US2606959A

US2606959A - Controller for inductive devices

Info

Publication number: US2606959A
Application number: US189442A
Authority: US
Inventors: John D Leitch; Richard G Widdows
Original assignee: Electric Controller and Manufacturing Co LLC
Current assignee: Electric Controller and Manufacturing Co LLC
Priority date: 1950-10-10
Filing date: 1950-10-10
Publication date: 1952-08-12
Anticipated expiration: 1969-08-12

Description

Aug. 12, 1952 J. D. LElTcH ET AL 2,606,959

CONTROLLER FOR INOUOTIVE DEVICES Filed Oct. 1o, 195o s sweets-snee#h 1 v INVENTORS JO//A/ .0. E/Tc/f .f

BY l WY 1: g. 4- Muzaw,

JWM-

Aug. 12, 1952 J. D. LElTcH ET AL 2,606,959

CONTROLLER FOR INOUOTIVE DEVICES Filed Oct. l0, 1950 I5 Sheets-Sheet 2 Aug. 12, 1952 J. D. L ElTcH ET AL 2,606,959

CONTROLLER FOR INOU'OTIVE DEVICES Filed Oct. 10, 1950 5 Sheets-Sheet 5 Patented Aug. 12, 1952 2,606,959 CONTR-GLLER. FOR INDUCTIVE DEVICES John 1).*'Iie`itch and Richard G. Widdows, Shaker Heights, Ohio, assignors to The Electric Controller & -ltlariufacturi'ng Company, Cleveland, Ohio, a corioration of Ohio AAppiranien october 10, 1950, serial No. 189,442

lS Claims.

This invention Yrelates to an improved system for, vand vmethod of, controlling inductive devices, and more particularly'to Van improved'dntrol systeni and. method for vautornatically effecting de- (mag'netization 4of large inductive devices such as large lifting'magnets in a short time.

YIt is common practice to inag'netize va lifting magnet by connecting the magnet toa source of constant unidirectional voltage and to demagnetize the magnet vby reversing the connections to the source through a resistor of Xedvalue which eventually limits the reverse current that lions to the Vinagii'et alter the self-induced magnet Voltage has 'decreased below the voltage oi' 'the sou e. The reverse current is interrupted automatically when the magnet fluir is approximately aero. Full "advantage of vthe Operating eillency oi the larger lifting magnets now `being used in in easing numbers :by the steel and allied industries ls not realized when the de'rna'gnet'latn lol the `larger magnets is controlled in the -foregoing manner` which has becomecommon for fsnfaller'magnets. This is because the -inductanee to vresi'st'ance ratio or time constant of the larger lifting magnets is so great that, 'when the 'larger magnets are 'controlled by conventional. magnet eontmiiei's, the Imtecf 'flux decay is 'so 'S10-w that vexcessivedribbling f scrap iron and similar loads results. Furthermore, when the larger magnets 'a'r'e controlled in the usual manner, the time req'ui-r'ed to build up lthe 'flux lto its maximum value and vthe time required to reduce the flux substantially to aero together constitute a relatively large portion of the overall operating cycle which generally includes "attracting, lifting, transporting, and rdrofpping of loads. g

The build-up or magnetization time of the larger magnets has been vdecreased in some fin#- stances :b'y cverexciting the magnet during the vattracting vperiod and the initial portion of the lifting period vof the operating cycle and reducing the excitation during the remainder of 4the lifting period andduring the transporting period to prevent damaging overheating of the magnet. rThe use 'of two voltage lvalues inthis manner also reduces the energy stored in themagnet prior to demagncti'z'ation and therefore the demagnetzation time is reduced slightly 'as 'compared to energizing with a lined voltage, vbut this reduction in time is insuiiicientto reduce appreciably the excessive dribbling of `the scrap material, n

The magnet control system 'or automatic lifting magnet 'controller fof lthe present invention ef# fects 'complete "demfagnetization of veven the largest lifting magnets in 'such a short time that scrap iron or 'similar 4loads do not dribble oli the masynet 'but instead drop quickly and substantially all at once. ln iact, the demagnetization time obtained 'with ythe new 'controller is so short that it becomes a relatively unimportant part of the overall `dut`y cycle. The improved magnet controller can provide a conventional energizing circuit 'or it can include equipment to kprovide the large initial voltage at the magnet for a time interval, thereby to cause the q'ui'ck build-up lof magnet followed by the period 'of reduced voltage prior to vdema'gnetiza'tionwhich prevents damaging'over-heatin'g of the magnet.

In'ne structure in which the present invention is embodied, demagnetiz'ation of a magnetized lifting magnet is initiated inthe usual manner .by closing a drop contactor just Ipriortc') opening of a vlift contacter thereby to connect the magnet to the source in areverse direction through a resistance in series therewith. In accordance with ourinventlonhowever, the value of this resistance 'is so selected in relation to the voltage of the source, "the inducta'nce and resistance of the Vmagnet Winding, and thearc quenching ability of the Vlift 'contacter that the voltage rise at the magnet terminals Vupon opening of the lift contacter apprloachesbut does not exceed a maximum safe value. When 'the magnet voltage approacl-les line voltage, a relay responsive tothe magnet voltage operates to reducethe resistance in series with magnet. This reduced value of resistance is so selected in relation to the voltage of the source andthe inductance and resistance of the magnet Winding that th'e reverse current through the magnet increases as rapidly as possible without 'exceeding the rate at which it would interfere With the proper operation of an automatic interrupting` means, lthereby insuring interruption of the reverse current circuit before the magnet flux reverses..I Injonjelmodication, the magnet circuit resistanc'ejis automatically increased before it is finally decreased;f Arfurther modification is a magnetcontroller which also provides a high initialfenergization to shorten the build-up time, the initial energiz'ationbeing subsequently reduced to prevent overheating of the magnet.

Although the invention is shown as an improvement of the type of magnet controller dis#- iosed and claimed in Wright Reissue Patent No. -20 ,V724 of May'lO, 1938, its applicability to other types of automatic controllers for inductive dvices is apparent.

