US2257361A - Material handling magnet control - Google Patents

Description

P 1941- w. R. YORKEZY 2,257,361

MATERIAL HANDLING MAGNET CONTROL Filed Sept. 12, 1939 2 Sheets-Sheet 1 125% VOLTS IOOV. VOLTS 100% NORMAL (I) t 87% VOLTS 3 TIME I007. VOL-rs -NORMAL (I) i: g 767 VOLTS O C E 50% A TUAL 1 60% VOLTS QC LU PRIOR ART I l a 3 TIME I jflgz INVENTOR.

WILLIAM R. YORKEY Patented Sept. 30, 1941 2,257,361 MATERIAL HANDLING MAGNET CONTROL William R. Yorlrey, Maplewood, N. 1.,

assignor The Electric Controller & Manufacturing Company, Cleveland, Ohio, a corporation of Ohio Application September 12, 1939, Serial No. 294,466

4 Claims.

This invention relates to a method or and apparatus for removing magnetic material from a pile of material and for handling magnetic material, and more particularly to a method of and apparatus for controlling the degree of energization of a material handling magnet throughout a complete working cycle.

An object 01' the invention is to obtain greater efliciency in the operation 01 electromagnets used to separate magnetic material from a pile of materlal.

A further object is to provide a method of lifting magnet operation which makes possible the separating and lifting of a given load from a pile with a smaller magnet than has been necessary heretofore, and conversely permits the use of an existing magnet of given size for separating and holding larger loads than heretofore possible with such existing magnet.

A more specific object of the invention is to provide a system of magnet control in which an overenergizing or higher than normal terminal voltage is applied to the magnet to create a large separating flux to attract material to the magnet and separate the attracted material from a pile, and, after the magnet and the attracted material have been separated, a reduced terminal voltage is applied to the magnet to create a holding flux which is less than the separating flux.

A further object is to provide a magnet control system whereby an electromagnet having insuiiicient capacity at normal excitation to separate a particular load of material from a pile of material may be caused to separate the particular load by over-exciting the magnet for an interval and then reducing the excitation to below normal to retain the load after it has been separated from the pile and to prevent deleterious heating of the magnet.

A more specific object is to provide a method of magnet operation in which the magnet is overenergized to such a high degree that the time required for attracting and separating a load, and consequently the period of overenergization and heating, is very materially reduced, thereby making possible the use of a smaller magnet for handling a given load and assuring that the magnet is subjected to excessive heating only for the shorter resultant period and thus is not deleteriously affected.

Other objects and advantages will become apparent from the following specification wherein reference is made to the drawings in which:

Fig. 1 is a graphical illustration oi the operason of the control system of the present inven- Fig. 2 is a graphical illustration of the operation of the prior art control systems, for purposes of comparison;

Fig. 3 is a wiring diagram oi. one control system which may be utilized in carrying out the invention; and

Fig. 4 is a wiring diagram of another control system which may be utilized in carrying out the invention.

Generally, material handling magnets and their associated circuits are designed for use at one of the standard direct current voltages. The resistance of the energizing winding of such magnets is selected relative to the voltage applied to the terminals of the magnet so that the temperature of the magnet is at a safe value throughout its operation over a predetermined duty cycle. Thus, thetemperature rise caused by the energizing currents is the limiting factor in the design.

Hereinafter, this limiting temperature is referred to as the normal operating temperature and the terminal voltage for which the magnet is designed to give the normal operating temperature is referred to as the normal terminal voltage.

By duty cycle, in the case of a lifting magnet operated in the usual manner, is meant a period during which the magnet is energized, lifts and transports its load, then is deenergized, drops its load and then returns for another load. The usual duty cycle for a lifting magnet is approximately one-half time energized and 0ne-hali time deenergized. In the case of a separating magnet,

- operated in the manner of this invention, the

duty cycle is the series of periods during which the overenergized magnet separates the load, is reduced in energization and holds the load, and, if idle between these periods, the idle or cooling period.

The amount of iron in the magnetic circuit of the usual material handling magnet is generally such that magnetic saturation of the iron does not occur even if the magnet is carrying a load which substantially completes the magnetic path between poles. In most cases, particularly in the case of the handling of scrap iron, the magnetic circuit of the magnet is far from being saturated during energization due to the numerous air gaps between the attracted pieces of magnetic material themselves and the pieces and poles of the magnet. Therefore, when operated at normal terminal voltage, there is present in the usual material handling magnet a great excess of iron capable of supplying flux to the usual load.

