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To be Presented at the 33d Annual Convention of the American Institute of Electrical Engineers, Cleveland, O., June 27-30, 1916.

Copyright 1916. By A. I. E. E. (Subject to final revision for the Transactions.)

ELECTRIC DRIVE FOR REVERSING ROLLING MILLS

BY W I L F R E D SYKES AND~DAVID HALL

ABSTRACT OF PAPER The manner in which the electrically driven reversing rolling

mill has been adopted especially within the last year, is surprising in view of the strongly entrenched position of the steam driven mill. Electric motors have been used for many years on mills running continuously in one direction, but many motor users have felt that the reversing mill· could be better handled with the steam engine. There are naturally many characteristics little understood, due to the limited use in this country today.

This paper answers some of the questions which are raised and describes the constructions that have been found desirable.

ÐÃÇÅ ELECTRICALLY driven reversing mill has been the * subject of a number of papers* before the Institute in

which the general scheme of operation has been described in detail. Since these papers were presented this type of mill has been considerably developed and a number of installations made. In addition, a great many new mills are being equipped, and within the next year there will be 15 reversing mills in operation in the United States. The great success that has been attained appears to warrant a review of this subject together with a discussion of some of the characteristics of this equipment.

Since the first installations were made and mill engineers have been in a position to personally check the operation and economy of equipment, the steam engine for reversing mills has been comparatively neglected. As an indication of the position that the electrically driven mill has attained, the engineers of one of the large steel companies upon making investigation regarding the type of drive to install for new reversing mills, stated that the electric drive would undoubtedly in the very near future entirely supplant the reversing steam engine except

�Electrically Driven Reversing Mills, by Wilfred Sykes. A. I. E. E. TRANSACTIONS, 1911.

Operation of a Large Electrically Driven Reversing Mill. By Wilfred Sykes, A. I. E. E. TRANSACTIONS, 1912.

Electrification of a Reversing Rolling Mill of the Algoma Steel Co. By B. T. McCormick, A. I. E. E. TRANSACTIONS, 1912.

Manuscript of this paper was received April 14, 1916. 739

740 SYKES AND HALL: [June 27

in perhaps certain peculiar cases. Practically all the new installations of reversing mills contemplated at present will be electrically driven. Although the electrically driven mill has not so far made the advance in this country that it has in Europe, it is characteristic of American practise to quickly adopt any device which has been demonstrated to suit the American conditions. The reversing mill as developed in this country and as shown by the existing successful installations, differs in many respects from European construction. Special attention has been given to the mechanical construction of the reversing motor and every care has been taken to insure that the machine will stand the much rougher handling which it receives in this country.

As pointed out in one of the papers previously read before the Institute, the reversing plate-mill drive installed at the South Chicago plant of the Illinios Steel Co. was the second drive of this type to be put into operation in the world and it was designed without knowledge of the fact that a similar arrangement was being constructed in Europe. It was a number of years later before a reversing blooming mill was electrified.

The first successful installation of a reversing blooming mill was that of the Steel Company of Canada at Hamilton, Ont. This installation consists of a double reversing motor capable of developing about 10,000 h.p. maximum, and is supplied with power from a flywheel motor-generator set with two generators. The complete electrical installation is shown in Fig. 1. This mill has been in operation for over three years with very satisfactory results. It is at present working at a rate very considerably in excess of the capacity specified when it was installed. The following are particulars of the mill and driving equipment.

Size of ingot 15 by 17 in. Weight 4000 lb. Finished material 4 by 4 in. Elongation 16 " Number of passes 19 Capacity, tons per hour 60 Roll diameter 30 in. Pinion diameter 34 in. Speed, full motor field 70 rev. per min. Speed, weakened motor field 100 rev. per min. Driven from motor direct Number of motors 2 Voltage across each a rmature 600 Maximum operating torque 900,000 ft-lb'. Maximum motor horse power 10,000 Number of generators 2 Ra ted power of driving motor of set 1800 h .p . Weight of flywheel 100,000 lb. Speed of flywheel set 500 rev. per min.

PLATE XIV. A. !. E. E.

VOL. XXXV, NO. 6

Y...

ISYKESJ F I G . 1 — G E N E R A L V I E W OF F L Y W H E E L M O T O R G E N E R A T O R AND

R E V E R S I N G M O T O R INSTALLED AT THE P L A N T OF THE S T E E L COMPANY OF CANADA.