An 'object of this invention is to `|provide lan improveusysteiu method for effecting rapid demagnetiaa'ti'on of a large inductive device.

Another object is to provide an improved control system for, and method of, controlling an inductive device in which the resistance of the discharge resistor is changed during the demagnetization period in order to shorten the demagnetization time.

A further object is to provide a control system for an inductive device in which the resistance of the discharge resistor is reduced automatically at approximately the instant that the current flowing in the winding of the device reverses or becomes substantially zero.

A more detailed object is to provide a control system for an inductive device in which a voltage relay controls the resistance of the discharge path in accordance with the voltage at the terminals of the device.

A more specic object is to provide a controller for an inductive device in which the value oi the resistance of the discharge circuit during reduction in the magnetizing current is so related to the energy stored in the device that the peak discharge voltage reaches but does not exceed a maximum safe value and in which the value of the resistance in series with the device during build-up of the reverse current is so related to the voltage of the source and the time constant of the device that the reverse current increases as rapidly as possible while still permitting automatic removal of the reverse voltage when the magnetic iux is approximately zero.

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

Fig. 1 is a wiring diagram of a preferred form of magnet control system;

Figs. 2 and 3 are wiring diagrams of respective modifications, and

Figs. 4 and 5 are graphs illustrating the operation of Figs. 1 and 2, respectively.

The illustrative embodiment of the control system shown in Fig. l is a lifting magnet controller comprising an electromagnetic lift contactor I0, an electromagnetic drop contactor II, an electromagnetic resistor control contactor I2, a voltage relay I4, and a two-position master switch I5. The controller is arranged to be supplied from a suitable source of unidirectional voltage I6 through supply leads I'I and I8 which are also arranged to supply power from the source I6 to an inductive device such as a large lifting magnet I9 provided with a winding |9w having terminals I9a and ISb. Instead of a lifting magnet, the inductive device may be a large electromagnetic brake, clutch, chuck or similar apparatus having a highly inductive operating winding to be energized by direct current.

The contactor I may have an operating winding Iw, normally open main contacts Ia and Illb, normally open auxiliary contacts I0c and Id, and normally closed auxiliary contacts IDe and IIlf. A suitable means, such as a copper sleeve Ig surrounding the core of the contactor I0, is provided to delay the opening of the contactor ID upon deenergization of the winding I Uw. The contactor I I may have an operating winding IIw, normally open main contacts IIa and IIb, and normally open auxiliary contacts IIc. The contactor I2 may have an operating winding I2w, normally open main contacts |2a and |2b, and normally closed auxiliary contacts |2c. The relay |4 may have an operating winding |4w and normally closed main contacts |4a and preferably is provided with suitable well-known means (not shown) to facilitate adjustment of its pickup and drop-out voltage values. The master switch I5 preferably has a pair of relatively stationary contacts I5a and |51) adapted to be bridged by a relatively movable contact I5c when the master switch is in the drop position as shown and indicated by a broken line D, and a pair of relatively stationary contacts |5d and |56 adapted to be bridged by the movable contact I5c when the master switch is in the lift position indicated by a broken line L. Any suitable form of master switch may be used which is capable of completing one circuit upon interruption of another, and vice versa, and which preferably is biased as by a spring (not shown) to the drop position.

A pair of resistors 2D may be connected in series with the winding I Iw and a pair of resistors 2| may be connected in series with the winding |4w. Suitable means, such as a xed resistor 24 and an adjustable resistor 25 connected in series with each other and in parallel with the winding IIw, are provided to control the drop out of the contactor II. Resistors 28 through 3| are interposed in series with each other in a discharge and reverse current circuit for the magnet I9. Preferably, the

resistors

28 and 30 have approximately the same resistance, and likewise the resistors 29 and 3| have approximately the same resistance.

Operation of Fig. 1

Further understanding of the embodiment of Fig. 1 can be had from the following brief and detailed descriptions of its operation. Briefly, the magnet I9 is magnetized by moving the master switch contact |5c to the lift position L which causes the contacts IIla and |01) to close and connect the winding lSw directly across the supply lines I'I and I8. The magnet I9 may be demagnetized by returning the contact I5c to the drop position D which causes closure of the contacts IIa and IIb followed by opening of the contacts Ina and Ib. This causes the voltage at the magnet terminals ISa and I9b to reverse and increase suddenly whereby current continues to now in the same direction through the winding |910, the circuit being completed through the contacts IIa and IIb, the

resistors

28, 29, 30 and 3| and the source I6. The increased voltage between the terminals I9a and ISb causes pick up of the relay I4 and thereafter decreases exponentially and, when it approaches the Voltage of the source I6, the relay I4 drops out to cause the contactor I2 to close its contacts |2a and I2b. This reduces the resistance in series with the magnet by shunting the resistors 29 and 3| so that after the voltage between the terminals I9a and I9b becomes less than the voltage of the source I8, the magnet current builds up rapidly in the reverse direction. The voltage between the terminals ISa and |9b becomes less than the voltage of the source because of the voltage drops across the

resistors

28 and 30 which increases with increase in the reverse current. When the voltage across the terminals I 9a and |91) reaches a low value indicative of zero flux in the magnet and determined by adjustment of the resistor 25, the contactor II drops out to interrupt the reverse current circuit.

