US2636156A - Limit control system for reversible motors - Google Patents

Description

April 21, 1953 A. H. MYLEs 2,636,156

LIMIT CONTROL SYSTEM FOR REVERSIBLE MOTORS Filed July 13, 1949 2 slams-4m: 1

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fig I P2 '4 INVENTOR. ASA MYLE-S gg-4, QKXeaMWL WK ATTORNEYS.

A ril 21, 1953 A. H. MYLES ,6

LIMIT CONTROL SYSTEM FOR REVERSIBLE MOTORS Filed July 13, 1949 2 SHEETS-SHEET 2 I I a x 47 7 Q5: 5'2 w I l 66cL l.L 48 62a T 6/\ ,66 .362 igz E37 626 i r" r I ,0 5'6 F21:- i 55 55 1 M I g am I l x56 1 62----4 65 W41! 1* i 3 3 i I s M i I i: g l 70 i I i 64? 5 52 AEV FWD 5 i 9 $8 1 l l a a i l I INVENTOR. ASA H Mags Patented Apr. 21, 1953 LIMIT CONTR QLSYSTEM FOR REVERSIBLE -MOTORS As v:H- Mrlesrfic oaas g c to Thames:-

- -ri Controlle 1-. nufac ur n Cement C evelan :Ohi o porat n f Oh o Application J uly13, 1949, Serial No. 104,575

- 9. Qlain s,

This; invention relates to controllers for electric .nwtor driven :ma-chinery, and a more particularly to amotorcontrol system including jmpro-ved limit control circuits of the variable impedance 'Whilenot limitedathereto, thepresent invention ,is .especial-ly .advantageous 'for controlling the --motor.-.Which drives. a reciprocating machine-elementebetween preselected limits. .In order-to increase the rate of production of many large machines :of this type, therspeedof the reciprocating element has been increased'to-suchan -extentthat :it'is no longer feasible to use track type limit switches 'orother limit switches requirlug-physical contactbetween the moving element a -switch operating lever. Furthermore, 1 in "many m'ac-hines having reciprocating elements,

suchas-reelerbars,for-example, the path of travel of the-reciprocating element-is not. sulfi- -cient1ydefinite to permit the useof 1 prior variable impedance limit control circuits 'not requiring such physical contact. Consequently, the problem of controlling such machines has not heretofore been adequately solved, particularlyin the. provision of limit control mean capable of providing' thedesired accuracy and-safety with reasonable endurance under extremely severe operating conditions.

Prior known variable impedance type limltcon- --trol circuits operating at low irequemyandwithout electronicdevices havecomprised only a series connected-reactor and relay. A magnetic operating member or vane when moved into the field. or-the-reactor increases the impedance of the =reactor-'-so'that the r lay current, which varies in-- inverse proportion to the circuit impedance, decreases toe-value permitting drop-out of the relay. It is because of the small-reduction in relaycurrent that is obtained,- particularly when considerableclearance must 'be provided between the vane and the reactor, that-such; prior limit control circuitscannot be-used for large industrial machines of the -type herein contemplated in whichjthe path oftravel of the moving magnetic member is-erratic so that the vane is apt to'strlke the'reactor or pass atso greatadistance-therefrom thatinsufiicient current change results. Additionally, since at constant voltage only a-small reduction in relay current occurs in the-se prior limit controlcircuits-normal increases in the supply voltage often cause a false-operation by maintaining the relay energizedeven though the magnetic-vaneis'in the reactor field.

1"It:is an object ofthe present invent-lento provide "a motor control system solving gthe aforementioned problems.

,Another Object is ,to .provide a motor control system including an improved limit control-circuit of, the. variable impedance type.

One of the important features 5 of the present invention is the provision ofa reacterofrelativelylowinductance having itspoles arranged to V define anv elongated concentrated leakagefield the reluctance of which can be materially decreased by movement of-a magnetic vane-therein at a considerabledistance fromthe pole iaces, combining the reactor in. a low. frequency tuned circuit with acapacitor and a'relay Which has ,a very small differential,between its pick-up and drcpout current values, and-supplying the tuned circuit with a low frequency voltage ofesuchJow value that the relay currentatresonanceis limited without. adding-resistanceto the'circuit-other thanthat-inherent in the reactor and relay wind.- ing or thatused. to'decrease the relay difierential.

