US2748322A - Lifting magnet control system - Google Patents

Description

May 29, 1956 LIFTING MAGNET CONTROL SYSTEM Original Filed May 31, 1951 2 Sheets-Sheet 1 57%;? 52 Maw/d.

May 29, 1956 J. V. OSWALD LIFTING MAGNET CONTROL SYSTEM Original Filed May 51, 1951 2 Sheets-Sheet 2 United States Patent F LIFTING MAGNET CUNTRGL SYSTEM Joseph V. Oswald, Chicago, Ill.

Continuation of application Serial No. 229,063, May 31, 1951. This application April 2, 1953, Serial No. 346,356

Claims. (Cl. 317123) This invention relates to an improved method of effecting and controlling the excitation and de-excitation of highly inductive electrical devices and more particularly relates to a method of controlling the complete operation cycle of lifting magnets such as are used by the steel industry in the handling of steel and iron in its various forms. It specifically relates, as herein set forth, to lifting magnet applications wherein the power for energizing the lifting magnet is derived from an individual generator driven by a suitable prime mover.

Such a generator is usually operated to serve as a source of constant potential direct current whereto the magnet is connected to be energized to pick up and hold the load material and then, through suitable switch means, is first disconnected from the generator and then again reconnected in reverse manner to effect a rapid de-energization and finally complete de-magnetization of the magnet to release the load material.

As is commonly known by those familiar with the related art, the operation of highly inductive devices from a constant potential source of power is fraught with difiiculties and problems which manifest themselves in such manner as severe arcing at the contacts used in the switch means, resulting in burning of the contacts thus shortening the life thereof, and puncturing of the insulation of the windings of the said device due to excessive voltage stresses produced therein by a too rapid collapse of the magnetic flux content, thereby resulting in premature destruction of the said windings. The high electrical inertia of some of such devices is also detrimental to satisfactory operation as is also the remnant flux left in the magnetic paths of the device at the end of a free or exponential decline of the said magnetic flux.

With specific reference to the operation of the lifting magnet from a constant potential power source, the initial connection of the windings thereof to the said source of energizing power produces no difiiculties from the standpoint of either arcing of contacts or voltage stresses in the winding. However, due to the high electrical inertia or inductance of the lifting magnet, the build-up of energizing current through the magnet is retarded considerably so that an appreciable portion of the operation cycle is invested in that function. This buildup of current and thus the magnetic flux through the magnet can be accelerated by impressing a higher than normal initial voltage on the magnet windings and then reducing said voltage to normal when normal full excitation has been attained in the magnet structure.

In the de-energizing phase of the operation cycle, however, the lifting magnet, under full excitation, is abruptly disconnected from the power source and reconnected again in reverse manner as previously described. It is during the transitional functioning of the switch means that objectionable arcing of contacts and production of high self-induced voltages in the windings occurs. Also, through the switch means, the voltage of the power source is placed in opposition to the self-induced voltage of the magnet to accelerate the decline of the said self-induced 2,748,322 Patented May 29, 1956 'ice voltage and the discharge current due thereto, but the reverse connection sequence of the said switch means generally introduces a limiting resistor into the circuit between the lifting magnet and the power source, the ultimate purpose of which being to limit the value of reverse current flow through the magnet winding for subsequent final disposition of the remnant magnetic flux. However, the presence of the resistor in the circuit during the discharge period impedes the flow of discharge current, lengthening the time required to bring the said discharge current to a zero value and causing the self-induced voltage in the magnet windings to maintain a value perhaps several times normal during the discharge period.

In this phase of the operation cycle also, the decline of the self-induced voltage in the magnet windings and the discharge current due thereto can be accelerated considerably by raising the opposing power source voltage substantially above normal during the discharge period of the cycle resulting in appreciably faster unloading of material from the magnet.

In the final or de-magnetizing step of operation the remnant flux in the magnet structure is disposed of by permitting a small value of current to build up through the magnet windings in a reverse direction thus reducing the remnant flux to zero, or, more generally, causing the value of the remnant flux to be passed through zero to a small value of opposite polarity at which time the fine load materal will be released as the remnant flux passes through zero or when the reverse excitation is finally removed.

