US2386149A - Electromagnetic control - Google Patents

Description

Oct. 2, 1945. w. L. SMITH ELECTROMAGNETIC CONTROL Filed Aug. 9, 1945 2 Sheets-Sheet l H N Ru V W m MN NS k W m N r ID A Oct. 2, 1945. w SMITH ELECTROMAGNETIC CONTROL Filed Aug. 9, 1945 2 Sheets-Sheet 2 SATURABL INVENTOR. VVAAD L. SMITH lOO E %N0/?MAL vamie BY cf 22% Arrow/5Y8 -referred to as the Patented Oct. 2, 1945 Yum-Tao STATES PATENT. OFFICE 2,386,149 ELECTROMAGNETIC CONTROL Ward L. Smith, Hudson,

Electric Controller & M Cleveland, Ohio, a corp Ohio, assignor to The anufacturing Company, oration of Ohio Application August 9, 1943, Serial No. 497,941 10 Claims. (01. 1754.75)

operating winding, or magnetic flux in the iron winding, current in the path which are suflicient to retain an electromagnetic device in operated position, hereinafter drop-out values of the respective quantities, are an important factor in the design of electrical apparatus embodying such devices.

For example, a disadvantage in the operation tofore has been the tendency of the armatures of such devices to remain in an intermediate position consequent upon a reduction in the supply overheated and may weld together. An important object of this invention is to provide an improved electromagnetic control for causing the armature of an alternating current electromagnetic device to move quickly and steadily to its fully open position consequent upon a decrease in the supply voltage to a value within a range but slightly below the drop-out value or consequent upon a gradual decrease in the supply voltage through said range.

Another disadvantage in the operation of electromagnetic devices having movable armatures and energized by alternating current is the inability of such devices to remain in operated voltage to a value slightly below the drop-out value or to hesitate in such intermediate position during an opening cycle caused by such a decrease in voltage. Momentary voltage variations of this magnitude without substantial change in frequency occur frequently in many installations due to the sudden application of a large load to the supply system or due to other causes resulting in a temporary current demand in excess of the normal circuit capacity. The undesirable operation results from the fact that the contact spring which, when the switch or relay is-closed, provides a uniform pressure at the contact surfaces, assists the opening movement of the armature only during the initial stage of the opening cycle.

= At the instant when the contacts start to separate,

the assisting force of the contact spring suddenly reduces to zero, and the remaining force or forces tending to cause opening movement of the armature are, in many instances, insuflicient to effect such additional opening movement unless supply voltage is further reduced.

It is thus seen that when the supply voltage is within a range slightly below the drop-out value that the armature of an alternating current switch or relay may open only partially or hesitate during its opening cycle. While the armature is in the intermediate position, the contacts are but lightly touching each other, and the contact surfaces therefore tend to become position consequent upon a reduction in the supply voltage to values in the neighborhood of 60% of normal.

For example, to prevent needless deenergization of electrical apparatus controlled by an alternating current electromagnetic relay due to opening ofthe relay as a result of a temporary decrease in the supply voltage to a value below the dropout value of the relay, special low voltage protective devices equipped with time delay mechanisms have been used. The time delay mechanisms serve to maintain the control relay contacts in closed position for an interval after the voltage falls below the drop-out value, so that, if the voltage increases again before the expiration of the time interval, the control relay does not operate to shut down the controlled apparatus. In many instances these time delay mechanisms could be dispensed with if the control relay armature did not tend to move to its deenergized position until the supply voltage reached a very low value. It is an object of this invention to provide an electromagnetic control for preventing opening movement of the armature of an alternating current electromagnetic device unless the voltage of the circuit supplying said device is reduced to a small percentage of its normal value.