Accordingly, a further object is to provide a limit control circuit operative at low .frequency inwhich a 'relayis operated directly by curegent change i-resulting from aedisturhance of, the ratio between capacitive and inductive reactance in the circuit.

'Another object is to provide a tuned'limit control circuit including a reactor, a capacitor, and a-relay which isheldclosed by thecurrent in the olrcuitpreferably .ator near the peakbf resonance and which drops out upon that impairment of the resonant conditions "resultingfrom movement :of a metallic-vane into -the; lea k age field of-t-hereactor. I

Another object is to provide an improvedvariable impedance limit control circuitinchidin a resonant .circu-it but not utilizing electronic devices.

"still another object is to 1 provide a low figequency limit control circuit including a reactor and operative when either amiagnetic or a- -nonmagnetic metallic vane is moved into thereactor field.

The limit control circuit in accordance with this invention includes a reactor having widely spaced.- poles defining a single elongated leakage field and connected in series with aflseries onnested capacitor and relay Winding ,across ,a source .of relativelylow voltage andfrequency. The circuit is tunedfor, resonanceavhen the leakagefield. containssubstantially no metal. The relay hasa small d-ifierential hetween itspick-up and dropout-current values and isheldcloscd by the. circuit current at resonance. 'The-small differential may be an. inherent feature ,of -the relay ormay. be obtained: by causing the relfav when the relay picks up. A moving element of the machine to be controlled carries a. metal vane which moves into the leakage field thereby impairing the resonance of the circuit and causing the relay to drop out. Preferably the vane is of magnetic metal, but a non-magnetic metal vane also can be used to obtain an accurate response. The control system is arranged to utilize relays responding as just described to slow down the motor as the machine approaches opposite limits of its travel and to stop the machine at the limits. A novel interlocking arrangement is provided which permits both slow-down and stop control functions to be performed at both limits of travel by using four reactors but only two relays in the resonant circuits.

A more specific object of the invention is to provide a limit control circuit comprising a reactor, a capacitor, and an electromagnetic relay having an operating winding all connected. in a series circuit arranged for connection across a source of alternating current of constant and low frequency, the electrical constants of the series circuit being so related-to each other and to the voltage and frequency of the source-that the relay'picks up when the circuit is connected to the source with the leakage field of the reactor substantially free of conducting material and the electrical constants of the series circuit being further so related to each other and to the voltage and freuuency of the source that the series circuit is substantially re onant when the relay is picked up, the current that flows in the circuit at resonance being sufficient to hold the relay in its picked up position, together with extraneous means for so varying the reactance of the reactor that the resonance of the circuit can be im aired sufficiently to permit drop-out of the relay.

Other obiects and advantages will become apparent from the following description wherein reference is made to the drawings, in which Fig. 1 shows the limit control circuit diagrammatically. the reactor with its cover removed and the machine to be controlled being shown schematically.

Fig. 2 is a schematic side elevation of the reactor and a portion of the controlled machine,

Figs. 3 and 4 are graphs illustrating features of the invention, and

Fig. 5 is a wiring diagram of a complete control system utilizing a plurality of the limit control circuits of Fig. 1.

Referring to Fig. 1 a limit control circuit in accordance with this invention comprises a reactor ill, a capacitor H which may be adjustable, and an operating winding i210 of an electromagnetic relay 1! all connected in a series circuit with each other across a tapped low voltage secondary winding i ls of a step-down transformer Hi having its primary winding Mp supplied from a source it) of alternating current at the usual low frequency of electric power supply systems. The constants of the circuit are so selected with respect to each other and the voltage and frequency of the source that when the relay i2 is closed the a circuit is resonant and the resonant current holds the relay closed. Means to be described are provided for changing the reactance of the reactor thereby to impair the resonant condition effecting a reduction in the current in the winding iZw to a value materially below the drop-out current value of the relay [2.