LSYKES]

F I G . 2 — R E V E R S I N G M O T O R B U I L T FOR B E T H L E H E M S T E E L COMPANY ASSEMBLED IN S H O P .

1916] BLECTRICALLY DRIVEN REVERSING MILL 741

The largest installation at present in operation is that of the Bethlehem Steel Co. which drives the 35-in. blooming mill at the Lehigh Plant. Fig. 2 shows the motors as assembled in the shop before shipment. Both of the above mentioned installa-

FIG. 3—SCHEMATIC DIAGRAM OF CONNECTIONS OF LARGE REVERSING MILL DRIVE

OCB —oil circuit breaker with no-voltage and overload trip SR —automatic liquid slip regulator A CM —alternating-current wound rotor induction motor DCG —direct-current separately excited generators DC M —direct-current separately excited roll motors CB —circuit breakers—1 generator field—2 main circuit R —relay for operating circuit breaker in generator fields FC —field controller F —flywheel SE —shunt exciter for generator and roll motor fields SeE —roll motor exciter the field of which is separately excited by the main d-c. circuit S AC M—alternating current squirrel cage induction motor V —voltmeter A —ammeter W —wattmeter

tions have double motors due to the amount of power required. The machines are arranged as shown by diagram, Fig. 3. A somewhat similar drive is installed at the plant of the Central Steel Co., Massillon, O., but a single motor is used for driving the mill, the capacity of the motor being approximately 8000 h.p.


This motor is shown in Fig. 4, which illustrates the machine as installed for driving the mill. Characteristics of these mills are as follows:

Bethlehem Central Steel Co. Steel Co.

Size of ingot 19 by 23 in. 18 by 20 in. Weight 10,000 lb. 5,000 lb. Finished material 4 by 4 in. up 4 by 4 in. up Elongation 10-12 av. Up to 20 Number of passes 17-21 19-21 Capacity, tons per hour 100 60 Roll diameter 30 in. 30 in. Pinion diameter 35 in. 34 in. Speed, full motor field 40 50 Speed, weakened motor field... 120 120 Driven from motor direct direct Number of motors 2 1 Voltage across each armature . . . . 600 700 Maximum operating torque 1,550,000 ft-lb. 750,000 ft-lb. Maximum motor horse power. . . 12,000 8,000 Number of generators 2 1 Rated power of driving motor

of set 2,000 kw. 1,500 kw. Weight of flywheel 100,000 lb. 60,000 lb. Speed of flywheel set 375 rev. per min. 375 rev. per min.

In the recent installations the reversing motor is arranged to have the characteristics of a compound machine. This is obtained indirectly through a series exciter. The current to be handled in the main circuit may be as high as 10,000 amperes, and it is obvious that it would be extremely difficult to reverse the series field each time the motor is reversed, which would be necessary to keep both fields in the same direction. A series exciter is therefore used, the voltage of wjiich is proportional to the current flowing in the main circuit. The armature circuit of the series machine supplies a separate winding of the field of the motor which may be readily reversed when the direction of rotation current is changed. The switches for reversing this field are operated from the same point on the master switch that reverses the field of the generator. The use of a motor with compound characteristics makes the operation of the mill a good deal easier on the mechanical equipment as the drive has more or less "give" to it. At the same time if there is an extreme load due to excessive draft or cold steel, the motor characteristics tend to compensate by automatically increasing the torque available and decreasing the speed.

Although the electrically-driven reversing mill has been practically adopted universally for all new installations there is still some misapprehension as to its operating characteristics.

PLATE XV. A. I. E. E.

VOL. XXXV, NO. 6

1

1

i W i ~~Γΐ̂ ~

| j J r -- i-J-

J I

J r ^ 1 1

Is..

[SYKES]

F I G . 4 — R E V E R S I N G M O T O R D R I V I N G BLOOMING M I L L OF C E N T R A L S T E E L COMPANY

[SYKES] F I G . 12—COMPENSATING AND COMMUTATING P O L E W I N D I N G S AS U S E D

IN R E V E R S I N G M I L L M O T O R S

1916] ELECTRICALLY DRIVEN REVERSING MILL 743

It is of course natural that engine builders will fight the development of the electric reversing mill drive as much as possible and the following advantages have been claimed by one of the prominent engine builders:

1 First Cost. The first cost of the reversing engine is only a small fraction of the aggregate cost of an electric drive (steam turbines, generators, converter sets, motors, field controls and auxiliaries).