Considering now the operation of the embodiment of Fig. 1 in more detail, movement of the master switch contact |5c to the lift position L completes an energizing circuit for the winding |0w from the supply line 'I through the contacts |5d, |50, |5e and |2c and the winding IDw to the supply line I8. The contactor I0 responds accpte 5 immediately upon energization of its winding `I Uw to 'close -its contacts Illa, Ib, I'Oc, and ld 'and to open its contacts Ie and Illf. Closure of Ythe contacts Ic and Id completes a portion of an energizing cir-cuit `for the winding I Iw later to be fully completed upon return of the contact ISc to the drop position D. Opening of the contacts ille disconnects the winding l Iw from the magnet terminal IQa so that no cur/rent can ilow in the winding IfIw while the contact I5c is in the lift position L. Opening of the contacts laf ifnsures that an energizing circuit for the winding |2w cannot be completed until after the contacter I has dropped out. Closure of the contacts I'a and Illb complet-es an energizing circuit for the magnet I9 from the supply line Il through the contacts Illa, the terminal |911, Vthe winding I'Sw, the terminal |91), and the contacts I 0b to the supply line I8. The current in the magnet winding |9112 now increases exponentially from zero in the direction of the arrow 32 at a rate dependent upon the inductance to resistance ratio off the winding Iiw and the magnitude of the voltage of the source I6 to a final steady value determined b'y the resistance of the winding Iflw and the voltage of the source I5. This nal steady current value is indicated by a horizontal line 3'4 in Fig. 4 wherein current in the winding ISw is plotted against time, The winding |920 preferably is so selected that when operated on a known duty cycle and carrying the current 34 during its energized period, the magnet I9 will eventually reach but not exceed a maximum safe temporature.

Although the windingl Mw is now connected across the supply lines I1 and I8 in series with the resistors 2|, the relay I4 is selected and adjusted so that it does not pick up at this time.

The energized magnet ISI may now attract, lift, and transport a load (not shown). When it is desired to dcmagnetize the magnet I9 to drop the load, the master contact I5c is permitted or caused to return to the drop position D. This interrupts the energizing circuit for the winding Illw at the contacts ld and I5e and 'completes the energizing circuit for the Winding IIw from the supply line I1, the contacts I'5d, i527, Ic, and IEa, a conductor 35, the still closed contacts Ic, the winding I iw, and the still closed contacts I @d to the supply line IB. In response to energization of its winding IIw, the contacter II closes its contacts I I a, lib, and Hc. Although the energizing circuit to the winding Iw has been interrupted, the action of the copper sleeve Ig causes the contactor IU to remain in its energized position for a time interval to insure that the contacts IIa and lib close before the contacts Illa, and Illb open. rlhe contacts llc are in the energizing circuit for the winding I2w but their closure does not complete this circuit because the contacts I'f are still open. Closure of the contacts Ila connects the series connected

resistors

28 and 29 in parallel with the magnet winding lw, and closure of the contacts I Ib connects the series connected resistors 39 and 3| in parallel with the winding 1920. This has no effect on the current in the winding I9w.

The copper sleeve I'Ilg delays drop out of the contacter I9 just long enough for the contacts I IU. and i'lb to close. Drop-out of the contacter I0 causes opening of the contacts Illa and IIJb which disconnects the terminal I9d from the supply line l1 and the terminal I'9b from the supply line I9, but, since the contacts IIa and II b are now closed, the winding I9w remains connected to the supply lines in a discharge and reverse 6 current circuit which is from the supply line I1 through the resistors 3l and 3U, the contacts IIb, the terminal i191), the winding lSw., the terminal Isa, the contacts I la, and the

resistors

28 and 29 to the supply line i8. The voltage between the terminals |911 and leb upon opening of the contacts Illa and 59h suddenly reverses and, dueto the self-inducta'nce of the winding lw., increases to a value far above that of the source I5, vand the current in the winding Iw continues to flow in the direction of the arrow 32 through the discharge and reverse current circuit just traced. Although the contacts luc and led open upon drop-out or" the contacter l, the winding IIw remains energized because the concurrent closure of the contacts ite connects the winding IIw across the terminals 19a and 59?; in series with the resistors 29.

The maximum. Value of the voltage at the ter'- minals I9a and lh upon opening of the `contacts lila vand I9?) is dependent upon the time required to extinguish the varcs at the contacts lila and 613, the voltage of the source I6, the inductance of the winding Iiiw, and the total resistancejof the winding lino and the series connected4 rcsistors 2d, 29, Sli, and 3|. Preferablyy the contacter kle is selected so that arcs persist at the contacts Illa and ich long enough to maintain the voltage at the terminals ISa and leb materially below the value it would reach if no or little arcing occurred. The arcs thus dissipate some of the magnetic energy stored in the winding i-Sw and permit the use of a higher discharge circuit resistance which permits quicker demagnetization.

Consequently upon the sudden reversal and increase of the voltage at the terminals lea and l9b, the relay I picks up to open its contacts Ma. .The contacts Ida are in the energizing circuit ior the winding I2w but they openy so -soon after the contacts IQJ close that the winding I2w does not become operatively energized even momentarily at this time.

Assuming that the arcs on contacts Illa and Illb are interrupted at time t1 in Fig. 4, the cuirent in the Winding Ilw starts at time ti to decrease exponentally as indicated by the curve 36. The resistors 29 and 3i are so selected that the total resistance of the

resistors

23, 29, 39, and 3i when al1 are in series is such as to permit the initial decrease of current in the Winding IS-w to be as rapid as possible without causing a damaging voltage rise. Thus, the slope of curve 36 is made as steep as possible by causing the resistance of the discharge and reverse current circuit to be as large as possible, the maximum value being determined by the maximum peak voltage that can be withstood safely by the insulation of the winding |910.

When the magnet discharge current reaches a value indicated at 31, the voltage between the terminals I9a and ISb has decreased to a voltage only slightly in excess of the Voltage of the source I6. The relay I4 is adjusted to drop out when the voltage at the terminals |911 and lllb reaches this value. Drop out of the relay I4 causes closure of the contacts Ma which completes the energizing circuit for the winding I2w from the supply line Il, the contac I5d, I5b, I5c, I5a, IDf, Ma, and IIc through the winding I2w to the supply line I8. The contactor I2, in response to energization of its winding I-Zw, closes its contacts I 2a and I2b and opens its contacts I2c. Opening of the contacts I2c prevents operation of the contacter I0 while the contactors II and I2 are picked up thereby to prevent large currents from flowing in the resistors 28 and 3U which are of relatively low oh-mic value. v Closure of the contacts |2a and |217 short circuits the resistors 29 and 3|, respectively. If the voltage between the terminals |9a and ISb has not become equal to the voltage of the source I5 when the contacts I2a and I2b close, the magnet current decreases further as indicated by the curve 38.