The effect of the condenser H on the current in the relay windinglfiw upon an increase inthe reactance of the reactor i0 is graphically shown in Fig. 3 wherein relay current is plotted against the inductive reactance of the circuit. A curve i 8 in Fig. 3 shows the operating characteristic of a circuit such as that of Fig. 1, whereas a curve i? shows the operating characteristics of a similar circuit but without a capacitor. Since the combined inductive reactance of a series connected reactor and relay Winding must have some finite value, a variable impedance limit control circuit without a capacitor must be operated along the flat portion of the curve ii at a material distance from the vertical axis. By adding a capacitor to the circuit, the operating characteristic is moved to the right as shown by the curve It, and the circuit can be operated along the steep part of the curve is from a maximum current value at X limited only by the resistance of the circuit to a lower value at the point Y where the curve it begins to flatten out. Although it is preferred that, when the relay is picked-up and the reactor is at its normal reactanoe value, the limit control circuit is exactly in resonance, an approximate resonant condition providing the desired degree of current change upon impairment of the approximate resonant condition is also satisfactory. It is seen, by comparing the curves l6 and ii, that a limit control circuit including a capacitor if made approximately resonant can be made to have a very much larger current variation in the working range for a given change in inductive reactance than can a circuit without such capacitor.v

Referring now to Figs. 1 and 2, the reactor It comprises a case it preferably formed of insulating material or non-magnetic metal and containing a U-shaped laminated magnetic core 19 having multi-turn windings 20 upon its respective legs. The windings 28 are connected in'series with each other and in such manner that the respective fluxes produced thereby are additive. The core i9 preferably has sufficient cross-section to minimize the effect of magnetic saturation. The legs of the core it are spaced apart a material distance and extend toward and substantially engage a relatively thin, gasketed, non-metallic cover 2! for the case [5, the cover 2! preferably being formed from Bakelite or other suitable insulating material. When the coils 20 are energized, an elongated leakage field extending lengthwise of the case it is produced which is concentrated in the region adjacent to and outside of the cover 2!. In order to minimize the voltage across the capacitor H at resonance, it is desirable to make the inductance of the reactor Hl relatively low as by using relatively few turns in the windings 2t. The relay [2 has a pair of normally-open contacts 52c and 52b and preferably has a very small diiferential between its pick-up and drop-out current values. In order to provide the desired small differential and to render the amount of diiferential adjustable, the contacts [2a may be arranged to connect a variable resistor 22 in parallel with the winding [2w when the relay I2 is in its energized position. Preferably, the secondary winding ids of the transformer i4 is provided with taps Ma so that the voltage applied to the limit control circuit can be adjusted readily to compensate for variations in the voltage of the source IE or for manufacturing tolerances in the reactor It, relay I2, and capacitor H. The contacts 12b of the relay may be used to control in connection with Fig. 5.

The constants I of the limit-control circuit'are sol-selccted witnrespectto each other and-to the voltage and frequenoy of thesouroe 15 that the limit. control": circuitv is; substantially resonant when? a magnetic vane 24 is not" in. the leakage field: of. the reactor I When" the limit" control circuit? is resonant; the: current'in'the circuit. is abovesthe pick-sup current valuerofathe relay l2 and :thexrrelay 112' is initsenergized position, the resistor 2-23 isconnected in: parallel with therwinding. 1 Zw through; the: contacts 12a, and the con tacts t2brarerclosed, Before: the relay It picks up-,; the inductivereactance of. the circuit" is slightlyilessthan the capacitive reactance, the two react'ances. .:prefere;blyr, becoming: I substantially equakwhenthe relay l2? closes; When: the vane 2411s inthe leakage field. of. the-reactor, the reactance: of? the reactor increased. and the resonance ofv the; limit control. circuit is impaired. Thewimpairment of: resonancercauses the current inathe: relay winding l eu-to bevcry much less than its value underresonant conditions-and the relay/1.2 iS'EdIODPQdt-Cllt. Whentherelay l2 drops out;.thetreactance ofiits windin-g tZw-isdecreased. Iniso; far as current flow in the limit control circult. is. concerned, the. decrease in' the reactance ofitherwindinglzwzupon drop-outof the rela [-2 iscompensatech for. the much. greater increase in. theareactanceof the reacton HT and also to. a-

lesserviex tentby the increase in resistance of the circuitresulting from. the disconnection of the resistonEZupon opening'ofthe contacts.v [2a. Due to! eddycurrents. induced. in the. magnetic vane, the effective resistance of the circuit. is. also increased. w hen.. resonance. is impaired.- The increase .in; eflfective. resistance also eiiects. a reduction in. the.noneresonantvcurrent.

Iflthevvane .24 is made. of non-magnetic condjuetingrmaterial the relay l2 also responds to movementoflthe vane into the reactor. field. This is because the conductingmaterial effects a decrease. in. the impedance of the reactor it) which also impairs the resonance. of the limit control circuit resulting in a reduced currentflo'w therein as indicatedby the. portion 2' of. the'curve l6 in-Eig. 3'." If the circuit did not include a capacitor; a nonemagnetic conducting vane" would cause an increased current tofiow to the relay.