2. Cost of Operation. The modern reversing engine uses no more steam to do the work required than an electrical drive.

3. Energy Saved During Reversal. In a properly designed engine and mill all of the energy required for acceleration early in the pass is utilized at the end of the pass ; while with an electric drive, due to the heavy rotating masses, only part is saved.

4. Low Power Consumption with Partial Load. High economy is obtained at partial load because a properly designed engine works with cut off. Low pressure control valves prevent all racing and speeding.

5. Greatest Economy of Time. A modern reversing engine accelerates in less time than will ever be possible with a motor on account of the smallness of the rotating masses of the reversing engine."

As these are points that can be directly answered from data already available on electrically driven reversing mills in the United States, the points are taken up in the order given.

1. The first cost of an installation does not consist only of the cost of the engine or of the motor driving the mill. In the case of steam drive there are a great many items to be considered which include boiler plant, coal and ash handling facilities, coal storage yards, steam piping, condensing system, water supply for the condensing system, and foundations. In the case of electric drive in addition to the reversing motor there is the flywheel motor-generator set to supply power to it and the generating equipment consisting of power house with its complete equipment, or if power is purchased the only items to be considered are the motor-generator set, reversing motor and the small amount of control apparatus. Many of the items entering into the cost of the drive depend upon the particular layout of the plant. For, instance the whole plant layout might have to be modified so as to enable boilers to be located within a reasonable distance of the steam consuming engines, and this


very often seriously restricts the arrangement of the mills and other units. I t may cost a very considerable amount of money to supply water to the engine condensers, which must be used if reasonable economy is desired, whereas in the case of generating station it would naturally be located close to the water supply. This would also be the natural location if blast furnaces are installed, as in this case the boilers would be close to the blast furnaces and the blast furnaces will of course be close to the dock on which the ore is unloaded, if water transportation is used. In any case the blast furnaces would be located close to the water supply which is also desirable for the boilers. It is of course immaterial from the distribution standpoint where the generating equipment is located. This is not so with the steam driven plant due to the length of piping and the consequent losses. The statement that the first cost the reversing engine is only a small fraction of the aggregate cost of an electric drive is certainly not correct as will be shown by the following figures. These figures are based on the actual installation cost and while some of the items would undoubtedly have to be modified to suit different locations, these figures give some idea of relative costs of the equipments.

C O S T OF E Q U I P M E N T FOR D R I V I N G 40 IN. BLOOMING M I L L TO R O L L 60,000 T O N S OF S T E E L PER M O N T H , 24 BY 24 TO

8 BY 8 IN.

Electric drive with purchased power. Complete cost of reversing motor, flywheel motor

generator set, exciters and control equipment . . . . $185,000 Foundat ions , wiring, etc 10,000

Total $195,000

Electric drive with power generated at plant. Complete cost of reversing motor, flywheel motor

generator set, exciters, and control equipment.. . . . $185,000 Foundat ions , wiring, etc 10,000 Proport ion of power house cost, 2500 kw. a t $50 per

kw 125,000 Transmission and outside wiring 5,000

Tota l $325,000 Steam Drive.

Compound reversing engine $125,000 Condenser, exhaust piping, including pumps 25,000 Foundat ions 10,000 Boilers, 2500 h.p., including stokers, coal and ash

handling plant a t $30 per h.p 75,000 Steam piping with covering, valves, etc 15,000 Water tunnel for condenser with discharge 8500

gallons of water per minute 50,000

$300,000


2. The statement that the modern reversing engine uses no more steam than the electric drive indicates the lack of knowledge of what the electric drive requires. So that there can be no misunderstanding on this point, in Tables I and II arepro-

TABLE I.—STEAM CONSUMPTION OF REVERSING STEAM DRIVEN BLOOMING MILL

POUNDS OF STEAM I-ER TON

No.

A B C D E F G H I J ê L M N

Size

Ingot

20 by 22 in. 20 " 22 in. 20 " 22 in. 20 " 22 in. 20 " 22 in. 20 " 22 in. 20 " 22 in. 20 " 22 in. 20 " 22 in. 20 " 22 in. 18 " 32 in. 18 " 32 in. 19 " 46 in. 19 " 46 in.

