe maximum speed at which this machine may be run, and the output is then limited only by the length of the armature core. As this length is increased, the voltage generated in each turn of the armature also increases until, when a value of about 6 volts between adjacent segments is reached, interpoles become necessary. With inter-poles supplied, the machine may be further lengthened until, when a value of about fifteen volts is reached between segments, the machine becomes sensitive to changes in load and is liable to flash over. Compensating windings must then be supplied«, if the output is to be further increased.

Evidently, then, the 500-rev. per min. European machine is run either with a greater peripheral velocity, or with a greater value of voltage between segments than in the case of the 375-rev. per min. American machine. From the fact that they are designed closer to the limit, much more care is necessary in the construction of the apparatus, so that the machine is not necessarily cheaper.

In regard to the rating of the motors, it must be clearly understood that while the motors in question have a rating of 10,000 h.p., at 100 rev. per min., they would become hot with a continuous output of about 4000 h.p. On the other hand, if the machines were large enough to dissipate the loss corresponding to a 10,000-h.p. load, they would have become so long as to operate badly with regard to commutation.

It is, therefore, rather hard to decide how they should be rated, because they are designed as 10,000-h.p. machines so far as commutations is concerned, and as 4000-h.p. machines so far as heating is concerned. Since the former is the more important limitation of the two, it seems reasonable to give them a rating of 10,000 h.p. maximum.

With regard further to the rapidity of reversal of the generator voltage, it is of interest to note that, since the machines have compensating windings, the air-gaps can be small, and the shunt excitation be almost negligible. It is, therefore, quite within reason to put a non-inductive resistance in series with the field coil circuit so as to reduce the time constant of this circuit and thereby allow the field to build up rapidly; and this would not reduce the efficiency of the machine to any appreciable extent, and, moreover, would allow the rate at which the machine builds up to be adjusted. It would seem from the discussion, however, that the machine reverses as rapidly as the mill engineer desires,

1490 ELECTRICALLY DRIVEN REVERSING- MILL [June 27

Wilfred Sykes: The discussion by Mr. Pauly indicates lack of experience with this type of apparatus. The question of rating has-been given considerable thought by engineers designing reversing mill equipments, both here and abroad, and it has been the practise to rate reversing mills on the maximum operating peak loads as it is these loads that fix the size of the apparatus, and not the continuous capacity based on heating. These loads do not represent the absolute maximum capacity of the machines without injury, but are the peaks that are ordinarily carried during the operation of the mill.

Mr. Pauly's remarks regarding the similarity of the reversing rolling mill and the mine hoist motors are correct as far as the general system used is concerned, but the operating characteristics of the reversing rolling mill motor, and the control problems are entirely different, and much more difficult than those of the mine hoist. The shocks which the reversing mill motor is subjected to are very much more severe than those of the non-reversing mill motor where the conditions are comparable, such as, for instance; the three-high blooming mill compared with the two-high blooming mill. In such cases the continuously running motor has a flywheel between it and the mill, and the shocks on the parts of the machine are very much less than with the reversing motor where the flywheel effect is reduced to a minimum.

Mr. Pauly's remarks about the speed of operation are answered very well by the discussion of Mr. Tschentscher. The same opinion was held ten years ago regarding the necessity for high rate of speed change, but after practical experience obtained with electrically driven mills it was obvious that extreme steps were not necessary to obtain high rate of reversal, and when Mr. Pauly has gained some actual experience with these mills, no doubt, his views will change.

The question of size of motor shaft, and upon which Mr. Pauly places so much stress, seems to be catering rather to the old idea of making a part massive without the appreciation of the conditions under which it operates. It is obviously useless to make the shaft many times stronger than some other part of the machine which is just as liable to be broken, and in designing equipments it has been our object to obtain the accumulated experience of motor and engine builders for this type of mill. A shaft diameter out of all proportion to the output of the equipment, or the size of the mill may be an excellent talking point in selling apparatus to an ignorant customer, but is not good engineering.

Regarding the discussion of Mr. Petty. It is not feasible to reduce the speed of the generator below certain limits, due to the fact that the flywheel diameter is limited by transportation facilities, and the flywheel effect necessary might mean prohibitive weights with speeds too low for economical design.

Mr. Lilgenroth brings up a number of points regarding the


design of European compared with American reversing mills. The question of using one instead of two motors for the large equipments has received a great deal of consideration, and it was with a full knowledge of the European practise that the equipments now being built were designed with double motors. The question of safety to attendants is one on which a great deal of stress has been placed by mill operators, and there was a very great objection to the use of voltage higher than 600 across an individual machine. In the United States we must also face less skillful attendants, and the use of a 600-volt machine with 1200-volt insulation, such as the Bethlehem machine is, gives us a greater factor of safety than would be possible if we used single motors with, say, 1500 volts on the windings. The design features brought out, having been found desirable in Europe, are all incorporated in the machines which have been built.

The question of generator speed is one that has been given a good deal of thought, and it might be of interest to mention that the 500-revolution generators running at the Steel Company of Canada's plant repeatedly carry loads of 5000 kw. each, and it is quite possible to build generators for 500 revolutions to supply power to the motors at Bethlehem. On the other hand, using a speed of 375 r. p. m., there is greater leeway, and a greater margin could be obtained to cover the uncertainty of operation which was a factor on the Bethlehem installation. This mill was installed for rolling a very wide range of products, and different classes of steels, and it was practically impossible to predict before hand just how much power would be required. For this reason a conservative design was adapted, the wisdom of which has been shown by the operating conditions of the mill since installed.

Regarding the question of flywheels, we have found it desirable to limit the operating peripheral speed to approximately 300 ft. per sec, although the stresses at this speed are very low. This is due to the uncertainties as to the quality of metal obtained in castings. The conditions in the United States are very different from those in Europe where cast steel wheels, for higher speeds, can be readily obtained.

Regarding the question raised by Mr. Lindemann, provision is made in the controller for the generator fields to prevent the motors being started in the way he mentions.

Documents

Discussion on “electric drive for reversing rolling mills” (Wilfred Sykes and David Hall), Cleveland, Ohio, June 27, 1916. (see proceedings for June, 1916)