Althoughthe limit. controlcir'cuit maybe applied. in various ways and: for. numerous" applications; its. operation is described herein in connection: with a machine havinga. base member 25 upon which the reactor 1 l1. ismounte'd. asbysuitable bolts. base member 25 carries a' moving maclrine el'einentt l6 slid'able' lengthwise *of' the ablysupporte'd on themachineelement 2G s'o't'hat astlieelement zfimoves back and forth along'the base ZFtlie-vaneid moves in a path parallel to andspaced'irom-the cover 21' and into and out of the leaka'ge" field oi the reactor Mi The vane 24" ispreferably the same size asthe cover 2-1 or slightlylargerrs Well known means (not shown) maybe employed-t'o provide for adjustment oftlie position of the rea'ctor' H2 lengthwise of'the' base 2 'or of thexposition of the vane 24 on the element zfiiorbothaz.

The contacts 12b. may be arranged t'o permit; wlieni' cldsed, operation of a motor (notshown)" driving the eleinent- 26 and" to prevent operation ofrthe motorfwhen openix- Thus; should there be a= -f'ailure;= of power in the limit control circuit, the relay l-12- opensvthe contacts-l tiolt off. the motor.

6 outas described above to en'ect stopning of th'e motor and the element 26-; dccrease in the current in the relaylwi-ndin'gt 'l 2 wi as: the vane: 24 moves into "the: leakage held of the reactor is graphically" illustrated by the curve: 29: of Fig 4'; wherein. the value of re'lay' current is plotted against the. distance between the approaching edge of the vane 24k and: the longitudinal center line of the: reactor lrflz- Because. of the: relatively great decreasein relayourrent.fora small change inv the reactan'cefi of themeactor HI "resulting. from tho-dmpairm'ent ofresonance, and 'beoause' of the smalldifierential: between the pick-up and: the drop-out current. values: of the-relayl 2, thewane 2423135? be so: mountedthat. its? path: of. movement isat-a considerable-distance from themeactor. l0 andistillv obtain accurate operation. of the relay ilk. Hence; it. is=poss-ib1e to use -the limit-cont'rol. circuit where: alignment between: the? base- 25 and the-element ZBis-very erratic. Although insome instances; the poles of. the-variable reactor couldbe arranged to define a slot through which the vane is moved, or could be aligned'witheachlother as. in.- a-bar magnet to operate in/conjunctiomwithi a U-shaped. vane, such arrangementsbecome-impracticalwhere the path of the vane isl-notaccurately predetermined: By providing a reactor having a lJ--shapedcore defininganelongated leakage field so-that its-reactance-can-be changed 1 even though large clearance is providedbetween the reactorand the vane, combining the reactor witl'ra capacitor to provide a tuned circuit,-and utilizing a close differential relay-,the applicant has obtained an. improved variable impedancelimit control circuit of. increasedutility.

When the voltage supplied to. 'the.limit'.control circuit of Fig. Lisashigh asthaLt-commonl'y used in. industrial plants. for. lighting service, for ex.- ample or 220 volts,.and.the.relay ofruggedl construction, it has been found that the voltage drop across the capacitor at resonance becomes too great for economical capacitors oilow voltage rating. By using. the step down transformer l4, thevoltage across the capacitor can be kept at a reasonably low value by making the reactance of the'react'or at resonance relatively low; The re"- sistance of'the circuitlis less" when the reactance is small because of the few turns required in the windings 20', hence the lowvoltageservesitoliniit the relay'current to a safe value at resonance.

'' Th'e'low resistance of thecircuit" causes the portion' of the curve; I6 of Fig. 3' between the. points X'and -Y to b'e'much steeper than it would be if the circuit had a higher resistance Reierringnow'to Fig; 5; apluralit'y of limit 1. control: circuits similar to the circuit of Fig. 1

and having a plurality of respective reactors: 30, 31", '32'and' 33 each'of which" is similar tot'he reactor H! are shown for controlling a motor 34 arranged to drive a cable drum 3'5 thru suitable shaftin'g' 35" and gearing 31-5 The motor 34 which may be of any suitabletype is shown a'sa direct current series machinehaving an arma-- ture winding 34a and a series field winding 34?." A cable 38- wound upon thedrum 35'and an idler drum 39 is connected to a carriage 40 diareeler' bai 'havin'g'a base member" 41. Operation 01" the motor iitdr'ives the drum 'as causin the cable 3s'tom'ovethe carriage EO IoaCK ahd -forthalong waysflforined on the-base member 41;

