US2919325A

US2919325A - Magnetic overload relay

Info

Publication number: US2919325A
Application number: US689013A
Authority: US
Inventors: Charles F Brauneck
Original assignee: Murray Manufacturing Corp
Current assignee: Murray Manufacturing Corp
Priority date: 1957-10-08
Filing date: 1957-10-08
Publication date: 1959-12-29
Anticipated expiration: 1976-12-29

Description

Dec. 29, 1959 c. F. BRAUNECK 2,919,325

MAGNETIC OVERLOAD RELAY Filed Oct. 8, 1957 3 Sheets-Sheet 1 IN V EN TOR.

C HARLE 8 F. 824 UNECK ATTORNEY Dec. 29, 1959 c, BRAUNECK 2,919,325

MAGNETIC OVERLOAD RELAY Filed on. a, 1957 :5 Sheets-Sheet 2 Fz' 2 69 r 7 5 7o 6 I I Fig 4 mmvrox.

CHIRLES R BRA UNECK ATTORNEY Dec. 29, 1959 c. F. BRAUNECK MAGNETIC OVERLOAD RELAY 3 Sheets-Sheet 3 Filed Oct 8, 1957 MAX.

DISTANCE or con: man POLE PIECE DISTANCE OF CORE FROM POLE P/CE a o t 1 I. u 2 1 4 3 2 I u 36 3.93 M23 wuuok 25KB .33 .3 12.252 qwkutwkwq xdbk T ll & A M

E c m F DISTANCE OF CORE FROM POLE PICE 1 2g: 6

FULL) PETRA CTED Ia PO51 TIOAI 0F CORE FORCE SPRING FORCE LINE MAX- DISTANCE OF CORE FBOM POL: PIECE F 8 INVENTOR.

CHARLES F1 BRAUIVECK BY Q4 flaw/ Q A TTORNEY 2,919,325 MAGNETIC OVERLOAD RELAY Charles F. Brauneclc, Glen Cove, N.Y., assignor to Murray Manufacturing Colporation, Brooklyn, N.Y.

Application October 8, 1957, Serial No. 689,013

' 11 Claims; Cl. 200-95) This. invention relates to overload protective devices such as magnetically operated time delay relays, and more particularly to such relays as used in motor starter arrangements.

Basically, a magnetic motor starter arrangement consists of a magnetically operated switch in series with one or more overload relays. The relays operate in response to an overload of predetermined level to interrupt the voltage supply to the operating coil of the electromagnetic switch.

For many years, thermal overload relays have been used in the magnetic motor starters However, such relays generally are subject to variations of ambient temperature. If the ambient temperature is lower than the temperature at which the relay was calibrated, the relay, in order to operate,- will require a higher current than the nominal rating of the heater element. Thus, the motor can be overloaded and the motor insulation damaged. Similarly, if the ambient temper'ature'is higher than the calibrated temperature, the relay will operate at lower current than the nominal rating of the heater element.

To obviate these difficulties, magnetic overload relays were developed.

Thereare now several typesof magnetic overload relays known. One type is a simple electromagnet, the armature of which is set to pull up at a certain current value. This type of relayoperates with no time delay. For reasons which will be explained later, time delay in an overload protective device is highly desirable.

A second type of magnetic overload relay consists of a solenoid type elcctro'magnet mechanically coupled to an oil filled dashpot. This type of relay is comparatively large-and cannot be hermetically sealed.