Such lifting magnet installations wherein an individual generator is used as a power source are generally susceptible to conversion from the constant potential system to the well known variable voltage system of operation by the addition of a suitable source of separate excitation for the exciting field of the generator and, by the further substitution of the switch means herein described for the constant potential switch means previously used, to provide a lifting magnet control method superior to methods now in general use.

This invention, therefore, proposes to apply the variable voltage principle to the operation of the lifting magnet with the object in view of providing a method of lifting magnet control wherein:

(1) The initial voltage applied to the magnet windings for energizing the magnet is substantially above the normal rated voltage of the magnet to thus accelerate the build-up of current flow through and consequently the flux content within the magnet to its normal value and wherein the said initial voltage declines to normal value as the excitation of the magnet attains its normal maximum value.

(2) The inherent volt-ampere characteristics of the type of generator employed may provide an appreciable regulatory function, so that the voltamperes of power supplied to the magnet when the windings are cold and therefore of lower resistance, is lower than that supplied when the windings have reached normal or even abnormal operating temperature. Thus, the attractive power of the magnet tends to remain more uniform during the steady state phase of the cycle over a protracted period of magnet operation.

(3) The de-energizing function of the control provides a voltage substantially above normal in opposition to the self-induced voltage of the magnet to accelerate the decline of the magnet discharge current to a zero value to expedite unloading of material from the said magnet.

(4) The de-energizing function of the control also provides a means of interrupting or commutating the circuit between the generator and the magnet at a time of minimum current flow and a minimum of potential a minimum of arcing at contacts and with a minimum of current and voltage disturbance within the windings of the generator and magnet.

(5) The flow of magnet discharge current occurring during the de-energizing phase of the operation cycle takes place through a circuit of negligibly low ohmic resistance, so that the self-induced voltage of the magnet does not exceed the opposing generator voltage by a value greater than that which is propo 'ional to the ampere-impedance drop through the components of the circuit between the magnet and generator.

(6) The de-magnetizing function of the control disposes of the remnant magnetic flux by permitting the build-up of a small value of reverse current through themagnet windings after the discharge current of the magnet has been reduced to a zero value and after a predetermined time and, upon attainment of certain electromagnetic conditions within the lifting magnet, automatically acts to remove the said reverse current.

The invention comprises other objects, advantages and capabilities which will become apparent upon perusal of the attached drawings and specification, which show a preferred embodiment of the said invention. Howeverfsuch preferred embodiment is not intended to preclude modification of the invention and therefore it is to be understood any such modification need not necessarily be regarded as a departure from the spirit of the invention as herein set forth.

In the interest of subsequent understanding of the nature of the invention, attention is directed to the drawings of which Fig. 1 represents a schematic diagram of the complete system;

Fig. 2 is a line diagram of the power circuit or, more specifically, that portion of the system involved in the direct conduction and commutation of the energizing and discharge currents which flow between the generator armature member and the lifting magnet windings in the'course of operation;

Fig. '3 is a line diagram of the control circuit or that portion of the system which controls the actuating function of the various switch means and also the application, reversible manipulation and final removal of excitation to and from the main exciting field winding of the generator;

Fig. 4 is a graphic illustration of the complete operation cycle of the control system wherein the variations of voltage, current and ampere-turns of field excitation occurring thruout the operation cycle are presented in synchronous sequence. Their respective ordinate values as shown, however, do not necessarily bear a fixed relationship to each other but are merely intended to illustrate their approximate magnitudinal variation thruou thecycle. To that end, the reference scale on the left of the graph serves as a comparathle means.

For a general description of the major components of the invention reference is made to Fig. l of the drawing, wherein a lifting magnet it is to be energized by current generated in the armature member of a generator comprising said armature member ill, a separately excited main field i2 and an auxiliary series field winding 13. The generator chosen for this preferred embodiment, comprising members 11 i2.i3 is of the type known to the art as a diii rentially compounded machine wherein, under normal operating conditions, the ampere-turns of the series, field wi 13 are opposed to the ampere-turns of the main field winding 12and the resulting excitation which is applied to the field structure of the generator is a net excitation of a value which is equal to the difference of the two values of ampere-turns. Thus, such a generator will have'a drooping voltage characteristic from no load to full'loa'd condition and the ratio of no load voltage to full load voltage is determined by the relative values of ampereturns developed by main field winding 12 and series field winding 13.