One of the functions of a low-voltage protective relay of the time delay described above or the of the more common type having no time delay feature is to control the deenergization of all of the electromagnetic switches in a control system so that, under conditions of reduced voltage, all

.of the switches open simultaneously. In order to accomplish this result the low-voltage protective relay must open before any of the switches or other relays in the system. Because of the smaller inertia of the movin parts of the lowvoltage protective relay, the relay opens before the switches consequent upon a very sudden reduction in the supply voltage to zero, but difiinetic device having a movable armature.

culty has been experienced in designing low-voltage protective relays which open before the other switches of the system consequent upon less sudden reductions in voltage or under conditions wherein the voltage remains constant at some reduced value. An important factor here in= volved is the fact that the heavier moving parts of the electromagnetic switches tend to cause the switches to start to open at values of supply voltage higher than the drop-out voltage of the lowvoltage protective relay, 1. e. the switches have a higher drop-out voltage than the relay. A iur= ther object of this invention is to provide means operable to cause the drop-out voltage of an alterhating current low-voltage protective relay to be higher than that of the electromagnetic switches or other relays in the-same control system.

Another object is to provide an alternating current electroetic control for causing unhesitating opening movement of .the armature of an alternating current switch or relay when a predetermined reduced value of supply voltage is reached which predetermined value is selectable from a wide range of voltage values.

A specific object'is to provide an alternating current electromagnetic control for preventing movement of an armature to an open position except when the supply voltage is close to zero.

A further object is toprovide an electromagnetlc control for changing the drop-out voltage value of an alternating current electromagnetic device.

- Yet another object is to provide an alternating current electromagnetic control which operates to cause an armatureto move quickly and steadily to-its open position regardless of the nearness of the supply voltage to the drop-out value of the device comprising the armature.

A more detailed object is to provide a means in the energizing circuit of theoperating winding of an electromagnetic device having a mem= ber movable to change the inductance of said windingwhich means causes the impedance of said energizing circuit to increase as said member moves to decrease the inductance of said winding.

In accordance with this invention a negative or capacitive reactance means such as a condenser is connected in series with a positive or inductive reactance means such asthe operating winding of an alternating current electromag= The condenser is proportioned relative to the inductive reactance of the winding so that the im= pedance of the supply circuit of the operating winding increases as the armature oi the device opens, whereas without the condenser the supply circuit impedance decreases, and consequent= ly there is a positively decreasing force rather 7 than a substantially constant force tending to prevent the armature oi the device from moving to its open position. Ina modification a saturable inductance and a condenser connected in series with each other and with the operating winding of an electromagnetic device are rela- ,tively proportioned so as to maintain the cur rent through the winding at a substantially con- 4 stant' value even though the supply voltage decreases greatly from its normal value and so that when the armature oi the device moves toward its open position the attracting force steadily decreases. In another modification, an adjustable resistor and a condenser connected in series with each other and with the operating winding v of an electromagnetic device are proportioned assaieo so as to increase the resistance component at the impedance of the supply .cucuit to thereby in crease the drop-out voltage above that enihting when the condenser and winding alone are By means of this invention it is possihlg to either increase the drop-out voltage or voltage protective relay of an alternating cur rent control system or to decrease the drcp eut voltage of the larger electromagnetic switches involved in the system or both, and icy vlrtueofi the same electromagnetic control mangement which accomplishes the changeor cleanses lathe dropout voltages, to cause a gulch unhesl= toting drop-out action or hath the relay and the switches when the respective new values oi p= out voltage are reached. That is, the electrce magnetic control oi this invention not only hav omits the designer to select ahcost voltage below the maximum for the drop=cut value oi a switch or relay, but also causes the relay or" switch to move to its open position without heel tation when the drop-out voltage selected lot that relay or switch is reached.