A third typeof magnetic overload .relay utilizes a hydraulic magnetic tubeof the type employed in the time delay electromagnetic circuit breaker. Although this type of relay is more satisfactory than the others, it suffers several shortcomings;

Generally, the time delay magnetic overload relay comprises a hydraulic-magnetic tube, a coil and an armature. The tube, filled with a liquid, contains a movable magnetic core and a spring which urges the core towards one end of the tube. At the opposite end of the tube, a magnetic pole piece is provided which cooperates with the armature. The coil is connected in series with the source of power and load, .whereby the load current traverses the coil. The stiffness of the spring and the viscosity of the liquid are selected so that the core is restrained from movement towards the pole piece until the coil current exceeds a predetermined value. At shortcircuit, the developed by the coil is suflicient to attract the armature irrespective of the position of the core. Under less extreme conditions of overload, the developed is not suffici ent to attract the armature until thecore is adjacent the pole piece. When the two magnetic members are adjacent to each other, a magnetic flux path in iron is substantially completed. A sufficient United States Patent "ice M.M.F. is then developed to attract the armature. Thus, it is apparent that the relay can tolerate overloads of varying amounts for diiferent periods of time depending on the characteristics of the tube, i.e., the time of core travel versus coil current.

For motors of difiere'nt requirements coils of appropriate ratings must be utilized. In the known relays, the coils are not mounted for interchangeability and therefore separate relays must be used for the different motors.

Further, in the known relays, the developed by the magnetic circuit for a givencur'rent is not amenable to variation, and therefore, the operating current which operates the relay is fixed for anyone coil, or to state it another way, the time delay characteristics are fixed for any one coil.

Further, in the known relays, there isalso the problem of nuisance tripping. During normal motor operation, the magnetic tube core is at some intermediate position in the tube. It the current is then momentarily interrupted and reapplied before the core has had time to move to the end of the tube, the starting current may be sulficient to operate the relay and disable the motor circuit. This is known as nuisance tripping.

A primary object of this invention is to provide a magnei'ic overload relay of novel and simple construction which is not subject to any of the above-mentioned shortcomings, and which is suitable for general motor control applications.

A feature of my invention is to provide a magnetic relay comprising acoil which is uniquely mounted for easy replacement and interchangeability.

A further feature of my invention is to provide a magnetic relay of the time delay type wherein the tube is axially movable within the coil, whereby the operating current for" a given coil may be selectively varied.

In accordance with a broad aspect of my invention, there is provided a magnetic overload relay comprising a tube having a pole piece ailixed to one end thereof. A coil, adapted to be connected in a circuit to be protected by the relay, is positioned around the tube. When an overload condition occurs, an armature is drawn to the pole piece and in so doing operates a switch to disable the circuit. This aspect of my invention is characterized by a unique mounting arrangement for the coil which permits it to be easily detached from the tube and another coil of a difierent rating substituted in its place.

In accordance with another aspect of my invention, I provide a magnetic relay for motor overload protection comprising a sealed tube having a pole piece affixed to one end thereof. The tube is substantially filled with a fluid and contains a magnetic core member and a spring urging the core member to the opposite end of the tube. A coil is provided, adapted to be connected in the motor circuit, and surrounds a given axial length of the tube. The characteristics of the spring are such that the force thereof exceeds that induced in the movable iron core by the M.M.F. produced by the normal motor operating current passing through the coil, but is less than that produced by overload current. Thus, during normal current flow the core is maintained at its fully retracted position. At the inception of overload current the core moves toward the pole piece. However, the movement is delayed by the action of the spring and fluid, the delay being inversely. proportional to the In response to a predetermined level of times time, means is provided to disable the motor circuit, thereby protecting the motor from dangerous overload.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a view, partly in cross-section, of the front of the relay with the cover removed;

Fig. 2 is a top view of the relay;

Fig. 3 is a perspective view of the tube and armature assembly;

Fig. 4 is a rear view of the relay with the cover removed; and

Figs. 5, 6, 7 and 8 are graphs illustrating characteristics of conventional and inventive time delay tubes.

Referring first to Fig. 1, the magnetic relay comprises an insulator housing 1. The housing is preferably molded in three parts. The main part consists of a hollowed rectangular housing having integral sides 2, 3, bottom 4 and rear wall 5. The remaining parts of the housing are front and rear panels 6, 7 respectively (Fig. 2). The main part of the housing is shaped to accommodate the several operating parts of the relay as will be explained in connection with such parts.