Excitation to the separately excited main field winding 12 is obtained from a separate source of direct current (not shown) such as an exciter, rectifier or battery, etc., but which is herein represented by the characters E1 and E2.

Excitation to main field winding 12 is applied through the medium of a first switch A, which comprises a plurality of contacts as shown'and is actuated either manually or, as shown in this preferred embodiment, electroresponsively by a magnet 15 when said magnet 15 is energized from separate source E1 and E2 by closing a pilot switch 14. Contacts 1, 2 and 3 of first switch A are normally held open and contacts 4 and 5 are normally held closed by the biasing action of a tension spring 16.

Separately excited rnain field winding 12 is connected into a network comprising winding 12 and contacts 1, 2, 4 and 5 as shown whereby main field'winding 1-2 may, in the course of operation, be reversibly connected to the separate excitation source E1 and E2.

Contact 3 of first switch A serves as a pilot contact for energizing the winding I18 of a second switch B from separate excitation source E1 and E2.

Second switch B may be of the type known to the art as a clapper type contactor, which may be provided with two windings as shown. to be excited by'separate source E1 and E2 and may consist of many turns of small section conductor, while winding 19 is adapted to be excited by the relatively heavy currents flowing between generator 11 and lifting magnet id; and thus may consist of comparatively few turns of comparatively large section conductor.

The-electrical proportioning of windings 18 and 19 is based on the observation that the number of ampereturns of magnetomotive force required to close and'seal the armature and contacts of such a switch as represented by the second switch B may be an average of ten times the number of ampere-turns of magnetomotive force required to hold such a switch closed with an effectiveness just short of the release point. Thus winding 18 may be'propo'rtioned to exert, when energized from source E1 and E2, sufficient attractive force to close and seal the armature'and contact 6 of second switch B. Winding 19 may be proportioned to exert, with a given value of current flowing therethrough and winding 18 tie-energized, the minimum necessary attractive force to hold the armature and contact 6 closed while the current flow therethrough is equal to or in excess of the above mentioned critical value. Thus, switch B will close upon energizing winding 18, flow of substantial current through co-acting winding 1% will augment the attractive force of winding 18, holding switch B closed more positively, switch 3 will remain closed upon de-energizing winding 18 if the current fiow through winding 19 is equal to or in excess of the aforementioned critical value, and decline of the current flow through winding 19* beiow the said critical vaiue will result in the opening of second switch B.

A third switch C is of the type referred to in the art as a magnetic time delay switch, wherein the variations in magneticfiux content of the actuating magnet are retarded by the action of a lag coil 22 consisting of a copper sleeve or other winding shortcircuited upon itself electrically, thus resulting in a delay in the response of third switch C to the variations of the ampere-turn force of its actuating winding The structural and functional characteristics of third switch C are such that a substantial flow of current through winding 29 in one direction serves to attract the armature of said third switch C and'close'contacts 7 and -53. The retarding action of lag coil 20 delays the closing somewhat, but in the sequence of operation 'ofthe control system herein de- Winding 18 is adapted scribed such delay is of negligible consequence. Subsequent decline of the current flow through winding 20 to a zero value does not serve to open third switch C but, due to the action of the lag coil 22 and the residual magnetic flux remaining in the frame of the actuating magnet of third switch C, it is required that a small value of current of some time duration be passed through winding 20 in a reverse direction in order to eliminate the said residual flux against the retarding action of lag coil 22 to thus release the armature of third switch C and open contacts 7 and 8.

Proceeding to the description of the operation of the control system, reference is again made to the Figs. 1, 2 and 3 of the appended drawings.

Energizing-Assuming that power of suitable nature and value is available at El and E2, and that armature 11 of the generator is rotating at required speed, the operation cycle is initiated by the closing of pilot switch 14, thus energizing winding 15 of first switch A from source E1 and E2, and closing contacts 1, 2 and 3 and opening contacts 4 and 5 against the action of biasing spring 16. Closure of contacts 1 and 2 applies excitation to separately excited main field winding 12 from source E1 and E2 in what may be assumed for purposes of description as the positive direction, as distinguished from the reverse or negative direction to be encountered later in the operation cycle.