Other objects and advantages will hecome ap parent from the following description where n reference is made to the drawings, inrvhich Fig. 1 illustrates the invention as applied to a the control of an electromagnetic switch shown in its closed position;

Figs. 2 and 3 show the switch oi intermediate and the open positions,

tively;

. Fig. 4 is a wiring diagram of a mofificaticn;

Fig. 5 is a vairhig diagram oi a iurther medi fication; and

Figs. 6 and '2 are graphs showing impedance and current variations, respectively, which cc= cur when the present invention is used ascent pared with prior art arrangements.

respec= Referring to Figs. 1, 2 and 3, one type of elec= tro-magnetic device to which this invention 'mcy be applied is shown as a switch it representative of a well-known construction and having a c-shaped magnetic frame it and a clapper-type armature H? which is pivoted at a knife edge 38 so as to he movable from a fully closed position bridging the open end of the frame it as shown in Fig. 1 to a fully open position a suit able stop to as shown in Fig. 3. Pivots-cl to the armature 62 at a knife edge it is a movable con tact arm ll biased in a counter clcchvlise tion about the knife edge l8 icy a helical spring 98 interposed between the contact arm ll a suitable stop near the-outer end of a spring post it. The post 09 is seemed to m upwardly extending bracket 29 of the armature i2 and passes loosely through an opening in the ill. in both the intermediate and the open pool? tions of the switch it shown in Figs. 2 and il, respectively, the spring is is estwded and the contact arm ll is thereby forced countercloclr wise against the bracket 28, and in the closed position of the switch it shownin i, spring i8 is compressed and lilacs g, contact 28 on the contact arm ll against a stationary can tact is suitably attached to an sup port 2:3 xtending upw from the home it,

The switch it i preferably provided "with a normally open auxiliary contact mechanism ilor a purpose to be hereinafter desci'ih'ed. illus trative auxiliary contact mechanism may com= prise a fiat-spring 28 having one end suitably at tached to but insulated irom the lower end oi the'armature i2 and having at its free ends movable auxiliary contact when the come the low ture- I2 is in the closed and intermediate positions 01 Figs. 1 and 2, respectively, the movable auxiliary contact 23 is in circuit completing engagement with a suitably supported stationary auxiliary contact ll, and when the armature I2 is in the open position of Fig. 3, the contacts 23 and 30 are disengaged.

The construction of the switch It has not been described in detail since its construction forms. no part of the present invention and is merely illustrative of one type of electromagnetic device with which this invention may be used.

The armature I2 is moved from the open position of Fig. 3 to the closed position of Fig. 1 and is held in the closed position by magnetic flux produced by energization of an operating winding, 34 on the frame H, is moved from the closed position to the intermediate position of Fig. 2 due to the combined action of the spring I8 and gravity consequent upon a reduction in the amount of magnetic flux, and is moved from the intermediate position to the open position of Fig. 3 due to gravity alone. It is obvious to those skilled in the art that the switch H3 may include an additional spring or other means augmenting the force of gravity throughout the opening cycle or replacing the force of gravity entirely.

The winding 34 is arranged to be energized from a source 33 of alternating current upon closure of a normally open, momentary contact type push button 36 through a pair of conductors 33 and 39 and the contacts of a normally closed push button-4| interposed in the conductor 33. Connected in parallel with the stationary contacts the push button 38 is a holding ircuit comprising a capacitive reactance means such as a variable condenser 42 and the auxiliary contacts 29 and 30 of the switch It, which holding circuit is described more in detail hereinafter. Closure of the push button 36 causes energization of the winding 34 from the source 33 through the conductors 33 and 39 and consequent movement of the armature I? to the closed position of Fig. 1 wherein the contacts 2i and 22 and the auxiliary contacts 29 and 30 are closed. Opening of the push button 36 does not effect deenergization of the winding 34 since the winding continues tobe supplied with current through the holding circuit 411 which is completed upon closure of the auxiliary contacts 29 and 30. Opening of the push button 40, however, causes complete deenergization of the winding 34 and the armature l2 consequently moves quickly to its open position.