Fundamentally, the relay comprises a coil 8 adapted to be connected in the circuit to be protected, a time delay magnetic tube 9 for controlling the speed at which the relay operates, an armature 10, and a switch 11 operable by the armature for interrupting the circuit.

The time delay tube 9, as best seen in Figs. 1 and 3, comprises a non-magnetic cylindrical member 12, made, for example, of brass and hydraulically sealed at both ends. The end 13 may be spun over and cold welded as described and claimed in copending application, Serial No. 646,425, filed March 15, 1957, and assigned to the Murray Manufacturing Corporation. The opposite end of the tube is sealed by a magnetic pole piece 14 rigidly fixed thereto by any suitable means. One means is described and claimed in copending application, Serial No. 638,241, filed March 15, 1957, and assigned to the same assignee.

Inside the tube, a movable magnetic core 15 is mounted and normally urged towards the end 13 by a spring 16. The remaining space is filled with a liquid 17 of desired viscosity.

The coil 8 is wound on a sleeve which is soldered, or otherwise firmly attached, to a collar 18. In accordance with one aspect of my invention, the coil and collar assembly is easily detached from the housing by a screw 19. One end of the coil 8 is connected, e.g. by soldering or brazing to the collar 18 and the other end of the coil is secured by a screw 20 to a connector plate 21. Load and power supply wires (not shown) are connected to the opposite sides of the relay by means of

connectors

22, 23 respectively. Connector 22 is fastened to connector plate 21 by a screw 24 and connector 23 is connected to connector plate 25 by a screw 26. The

connector plates

21 and 25 are shaped with extensions 27 along the front and rear edges thereof (Fig. 2). These extensions fit snugly in grooves provided in the rear wall and front panel 6 of the housing. Therefore, when the housing is assembled the connector plates and the parts attached thereto are rigidly secured.

The diameter of the coil sleeve is slightly larger than the diameter of the tube 9, whereby the coil assembly may easily be removed from the tube. Thus, if it is desired to use the relay in another circuit having different load characteristics, the coil assembly 8 is removed by loosening the screws 19 and 20 and simply lifting the coil and collar 18 from the tube 9. A different coil of a desired rating may then be substituted in its place.

The operating current, or the time delay operation, of the relay is dependent on the characteristics of the magnetic circuit, as will now be described.

The core member and pole piece 14 define one part of the magnetic circuit. Externally of the tube, the collar 18 is made of magnetic material. An end of the connector plate 25 is turned towards the pole piece 14 and is positioned by a wall 28 integral with the insulator housing. Cooperating in sliding engagement with the turned end is a magnetic member 29 attached to the pole piece 14 by a bracket 30 made of non-magnetic material. The bracket 30 may be brazed or otherwise attached to the pole piece and adjoining member 29.

The axial length of the core 15 is approximately equal to the axial length of the coil 8 so that when the core is adjacent to the pole piece a magnetic path substantially of magnetic material is completed.

As best seen in Fig. 3, the magnetic member 29 includes lugs 31, turned at right angles to the plane of the member. The lugs are provided with aligned holes 32 for supporting a pin 33 on which the armature 10 is pivotally mounted. The armature comprises similar lugs 34 with aligned holes through which the pin 33 passes.

An armature stop member 35, having an extension 36, is located in the path of the armature for limiting its travel away from the pole piece. The extension is made of a malleable material and its distance from the pole piece may be adjusted by simply bending it into the desired position. The armature is normally urged against the extension 36 by a helical spring 37. The spring 37 is mounted on the pin 33 and has one end bearing against an angular extension 38 of the armature. The tension of the spring may be adjusted by positioning the opposite end of the spring in one of the teeth of a serrated strip 39 extending from the stop member 35.

A trip blade or spur 40 is rigidly attached by any suitable means to the angular extension 38 and serves indirectly to trip the switch 11.