Simultaneous closure of contact 3 energizes winding 18 of second switch B, closing contact 6 to establish the circuit between generator armature 11 and lifting magnet 10. Reference to Fig. 2 of the drawing shows that at this stage of the operation cycle, armature 11, series field winding 13, lifting magnet winding 15), winding 20 of third switch C, the now closed contact 6 of second switch B, and winding U of said second switch B are all connected serially into a power loop (so called in the art) and are acted upon simultaneously by the ensuing flow of current originating in armature 11. Flow of said ensuing current through winding 19 of second switch B serves to augment the attractive force of winding 18 of said second switch B to hold contact 6 closed.

Flow of said current through winding 20 of third switch C attracts the armature of said switch C after a slight delay, due to the retarding action of lag coil 22, to close contacts 7 and 8. Closure of contact 7 establishes a subsidiary circuit through a resistor 21, the function of which will become clear in the final phase of the operation cycle. Closure of contact 8 establishes a portion of the circuit for the subsequent reverse or negative excitation of separately excited main field winding 12 (as distinguished from the previously mentioned forward or posi tive direction of excitation), said circuit for negative excitation being presently maintained open by the now open contacts 4 and S. The lifting magnet is now energised and ready to be applied to the load.

Steady state.The so called steady state of the operation cycle is that period immediately following attainment of full energization of the magnet and immediately preceding the de-energizing phase of the cycle. During the steady state period the magnet is applied to the load material, which is then picked up and transferred to the point of deposit.

Abrupt application of the energized magnet to the load material produces a momentary dip in the current flow to the magnet. It should be noted that the switch means does not respond to such current dips, even if extreme in magnitude, since second switch B is held closed by winding 18, which is energized from source E1 and E2 and, due to the predominance of the attractive force of winding 18 over that of winding 19, it would be necessary for the current flow to swing considerably to the reverse or negative direction to cause a neutralization of winding 18 by winding 19, a condition which is not normally encountered in practice. Such a reversal in current flow, if of consequential magnitudes and duration, might conceiv- 6 ably cause third switch C to open, but opening of contacts 7 and 8 at this time is of no consequence and upon resumption of normal current flow, switch C would again close.

Loss of all or a substantial part of the load in transit such as a large casting, etc. produces a momentary rise in the current flow to the magnet and a rise in current flow would obviously tend to increase the attractive force of windings 19 and 20 of switches B and C respectively, thus precluding their opening under such a condition.

De-energizing.With the load material picked up and transferred, the de-energization of the magnet and con sequent release of the load material is initiated by the opening of pilot switch 14, de-energizing winding 15 of first switch A, opening contacts 1, 2 and 3 and closing contacts 4 and 5 by the action of biasing spring 16. Opening contact 3 de-energized winding 18 of second switch B, but due to the presence of current in winding 19, switch 13 and consequently contact 6 remain closed.

Opening of contacts 1 and 2 and closing of contacts 4 and 5 of first switch A transfers separately excited main field winding 12 from a forward or positive excitation to a reverse or negative excitation, resulting in an almost abrupt reversal of the generator voltage. This reversal of generator voltage initiates a reduction of current flow to the magnet, with the result that a self-induced voltage; arises in the magnet winding in opposition to the now reversed generator voltage. During the de-energizing, stage of the cycle, this self-induced voltage exceeds the opposed generator voltage to the extent that the current flow in the power loop continues in its forward direction, though now under the impetus of the self-induced voltage of the magnet windings instead of the voltage induced in armature 11 of the generator. The sum total of this is that during the de-energization process the magnet windings become, in effect a generator and the generator proper becomes a motor against whose counter-electromotive force the magnet windings discharge the current of selfinduction.

This fiow of discharge current in the original forward direction acts upon winding 19 of second switch B to keep the power loop closed at contact 6 and also acts upon winding 20 of third switch C to keep contacts 7 and 8 closed for subsequent functioning at the end of the cycle.

This flow of discharge current against the opposing generator voltage continues in a progressively declining manner until the critical value is reached which will cause winding 19 of second switch B to release the armature of said switch B and open contact 6 to commutate (or alter) the power loop to a new condition of electrical composition.