In order to facilitate the description of the function of the condenser 42, an explanation of the operation of Figs. 1, 2 and 3 is given here with the assumption that the condenser 42- is short circuited. With this assumption, which renders the circuit of Fig. 1 the same as the usual prior art circuits, and with the push button" 36 open and the armature H in the closed! position, the impedance Z of the energizing circuit 4| for the winding 34 is given by and X1. is the inductive Due to the substanthat the resistance may be neglected in determining the impedance. Thus equation (1) may beconsidered approximately to be (2) Z=XL The magnitude of the current through the winding 34 is inversely proportional to the impedance of the energizing circuit, and consequently a decrease in the impedance results in a proportional increase in the current through the winding 34 if the voltage of the source 35 remains constant. Since the introduction of an air gap into the magnetic circuit of the switch 10 causes the inductive reactance of the winding 34 to decrease, the impedance of the energizing circuit under the assumed conditions decreases in accordance with Equation 2, and consequently a current of larger magnitude flows through the winding 34 upon movement of the armature l2 toward its open position. Although the introduction of the air gap has increased the reluctance of the magnetic circuit, the increase in current tends to maintain a substantially constant flux irrespective of the position of th armature l2.

Assume now that the armature I2 is held in the closedposition of Fig. 1 by iiux produced by the winding 34 (the condenser 42 still being short clrculted) and that the voltage of the source 35 decreases while the frequency remains substantially constant. As the voltage of the source 35 decreases, the current in the winding 35 decreases, and consequently the flux holding the armature It decreases. At a predetermined reduced value of voltage (the drop-=out value) the armature I2 starts to move toward its open position due to the fact that the holding force of the flux becomes less than the combined forces of the spring i8 and gravity. However, a slight opening movement of the armature l2 causes the current in the winding 34 to increase from its reduced value since the introduction of the air gap in the magnetic circuit reduces the inductive reactance and hence the impedance of the energizing circuit. Assuming that the voltage now remains constant at the drop-out value or slightly below, the increasing current tends to maintain a constant flux in the magnetic circuit irrespective of the position of the armature it, but the flux is slightly less effective in its attraction on the armature i2 due to increased leakage and fringing" at the air gap. The armature it, therefore, continues to move toward its open position due to the difference between the pull of the flux and the combined forces of gravity and of the spring 18. However, when the armature i2 reaches the intermediate position of Fig. 2, the contacts 2i and 22 are just barely in contact with each other and the opening force of the spring i8 is consequently zero and the force of gravity only remains to oppose the pull of the flux. When the armature i2 is in the intermediate position it is desirable for proper closing action of the switch It that the force of gravity on the armature it be as small as possible. Consequently, there is a very strong tendency under the assumed conditions of reduced voltage for the armature ii. to remain in the intermediate position, and, since the contacts 2i and 22 are very lightly in contact with each other over a small area, undue heating of the contacts occurs and the possibility of the contacts welding together exists. Reduced values of supply voltage throughout a range immediately below theminimum drop-out value which are insuflicient to maintain the armature l2v in its closed position thus are suiiicient to prevent the only greater then the inductive reeoteuce oi the circuit when the armature E2 is closed. With the condenser 62 in the holding circuit ll, the push button 35 ogen, end the armature B2 in its closed position, the impedance Z of; the energizlug cir= cult for the winding is given by where R and Z1. refer to the some quantities as before end. & is the cepscltlve reectence of the condenser the euerglmig circuit is vety much greater than the resistance, and the capacitive reectence is considerably greetertheu the inductive reecteuce when theeroseture i2 is closed, the oifierenoc between the inductive reectence and the cepeci= tive reaoteuce is also much greater than the resistance (the negative sign of the difierence being of no consequence) If the capacitive re= ectence introduced into the energizing circuit is approximately twice the inductive resctuuce of the energizing circuit when the ature i2 is closed, the impedance of the circuit has not been changed, materially by the inclusion of the condenser til therein so long as the structure i2 is in its closed position, as is clear tram e comparison of Equations 1 and. 3.