The switch 11 is preferably of the snap action type comprising a Hat rectangular frame. One end of the frame is wedged at 41 in a slot provided in the housing. A tongue 42, preferably integral with the wedged end of the frame, extends towards the opposite end, above the plane of the sides 43 of the frame. An arcuate spring 44 is coupled at its ends to the free end of the frame and to the end of the tongue respectively. For this connection, the spring may be slotted and the tongue and frame ends provided with projections which fit into the slots. A pair of contacts 45 are attached preferably to the opposite corners of the free end of the frame. The switch is operated into open position by depressing the tongue 42. Depression of the tongue causes a compression of the spring 44 and also a slight rotation thereof. When the spring is rotated past the equilibrium position the force of the spring snaps the free end of the frame into a raised position. That is, raised with respect to a pair of fixed contacts 46, insulated from each other and attached to the feet of a pair of connectors 47. As shown, the connectors are wedged in slots provided in the housing. The travel of the free end of the switch is limited in one direction by a partition 48 and in the opposite direction by the fixed contacts 46.

The free end of the switch is closed by simply depressing the end past the equilibrium position of the spring 44. For this purpose a plunger 49 is provided mounted in a suitably shaped channel 50 in the housing. As best seen in Fig. 2 the front panel 6 maintains the plunger inthe channel. The channel is shaped with upper and lower stops 51, 52 to limit the travel of the plunger. The lower stop 52 is so located that while the plunger is abutting it, the end 52 of the plunger is at a position which prevents the free end of the switch from reaching the equilibrium position. A manually adjustable plunger spring 54 is mounted in a groove in the housing. This groove is channeled for positioning the plunger for either manual or automatic operation of the switch 11. if the handle of the spring 54 is in the upper channel, the plunger is positioned against the stop 52 and the switch is adjusted for automatic operation. That is, if in response to a particular circuit condition, the switch is caused to open, it remains open only as long as the condition prevails. Upon the condition terminating, the free end of the switch automatically returns to the closed position because it never exceeded the position of equilibrium. For' manual Operation, the spring handle 54 is moved to the lower channel, as shown, whereby the plunger is urged against the upper stop 51. In this position, the free end of the switch is fully raised, and to reclose the switch, the plunger must be depressed to its lowermost position.

The switch is actuated into the open position by means of a' trip member 55 pivotally mounted to the housing at 56. The trip member comprises a first extension 57 adapted to be contacted by the. spur 40 and a second extension 58 positioned to contact the tongue 42.

Thus, the coil 8 is serially connected in the circuit to be protected and the switch 11 is connected in a circuit adapted to control the coil' circuit. For example, if the relay is used inan electromagnetic motor starter, the coil is'connected'in the feed lines to the motor and the switch contacts may be connected in a relay circuit which controls an electromagnetic switch; the electromagnetic switch connecting the feed linesto the motor circuit.

Let us'n'ow assume that the relay is connected in a circuit. The switch "11 is closed and the current traversing the coil 8 is normal. The movable core 15 is urged to the end 13 of the tube 9 by the spring 16 and the armature is separated from the pole piece 14. The armature is not drawn tothe pole piece because the force of the spring is greater than the developed by the current flowing through the coil.

If now there is an'overload, so that the exceeds the spring force, the core is caused to move towards the pole piece; the spring and liquid retarding or delaying the rate of movement of the core. The time that it takes the core to reach the pole piece is inversely proportional' to the amount of overload. When the core reaches the pole piece, the reluctance of the magnetic circuit is 'at a minimum and the is sufiicient to draw-the armature. At extreme conditions of overload such as short circuit, the developed is sufficient to drawnthe' armature even if the core is at the end 18 of the tube.

In accordance with another aspect of myinvention, I have found-that the operating or tripping current can be variedup to approximately 20'perce11t' by shifting axially either the tube or the coil relative to the other. I prefer to displace the tube relative to the coil.

In. order to eifect this axial shifting of the tube, the bracket 30' which attaches the magnetic member 29 to the pole piece 14' includes a pair of oppositely disposed dependingears'5'9. The ears '59 guide the movement of theztube by sliding. in a channel 60 provided in the rear wall :5.