For a theoretical illustration, it may be assumed that second switch B is adapted to open at a zero current flow through winding 19. This value of zero current will obtain when the self-induced voltage of the magnet windings is exactly equal to the opposing generator voltage. Under such an ideal condition contact 6 would open with a state of zero-volts and zero-amperes existing at its contact surfaces at the instant of opening, and an arc would be non-existent.

Actually, such a response is not practical of attainment, but any more or less minor deviation therefrom under practical conditions does not detract from the effectiveness of the arrangement materially. If switch B is adapted to open at a discharge current value equal to approximately 2% of the normal energizing current of the magnet, the are at contact 6, is almost imperceptible. This has been determined to be due in some measure to the inherent mechanical lag in the switch mechanism and since the discharge current is in a state of rapid decline at the time of opening of contact 6, a small amount of mechanical lag tends to delay the instant of opening toward the zero-current point.

De-magnetizing.-When the self-induced voltage in the magnet windings and the counter-voltage of the motorized generator strike abalance, the discharge current has declined to a zero value and contact 6, having opened, in-

troduces'into the power loop the subsdiaiy circuit containbalance and zero discharge current, theself-indu'ced volt-' age'of the magnet windings declines below the counter-' voltage of the motorized generator, whereupon the gen-' erator again resumes its normal function as such and initiates a build-up of current through the power loop in a reverse direction. This build-up of reverse current is paced by the still declining self-induced voltage in the magnet windings and reaches its final value upon attainment of a slight degree of reverse magnetization of the lifting magnet structure. The final steady state value of the reverse current is determined by the combined resistance of the lifting magnet and resistor 21 and the generator voltage obtaining at that stage of the operation cycle.

The build-up of the reverse current to its final value also acts upon winding of third switch C to eliminate the residual flux in the actuating magnet frame of said switch C and this process is opposed and retarded by the lag action of winding 22. After a time interval, the residual flux in switch C is reduced to the extent that the armature thereof is released, opening contacts 7 and 8. Opening of contact 7 opens the power loop, finally disconnecting the magnet from the generator to remove the reverse current from the lifting magnet, and the opening of contact 8 opens the reverse excitation sequence of the network involving separately excited main field winding 12, thus removing excitation from the generator and causing the generator voltage to fall to a zero value, completing the operation cycle.

The system may be made adaptable to virtually all sizes of magnets falling within the capacity of the generator by providing an adjustable resistor 21 and a multitapped winding 20 for third switch C to permit of obtaining a suitable time interval for the demagnetizing function.

The foregoing description of the operation cycle served to illustrate the operation of the switch means A-B-C and no account was taken of the voltage and current transients occurring within the power loop during the said cycle.

Reviewing the operation cycle from the standpoint of the graphical representation of Fig. 4, which is based on the characteristics of the type of generator used in the herein disclosed control system, identification of the various curves shown discloses that:

The curve A denotes a constant value of excitation applied to separately excited field winding 12 from source E1 and E2. For the sake of clarity, this valueis shown as being equal in both the forward or positive and the reverse or negative directions as comparison by suitable A means Will show.

The curve D represents the value of ampere-turns of excitation developed by separately excited winding 12 for between generator armature 11 and lifting magnet wind ing 10.

The curve B represents the resultant or net excitation impressed on the generator field structure as a result of the combined action of the ampere-turns of windings 12 and 13, curves D and C.

The curve E represents the voltage outputof the 'gen-' erator as a result of the excitation represented by curve B' i and is proportional to said curve B'throughou't the operation cycle.

The curve F represents the variation in magnitude and polarity (direction) of the current flow between generator armature 11 and lifting magnet winding 10.

The curve G represents the value of voltage appearing at the lifting magnet terminals in the final or de-magnetizing phase of the cycle.

The curve H is an approximation of the magnitude of the self-induced voltage occurring in the magnet windings during the deenergizing phase of the operation cycle.

Energizing-Closing of contacts 123-6 and opening of contacts i5 applies excitation to separately excited field winding 12 and also closes the power loop as described. Excitation of the ampere-turn value of curve D is applied to field winding 12 and rises to its full valve almost immediately because of the relatively low electrical inertia of such a winding. The resulting generator voltage initiates the build-up of current flow to the magnet (curve F) but, due to the high electrical inertia of the magnet, the build-up of current therethrough and consequently the rise of the counter-acting the magnet to energize it starts from an above normal value and recedes to normal as the magnet attains full energization.