Bloom

7 by 6 in. 7 " 6 in. 7 " 6 in. 7 " 6 in. 7 " 6 in. 7 " 6 in.

7} " 3} in. 7} " 31 in.

11} " 3 in. 11} " 3 in. 23} " 4* in. 23} " 4 i in. 36* " 4} in. 3 6 | " 4} in.

Elongations

9.04 9.04 9.04 9.04 9.04 9.04

15.1 15.1 10.75 10.75 5.13 5.13 4.63 4.63

Lb. steam per ton

587 490 497 520 518 575 767 610 694 625 522 423 356 292

Remarks

Cold ingot Hot ingot Good rolling New engineer New engineer . Bad rolling New engineer. Good manipulation New engineer Good manipulation Good rolling-cold Good rolling-hot Bad lolling Good rolling

TABLE II.—STEAM CONSUMPTION OF REVERSING STEAM DRIVEN BLOOMING MILL

Size

Ingot

20 by 22 in. 20 " 22 in. 20 " 22 in. 18 " 32 in. 16 " 32 in. 19 " 46 in. 18 " 32 in.

Bloom

11} by 3 in. 7} " 3}in. 7 " 6 in.

23} " 4* in. 29 " 5 in. 36J " 4 } in. 23è " 3 in.

Numbei of elonga

tions

11.5 15.0 9.0 5.0 3.25 4.75 7.5

Lb. steam pei ton

643 600 495 420 280 300 410

Lb. steam per ton at

5-Elong.

444 375 350 420

256

9-Elong.

591 505 495

duced the figures from a paper read before the Engineers Society of Western Penna, by Mr. Karl Nibecker giving the results of tests on a reversing engine. This engine is one of the most modern installed in the United States and comparison between it and the electrically driven mill can be justly made, i t will be noted


that these are tests of single ingots, but Table I shows the results of a series of six ingots rolled from the same size bloom from which a fair average can be obtained. Table III gives the results of tests made upon electrically driven reversing mills which are shown graphically in Fig. 5. These figures are not the results

TABLE III . ELECTRICALLY DRIVEN REVERSING MILL.

Ingot

18 in. round 18 by 20 in. 18 " 20 in. 18 " 20 in. 17 " 15 in. 20 " 20 in. 20 " 20 in.

Bloom

7 | by 7i in. 3 " 8 in. 2 " 16 in. 4 " 4 in. 4 " 4 in. 5 " 5 in. 8 " 8 in.

Elongation

4.66 12.2 9.2

18.5 16. 16. 6.25

h.p-hr. per ton.

11.4 23 19.4 26 24 25.5 17.

Remarks

High carbon " "

Soft steel

of tests of individual ingots but are based upon the power consumption of a large number of ingots rolled during the regular operation of the mill and they do not in any way represent figures made under ideal test conditions. They have been obtained by reading the watt-hour meter in the line supplying power to the reversing mill equipment including all losses and

TIMES ORIGINAL LENGTH TO WHICH STEEL IS ELONGATED

FIG. 5—CURVE SHOWING RELATIONS BETWEEN INPUT TO ELECTRIC DRIVEN REVERSING MILL AND ELONGATION OF STEEL ROLLED

represent the total power required to drive the mill. Fig. 6 shows the steam consumption of a 5000-kw. turbine, which is a common size in steel mills, operating under the same steam conditions as the steam-driven reversing mill. These steam conditions are not altogether ideal for a turbine as a higher pressure and superheat might be used, in which case still lower

1916] ELECTRICALLY DRIVEN REVERSING' MILL 747

steam consumption and better thermal efficiencies would be obtained. This curve of steam consumption includes the power necessary to operate the condenser circulating water pump and the air pump. Under normal operating conditions the turbine would run at 70 per cent of load and taking the steam consumption at this point, we find that one h.p-hr. can be generated for 13.6 lb. of steam. From the power requirement the total steam consumption can be calculated and it will be seen that this does not amount to more than from 50 per cent to 60 per cent of the best engines installed in the United States to date.

3. The question of saving accelerating energy is one that is given a good deal of thought by the engine builders as there is no way of storing it in the engine. The characteristics of the motor and engine are entirely different. It is true that if a mill is operated in an ideal manner the metal will leave the rolls at practically zero speed so that ali the energy stored in the rotating

of ß 16 x ce CL·

3 14

12 0 1000 2000 3000 4000 5000 6000 7000 8000

GENERATOR LOAD IN HP.