Theear: which slides in the rear wallS, as best seen inFi'g. 4,,includes a pin 61 which is slidable in a linear slot 62, inthe housing. The pin 61 serves as a cam follower; The-tube is manually adjustable by means or" a cam 63 which'is rotatably mounted on a pin 64 extending frorn the rear surface of the wall *5. The cam is provided with a-slot' 65 which is eccentric with respect to-the center= of rotation. The cam is provided with a lever 66'for hand operation. The top of the lever is bent over to serve as an indicator 6'! (Fig. 2). Precalibrated slots 68 may be provided on the running surface 69 and the indicator may be indented to register with the slots, for accurate and stabilized adjustment.

The rear surface of the wall 5 is shaped as shown at 7% to accommodate the cam 63. The rear panel 7 maintains the cam-in position.

The relay as thus described has utility in many applications.-- However, it has particular utility in a motor control starter. For'example, the same basic relay may beadapted for use with differently rated motors, simply bysubstituting one coil for another.

. l The desirability of this interchangeability feature is readily appreciated-when it is realized that the National lilectricalCode; requires 'that: integral horsepower (H'.P.)

motors and certain installations of fractional H.P. motors shall be protected by separate overcurrent devices responsive to the motor current. Complete coverage between .75 ampere and 100 amperes motor current ratings, requires between 35 and 45 difierently rated overload relays. In accordance with thisaspect of my invention, one basic relay and separately rated, and interchangeable coils may be used in place of a large number of different- 1y rated, factory calibrated relays.

Regarding now the advantages to be derived from a movable tube, it is to be realized that the magnetic tube and associated magnetic circuit are adapted to operate at some constant value of ampere turns. For example, if it is desired to trip a relay at 300' ampere-turns, a three turn coil would have a trip rating of 100 amperes. In accordance with standard practice, a trip rating of 100 amperes would be suitable for motors having:

Minimum full load amps.=%= am s.

Maximum full load amps.=%= 87 amps.

Minimum full load amps.= =60 amps.

Maximum full load amps.= i 5=65 amps;

A five turn coil would have a trip rating of 60 amperes and would be suitable for motors having:

0 =48 amps.

6 Minimum full load amps.-

Maximum full load amps.=%=52.2 amps.

Thus, in the range of 48-100 amperes, any of three coils may be usedhaving full loadcurrent ratings of between 48 and 52 amperes, 60- and 6S amperes, and

80 and 87 amperes respectively. It is observed that these ratings exhibit large gaps between the respective ranges.

In accordance with this aspect of myinvention, the gaps are reduced by shifting the tube relative to the coil. As previously mentioned, the trip rating can be altered by 20 percent; the trip current decreasing as the tube is moved into the coil. The-eifect of shifting the tube is illustrated in Fig. 5.

In Fig. 5, curves I 1.11 and 1.21 are the full load currents corresponding to three different positions of the tube relative to the coil. The curves I 1.11 and 121 are tripping currents relating to currents I 1.11 and 1.21 usually 1.15 to 1.25 times the full load motor current. The critical points: on the curve are where the current force lines cross the spring force line, e.-g., d d and d Beyond this point the exceeds the spring force and the relay is tripped. It is seen that the trip rating may be selectively'varied between l and 1.2I by shifting the tube relative .to the coil.

In accordance with a further aspect of my invention, I have found that nuisance tripping can be eliminated by a novel design of the coil.

Fig. 6 illustrates force curves for. the conventional time delay tube. Fig. 7 shows the values-of current required for producing a force capable of drawing the armature even though the core is separated from'the pole piece.

