Control of direct current hoists in iron and steel mills
Post on 09-Apr-2017
Presented at the 308th meeting of the American Institute of Electrical Engineers, Pittsburgh, Pa., April 16, 1915. Copyright 1915. By A. I. E. E. (Subject to final revision foi the Transactions.) CONTROL OF DIRECT CURRENT HQISTS IN IRON AND STEEL MILLS BY G. E . STOLTZ AND W . O. LUM ABSTRACT OF PAPER The control problem discussed in this paper is confined entirely to a typical example of a skip hoist. An outline of the conditions existing in the case of this particular application is given in which the load on the hoist is divided into three parts, namely, the friction load, the inertia of moving parts, and the useful work in hoisting the net load. The proper cycle of operations is determined, which in turn determines the required characteristics for the motor and controller which are together considered as a unit. The details of this d-c. skip hoist control are described and illustrated. THIS SUBJECT would, in general, confine our discussion to skip hoists, cranes and ore handling machinery, the operation of each being sufficiently different from the others to warrant individual consideration. To treat each of these applications properly would require too much time, so it has been decided to confine our discussion to a typical example of a skip hoist. In order to design control apparatus intelligently for an application of this kind, it is necessary for the engineer to understand fully the function of the various parts of the driven apparatus as well as the operating characteristics of the electrical machinery. We will therefore outline the existing conditions of this particular application, and then suggest a control which seems best to meet the definite requirements set forth. SKIP HOISTS Fig. 1 is an illustration of a typical hoist which -has been selected for consideration in this paper. The load on a skip hoist can be divided into three parts, namely, friction load, inertia of the moving parts, and useful work in hoisting the net load. The static friction at the moment of starting is, of course, Manuscript of this paper was received March 30, 1915. 723 724 STOLTZ AND LU M: [April 16 greater than the running friction, but it is a comparatively small item, and with the machine in approximately continuous operation the static friction is not excessive, so that it is ordinary practise not to differentiate between static and running friction in calculating the load cycle. No formula can be used to determine definitely the friction, but various empirical rules are employed which approximate the true values sufficiently close to that obtained on hoists under normal operating conditions. F I G . 2 C H A R T SHOWING R E L A T I V E I N E R T I A V A L U E S OF M O V I N G P A R T S AMotor armature BDrums CCars and ropes F I G . 1SKETCH OF TYPICAL S K I P H O I S T - - i l l 1/ f 0 5 1 No D 1 SECO ..-.ad 5 ft NDS 0 2 ~1 K 5 3 \| 0 3 5 F I G . 3 The next item involved is the inertia of the moving parts. The apparatus involved consists of the drums, sheaves, gearing, armature, ropes, skips, and net load. The energy required to overcome the inertia of a moving body varies directly as its weight and as the square of its velocity. Therefore to minimize the starting and braking peaks, care should be exercised in the design of the apparatus. The diameter of the rotating parts should be kept as small as possible consistent with good mechanical design, as the velocity of these parts is a direct function of the diameter. This feature is characteristic of reversing mill motors, which are designed long and small in diameter to reduce their flywheel effect. 1915] CONTROL OF DIRECT-DURRENT HOISTS 725 The weight of the armature is comparatively small, but its high velocity makes it an important item as is indicated in Fig. 2. The skip will occasionally be operated empty, the cycle being approximately as indicated in Fig. 3. The normal operation of this particular skip is two loads of coke, net weight 3000 lb. each, two loads of limestone, 3000 lb. each, two more of coke and four loads of ore, 6000 lb. each. This cycle will be varied to obtain satisfactory distribution of the material in the furnace, but is sufficiently near the average operating conditions for our purpose. Occasionally a load of scrap weighing 9000 lb. will be hoisted, but this is rare and need not be taken into consideration except as an overload for a few seconds. Fig. 4 graphically illustrates the load cycle when hoisting a 3000-lb. F I G . 