Steady state.ln the steady state of energization, the positive main field ampere-turns (curve D) combined with the negative series field ampere-turns (curve C) produce a positive net field excitation (curve B) of a value required to produce the normal positive generator voltage (curve E) in turn required to pass the normal current (curve F) through the lifting magnet.

In the practice of lifting magnet operation, the energization of the magnet is generally done prior to applying the magnet to the load it is then customary to plunge the magnet into the load material, presumably for more efficient loading of the magnet. in plunging the magnet into a dense mass of load material, the abrupt-closing the working gap of the magnet results in an equally abrupt reduction or" the rel ctance of the magnetic paths of the now combined magnet frame and load material, requiring an additional braid-up of magnetic flux therein.

This additional build-up of magnetic flux causes a voltage se in the magnet windings in oppoot' self-induction to ri sition to the generator voltage, causing a dip in the current drawn by the magnet. The current has to then recover to its normal value while building up the new value of magnetic iiux required to hold the load material.

Therefore, in the steady state portion of the operation cycle (though not so illustrated in Fig. 4), any temporary reduction of the magnet current (curve F) caused by the above described practice results in a reduction of opposing ampere-turns of series field winding 13 (curve C),

with the further result that the net field excitation (curve B) and consequently the generator voltage (curve E) both rise to speed the recovery of the magnet current'to normal, said net field excitation and generator voltage returning to normal upon establishment of the new value of magnetic flux in the magnet and load material.

When a loss of all or a substantial part of the load occurs in transit, the reluctance of the magnetic paths is abruptly increased, causing a decrease in the magnetic flux content of the magnet, and a self-induced voltage again rises in the magnet windings but in this instance it coincides with the generator voltage and a rise in magnet current ensues. The rise in magnet current (curve F) causes an increase in opposing series field ampere turns (curve C) resulting in a decrease in the net field As the current fiow to the mag-- excitation (curve B), lowering the generator voltage (curve B) to thus reduce the total voltage in the power loop tending to maintain this abnormal current flow. As the magnet flux becomes stabilized at its new value, the voltage (curve E) and the current (curve F) again return to normal. Thus, the generator herein used exerts a stabilizing influence upon the system during the steady state period of the operation cycle.

De-energizing.pening of contacts 123 and closing of contacts 45 results in reversal of excitation to separately excited winding 12 and the generator voltage (curve E) abruptly reverses polarity, initiating the decline of the current (curve F). Continued flow of the current (curve F) is at a declining rate and is compelled by the self-induced voltage (curve H) in the magnet windings, which voltage is in opposition to and greater than the generator voltage (curve E) by a value necessary to send the said current (now referred to as the discharge current) through the power loop in the original forward or positive direction. Thus, during the de-energizing period of the cycle, the differentially compounded generator now becomes a cumulatively compounded motor whose back-voltage or counter-E. M. F., as it is referred to in the art, is the product of the now co-acting ampere-turns of winding 12 (curve D) and winding 13 (curve C) and the said back-voltage may rise to its maximum or saturation value.

Decline of the self-induced voltage and consequent discharge of magnet current against the above normal opposing voltage of the motorized generator proceeds to the point where the tension of the magnetic field of the lifting magnet has been relieved to the extent that the value of self-induced voltage in the magnet windings will be equal and still opposed to the voltage of the still more or less motorized generator at which time the discharge current will have declined to a near zero value (point 0). At this time, contact 6 will open to introduce resistor 21 into the power loop in preparation for the reversal of the current (curve F) for subsequent demagnetization of the lifting magnet.

Thus, during the de-energizing phase of the operation cycle the voltage applied to the magnet to accelerate the decline of the self-induced voltage and the discharge current due thereto is again of higher than normal value.

De-magnezfizing-Contact 6 having opened at approximately the instant of voltage balance and zero discharge current in the power loop, point 0, the generator now resumes its function as such and passes a reverse current through the magnet and the third switch C to complete the cycle of operation as previously described.