In Fig. 6, I represents the normal motor operating current. 1 and 1 represent overload currents and 61 represents the starting current required to' operate the motor. As seen in Fig. 7, the starting current 61 is insufiicient to draw the armature while the core is fully retracted from the pole piece. Thus, when the switch is just closed, a current 61 traverses the coil. The developedby the current pa'ssing-throughthe coil causes the core to move towards the pole piece. However, the viscosity of the liquid and the force of the spring are selected to delay the core so that the current value drops below I (Fig. 7) before the core reaches the distance (1 in the tube. Let us assume now that the current is normal. Referring to Fig. 6, it is seen that the core will come to rest at a distance d where the spring force is equal to the developed by the current passing through the coil. If, as a result of the starting current, the core is forced to the distance d it is seen from Fig. 5 that the spring force at d exceeds the and, therefore, the core is forced back to the distance d If now the motor circuit is momentarily interrupted and reclosed before the core has reached position d the relay will .trip. This is known as nuisance tripping which, as can be observed from Fig. 6, is a result of the secondorder force curves.

In accordance with this aspect of my invention, I have designed a coil-and tube having substantially linear force curves except for the motor starting region. The curves for the novel coil and tube are shown in Fig. 8. Referring to Fig. 8, it is seen that the spring force is constantly greater than the force developed by the normal current. Therefore, during normal current flow, the core piece is positioned at the end of the tube and the possibility of nuisance tripping is eliminated.

By way of example, I have constructed a relay for operation at approximately 275 ampere turns with the tube fully inserted in the coil. By moving the tube partially out the trip rating was increased .to approximately 300 ampere turns. Fully .retracting the tube increased the trip rating to 330 ampere turns. The coil was designed to operate at 27.5 amperes minimum having turns of number 12 wire wound in one layer. The axial length of the winding was approximately inch with a 4; inch thick insulating washer between the coil and the collar. By varying the position of the tube, a maximum operating current of 33 amperes was achieved.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What I claim is:

1. A magnetic overload relay, comprising a sealed tubular member, a coil member surrounding a given axial length of said tubular member, means for moving one of said members relative to the other, a pole piece aflixed to one end of said tubular member, a core member movably mounted within said tubular member, and approximately equal in length to the axial length of said coil, a spring within said tube urging said core towards the opposite end of said tube, fluid of given viscosity within said tube, a first magnetic member attached to said pole piece and extending towards said opposite end of said tube, a second magnetic member slidably engaging said first magnetic member and mechanically coupled to said coil at the end thereof remote from said pole piece, whereby a magnetic path of adjustable length is defined through said first and second members, an armature pivotally mounted in cooperative relation with said pole piece, the characteristics of said spring and fluid being such that the core is restrained from moving a substantial distance towards said pole piece during normal current flow through said coil member, and upon the current exceeding a predetermined value, the magnetomotive force produced thereby being sufficient to cause said core to move towards said pole piece, thus substantially completing the mag netic circuit of magnetic material, whereby the flux is in creased sutliciently so that said armature is drawn to said pole piece.

2. The relay according to claim 1 and further comprising means for detachably attaching said coil member around said tubular member, whereby the coil may easily be replaced by another of a ditferent rating.

3. The relay according to claim 1 and further comprising manually adjustable means for moving said tubular member relative to said coil member.

4. The relay according to claim 3 wherein said manually adjustable means comprises a cam follower in the form of a pin attached to said tubular member, means guiding said pin so that the movement thereof is linear, a rotatable cam having a slot eccentric to the center of rotation, said pin fitting into the slot, whereby rotation of said cam produces linear movement of said tubular member.