FIG. 6—STEAM CONSUMPTION OF 5000 KW TURBINE, 150 LBS. STEAM PRESSURE, 28-IN. VACUUM

parts will have been returned to the mill and usefully consumed. Mills, however, are not operated in this way and neither the electric drive or the engine drive is operated in an ideal manner. Mills are handled by workmen and not by designers, and production is the object arrived at. The workmen are not interested nor do I believe it is possible to interest them, in the best conditions for obtaining low power consumption. This is a condition that must be reckoned with, and if possible the design of the equipment should be such that the power consumption cannot be affected materially by unskillful operation.

Due to the fact that the speed of the reversing motor is proportional to the throw of the controller handle and does not vary appreciably with the load, ideal conditions can be more nearly approached than with a steam engine. In the case of steam drive, it is quite common for the engine to race after the metal has left the rolls, especially if the draft has been a

»


heavy one, when there may be a large volume of steam in the cylinders which is not expanded, and which accelerates the engine parts. The engine must then be stopped and energy is required to do it. Fig. 7 shows the speed curve of a reversing engine taken from a recent test. I t will be seen that in quite a number of cases the engine has raced after the metal leaves the rolls. For comparison a similar speed curve is reproduced of an electrically driven mill taken from the motor at the plant

30sec. not shown-shearing billet TIME IN SECONDS

F I G . 7 — S P E E D C U R V E OF R E V E R S I N G E N G I N E . (Note: O N L Y A L T E R NATE P A S S E S SHOWN TO OBTAIN L A R G E D E F L E C T I O N ON R E C O R D I N G INSTRUMENT. )

of the Central Steel Co. In the case of electric drive, the motor is stopped by reversing its function and making it act as a generator. This is a natural characteristic of the equipment and enables the braking to be done very rapidly and also economically, as the energy stored in the rotating parts is returned to the flywheel of the set. The losses are only those due to the resistance of the windings. Whatever energy might therefore be lost due to the fact that the mill is not operated in an ideal

^ _ ^ PASS NUMBER AND DURATION OF PASSES j - N0T9 No.18 NoÎ7 lTl5*14T3'Î2Tri0 9^8VNo.6"5*4"3No.2NÔ.l

90 80 70 60 50 40 30 20 10 0 l w

TIME IN SECONDS

F I G . 8 — S P E E D C U R V E OF R E V E R S I N G M O T O R D R I V I N G M I L L

manner is returned to the flywheel and is available for the next pass. The whole point, however, is of little importance as it shows up in the relative power consumption of the two methods of drive which after all is the only criterion as to which is the better system to use.

4. With electrically driven mill the economy of course falls off somewhat as the output is reduced due to the continuous windage and friction losses of the flywheel motor-generator


set which are independent of the load on the machine. Outside of these losses it makes practically no difference whether one or 30 ingots are rolled per hour as far as the unit power consumption is concerned. In other words, the net power, (leaving out the constant losses) per ton of steel is practically independent of the quantity rolled. A somewhat similar condition exists with the steam engine inasmuch as it has certain constant losses due to leakage, piping and auxiliary power. However, outside of these losses the steam consumption will not be constant per unit of work done as the expansion conditions vary.

TABLE IV. TIME STUDY OF REVERSING ENGINE

Pass No.

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15

Leaving press

Time of entering pass-sec.

0. 3.5 9.7

13. 17.2 20.8 27.2 31.8 37.0 42.0 50.0 55.0 62.0 67.7 75.8 79.3

Manipulation

Turn

Turn

Turn

Turn

Turn

Turn

5. The question of the time required for operation is liable to be clouded very much by conditions that have nothing whatever to do with the time required for rolling the metal. I t is undoubtedly true that the steam engine can reach certain given speeds quicker than the reversing motor. At the same time it does not necessarily follow that the reversing engine will roll any greater amount of metal than the reversing motor. It depends upon many other conditions such as the way the metal is handled on the tables, the maximum speed reached, and also time lost in the manipulation of the driving unit. Table IV shows the results of a time study brought .out in a discussion


of the paper already referred to. Table V shows the results of similar figures taken from the reversing motor on the Central Steel Co. plant at Massillon, O. It will be seen that the time of entering the 15th pass in the case of the steam-driven mill was 75.8 seconds from the beginning of rolling and in the case of the motor driven mill 59.8 seconds. While it is not claimed that these figures show the advantages of one over the other system of drive yet they are sufficient to indicate that in practise the reversing motor will operate just as quick, if not quicker

TABLE v. TIME STUDY OF REVERSING MOTOR

Pass No.