When the contacts I2d and |2b close or shortly thereafter, the voltage between the terminals I9a and |91) becomes equal to the voltage of the source I6 and the magnet current reaches zero as indicated at t2 in Fig. 4. The current in the winding I9w then starts to increase in the reverse direction or in the direction of the arrow 3| in Fig. l. The resistance of the

resistors

28 and 29 is sufficiently low so that the reverse current builds up as rapidly as possible thereby to reduce the remanent magnetism to zero in as short a time as possible consistent with proper operation of the contactor II. The rise in reverse current is indicated by a curve 39 in Fig. 4. The rise in reverse current is accompanied by a further decrease in the voltage between the terminals I9@ and |917. The resistor 25 is so adjusted according to the magnetic properties of the load being handled that the contactor I drops out to open its contacts Ilia, IIb, and IIc when the reverse current reaches a value corresponding to substantially zero flux in the magnet I9 which is indicated at t3 in Fig. 4. Opening of the contacts IIc causes deenergization of the winding I2w and opening of the contacts Ira and IIb disconnects the winding I9w from the supply leads I'I and IB. The reverse current circuit is thus interrupted at or near the point of zero flux and the magnet I9 remains substantially demagnetized until the contactor I is again operated.

By properly selecting the value of the resistance in series with the magnet at the initiation of discharge and causing the contactor I2 to reduce this resistance to a very low value at the proper time during the demagnetizing cycle, the time required for complete demagnetization is made very short and dribbling of loads is thereby prevented. For example, a 230 Volt magnet having a resistance of 2.5 ohms and an inductance of approximately 2O henries when loaded with scrap requires about 6 seconds to completely drop the `load when a discharge resistance of l ohms is used throughout the demagnetization cycle, whereas, when the discharge resistance is changed from 20 to 4 ohms during the demagnetization cycle as herein described, the demagnetization time is only about 2 seconds. This shorter time prevents dribbling of the load, permits it to be accurately placed, and speeds up the material handling operation.

Although in Fig. 1 the closure of the contactor I2 is controlled by the voltage relay I4, it is apparent that a relay directly responsive to the magnitude of the magnet current could be used instead if desired. 1t is also apparent that other well-known means could be used to interrupt the reverse current circuit at the proper time.

Modification-Fig. 2

In the modication of Fig. 2, wherein control elements which may be the same as those of Fig. 1 are referred to by the same reference characters, the resistor 29 of Fig. l is replaced by a pair of series connected resistors 40 and 4| and the resistor 3| is replaced by a pair of series-connected resistors 42 and 43. A lift contactor 45 is similar to the contactor I0 of Fig. 1 in that it may have main contacts 45a and 45h interposed in the energization circuit for the magnet I9, normally closed auxiliary contacts 45e and 45j, an operating winding 4520, and a copper sleeve 45g. The contactor 45 differs from the contactor l0 in that in addition to normally open auxiliary contacts 45e and 45d it may have also normally open auxiliary contacts 45h. The contacts 45h control the energization of a winding 46w of an electromagnetic resistor control contactor 46 which may have normally open main contacts 45d and 46D and normally closed auxiliary contacts 46c. Drop out of the contactor 46 is preferably delayed for a time interval after deenergization of its winding 46111 by any suitable means shown as a resistor 48 and capacitor 49 connected in series with each other in parallel with the winding 46w. The contacts 46a and 45h when closed short circuit the

resistors

40 and 42, respectively.

The contactor I2 of Fig. 1 is replaced in Fig. 2 by an electromagnetic resistor control contactor 59 which may have an operating winding 50w, normally open main contacts 50a and 59h, normally closed auxiliary contacts 59C, and normally open auxiliary contacts 50d. The contacts 50a and 50h when closed short circuit the resistors 4| and 43, respectively.

Operation of Fig. 2

Further understanding of the embodiment of Fig. 2 can be had from the following brief and detailed description of its operation. The magnet I9 in Fig. 2 is energized by moving the master switch contact |5c to the lift position L which causes closure of the contacts 45a and 45D connecting the winding I9w directly across the supply lines Il and I8. The magnet I9 may -be deenergized by returning the contact I5c to the drop position D which causes closure of the contacts IIa, lib, 46a, and 46D followed by opening of the contacts 45a and 45o. The magnet I9 now starts to discharge through the resistors 28, 3B, 4I and 43 in series. The contacts 45h open when the contacts 45a and 45h open, and shortly thereafter the contacts 46a and 46h open to insert the

additional resistors

40 and 42 in series with the magnet I9. When the voltage across the terminals I9c and |9b approaches the voltage between the supply leads I1 and I8, the relay I4 drops out to cause closure of the contacts 50a and 50h which is followed immediately by reclosure of the contacts 46a and 4613. This reduces the resistance in series with the magnet by shunting the resistors 40, 4|, 42, and 43 and the magnet current builds up rapidly in the reverse direction. When the voltage across the magnet terminals I9a. and |92) reaches a predetermined low value, the contactor II drops out to interrupt the reverse current circuit.

Considering now the operation of the magnet controller of Fig. 2 in more detail, movement of the master switch contact I5c to the lift position L completes an energizing circuit for the winding 45w from the supply line |'I through the contacts I5d, |50, |5e, 50c, 46c, and the winding 45u) to the supply line |8. The contractor 45 in response to energization of its Winding 45w closes its

contacts

45a, 45h, 45e, 45d, and 45h and opens its contacts 45e and 45] without time delay. Closure of the

contacts

45a, 45h, 45e, and 45d and opening of the contacts 45e and 451 results in completion and interruption of circuits as in the case of the corresponding contacts of the contractor I in the embodiment of Fig. l. Closure of the contacts 45h, which are in an energizing circuit for the winding 4610 permits that winding to be energized upon return of the contact Ic to ther drop position D. With the contacts 45a and 4519 closed, the current in the magnet Winding I9w increases exponentially from zero in the direction of the arrow 32 until it reaches a iinal steady value indicated at 34 in Fig. 5 wherein current in the Winding IS-zu when controlled by the controller of Fig. 2 is plotted against time as in Fig. 4.