5. A magnetic overload relay, comprising a housing, a sealed tubular member, a pair of oppositely disposed guide members attached to one end of said tubular memher and adapted to slide in corresponding channels provided in said housing, the channels being shaped to guide the movement of said tubular member in the direction of the tube axis, a coil surrounding a given axial length of said tubular member, magnetic means secured by said housing for rigidly supporting said coil at one end thereof, a pole piece afi'lxed to said one end of said tubular memher, a core member movably mounted within said tubular member and approximately equal in length to the axial length of said coil, a spring within said tube urging said core towards the opposite end of said tube, fluid of given viscosity within said tube, means for moving said tubular member relative to said coil comprising a pin extending from one of said guide members into a longitudinal slot provided in said housing, a manually operable cam member rotatably mounted on said housing including a slot eccentric to the center of rotation thereof, said cam slot slidably engaging said pin, whereby rotation of said cam member causes axial movement of said tubular member, an adjustable magnetic path of magnetic material around said coil comprising a first magnetic member attached to said pole iece and having a portion extending towards said one end of said coil, said coil supporting means including a portion overlapping said portion of said first magnetic member in slidable engagement therewith, whereby movement of said tubular member varies the length of the magnetic material enclosing said coil, an armature pivotally coupled to said first magnetic member and adapted to be drawn to said pole piece, a trippable switch normally in the closed position mounted in said housing and adapted to control the circuit in which the relay is connected, tripping means pivotally mounted to said housing and operable by said armature to trip said switch when said armature is drawn to said pole piece, the operation of said armature being determined largely by the characteristics of said spring and fluid, the characteristics being such that the core is restrained from moving a substantial distance towards said pole piece during normal flow through said coil, and upon the current exceeding a predetermined value, the magetomotive force produced thereby is sufiicient to cause said core to move towards said pole piece, thus substantially completing the magnetic circuit of magnetic material, whereby the flux is increased sufficiently so that said armature is drawn to said pole piece.

6. A magnetic relay for motor overload protection, comprising a sealed tube, a pole piece afiixed to one end of said tube, a magnetic core member mounted within said tube, a spring within said tube urging said core towards the opposite end of said tube, fluid of given viscosity within said tube, a coil adapted to be connected in the motor circuit surrounding a given axial length of said tube, the characteristics of the spring being such that the force thereof exceeds the magnetomotive force produced by the normal motor operating current passing through said coil over the normal operating range of the motor, but is less than the motor overload current, whereby during normal current flow the core is maintained at its fully retracted position and at the inception of overload current the core moves toward said pole piece, the movement being delayed by the action of said spring and said fluid, the time delay being inversely proportional to the magnetomotive force, and means operative in response to a predetermined level of magnetomotive force for disabling the motor circuit.

7. The relay according to claim 6, and further comprising means for detachably attaching said coil around said tube, whereby one coil may be replaced by another of a difierent rating.

8. The relay according to claim 6, and further comprising means for axially moving said tube within said coil, whereby the coil surrounds different selected portions of said tube, the position of said coil affecting the operating current of said relay.

9. The relay according to claim 6 wherein said means for disabling said motor circuit comprises an armature pivotally mounted and positioned to be drawn to said pole piece when said predetermined magnetomotive force is attained, a normally closed snap-action switch adapted to be connected so as to control the motor circuit, and tripping means operative in response to the movement of said armature to operate said switch.

10. The relay according to claim 8 and further comprising a frame of magnetic material positioned in a magnetic path of said coil, and means for adjusting the length of said frame in correspondence with the movement of said tube.

11. A magnetic overload relay comprising a tube, a movable pole piece located within said tube, means in said tube normally urging said pole piece towards one end thereof, a fixed pole piece attached to the opposite end of said tube, a coil surrounding a given axial length of said tube, less than the complete length thereof, and adapted to be connected in a circuit to be protected by said relay, magnetic path means adjustable in length connecting said fixed pole piece and the end of said coil remote from said fixed pole piece, and means for axially moving said tube relative to said coil, whereby the reluctance of the magnetic circuit is affected by the length of said movable pole piece surrounded by said coil.

References Cited in the file of this patent UNITED STATES PATENTS 733,563 Sundh July 14, 1903 1,322,097 Dunham Nov. 18, 1919 1,981,681 True Nov. 20, 1934 2,282,933 Cahill May 12, 1942 2,661,451 Tamm Dec. 1, 1953 2,690,528 Wilckens Sept. 28, 1954 2,756,302 Baltuch n July 24, 1956