The ampere turns of a material handling magnet operating at a given terminal voltage are inversely related to the resistance or the winding, and, since the resistance is directly related to the temperature, the ampere turns for a given terminal voltage are inversely related to the temperature of the winding. The amount of magnetic flux, and therefore the amount of ampere turns, necessary to hold a piece of magnetic material against a pole piece of a magnet, is less than that required to attract the same piece to the pole piece across an air gap. For this reason, after a magnet has attracted its load, it is possible to reduce the amount of ampere turns without permitting any of the attracted material to drop.

Also, in the case of separating a load of loose magnetic material, such as scrap iron, from a pile of the same material, or other material, an additional differential in ampere turns between those necessary to pull the load from the pile and those necessary to retain the same load on the magnet after it has been lifted is present due to the pull exerted by material attracted by the magnet but anchored mechanically or magnetically in the pile, or restrained thereby. It is known that the effectiveness of an electromagnet operating upon a movable armature can be greatly increased by over-exciting the winding during the period of initial movement of the armature, and then decreasing the energization after the reluctance of the magnetic circuit has been reduced by closure of the armature. The increase in effectiveness of a material handling magnet resulting from greatly overenergizing it while it is separating magnetic material from a pile of magnetic material is much greater than would be expected from the usual change, occasioned by the magnetically attracted material, in the reluctance of the magnetic circuit. This was heretofore unappreciated due to lack of knowledge of the extent of the mutual attraction between pieces in the pile and which have been magnetized by the presence of the magnet. The fact that such attraction between pieces is present prevents lifting magnets, operating in the ordinary manner, from separating and lifting as large a load from a pile as is warranted by the degree of completion of the magnetic circuit by the attracted and hoisted material itself. For example, in breaking loose from a pile of magnetic material, a lifting magnet and its load when the magnet is energized in the ordinary manner, two forces are present-one the weight of the material being lifted-the other the mutual attraction between pieces being separated and lifted and the pieces remaining in the pile. These forces cause a certain zone of cleavage and a given pull at such zone resisting the separation of the load. It would seem that when the magnet is overenergized this resisting pull would also more greatly resist the separation of the load. in fact, it would do so were it not that the zone migrates downwardly away from its original position as the energization increases. This migration continues until the resisting pull at the new zone is substantially the same with an overenergized magnet as it was at the original zone with a normally energized magnet. Since the resisting pull remains the same, all of the additional attractive force of the overenergized magnet is useful in separating the additional material which was between the two zones of cleavage. After separation, a lower energization can retain the increased load, as explained above. Also it was unappreciated that in s p a i g magnetic material from a pile of material the mechanical interlocking of the pieces in the pile prevents the separation of as great an amount as otherwise would be possible, were it not for such mechanical anchorage. It was not until tests had actually been made by overenergizing the lifting magnet in the manner of this invention that it was realized that a magnet was capable of handling much more piled material than was heretofore thought possible and the controlling factor was not the lifting capacity but the separating capacity.

The ampere turns may be reduced most easily by reducing the terminal voltage applied to the magnet. A reduction in the applied terminal voltage causes a reduction in the current in the magnet winding and consequently results in a reduced operating temperature. The reduction in temperature effects a corresponding reduction in the resistance of the winding. Because of the reduction in the temperature of the winding and consequent reduction in resistance, the next time that normal terminal voltage is applied to the magnet to attract a load, a greater amount of ampere turns is produced and a larger attractive force is available than if the normal terminal voltage had been applied continuously throughout the preceding lifting and transporting portion of the duty cycle.

This method of magnet energlzation and operation is described in Wright Patent No. 895,135 and Younghusband Patent No. 1,915,566, and although by its use somewhat larger lifts are obtainable, in most cases the added expense and complication of the control equipment for this purpose is not warranted.