During the de-magnetizing stage of the cycle the generator voltage will again be above normal since the again opposing ampere-turns of winding 13 (curve C) are now of smaller value and the net field excitation (curve B) is above normal. Upon attainment of the steady state in the de-magnetizing function, the voltage at the generator coincides with curve E but the voltage at the magnet terminals falls to the value represented by curve G, the difference between the two representing the voltage drop across resistor 21.

Thus, upon final disconnection from the generator, the voltage at the magnet terminals and the value of current flowing therethrough are of such low order as to be easily handled by comparatively small switch means and the voltage transients occurring at that time are of negligible consequence.

With reference to the regulatory function of the type of generator employed in this control system, it is commonly known that the power rating of a lifting magnet, as noted on the nameplate thereof, is generally that power required to fully energize the magnet at its normal operating temperature. A cold magnet will have a lower ohmic resistance than a hot magnet and will, therefore, require a lower voltage at its terminals to pass the rated current therethrough.

The type of generator used in the control system herein disclosed is capable, to some extent, of compensating for temperature changes, both in its own windings and those of the lifting magnet. The degree of compensation is dependent upon the volt-ampere characteristics of the generator and that is in turn determined by the propertioning of windings 12 and 13 and the degree of magnetization attained in the magnetic paths of the machine.

This feature is, however, not essential to the operation of this control system, the chief objective of the use of such a generator being the attainment of the above normal voltages during the transitional stages of the operation cycle, namely energizing, de-energizing and demagnetizing, resulting in accelerated functioning of the system and a correspondingly rapid response of the magnet in the handling of load material.

This is a continuing application of my application Serial No. 229,063, filed May 31, 1951.

The invention having thus been disclosed and described, what is claimed as new and desired to be covered by Letters Patent is:

1. In a lifting magnet control system, a lifting magnet having windings energized from a generator driven by a suitable prime mover, said generator having a separately excited main field winding; a separate source of electrical power suitable for the excitation of said separately excited main field winding; a first switch means operable to reversibly connect said separately excited main field winding to said separate source of electrical power to effect a reversal of the polarity of the potential of said generator and consequently etfect a stoppage or reversal of the direction of the current flow between said generator and said lifting magnet windings, a second switch means electro-responsive to the current fiow between said generator and said lifting magnet and operable to close upon substantialcurrent fiow from said generator to said lifting magnet in one direction, said second switch operable to open after a predetermined time-current interval of current flow from said generator to said lifting magnet windings in a reverse direction, a first winding on the actuating magnet of said second switch adapted to be excited by the current flow between said generator and said lifting magnet windings, a second winding on said second switch comprising a continuous winding of electrically conductive material short-circuited upon itself and placed in mutually inductive relation to said first winding, such arrangement effecting a delay in the response of said second switch to de-energization of said first winding; contact means on said second switch connected into the reverse sequence connection of said first switch, opening of said contact means adapted to disconnect said separately excited main field winding from said separate source of electrical power.

2. In a lifting magnet control system, a lifting magnet having windings energized from a generator driven by a suitable prime mover, said generator having a separately excited main field winding; a separate source of electrical power for excitation of said separately excited main field winding and other components of said control system; a first switch operable to reversibly connect said separately excited main field winding to said separate source of excitation to effect a reversal of the polarity of the potential of said generator and consequently to effect a stoppage or reversal of the direction of the current flow between said generator and said lifting magnet windings; a second switch adapted to establish the circuit for, and commutate the current flow between said generator and said lifting magnet windings, said second switch electroresponsively operable to close upon excitation of a closing winding provided on the actuating magnet thereof, said second switch electro-responsively operable to open upon the de-energization of said closing winding and a decline of current flow below a predetermined value through a holding winding provided on the actuating magnet of said secondswitcn'said closing winding on said second switch adapted to be excited from the said separate source of electrical power, said holding winding adapted to be excited by the current flow between said generator and said lift magnet windings; and a resistance means for limiting the current flow between said generator and said lifting magnet windings to a value suitable for disposition of the remnant flux in the lifting magnet structure.