When the contact I5c is returned to the drop position D, the energizing circuit for the Winding 4510 is interrupted at lthe contacts I5d and I5e and an energizing circuit for the Winding IIw is completed from the supply line I'I, the contacts I5d, I5b, |50, and I5a, a conductor 35, the now closed contacts 45C, the Winding I I'w, and the now closed contacts 45d to the supply line I8. The energizingvcircuit for the Winding 46u: is concurrently completed from the now energized conductor 35 through the contacts 45h, and the winding 46u) to the supply line I8, and the capacitor 43 in parallel with the winding 46w quickly accumulates a charge.

In response to energization of the windings I Iw and ltw, the contacts IIa, IIb, IIc, 46a and 4Gb close and the contacts 46c open. Closure of the contacts I Ic does not complete an energizing circuit for the winding 5w since this circuit is open at the contacts 45j.. Closure of. the contacts IIa connects the series connected resistorsv 28' and 4I in parallel with the magnet winding IQw, the concurrent closure of the. contacts 46a short circuiting the resistor 40, and similarly closure of the contacts IIb connects the series connected resistors 30 and y43in parallel with the magnet winding I9w, the concurrentr closure of the contacts 4Gb short circuiting theY resistor 42.

Shortly after the contacts IIa and IIb close, the contactor 4.5 drops out-to open itscontacts 45a and 45h which disconnects the terminal Isa from the supply line I'I and the terminal |51) from the supply line I 8. The Winding I19w, however, remains connected to the supply lines in a reverse sense in a discharge and reverse current circuit from the supply line Il through the contacts 46h, the resistors 43 and 30, the contacts IIb,

the terminal |91?, the terminal IS, the contacts Ha, the resistors 28 and 4I, and the contacts46c to the supply line I 8. The

resistors

28, 30, 4I, and 4-3 are so selected that their combined' resistance when connected inseries is such that, with. arcs existing for a time interval at the contacts 45a and 45D, the self-induced voltage at the terminals I9@ and |917 substantially reachesl but does not exceedy a maximum safe voltage. After the arcs are interrupted, the current in the Winding I 9u; starts to decrease at a rate;

indicated by a curve 52 in Fig. 5.

The contacts 45h open when the contacter 45 drops out to disconnect the winding 4Sw from the supply lines I`I and I8, but the contactor 46 remains in' its picked upposition for a time interval While the capacitor 49. discharges through the Winding 45211. The time delay period of' the contactor 46 and the resistance of the resistors ltand 42 are so selected as to permit the' energy f in the winding ISw to decrease to a value such that upon opening of the contacts 45d and 4Gb the resistance in the magnet circuit is increased suiiiciently to cause the self-induced voltage at the terminals ISa' and I 9b to again reach a maxi- 10 mum safe value. When the contacts 46a and 461) open, the magnet current has decreased to a value indicated at 54 in Fig. 5 and thereafter the magnet current starts to decrease more rapidly as indicated by the curve 55.

When the magnet current reaches a value indicated at 56, the relay I4, which picked up when the contacts a and 45h opened, drops out to reclose its contacts I4a to complete an energizing circuit for the winding u; from the energized conductor 35 .through the contacts 45j, I4a, and IIc and the Winding 50m to the supply line I8. The contactor 55 thereupon closes its contacts 50a, 50h, and 50d and opens its contacts 50c. Opening of the contacts 50c prevents inadvertent re-energization of the Winding 45u). Closure of the contacts 56d completes an energizing circuit for the winding 46u) from theV energized conductor 35 through the contacts 55d and the winding 46w to the supply line I8. Consequently, closure of the contacts 56a and 50h is quickly followed by closure of the contacts 45a and 46o, and the

resistors

45, 4I, 42, and 43 are all short circuited practically at the same instant. This causes the change in rate of current delay indicated at 56 in Fig. 5. When the voltage at the terminals Illa. and Iib` becomes less than the sup-ply voltage, the current which had `decreased from the point 5 6 reverses direction and increases as indicated by a curve 58. The resistance of the

resistors

28 and 29 is such that the rate of increase of the reverse current is a maximum consistent with proper drop out of the contactor Il when the voltage across the terminals Ia and IBb reaches a predetermined low value.

Opening of the contacts IIaI and IIb uponk drop out of the contacter II disconnects the winding I9w from the supply conductors I7 and I8 and opening of the contacts IIc deenergizes the winding 50w. The contacter 50 then opens its contacts 56d to deenergize the winding 46w. The contacts 50c reclose immediately, but the contactor 45 cannot be operated again until the contacts 46c reclose.

When the embodiment of Fig. 2 has been used with the specic magnet hereinbefore reierred to, it has been found desirable to use an original discharge resistance of about 20 ohms which is increased to about 50 ohms before being reduced to a nal value of about 4 ohms.

Since in Fig. 2 the resistance of the discharge circuit is increased and then decreased during the demagnetization period, the embodiment o1' Fig. 2 provides a somewhat shorter demagnetization ltime ythan does the embodiment of Fig. 1. The latter is preferred, however, because of its simplicity.

Modification-Fig. 3

The embodiment of Fig. 3 shows how the embodiment of Fig. 1 may be modied to .provide a decreased rbuild up time for the magnet I9, and control elements which may be identical in lthe two embodiments are referred to by the same reference characters. It will become apparent from 'the description of Fig. 3 that the yembodiment of Fig. 2 can readily be modied in a like In Fig. 3 the voltage applied to the magnet I9 is varied somewhat in the manner disclosed in Yorkey Patent No. 2,257,361 and the voltage values relative to the resistance and heat dissipating ability of the magnet are preferably' chosen as therein described to obtain the advantages of that patent in addition tov t-hosesetl l forth herein.