thereby agelitistinolvremout?along:the-:bsrset when? .in doekednositicmxazbilletz'ofiliot uretal over the mandrel 43 by suitable means (not shown). After a sufficient length of the billet has been pierced by the mandrel 33, the motor 34 is operated to reverse the carriage W and return it to the right to withdraw the mandrel and to await another piercing operation. In order to increase the production of the reeler bar it is necessary that the carriage 40 travel with great rapidity between its two extreme limiting positions. Heretofore, track type limit switches have been used to efiect slow down and stopping of reeler bar cariages. It has been found, however, that track type limit switches are not rugged enough to withstand repeatedly for long periods the'severe duty imposed upon them by the high speed desired of modern reeler bar carriages. Because the limit control'circuit of the present application operates with no physical contact between the moving and the stationary elements and with wide variations in alignment therebetween, many advantages result from its use. 4

' The reactors 32 and 33 are arranged to be connected to completeforward and reverse slow down limit control circuits, selectively, and are adjustably mounted near opposite ends of the base 4! in the proper position to efiect slow down of the motor 34 as it drives the carriage 29 with its vane 44 toward its opposite extreme limits. Thereators 3B and 3! are arranged to be connected to complete forward and reverse final limit control circuits. selectively, and are adjustably mounted substantially at the extreme limits of travel of the carriage ill adjacent the path of travel of the vane id to effect final stopping of the motor 3% after it has been slowed down.

The various limit control circuits of Fig. are supplied from a transformer 416 similar to the transformer i and arranged to be connected to a source of low frequency alternating current ll by a knife switch 48. Depending upon the direction of motor rotation, the reactors 32 and 33 are arranged to be connected in series selectively with a series connection comprising an adjustable capacitor 49 and an operating winding 55911) of an electromagnetic slow-down relay 58, and the reactors 3i) and 3! are arranged to be connected in series selectively with a series connection comprising an adjustable capacitor 5! and an operating winding 5210 of an electromagnetic fznal limit relay 52. The relay es has normally-open contacts 50a and 5M) and the relay has normally-open contacts 52a and 52b. The relays {it and 52 preferably have a close differential between their respective pick-up and dropout current values Which-close differential may be provided in any suitable manner or as shown in Fig. 1. f

The-motor 34 is arranged to be connected in series with the operating winding of a springapplied, electromagnetically released friction brake 5 1, an acceleration resistor 55 having a plugging section 552), and an operating winding 5610 of a suitable overload relay 56 having normally closed contacts 56a.

A forward electromagnetic contactor 6&3 has normally open contacts tea and 60b arranged to connect the motor 34 to a suitable direct current source 6| for forward operation and a reverse electromagnetic contactor 62 has normally open contacts 62a and 62b arranged to connect the motor 34 to the source 6| for reverse operation. A-

main line electromagnetic contactor 63 has a .normally..open contact 63a arranged to complete the motor, connection .to. one .side of. the source 6! for both directions of rotation. Slow down of the motor 3- prior to stopping is effected by an armature shunt contactor 66 having normallyclosed contacts 54a arranged to connect a resistor in parallel with the armature winding 34a. A plugging contactor 65 has normally open contacts 65a arranged to short circuit the plugging section 55p of the resistor 55, and an acceleration contactor 58 has normally open contacts 68a arranged to short circuit the remainder of the acceleration resistor 55. Operation of the contactor 6'5 may be controlled automatically by a suitable normallyclosed plugging relay 69 and the operation of the contactor 68 may be 'controlled by a suitable normally-closed acceleration relay l0. Only the respective normallyclosed contacts of the relays 59 and It have been shown in detail since such devices and their operating connections are well-known in the art.

The contactors (iii, 62, 63, 8 3, S5, and 68 each has an operating winding referred to by the reference numeral of the respective contactor followed by the letter w. The contactor also has normally-open auxiliary contacts Etc, 60d, and 69s and normally-closed auxiliary contacts 68f, the contactor 62 also has normally-open auxiliary contacts 52c, 62d, and 62c and normallyclosed auxiliary contacts 62f, the contactor 53 has normally-open auxiliary contacts 631: and normally-closed auxiliary contacts 630, the contactor also has normally-open auxiliary contacts 6%, and the contactor 66 also has normally-open auxiliary contacts 6% and normally-closed auxiliary contacts 660.