In accordance with the present invention. advantage is taken of the fact that the energlzation of a material handling magnet may be reduced after its attracted load has been separated from a pile of material as hereinbeiore described. Also advantage is taken of the fact that an electromagnet may be over energized by a terminal voltage greatly in excess of its normal terminal voltage and thereby create a separating flux for a relatively short interval of time without deleterious effects, whereas were such overenergization and resultant separating flux continued for a somewhat longer interval, the resultant heating would damage and rapidly deteriorate the magnet. Furthermore, advantage is taken of the fact that, by overenergizing a magnet to a high degree while it is separating magnetic material from a pile of such material, the tendency for materials which would be attracted by the magnet under ordinary energization to be retained in position due to mutual attraction with other magnetic material in the pile is eliminated. Again, advantage is taken of the fact that the greater the degree of overenergization the greater are the ampere-turns and accompanying hysteresis effect with its longer period of flux decay. Therefore, the greater the overenergization of the magnet, the shorter the time during which the overenerglzing current needs to be maintained because the hysteresis effect can be utilized to sustain the flux after cessation of the original exciting current. The decaying hysteresis eiiect produces no appreciable heating effect but does sustain the flux. Thus the heating period is further decreased as the overenergization is increased.

Further, advantage is taken of the fact that, because of the excess iron in the magnetic circuit of the magnet, the overenergization creates a great excess of flux which is efl'ective in attracting large quantities of magnetic material and separating a load thereof from the pile. It has been found that after such overenergization, a subsequently reduced applied terminal voltage of a value which creates a holding flux and is sufficiently low to permit the magnet to cool to its maximum permissible temperature. or the usual normal temperature, iscapable of causing the separated load to remain attracted.

Therefore, the present invention is a new combination of the circumstances and phenomena of magnet operation, some new and some existing in the prior art, and new cooperative relations therebetween, such as the facts that the magnetic path of a magnet is not saturated during normal operation, that overenergization of a magnet for a relatively short interval of time can be maintained without danger to the magnet, that a high degree of overenergization reduces to a minimum any tendency for materials magnetized by the magnet under ordinary energization to be retained in the pile by mutual attraction with other material remaining in the pile, that the interval during which material is being attracted to the magnet and separated from a pile by a magnet is shortened directly as a result of overenergization and thereby permits the separation of a much greater loadby a smaller magnet than heretofore thought practicable, and that since a smaller magnet can be used and inherently reaches its full magnetic strength in a shorter interval of time than a larger magnet, the period during which overenergization is required is additionally shortened, wherefore the period of application of the overenergizing terminal voltage and resultant heating is further reduced. By proper correlation of these facts in a lifting magnet and its method of control, the magnet can be so greatly over-energized that a relatively small magnet can be used for accomplishing that which heretofore required a much larger magnet.

These facts and their new correlation may be explained more clearly by reference to Fig. 1.

If a normal terminal voltage, or 100% volts, is applied to a magnet it causes normal wattage input, or 100% watts. This wattage input, if continued during the energized portion of the usual duty cycle, maintains the magnet at its normal temperature. In the illustrative example in Fig. 1, the lifting magnet is so designed in relation to the voltage of the source that when connected directly thereto, 156% watts are put into the magnet. This corresponds to a voltage which is 125% of the normal terminal voltage. If this degree of overenergization is maintained for nearly of the total energized time of the duty cycle, the wattage input for the remaining portion of the energized time must be reduced to 75%, corresponding to a voltage of 87% of the normal terminal voltage, in order to prevent destructive heating of the magnet. In such case, the average heating during the combined overenergized and energized portions of the duty cycle is exactly equal to the average heating occasioned by normal energizationof the magnet for the energized portion of the usual duty cycle. The average heating for a complete duty cycle is likewise the same ineach instance.

Because of the excessive iron in the magnetic circuit of the magnet, all of the ampere turn produced by the 125% applied terminal voltage is effective to produce a separating flux capable of attracting material to the magnet and sepamaterial.

rating it from the pile. The subsequent reduction in applied terminal voltage is as great as is possible without permitting any of the load to fall or drop from the magnet.