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 i9

Duration of

pass

1.5 1.2 1.7 1.4 1.7 1.7 2 . 0 2 . 0 2 . 5 2 . 2 2 . 5 2 . 6 2 . 9 2 . 6 2 . 8 3 . 0 5 . 0 5 . 0 6 . 5

Interval after pass

1.7 5 . 1.6 1.5 1 4 5 . 5 1.2 0 . 6 1.4 3 . 7 0 . 8 1.2 2 . 0 3 . 8 2 . 0 4 . 2 4 . 2 5 . 0 2 . 5

Time of entering pass integrated

0 3 . 2 9 . 4

12.7 15.6 18.8 26 29.2 31.8 35.7 41.4 44.7 48.5 53.4 59.8 64 6 71.8 81 91

Rev. pet min. at

entrance of ingot

15.5 36 12.5 23

7 . 8 23.5 11 23.5 15.5 23.5 18.5 23.5 23 5 28

7 . 8 15.5 23.5 23.5 22

Maximum rev. per

min.

39 47 47 48 44 55 50 62 56 59 55 78 62 67 64 76 74 70 80

Rev. per min. as ingot

leaves i oils

26.5 31 * 28 27.5 20 31 * 18.5 12.5 31 31 * 14 31 34 51 * 31 55 * 31 62 * 12.5

* Piece manipulated. Rolling 18 by 20 in. Ingot to 3 by 8 in. sheet bar blooms.

than the engine. Due to the ease of control of the motor drive is a good deal lighter on the operator and consequently he is able to continue running the mill at a maximum capacity with less fatigue than in the case of a steam-driven mill.

The design of the reversing motors and the generators supplying them with power presents many problems not encountered with ordinary direct-current machines. As the success of this type of mill depends upon the machines meeting the severe operating conditions without injury or deterioration, a brief review of the principal characteristics may be of interest.


There is no other class of service which might be properly compared with the requirements of a large reversing mill. The heavy torques, the sudden peak loads, and the quick reversals all call for apparatus of substantial mechanical design, and flexibility in electrical characteristics. The exchange of energy between the driving motor and the generator undergoes changes at a very rapid rate. In fact, the driving motor must perform the functions of a generator as will as that of a motor, and the supply generator may at one instant be furnishing current to the driving motor, and at the next instant it may be receiving electrical energy from the driving motor, and delivering mechanical energy to the flywheel. For example, the equipment at Bethlehem, which consists of two 600-volt motor armatures supplied by two 600-volt generator armatures all connected in series, may at one instant show a swing of 10,000 amperes, and at the next instant the swing may be of an equal value in the opposite direction. There must be rapid adjustments of flux conditions in both motors and generators in order to meet these reversals without showing harmful sparking at the brushes, and as the swings which occur many times a minute represent over-loads of 200 to 300 per cent, the design of the direct current machines must be suited to these over-loads, both in current carrying capacity and in flux conditions. Especially must the machines be designed for good commutation. This cannot be obtained at such overloads without making liberal allowances for the commutating flux, as the ratio which the commutating flux bears to the load must not be disturbed by leakage conditions, even at the overloads. This feature is more readily obtained by compensating the armature reactance to which further reference will be made, and this type of construction is of greatest importance, to successful results.

The choice of voltage per commutator and the use of two armatures, the commutators of which are connected in series for large powers, is deserving of a careful analysis, and this voltage should be selected with due consideration of the motor, the generator and the auxiliary apparatus. The following arguments are to show that, other things equal, it is desirable to adopt a relatively high voltage; viz. 600 volts and by series connection, alternating a motor with 'generator, derive all the benefits of 1200 volts.

The electrical equipment of a reversing rolling mill includes both the d-c. generator as well as the d-c. motor, and the gen-


erator does not furnish power to any other apparatus except the roll mill motor. The voltage should be of such a value as will give the best balanced equipments and the choice of voltage becomes a very important factor as the design of the entire equipment may be said to depend in a great measure upon the voltage selected.

The use of 250 volts has been common practise in rolling mills and it is natural that this voltage should be considered. However, for large capacities, there are many objections to so low a voltage, among which may be mentioned heavy currents, large commutators, larger machines, lower efficiencies, increased cost of auxiliary apparatus, higher maintenance charges and increased mechanical difficulties.