The magnet controller of Fig. 3 comprises an electromagnetic lift contacter 60, an electromagnetic drop contactor 6I, an electromagnetic resistor control contactor 62, an electromagnetic voltage control contactor 64, an electromagnetic time delay relay E5, and the relay I4. Instead of the master switch I5, the controller of Fig. 3 includes a lift push button 66 having normally open contacts 66a and a drop push button 68 having normally open contacts 68a and normally closed contacts 5817 and 68C. Preferably, the push button SS is biased to its open .position by a strong spring 66s making it difficult to hold the push button 66 in closed position. The supply conductors II and I8 in Fig. 3 are arranged to be connected to a suitable direct current generator 'IIl through a circuit including series connected resistors 'lI and 'I2 when a d-ouble pole knife switch 'I4 is closed.

The lift contactor BU may have an operating winding llw, normally open main contacts 60a and Gb, normally open auxiliary contacts Gc, 60d, and 66h, normally closed auxiliary contacts 60e and 661, and a copper sleeve 60g. The drop contactor 6I may have an operating winding SIw, normally open main contacts Bla and SIb, normally open auxiliary contacts 6Ic, and normally closed auxiliary contacts Gld. The resistor control contactor B2 may have an operating winding 62m, normally open main contacts 62a and 62h, normally closed auxiliary conta-cts 62e, and normally open auxiliary contacts 62d and 62e. The voltage control contactor 64 may have an operating winding lllw, normally open main contacts 64a, and normally open auxiliary contacts 64b. The time delay relay 65 may have an operating winding 65u), normally closed main contacts 65a, and a suitable time delay means such as a dash pot 65D for delaying reclosure of the contacts 65a.

Operation of Fig. 3

The following brief description of the operation of Fig. 3 is followed by a more detailed description.

With the knife switch 'I4 closed, closure of the lift push button 65 causes pick up of the voltage control contactor 64. This results in an increase of the voltage between the supply conductors Il and I8 and causes pick up of the lift contactor 60. The increased voltage is so related to the resistance of the winding ISw and the heat dissipating ability of the magnet I9 that if it were continued throughout the energized portion of the duty cycle of the magnet I9, the magnet I9 would become overheated in a relatively short time as explained in the Yorkey patent. The magnet when overenergized by the increased voltage builds up to its normal full voltage strength in a very short time. Depending upon the type of load being handled, the contacts 66a may be held closed for an interval suilcient to allow the magnet to attract and separate a large load. Release of the push button B6 causes drop out of the contactor S4, but the contactor 60 remains in its picked up position. The voltage between the supply conductors I7 and I8 is now sufliciently reduced so that the magnet I9 cannot become overheated but not to the extent that any of the attracted load is dropped as explained in the aforementioned Yorkey patent.

Demagnetization of the magnet I9 is effected by operating the push button 68. This causes the drop contactor 6I to pick up followed by drop out of the contactor 60. The current in the winding I9w now decreases, but, before it reverses, the contactor 62 picks up to reduce the resistance in series with the winding ISw. Pick up of the contactor 62 also causes reclosure of the contactor 64 for a time interval determined by the relay 65. The resultant momentary increase of reverse voltage causes a rapid rise of reverse current. Before the magnet ux reaches zero, the contactor 64 drops out to reduce the reverse voltage so that the reverse current thereafter increases more slowly to insure proper operation of the contactor 6I which is adjusted to drop out to disconnect vthe magnet from the source I0 before the magnet flux can reverse.

Considering now the operation of the embodiment of Fig. 3 in detail, closure of the push button 66 with the switch 14 closed completes an energizing circuit for the winding 64w from the supply lead I'I through the contacts 68D, 66a, and 65a and the winding 64m` to the supply lead I8. The contactor 64 thereupon responds to close its contacts 64a and 64b. Closure of the contacts 64a short circuits the resistor 'I2 and the voltage between the supply leads Il and I8 immediately increases. Closure of the contacts 64b completes an energizing circuit for the winding 60u; from the supply line II through the contacts 68e, 64b, and 62o and the winding 60u: to the supply line I8. The contactor 6U thereupon picks up and its contacts Sa, 60h, 60e, 60d, 60e, and Gf function like the corresponding contacts of the contactor I0 in Fig. l. The contacts 80h also close to complete a holding circuit for the winding 6010 from the supply lead I 'l through the contacts 60h, 6 Id, and E2C to the Winding 601e. The increased voltage between the supply leads IT and I8 is impressed on the winding I9w and the magnet flux increases rapidly. The relay I4 is adjusted so that it does not pick up at this time.

When a load (not shown) has been attracted and separated from a pile by the magnet I9, the push button 66 is released to permit the spring 66s to open the contacts 66a which deenergizes the winding 64u). The contactor 6I! remains in its energized position after thevcontacts 64b open because of the previously traced holding circuit, but opening of the contacts 64a reduces the voltage applied to the Winding ISw by inserting the resistor 'I2 in series with the generator 1U. The reduction in voltage at the terminals I9a and I9b is suiiicient to prevent overheating of the magnet I9 but is insufficient to permit the magnet I9 to drop its load.

Automatic demagnetization of the magnet I9 is initiated by operation of the push button 68 to close the contacts 68a and to open the contacts 68o and 68o. Opening of the contacts 68e prevents reenergization of the winding Gw by subsequent closure of the contacts 64b and opening of the contacts 68o prevents reenergization of the winding 64w through the contacts 66a should they be inadvertently reclosed. Closure of the contacts 68a completes an energizing circuit through the conductor 35 for the winding 6Iw which is like a similar circuit in Fig. 1. The contacts 6 Ia, SIb, and Sie function like the corresponding contacts of the contactor I I in Fig. 1, and the contacts Gld open to interrupt the only remaining circuit for winding llw. The contactor 60 drops out after a brief time delay period and the magnet I9 starts to discharge through the

resistors

28, 29, 30, 3|, I2 and 'II and the source 1B.