5 C H A R T S H O W I N G D I S TRIBUTION OF E N E R G Y W H E N ACCELERATING 3000 L B . LOAD ARheostatic loss CFriction BInertia DUseful work load. The curve is plotted between time, expressed in seconds, and torque expressed in equivalent horse power. During the accelerating period the dash line AB represents the actual mechanical output of the motor, but, since we are interested in the current taken from the line, all energy values will be expressed in the horse power equivalent of torque. The area enclosed in the triangle ABC represents the energy dissipated in the starting resistors during the accelerating period. This is one-half of the total input while coming up to full speed, as indicated in Fig. 5. It will be noted in Fig. 4 that the initial peak is only 66h.p., whereas we would naturally expect it to be 105 h.p. This is the result of lifting the top skip off its track. Before dynamic braking was introduced, with magnetic control, it was necessary to lift the skip two to four inches off the track when hoisting light loads to make sure the heavy loads F I G . 4 726 STOLTZ AND LU M: [April 16 would rise to a sufficient height to dump. At present dynamic braking can be made severe enough to practically stop all loads at one position. There is nothing to gain by doing this, and the rope is therefore adjusted normally to lift the top car off the track a few inches. This utilizes the whole car as a counterbalance and helps to land the skips more carefully. When the cars are again started the top skip still acts to its full capacity as a counterbalance and reduces the initial current peak accordingly. On a vertical hoist the line CB would be straight, the difference in height of the two points being due to the transfer of the weight of the rope from the load to the counterbalance side of the hoist. During the acceleration period the top car comes down over the knuckle, which changes its effect as a counterbalance, and accounts for the shape of the curve CB. 100 a^b L S J I x 2 5 0 C k h / 1 1 i / ) \ E ) ' 1 b "i 6000 L 5 i SECO 2 NDS = 5 K X 0 3 \ \ 5 4 U 125 100 g 75 50 o 0 t k \ / / / / / / ) i v L 1 u 1 9000 L b SECC Load 2 )NDS b 3 \ W \ (f j \ \ N b 4 J FIG. 6 FIG. 7 The line DE represents the load when hoisting at constant speed. As the loaded car rises and the empty skip lowers, the net weight of the rope gradually favors the counter-balance and decreases the energy taken from the line. During the period F it is proposed to operate the control to obtain a gradual slow-down. When the loaded car goes up over the knuckle and starts to dump very little energy is required to continue its travel, as indicated by the shape of the slow-down curve. During this period we have friction and net weight acting against the inertia of all moving parts. If the latter is not sufficient to carry the car to its destination the motor will take energy from the line. Just before the cars reach their final position, dynamic braking should be introduced to bring the apparatus to a stop, at which time the line switches should be opened and the holding brake applied. 1915] CONTROL OF DIRECT-CURRENT HOISTS 727 Fig. 6 represents the 6000-lb. ore load, and Fig. 7 the occasional 9000-lb. scrap load. Both are developed in the same manner as Fig. 4. The time of acceleration increases with the load, as the series switches will not operate so rapidly as with light load. The interval of time between hoisting varies considerably, but the capacity of the control installed is determined by the maximum demand which generally occurs after a breakdown. if the skip is not in operation for a period of time, the burden in the furnace may become so low that two or three hours are required to bring the furnace back to its normal condition. The rating of the control should therefore be based on the regular cycle of hoisting the 3000-lb. and 6000-lb. loads with an average interval between trips of eight seconds. The ap:? paratus will then be somewhat liberal for ordinary operating conditions, but this is a point which should be received with favor, as low maintenance and continuity of operation are important in an application of this type. The root-mean-square current will determine the carrying capacity of the switches with reference to heating. For the different loads these values, as well as the maximum values, are as follows : R. M. S. Maximum Load H.P Current H.P. Current 9000 6000 3000 Friction 76 57 42 24 300 240 182 116 128 110 102 83 500 430 400 334 The average heating current taken from the line during the heaviest cycle of hoisting the 3000-lb. and 6000-lb. loads is 210 amperes, and the maximum is 430 amperes. The accelerating switches can open immediately after the last switch is closed so that they only need to carry an average cuirerit of 380 amperes, for a short time during each cycle. The slow-down switches should be designed to handle th current adequately during this period of the cycle. The values actually obtained depend somewhat on the method employed of bringing the apparatus to rest. -Resistance will be required during the starting and retarding periods. An average of 220 h.p-sec. must be absorbed during the starting period of each cycle, and 600 h.p-sec. when retarding. 