In the prior art systems, the operation of which is illustrated graphically in Fig. 2, no attempt is made to go above the normal terminal voltage, but the subsequent reduction in voltage from the normal is employed only to reduce the total heating in order to permit more ampere turns per unit of applied voltage. Lifts of scrap of 5% greater than with ordinary constant voltage operation are possible with the prior art systems due to the lower operating temperature of a magnet energized only to normal for but a limited period of time and to below normal or completely deenergized for the remainder of the time. However, upon adoption of the system of the present invention, lifts of 30% above the usual are obtainable with the same size magnet, or, for the same load, a smaller magnet may be employed.

Referring to Fig. 3 of the drawings, the conductors l0 and II are connected to a source of voltage indicated at 9. A lifting magnet I 2 is arranged to be connected across the conductors l0 and II by means of an electromagnetic contactor H. The contactor [3 .has normally open contacts [3a and is operated by a winding I 310 which is energized from the conductors l0 and l I when a manual switch It is in the closed position. A resistor I5 is normally connected in series with the magnet l2. An electromagnetic contactor l6 having normally open contacts lia is arranged to short circuit the resistor l5 when an operating winding .liw is energized. The energization of the winding lGw is controlled by a time delay relay I! having an operating winding llw, normally closed contacts Na, and a retarding device llb which is adapted to delay the opening of the contacts Ila for a predetermined time interval after the winding l'lw is energized.

The control system of Fig. 3 operates as follows: Closure of the switch It connects the windings l3w, I'lw, and i610 across the conductors I0 and H for energization from the source of power 9. The contactor'i3 in response to the energization of its operating winding l3w closes its contacts l3a to connect the magnet I! in series with the resistor l5 across the conductors l0 and II. Simultaneously, the contactor l5 due to the energization of its operating winding I610 closes its contacts I6a to short circuit the resistor i5 to cause the full voltage of the source 9 to be immediately impressed directly upon the magnet I 2.

' The voltage of the source 9. is greatly in excess of the normal terminal voltage for which the magnet i2 is designed. Therefore, if the magnet I2 were connected directly to the conductors I0 and II and used in the ordinary manner, it would soon reach a temperature which would be damaging to the winding. To prevent this from happening, the winding I'Iw of the time delay relay ll, which wasenergized concurrently with the windings i310 and Him, causes the contacts Ila to open after a predetermined interval. The interval is just sufficient to permit the magnet I2 to attract and separate a load from a pile of Opening of the contacts i'la deenergizes the winding l6w of the contactor i6. The

oontactor IS in response to the deenergization of its winding I610 opens its contacts ifia to insert the resistor I 5 in series with the magnet I2 to thereby reduce the terminal voltage applied to the magnet l2 and create a holding flux. The

value of the initial applied voltage in relation to the thermal characteristics of the magnet i2, the length of time it is applied, the value of the re duced voltage, and the length of time that it is applied are so selected that over a complete working cycle the normal permissive heating of the magnet is not exceeded. Furthermore, the dinerence between the initial voltage and the reduced voltage is such that no material is dropped from the magnet whenthe voltage is reduced.

In Fig. 4 is shown a control system for carrying out the invention and suitable for use with magnets energized from a separate generator, as is common in the case of magnets operated from locomotive cranes. A direct current generator 20 having a field winding 23 and an armature winding 24 is driven by a prime mover 2|. Th armature winding 24 of the generator 20 is connectible to the terminals of the magnet l2 through the normally open contacts 22a of an electromagnetic contactor 22. The field 23 of the generator 20 is connected across the conductors 28 and 29 in series with an adjustable resistor 30. The conductors 28 and 29 are connected to a source of voltage 32. The resistor 30 is normally short circuited by the normally closed contacts 25aof an electromagnetic contactor 25. The electromagnetic contactor 25 has an operating winding 2510 which is connected in series with the normally open contacts 26a of a time delay relay 26. The relay 26 has an operating winding 26w and includes a time delay device 26b shown as a dash-pot. A knife switch 21 is manually operable to control the energization of the operating windings 2210, 2510, and 26w.

, The control system of Fig. 4 operates as follows: The closure of the switch 21 causes simultaneous energization of the windings 22w and 26w. The contactor 22 in response to the energization of its operating winding 22w closes its contacts 22a to complete a circuit from the generator 20 to the magnet 12. At this time the resistor 30 is short circuited by the contacts 250 of the contactor 25 and consequently the strength of the field 23 is at its maximum value. Therefore, the voltage of the generator 20 is at its maximum value, and a voltage in excess of the normal voltage is impressed upon the terminals of the magnet l2 to create a separation flux.

