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.
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
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.
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. 3SCHEMATIC 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 breakers1 generator field2 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 Malternating 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.
742 SYKES AND HALL: [June 27
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
i W i ~~ ~
| j J r -- i-J-
J r ^ 1 1
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 . 12COMPENSATING 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
744 SYKES AND HALL: [June 27
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
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
1916] ELECTRICALLY DRIVEN REVERSING MILL 745
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
A B C D E F G H I J L M N
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.
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.
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
Cold ingot Hot ingot Good rolling New...