728 STLTZ AND LU M: [April 16 ^toooooov First Step etarting-AII Res. l a Fig. 8 shows a sequence of control, starting at rest and accelerating the motor to full speed, then the slow-down and stopping of the load. The acceleration of the motor is controlled by a current-limiting device. For this reason the amount of current taken by the motor on the first notch must be sufficient always to start the motor with the maximum load of 9000 lb. This means that the starting current should be something more than the value required to start the load under ordinary conditions, so as to take care of tight bearings and other variations in the load. The number of accelerating points that is required will be determined by the maximum current peaks permissible. The number will also depend upon the characteristics of the particular motor used. It has been found in practise that six accelerating switches usually give satisfactory results on hoists of this character, with motors of 100 to 200 h.p. Series-wound switches may be used for all but the last accelerating notch. This last notch should be a shunt switch or its equivalent. This switch is used for short circuiting the series field of the motor, and it is very important that the switch remain closed after the motor has been brought up to full speed. There are conditions of operation which may cause the motor to regenerate, which will reverse the current and cause the series switch to open. After the skip has reached full speed and approaches the end of its travel, the motor must be slowed down in order to make an accurate stop. The diagram shows the slow-down is effected by connecting a resistance in shunt with the armature of the motor and opening all of the accelerating switches, which inserts the series field and starting resistance between the motor and th line. The second slow-down step consists in reducing All Res. cut out Step by Step All Res. and Series Field Short Circuited IstSlow-Oown 2nd. Slow-Oown with less Stwnt Res. Off Position leaves Dyn. Brake on Motor FIG. 8 PLATE XXXIX. A. I. E. E. VOL. XXXIV, NO. 5 FIG. 9 F R O N T V I E W OF SKIP HOIST P A N E L STOLTZ AND LUM] FIG. 1 0 R E A R V I E W OF SKIP HOIST P A N E L [STOLTZ AND LUM] 1915] CONTROL OF DIRECT-CURRENT HOISTS 729 the resistance in shunt with the armature. This shunt resistance causes a definite current to flow through the starting resistance and series field of the motor. This shunt current is independent of the load and the current taken by the motor, so that a low voltage is impressed across the motor terminals, This slow-down should exist for a period long enough to enable the motor to reach a constant speed before it is finally stopped. The motor is stopped and the mechanical brake applied when the line switch is opened ; th resistance is maintained across the motor armature and the shunt field is energized so that a dynamic brake is obtained in addition to the mechanical brake. The acceleration and slow-down are obtained automatically by means of proper switches attached to the hoist mechanism. The motor, however, can be controlled entirely from the master switch when desired. If the skip hoist should fail for any prolonged period, while the furnace is in service it will result in serious damage, and for that reason every precaution should be taken to prevent a failure with the electrical equipment. The master controller and limit switches should open both sides of all coil circuits and the shunt brake circuit, so that a ground or other disturbance on the control wire will not prevent the motor being brought to rest. In equipping the skip hoist, or for other applications where it is necessary to perform a definite cycle of operations and insure the customer against failure, it is very necessary to consider the motor and controller as a unit. The amount and kind of material forming the magnetic circuit of the motor will introduce a time element. The design of the motor itself may fit the amount of current used during, different parts of the cycle. The function of the controller is solely that of furnishing the motor with the assistance the motor requires in going through its cycle of operation. As the design of the motor is improved, some of the functions of the controller may be incorporated in the motor, and the controller simplified. In slowing down and stopping the motor, most of the stored energy is in the armature of the motor. This item must be considered carefully so that the design will give a positive and accurate stop without imposing too great a strain upon the brake.
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