Although the winding 2610 is energized at the same time as the winding 22w, the contacts 260 of the relay 26 do not close when the contacts 22a close since they are delayed in their closing movement by means of the dash-pot 26b. After elapse of a predetermined time interval depending upon the adjustment of the dash-pot 2617, the contacts 26a close to cause energization of the winding 25w. The contactor 25 in response to the energization of its operating winding 25w opens its contacts 250 to remove the short circuit from the resistor 30. The resistor 30 is thus inserted into the circuit in series with the field winding 23 and greatly reduces the voltage applied to the field winding 23. As a result the strength of the field of the generator 20 .is greatly reduced and consequently the output voltage of the generator 20 is reduced. Therefore, a much lower voltage is impressed upon the terminals of the magnet i2 during the remaining time of the energized portion of the duty cycle.

The voltage of the generator 20 when the resistor 30 is not in series with the field winding 23 may be as high as 125% of the normal magnet voltage. Insertion of the resistor 30 into the circuit may reduce the voltage of the generator 20 to 87% of the normal voltage of the magnet i2. In this way, an operation similar to that described in connection with Fig. 1 is obtained. The system of Fig. 4 is more economical than that of Fig. 3 in that there is a very small power. loss in the resistor 30 as compared with the power loss in the resistor l5 of Fig. 3.

The illustrated control circuits are greatly simplified for purposes of clarity. It will be understood that in order to safely control material handling magnets of the usual size some form of lifting magnet controller must be used. For example, the one disclosed in Wright Reissue Patent Number 20,724, issued May 10, 1938, could be readily incorporated with the system of either Fig. 3 or Fig. 14.

Having thus described my invention, I claim:

1. The method of removing magnetic materials from a pile of loose material by means of an electromagnet which is placed on the pile and the magnet and magnetically attached material are then freed and lifted from the pile, and which method comprises creating with the magnet a separating flux effective on the material by temporarily overenergizing the magnet to a degree greater than that which, if continuous, would cause. normal heating, maintaining said separating flux continuously while the magnet is in contact with material in the pile and the magnet and magnetically attached material are being freed and lifted from the pile, and, promptly after freeing and lifting the magnet and freed magnetically attached material, reducing the energization of the magnet to a value below normal and suflicient to create a holding flux capable of retaining the freed and magnetically attached material, and sufficiently low so that the reduced heating effect of the lowered energization compensates for the prior overheating occasioned by the temporary overenergization of the mag-net.

2. The method of lifting magnetic material from a pile thereof by means of an electromagnet which is placed on the pile and the magnet and magnetically attached material are then freed and lifted from the pile, and which method comprises creating a separating flux for separating a load of the material from the pile by temporarily overenergizing the magnet to a degree greater than that which, if continuous, during the energized periods of the usual duty cycle, would cause normal heating. separating the magnet and the attracted load away from the remainder of the pile while maintaining said separating flux continuously while the magnet is in contact with the material in the pile and the magnet and the attracted load are being freed and lifted from the pile, and, promptly after freeing and lifting the magnet and the attracted load, reducing the energization of the magnet to a value below normal and sufdcient to create a holding fiux capable of retaining the attached load and sufiiciently low so that the reduced heating effect of the lowered energization compensates for the prior overheating occasioned by the temporary overenergization of the magnet.

3. The method of lifting magnetic material from a pile thereof by means of an electromagnet which comprises placing the magnet on the pile of material, creating a separating flux for attracting a load of the material from the pile by temporarily overenergizing the magnet by applying to the magnet a terminal voltage greater than that which, if continuous during the energized portions of the usual duty cycle, would cause normal heating, separating the magnet and the attracted load away from the remainder of the pile while maintaining said separating flux, and, promptly after separating the load from the pile, reducing the terminal voltage applied to the magnet to a value suillcient to create and maintain a holding flux capable of retaining the load but suillciently low so that the reduced 10 heating eilect o! the lowered energization compensates for the prior overheating occasioned by the temporary overenergization of the magnet.

4. The method 01' controlling a magnet wherein the magnet is initially energized and the enerl5 energlzation.

WILLIAM R. YORKEY.

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