3. In a lifting magnet control system, a lifting magnet having windings operated from a generator driven by a suitable prime mover, said'generator having a separately excited main field winding, said generator having an auxiliary main field winding adapted to be excited by the current flow between said generator and said lifting magnet, said auxiliary main field winding being connected to counteract said separately excited main field winding in course of normal operation of said generator; :1 separate source of excitation for said separately excited main field winding and other components of said control system; a first switch operable to reversibly connect said separately excited main field winding to said separate source of excitation to effect a reversal of the polarity of the potential of said generator and consequently effect a stoppage or reversal of the direction of the current flow between said generator and said lifting magnet; 21 second switch adapted to establish a circuit for, and to commutate the current flow between said generator and said lifting magnet windings; said second switch electro-responsively operable to close upon excitation of a closing winding provided on the actuating magnet thereof; said second switch electro-responsively operable to open upon de-energization of said closin winding and decline of current flow below a predetermined value through a holding winding on the actuating magnet of said second switch; said closing winding adapted to be excited from said separate excitation source, said holding winding adapted for excitation by the current flow between said generator and lifting magnet windings; and a resistance means for limiting the current flow between said generator and lifting magnet windings to a value suitable for disposition of the remnant flux in the lifting magnet structure.

4. In a lifting magnet control system, a magnet having windings operated from a generator driven by a suitable prime mover, said generator having a'separately excited main field, said generator having an auxiliary main field winding adapted to be excited by the current flow between said generator and said lifting magnet windings, said auxiliary main field winding being connected to counteract said separately excited main field Winding in the course of normal functioning of said generator; a separate source of excitation for said separately excited main field winding and other components of said control system; a first switch electro-responsively operable to reversibly connect said separately excited main field winding to said separate source of excitation to reverse the potential polarity of said generator to effect a stoppage terminedvalue through a holding winding-thereupon to open said actuating magnet of said second switch; a resistance for limiting the current flow between said generator and said lifting magnet to a value suitable for disposition of the remnant flux in the lifting magnet structure; a third switch electro-responsive to the current flow between said generator and said lifting magnet and operable to dose upon substantial current flow from generator to said lifting magnet in one direction, said switch operable to open after a predetermined timecurrent interval of current flow from generator to said lifting magnet in a reverse direction, a first Winding on the actuating magnet of said third switch adapted to be excited by the current flow between said generator and said lifting magnet windings, a second winding on said third switch comprising a continuous winding of electrically conductive material short-circuited upon itself and placed in mutually inductive relation to said first winding, such arrangement effecting a delay in the response of said third switch mechanism to de-energization of said first winding; a contact on said third switch connected into the reverse sequence connection of said first switch, opening of said contact adapted to effect the removal of excitation from said separately excited main field winding.

5. In a lifting magnet control system, the combination of the lifing magnet windings operated from a generator driven by a prime mover, said generator having 21. separately excited mainfield winding, said generator having a series field winding connected to counteract said separately excited main field winding in the course of normal generator function; a separate source of excitation for said separately excited main field winding and other control systemv components; an electro-responsive first switch energized from said separate source of excitation and operable to reversibly connect said separately excited mainfieldwinding to said separate. source of excitation; an electro-responsive second switch having contacts to initially establish and subsequently commutate the circuit between said generator and lifting magnet; said second switch having two actuating windings, one of which is adapted to be excited by said separate source of excitation and the-other adapted to be excited by the current flow between said generator and lifting magnet; an electro-responsive third switch having an actuating winding adapted to be excited by the current flow between said generator and lifting magnet, said third switch having a short-circuited winding placed in mutually inductive relation to saidactuating winding to effect a delay in the response of saidthird switch to the rise or decline of current in said actuating winding; said third switch having contacts adapted to initially establish and subsequently interrupt circuits controlling excitation to said separately excited main field winding, said third switch having contacts. adapted to initially establish a subsidiary circuit through aresistance to limit the value of de-magnetizing current for disposal of the remnant flux within the lifting-magnetstructure and finally to interrupt the circuit between said generator and liftingmagnet.

References Cited in the file of this patent UNITED STATES PATENTS 2,206,823 Wertz July 2, 1940 2,257,361 Yorkey Sept. 30, 1941 2,287,745 Morawetz June 23, 1942 2,390,377 Lillquist Dec. 4, 1945

Images