The magnitude of the current becomes a factor when the power required on peak loads may reach 15,000 to 20,000 h.p. and not only are the connections, cables and switching apparatus expensive, but the losses in these parts are roughly proportional to the currents. The approximate cost of these parts will vary inversely as the voltage.

The size of commutators and the number of brushes will be a direct function of the current and it is desirable to keep down the size of the commutators from at least three points of view —mechanical difficulties of construction, over-all length and cost. No part of a direct-current machine is so difficult to construct as is the commutator, and for this reason the construction of commutators has received, and will continue to receive the most careful consideration from both the design and the manufacturing points of view. So all important is the commutator and the commutation that when these are right, there is seldom any cause for complaint. Increased voltage not only reduces commutator length, but insures less overall length—an extremely desirable factor.

The efficiency of the equipment as a whole will be higher with increase of voltage within certain limits, as in such installations the peak loads are relatively high as compared with the average or mean load.

The commutating conditions of the generator is one of the items which must be carefully considered in the selection of voltage, especially as economies can be effected by operating the flywheel set at a reasonably high speed. It is desired to consider whether a generator can be designed better for one voltage than for another, and what is a safe operating speed for

1916] BLECTRICALLY DRIVEN REVERSING MILL 753

a generator of a given output and voltage. With a view of setting forth the relations of kilowatt capacity and speeds, the writer has chosen familiar voltages of 125, 250, 600, 1200 and 2500 volts, and has plotted a curve for each one of these voltages. These curves are shown in Fig. 9. There are certain fairly well established relations and limits in direct-current machines, which lead to limits of output. However, it is not so much these limits that we would direct attention to at the present moment, but the relation of possible outputs of different voltages. I t will be observed that at 600 volts the possible outputs are greater than at either 250 volts or 1200 volts. This means that with the same degree of safety it is possible to make a larger 600-volt generator than a 250-volt or 1200-volt generator, for a given

800

600

CL

tr400

200

0 1000 2000 3000 4000 5000 K.W.

FIG. 9—CURVE SHOWING THE LIMITS WITHIN WHICH DIRECT-CURRENT GENERATORS CAN BE BUILT

speed. This relation applies at all speeds. I t will be observed that the product of kilowatt output and rev. per min. is approximately a constant for a given voltage. This relation is frequently lost sight of in considering the possibilities of machines for large output, at high speed. The basis on which these curves have been made up depends upon setting certain limits. These limits are not definitely fixed quantities, as each designer will set limits depending upon the experience which he has had with various machines. The relation of these limits will in a measure determine which curve will be highest, and some of these limits are entirely independent of each other. The limits which determine the possibilities of high-voltage machines are entirely different from those which determine the possibilities in low-voltage machines. Curves could be drawn for all voltages in

\ \

\ \ \ \

\ \

\ 125V >ltsX,

Volts

50 Vol

-1200

—<! Volte*

00 Vol *


a similar manner as these have been determined, but the more usual voltages are used for the sake of illustration, and they serve the purpose of showing the desirability of using a relatively high voltage for roll mill motors, such as we are considering.

As these curves represent limits it is of course understood that most machines will fall under them and the extent to which a machine falls within the curve will represent in a measure the ease with which that machine can be designed. We would emphasize the fact that the minimum cost for a given rating does not necessarily call for the highest possible speed, and the present day tendency of going to extreme speeds should be discouraged.

From these curves we deduce that 600 volts is a desirable selection per commutator for the generators and it is also evident that 1200' volts per commutator would be possible. A voltage

F I G . 10—COMMUTATING P O L E W I N D I N G

of 600 per commutator has proven wrell suited for the motor, as a lower voltage would lead to relatively large armature diameter. A very much higher voltage would require few poles with correspondingly heavy rotors, and either of the conditions is undesirable as low inertia effect is important.