A suitable master switch ll having an on position intermediate of a reverse and a forward position is arranged to control the operation of the several contactors selectively and has a plurality of movable contact buttons i2 through H and a plurality of relatively stationary contact segments is through 33. I'he master switch H also controls the energization of an electromagnetic under-voltage relay 8 1 having an operating winding $415 and normally-open contacts 84a.

Further understanding of the control system of Fig. 5 and the cooperative functioning of the various limit control circuits will be obtained from the following description of its operation:

, With the knife switch 48 closed and the master switch I! in its oil position, low frequency alternating current at relatively low voltage is available at the secondary of the transformer 46 but not circuit is completed. It is assumed that the carriage e13 is in itsextreme right-hand positionwith the vane A i covering the reactor 3| as indicated by dotted lines 88. When the master switch H is in its off position and power is available at the source conductors 3 an energizing circuit for the winding 84111 of the under voltage relay S is completed through the master contacts 72, H3, and 13 and through the normallyclosed contacts fita and E30. Closure of the contacts 8 3a connects the master contacts M and 19 to the left-hand source conductor 6|.

If the master switch H is moved into its reverse position at this time, an energizing circuit for the winding 62w of the reverse contactor 62 is completed through the master switch contacts E8, 3i, and Ti and through the normally-closed contacts 59]. The reverse contactor 62 thereupon closes its contacts 620 and 62d to complete the reverse slowdown and final limit control circuits. The slow-down relay 55w responds to close its contacts 50a and 5% upon closure of the contacts 62c sincethe reverse slow-down limit ciraeeaise the motor for forward operation, reverse switching means operable for connecting the motor for reverse operation, a forward approximately tuned circuit operative when approximately resonant to permit operation of said forward means and operative when its approximate resonance is impaired to prevent operation of said forward means, a reverse tuned circuit operative when approximately resonant to prevent operation of said reverse means and operative upon impairment of its approximate resonance to prevent operation of said reverse means, and means driven by said motor for impairing the approximate resonance of said circuits selectively depending upon the direction of rotation of said motor.

3. A limit control means comprising a pair of reactors mounted near opposite ends of travel of a machine element movable by an electric motor, forward and reverse electromagnetic switching means for said motor, a control circuit for said switching means, a series circuit including a capacitor and an openating winding of a relay having normally open contacts in said control circuit, and said switching means including means for connecting said series circuit to said reactors, selectively, depending upon the direction of rotation of said motor.

4. A limit control means comprising a reactor having an exposed external leakage field path,

a conducting vane, a motive means for driving said'vane and operative to move said vane into said leakage field path thereby to increase the reactance of said reactor, an electromagnetic device operative when energized to maintain said motive means operative at a relatively high speed, an energizing circuit for said electroma netic device operative only when completed to maintain said device operative, an electromagnetic relay having an operating winding, a capacitor having a capacitive reactance at a predetermined frequency approximately algebraically equal to the sum of the reactance of said winding and said reactor at said frequency and when said vane is out of said leakage field path, means connecting said reactor, winding, and cap'acitor in series with each other in a circuit between a pair of terminals adapted to be connected to a source of power, the constants of said series circuit and relay being so related to each other and to a predetermined voltage of predetermined frequency that, when said terminals are connected to a source of said voltage and frequency, said relay is picked up when said vane is out of said leakage field path and said relay is dropped out when said vane is in said leakage field path, and said relay having contacts operative upon drop out of said relay for interrupting said energizing circuit. 7

5. A limit control means comprising a reactor having an exposed external leakage field, a conducting vane operatively connectedto a movable machine element so as to be moved thereby in predetermined relation thereto in a path into and out of said leakage field thereby to change the reactance of said reactor, a motor for moving said element, an electromagnetic relay having an operating winding, a capacitor having a capacitive reactance at a predetermined frequency approximately algebraically equal to the sum of the reactance of said winding and said reactor at said frequency and with said vane out of said field, means connecting said reactor, winding, and capacitor in series with each other in a circult and adapted to connect the circuit across a source of substantially constant voltage of said frequency, means responsive to said relay for slowing down said motor, a second approximately tuned circuit including a second capacitor, a winding of a second relay, and a second reactor havin a leakage field which is in said path spaced from said first field, means rendered operative upon operation of said second relay for stopping said motor, and said second relay being rendered operative upon impairment of approximate resonance of said second tuned circuit resulting when said vane enters the leakage field cf said second reactor.