The resulting rise in voltage at the terminals ISa and IQb causes pick up of the relay I4. When the magnet voltage decreases to a value nearly aeoaese equal, to the' reduced: voltage` betweenthe supply leadsV .l 1, and i8;` they relay: I4' drops out tocom.- pletexan yobvious energizing circuitv forft-he vwinding 6210 and the'contactorfl then responds tdshort circuit the resistorsZS and 3|. The contacts 62d and 62eY close upon piclr upA of the contactor 6,2 to; complete, energizingj c ircuitsi the windings 6.41.0. and 65u). The circuit for the winding 6 4w is from the supply lead I1; through; the contacts 62d anda.- andthe winding; Mw to the supply leadl8. The circuit for thewinding-'w is from thesupply lead l1 through the contacts` 62d and 62e and the winding` Bw to the supply lead I8. The contacter (itV respondsqimmediately to close: its contacts 64a which effects an increase; inthe voltage between the leads Il; and I8;` This increased voltage causes thereverse` current to` increasev very; rapidly. Thefrelay 85 responds an instantl later to open its contacts G'awhich de-y energizes the. winding tdw. The contacter 64. again. drops out; andthe voltage atthe terminals 19a`v and |917 againdecreasesy sov that thev rate of` increase of reverse magnetv current is not too great to prevent propen operation ofl the contactor El which subsequently drops outy as described in connectionV withv the contactor- IlA in Figs. 1 andz.

We-claim:

1. A system for controlling the,A operation of.

an inductive.4 device designed forv operation at a predetermined value of' unidirectionalvoltage comprising a magnetizingcircuit, ahighly resistive discharge circuit, a low resistive reverse current circuit, magnetizing switch means operable to connect said device to a source of said unidirectional voltage through; said magnetizing; circuit for magnetizing said device, discharge switch` means operable to connect said device to said source in a reverse direction through said highly resistive discharge circuit, means for closing said magnetizing switch means, means for operating said discharge switch means to complete said dischargecircuitA fromA the source cof, the magnet while said magnetizing switch means is closed and for subsequently effecting interruption of said magnetizing circuit while the discharge circuit remains completed, whereby the voltage existing across said, device suddenly reverses to become opposedv to theV voltage of the source, then increases, and then declines exponentially, additional switch means operative for connecting said device to said source through said reverse current circuit while maintaining said opposed relation of said voltages, whereby, upon decline of said reversed voltage below the voltage of said` source, a reverse current can flow through said reverse current circuit from said source to said device, and means operative to eect disconnection of said device from said source when the magnetic flux of said device is approximately Zero.

2. A control system in accordance with claim 1 characterized in that electroresponsive means are provided which are connectible in a circuit with said device and, ywhen so connected, are rendered operative in response to the electrical condition of said device while said reversed voltage is declining, and, when operative, render sistance of said discharge circuit is so related to the; magnetic energy that cani be stored inasaid` device,A when the device isk magnetized through saidmagnetizing circuit, `thatthe maximumvalue of said reversed voltage can closely approach butnot exceed thecmaximum peak voltage capa'-A ble of safely being withstood by said` device.

5. A control system in accordance with claimV 4 characterized in that interruptionof said mag.- netizing circuit is accompanied by arcing, and the relatively large resistance of saidr discharge circuit is so: related to the; magnetic. energy that can rem-ain stored in said device after said arcingz terminates, that the maximum value of said reversed lvoltage, can4 cl-osely approach. but not exceed the maximum peakvoltagecapablezof; being safely withstood. by, said device.

6. A control systemy in accordance with claim; 1 characterized in` that said means for interrupting said; reverse current circuitis responsive to a predeterminedmagnitude. of` said reverse current; and is positively operativeA only when the rate of riseof saidreverse current is below. apredetermined maximum rate of rise, and the low resistanceV of said reverse current circuit; is. so, related to the resistance and' inductancei ofsaid device and to said. value, ci unidirectional voltagev that the rate of risev of said reverse current can approach but not` exceed said prede@ termined maximum rate ofrise.v

7., A; control systeml in accordance; with claim: lcharacterized in that the relatively large resistance of said discharge circuit; is so related toY the magnetic energy; that can` be; stored vin said device, when thevr device is, magnetized through said magnetizingvcircuit, thatthe max;

imumv valuev` of said reversed voltage can; closelyr approachbut not; exceed the maximumgpeak voltagelcapahle of safely being Awithstood/bysaid device, saidv means for interrupting saidreverse; current; circuit-,is responsive to. a predetermined magnitude ci: said reverse-- current and, nositivelyoperative only where the rate` of; rise of reverse current; is, below a; predeterrnined* maximum rate of rise,` andthe low-resistancel ofv said' reverse current circuit is sorelatedr to the resistance and inductance of` sai-d device and to said value of unidirectional voltage that the rate off rise of,y said reverse current can approach but not exceed; said; predetermined maximum rate of; rise.

Si A system forgccntrolling an inductive device designed to operate at a predetermined value of unidirectional voltage, comprising magnetizing switch means for connecting said;v device to; a source of said voltage in a magnetizing circuit tormagi-ietiaing said device, resistance means, switch means operable to ccnnectsaid; device inv a discharge and reverse current circuit including4 said resistance means, means for operating said discharge; switch means to complete said dis-` charge and reverse current circuit` while saidmagnetizingcircuit is completed, and for subsequently interrupting said magnetizi-ng circuit` whereby the voltage` existing across said devicesuddenly reverses and rises in value and then decreases in value exponenftially,l andy means op. erative to reducetheresistanee ci saidr resistance means while said reversed voltage is decreasing.

9. A system forcontrolling an inductive device comprising a switchV for admitting an energizing current tothe device to produce a magnetic flux in the device, a second switch for setting upy reverse current connections to the device, and magnetically operated means forV causing opening; ci; said first mentioned switch, and for causingclosing .oi Said second mentioned; switch. te set..

up said reverse current connections before said first mentioned switch can open and interrupt said energizing current, a variable resistor in said reverse current connections, said reverse current connections being operative to admit a reverse current to said device for neutralizing said magnetic flux after said magnetic flux has decreased a predetermined amount, and means operative to reduce the resistance of said resistor prior to admission of said reverse current.