TYPE OF WINDING To obtain the best operation under heavy peak loads, which

are subject to very rapid changes, for example, three times normal load, reversing at the rate of 30 times a minute, it is not only desirable, but it is necessary to neutralize to the fullest extent the distorting effects of the currents in the armature winding. The method of obtaining this result is to slot the pole face, and secure in these slots a bar winding which is connected in series with the armature, and making a number of conductors in the pole face just sufficient to neutralize the armature conductors


covered by the pole face. The excess winding necessary to produce a commutating flux is concentrated on the commutating poles, located midway between the main poles. The difference between the compensating winding and the interpole winding is illustrated in Figs. 10 and 11, which show the same number of conductors in both cases, but the conductors are shifted in position. In the plain commutating pole machine, having all the commutating winding located on the commutating pole, the conductors are as shown in Fig. 10, whereas in the compensating pole machine, which has part of the compensating winding located in the main pole face and the balance located on the commutating pole, the conductors are as shown in Fig. 11. By locating in the pole face the ampere conductors which neutralize the armature reaction under the pole face, the distortion of the flux at the main pole face is prevente º . has a beneficia

F I G . 11—COMMUTATING P O L E AND COMPENSATING W I N D I N G

effect in two ways. First, it prevents under sudden changes of load, a sweeping across the pole face of the main flux under the pole, and it prevents a distortion of the main flux which is a very important consideration, as this lowers the· maximum voltage between adjacent commutator bars, which would otherwise obtain. As it is the maximum voltage between commutator bars, rather than the average voltage, which determines the design, the importance of this type of construction becomes evident for this class of service. Another very important consideration in this arrangement of the compensating winding is that the leakage from the commutating pole is very much less than would be the case if all of the windings were concentrated on the interpole. This fact permits the carrying of heavier overloads, and it is the overload capacity which determines to a great extent the suitability of machines for this class of service. I t is evident that the leakage is very much reduced, when one


considers that the leakage is mainly between the main pole tip and the interpole, and that the magnetomotive force producing this leakage is made very much less by locating a large proportion of the ampere conductors in the main pole face.

This construction permits the operations of the machines through a wide range of voltage; in other words, the stability of the main flux is insured without regard to the strength of the main field winding. This is an important factor in the general scheme of control, as the speed of the motor and the direction of rotation of the motor depend upon the generator voltage and upon the field strength of the motor. These two factors must be susceptible to rapid changes and wide variations in order to effect the desired result.

In referring to the relative merits of commutating pole machines, versus machines with compensating winding, for this class of service, attention may be called to the fact that compensating windings are difficult of construction, in machines having very large current, as there is a limit to the desirable physical dimensions of a single pole face conductor. With heavy current machines, a suitable arrangement of compensating conductors is often quite a problem in a specific design. The most desirable arrangement is to have all compensating conductors connected in series, and the possibility of such an arrangement is limited by the capacity and voltage of the machine, and here again there is a decided advantage in not having the voltage too low, for large power capacities. For example, a single conductor having a cross section of more than two sq. in., is seldom used in the pole face winding. Assuming a current density of 1500 amperes, a single conductor, with all conductors in series, can be used on a 3000 ampere machine, which, at 600 volts, represents an 1800-kw. capacity. For larger current capacities, it is necessary to connect the conductors in parallel, and frequently the circuits are in parallel. In such cases, great care must be exercised in the building of the machines so as to have good joints, in order to insure the proper division of the current.

Generally speaking, the compensating winding of large capacity machines consists of a relatively small number of conductors, and simpler, better mechanical arrangements can be effected than are possible with smaller capacity machines. It is also desirable to design the compensating winding and the commutating pole winding so that no shunts will be necessary. This can readily be accomplished as it is possible to calculate very


closely the required ampere conductors, for compensating the armature reaction and furnishing the necessary excitation for the commutating pole. By avoiding shunts, one is insured of the simultaneous change of current in the compensating winding and the armature winding.

FIELD WINDINGS The type of field winding used on roll mill machines should

be very simple, and such as not to be easily damaged. If a low voltage is used for excitation of the d-c. generator and d-c. motor field coils, these coils can be made of copper strap winding with a layer of asbestos between turns, and the bare edges exposed to the air. Such coils are almost indestructr'ble by heat. Strap wound field coils arranged in two or more concentric sections insure a natural and easy ventilation.

INSULATION As this type of machinery is exposed to mill dust, and as this

dust is likely to contain a large percentage of conducting material it is advisable in the design to embody more liberal creepage distances than are necessary for ordinary service. In order to combat, to a certain extent, the bad effects of dust, the armatures may be given a finish by rolling them in varnish and baking them. This produces an insulating film over all parts, and fills all the pores and small crevices in the insulation, and insures a slick finish which will shed the dust.