6. The limit control means of claim 1 characterized in that said means responsive to operation of said relay comprises an electromagnetic device operative when energized to permit said motor to operate at a high speed and operative when deenergized to cause said motor to slow down, and said relay has normally-open contacts in an energizing circuit for said device.

7. A limit control system comprising a conducting vane, a reactor having poles defining an exposed external field for leakage flux and which is adapted to receive said conducting vane, means operable to move said vane into said leakage field, an electromagnetic device operative when energized to render said means operative to move said vane into said leakage field and operative when deenergized to render said means operative to reduce the speed of said vane, an

electromagnetic relay having an operating winding and operative when its winding is energized to cause energization of said device and operative when deenergized to cause deenergization of 'said device, a capacitor, means connecting said reactor, capacitor, and winding in a circuit across a source of alternating current of a given frequency, the constants of said circuit at said frequency and when said vane is out of said field being so related that said winding is energized, and said constants being so related that when said vane is in said field said winding is deenergized.

8. A limit control means comprising a reactor having an exposed external leakage field, a metallic vane operatively connected to a movable machine element so as to be moved thereby in predetermined relation thereto into and out of said leakage field, a motive means connected to said element and operative to move said element, an electromagnetic device operative when energized to maintain said motive means operative at a high speed, an electromagnetic relay having an operating winding, a capacitor having a capacitive reactance at a predetermined frequencyapproximately arithmetically equal to the sum of the reactance of said winding and said reactor at said frequency and when said vane is out of said leakage field, means connecting said reactor, winding, and capacitor in series with each other in a circuit and connecting the circuit across a source of alternating current of substantially constant voltage of said frequency, the electrical constants of said series circuit being so related to each other and to the voltage and frequency of said source that said relay is picked up when said vane is moved out of said leakage field and dropped out when said vane is in said leakage field, an energizing circuit for said electro-magnetic device arranged to be controlled by said relay and completed by said relay to maintain said device energized while said relay is picked up and interrupted by said relay to deenergize said device while said relay is dropped out, where- 13 by the speed of sad motive means decreases when said relay drops out.

9. A variable impedance limit control circuit comprising a reactor having poles defining an exposed external field for leakage flux and which field is adapted to receive a conducting vane, a conducting vane arranged to move in a path including said external field, a motive means connected to said vane for driving said vane in said path, a controller electrically connected to said motive means for controlling said motive means and including an electro-magnetic device having a control winding and operative when said control winding is energized to permit said motive means to move said vane into said external field and operative when said control winding is deenergized to reduce the speed of said vane, a capacitor, an electro-magnetic relay having an operating winding, a series circuit connecting said reactor, capacitor, and winding in series with each other and arranged for connection across a source of alternating current of constant frequency, the electrical constants of said circuit being so related to each other that, if connected to a source of predetermined voltage and low and constant frequency with said leakage field substantially free of conductive material, said relay will pick up, the electrical constants of said circuit being further so related to each other that,

if connected to said source, said circuit will become approximately resonant with said relay picked up, said relay being capable of being held in its picked up position by the current that flows in said circuit at approximate resonance, and the electrical constants of said circuit being further ,an energizing circuit for said control winding having said contacts in series with said control winding.

ASA H. MYLES.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,162,031 Carpenter Nov. 30, 1915 1,561,232 Higbee Nov. 10, 1925 1,699,625 Olofson Jan. 22, 1929 1,728,790 Eames Sept. 17, 1929 1,871,656 Bouton Aug. 16, 1932 1,961,133 Brown June 5, 1934 1,970,333 Muehter Aug. 14, 1934 2,228,163 Cohen Jan. 7, 1941 2,234,421 Turner Mar. 11, 1941 2,341,526 Breitenstein Feb. 15, 1944 2,411,247 Cohen Nov. 19, 1946 2,440,849 Defandorf et a1 May 4, 1948 2,441,348 Ducret May 11, 1948 2,443,122 Smith June 8, 1948 2,515,850 Benson July 18, 1950 FOREIGN PATENTS Number Country Date 379,711 Great Britain Sept. 5, 1932

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