10. The combination with an inductive device, a source of unidirectional voltage, two resistors, a pair of contacts, a circuit connecting the contacts and device in series to said source, means for connecting the device and one of its said series connected contacts in parallel with one of said resistors, means for connecting the device and the other of its series connected contacts in parallel with the other of said resistors, and means for opening said contacts whereby the winding remains connected to said source in series with said resistors, of means operable for short circuiting a portion of each of said resistors, and means rendered operative, by an electrical condition of said device resulting from opening of said contacts, to operate said last mentioned means prior to the reversal of the ilow of current in the device.

l1. In a control system for a lifting magnet, a source of power, switching means for connecting the winding of the lifting magnet directly to the source of power, resistance means, reverse switching means operable to connect the winding to said source, through said resistance means, for applying reverse power to said winding, electromagnetic switching means operable to reduce the resistance of said resistance means, electromagnetic operating means for said switching means connected to the winding and being responsive to an electrical condition in said winding, resulting from the said application of reverse power, for operating said electromagnetic switching means, and means for rendering said reverse switching means inoperative.

l2. In a magnet and control system, a lifting magnet to be connected to and disconnected from a source of power, a variable discharge resistor so related to the magnet as to be capable of preventing the occurrence of high induced voltages upon deenergization of the magnet resulting from disconnection of the magnet from the source of power, electromagnetic switching means selectively operable to connect and disconnect the magnet to and from the source of power, means for connecting said discharge resistor between the magnet and the source of power, magnetic operating means responsive to the voltage at the magnet while the magnet is connected to the source of power through said discharge resistor for reducing the resistance of said resistor, and magnetic operating means responsive to the voltage at said magnet after the resistance of said resistor has been reduced for disconnecting said magnet from the source of power.

13. A control system in accordance with claim 1 characterized in that means are provided for reducing the voltage impressed on said magnetizing circuit from said source after said magnetizing circuit is completed and before operation of said discharge switch means.

14. The method of operating a lifting magnet which comprises the steps of connecting the magnet to a source of unidirectional voltage for magnetizing the magnet to lift a load, subsequently, while maintaining the first connection, connecting the magnet in a reverse direction to said source through resistance, breaking the rst connection after the second connection is completed, whereby the voltage at the magnet suddenly reverses and increases to a maximum value and then starts to decrease exponentially, and, when said voltage at the magnet is approximately equal to the voltage of said source, decreasing the said resistance, and then breaking the second connection after said voltage at the magnet becomes less than the voltage of said source.

15. A system for controlling an inductive device designed to operate at a predetermined value of unidirectional voltage, comprising magnetizing switch means for connecting said device to a source of said voltage in a magnetizing circuit for magnetizing said device, resistance means, switch means operable to connect said device in a demagnetizing and reverse current circuit including said resistance means, means for operating said demagnetizing switch means to complete said demagnetizing and reverse current circuit while said magnetizing circuit is completed, and for subsequently interrupting said magnetizing circuit whereby the voltage existing across said device suddenly reverses and rises in value and then decreases in value exponentially, means operable to increase and then decrease the resistance of said resistance means in sequence while said reversed voltage is decreasing, and means rendered operative in response to the electrical condition of the magnet to operate said last mentioned means.

16. The method of operating a lifting magnet designed for operation at a predetermined normal value of unidirectional voltage which will cause a predetermined normal safe current to ow in the magnet depending upon the temperature of the magnet, said method comprising the steps of connecting the magnet in a magnetizing circuit to a source of power to impress on the magnet a unidirectional voltage higher than said normal value of voltage for causing the magnetizing current in the magnet to increase rapidly from zero, reducing the voltage impressed on the magnet to said normal value of voltage approximately when the magnetizing current reaches the value of said normal current, subsequently connecting the magnet in' a discharge and reverse current circuit to the source through a, resistor, interrupting said magnetizing circuit after said discharge and reverse current circuit is completed, whereby the voltage at the magnet suddenly reverses, increases to a maximum Value and then starts to decrease exponentially, then, when said magnet voltage is approximately equal to said normal value of Voltage, decreasing the resistance of said resistor and then, after said magnet voltage becomes less than said normal Value of voltage, interrupting said discharge and reverse current circuit.

17, The method of operating a lifting magnet designed for operation at a predetermined normal value of unidirectional voltage which will cause a predetermined normal safe current to flow in the magnet depending upon the temperature of the magnet, said method comprising the steps of connecting the magnet in a magnetizing circuit to a source of power to impress on the magnet a unidirectional voltage higher than said normal value of voltage for causing the magnetizing current in the magnet to increase rapidly from zero, reducing the voltage impressed on the magnet to said normal value of Voltage approximately when the magnetizing current reaches the value of said normal current, subsequently connecting the magnet in a discharge and reverse current circuit to the source through a resistor to impress said normal Value of voltage on the discharge and reverse current circuit, interrupting said magnetizing circuit after said discharge and reverse current circuit s completed, whereby the voltage at the magnet suddenly reverses, increases to a maxi-f mum value and then starts to decrease exponentially, increasing' the voltage impressed on said discharge and reverse current circuit when said magnet voltage is in the neighborhood of said increased voltage, and interrupting said discharge and reverse current circuit after said magnet voltage becomes materially less than said normal value of voltage. k

18. A system for controlling an inductive device designed to operatevat a predetermined normal value of unidirectional voltage which will cause a predetermined normal safe current to flow in the device depending upon the temperature of the device, comprising a magnetizing circuit, means for connecting thedevice in said magnetizing circuit and to a source of power to impress on the magnet a voltage higher than said normal value of vo1tagemeans operable upon the magnetizing current in the magnet reaching the value of said normal current to reduce the voltage impressed on said magnet approximately to said normal value of voltage, a discharge circuit REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 967,186 Hall Aug. 16, 1910 1,915,566 Younghusband June 27, 1933 1,923,311 Hodgson vAus. 22, 1933 2,020,670 Wright Nov. 12, 1935 2,126,775 Hodgson Aug. 16, 1938 2,206,823 Wertz July 2, 1940