Assume now thettiie voltage of the source 85 decreases to or slightly below e value at which the armature ill starts to move toward its open positlon. (its will be explained subsequentlyfthis v'elue of drop-out voltage is not necesslerlly tl le Since the inductive recctence ofv .oeeeise duced current causes 9. reduction oi flux, the

inductive i-eectcnce end consequently the imyed= once senerolly increase at lower flux densities as indicated by the curve as oil Fig.6 so that there is not s proportlonel drop in current with de= creased voltage, In Fig. 6, wherein impedance of the energizing circuit is plottediegeinst percentege oi normal supply voltage, the curve .63 indicates the variation in impedance of the em erglzing circuit for the opereting winding 3G with the condenser 62 short circuited end the curve his indicates the variation in impedance of the some as the drop-out voltage without the con- I denser 32 in the circuit.) when the armature it now starts to move. toward its open position, the inductive reactance of the winding 3 E decreases as before, but the impedance Eat the energizing circuit increases as is clear from an examination of Equation 3. Consequently, instead of the current through the winding 3% increasing as before, the current decreases as the armature it moves toward its open position. Since the flux varies as the current, thefiui: also decreases and the arms.- ture 52 moves rapidly and steadily to the open position of Fig. 3 and the contacts 28- ond 22 separate quickly and fully without hesitation at the intermediate position of Fig. 2.

It is thus seen that with a. condenser of suitable capacity connected in series with the operating winding of an alternating current electromcg netic device, a decrease in the supply voltage to values slightly below the drop-out value permits e rapid and unhesitcting opening movement oi the armature. However, if the condenser hes e 4 capacitive reoctence approximately twice-the inductive reectence of the operating winding 36 when the armature Q? is closed, the drop-out voltage is much lower than it would be if the condenser were not present. Without e condenser connected in series with the winding St, a reductlon in the voltage of the source Blicouses edecrease in current through the winding 36 since i the current I given fey V XL th at cc to mslected, out. since the energizing circuit when the condenser 32 is not once with this invention in the energizing circuit A the current in the winding so also decreases with the voltege since the current is then given by the resistence R being neglected. This reduced current causes a reduction in; the flux, and since the inductive reoctonce of the circuit generelly increases due tothe increesed permeebllity of the iron at low flux densities, the impedance of the circuit decreases, in eccordeuce with Eouo= tion 3 end as indicated by the curve 321' Fig. i, and the current foils less rapidly with declining voltage. The rate of decrease in current through the operating winding with declining voltage when the capacitive reuctonce oi the emerge; circuit is approximately twice the inductive rccctence is shown by the curve as of Fig. 7. From Fig. 2 it will be noted that o point 66'. on the curve 5 3 which polntis at the drop-mot current value occurs at a higher value .02 voltage then a. similar point 65' on the curve-6E5.

Thus the addition of the condenser 32 in series with the winding 8 not only causes orepld, end unhesitctlng opening operation ol the switch to consequentupon a. reduction in the supply volt age, but also affects the drop-out voltage ol the switch it. ln'some applications it may l e desir able to reduce greatly the value of the drop-out voltage and in other applications ltmoy be deem-=- glble to have a steady and unhesltstiuu drop-out at o-relatively high value of reduced voltage.

To further reduce the velue of the dropout voltage, a circuit such as Fig. 4 may be used, wherein a setureble reactor to is connected in series with the condenser 62 in the holding circuit ill. Due to the non-linear characteristics of the reactor 38 the inductive reectence of the energizing circuit increases greatly as the voltoge decreases, so that the impedance decreases vegy rapidly, and substantially constont current-flows through the winding 3% until extremely low volts ages are reached. If the resistance of the euer gizing circuit is made very smell and the electrical constants of the lnductsnces and the are properly selected so that a, condition of series resonance is approached at low voices of supply I 4 The combination with an electromagnetic device having a winding arranged to be energized reactance to cause an increase in the impedance.

To increase or to maintain the drop-out volt v age of the switch I. at its usual value, but to provide for unhesitating opening movement of the armature l2 consequent upon a decrease in the supply voltage'below the drop-out value, the condenser 42 may be adjusted to have a capacitive reactance greater than twice the inductive reactan'ce of the winding 34 when the armature i2 is cloud, or the circuit of Fig. 5 may be used, wherein an adjustable resistor 5| is connected in series with a fixed condenser 50 in the holding circuit ll. When the voltage or the source I5 is at its normal value, the resistor Si is so adjusted that the current in the winding 34 is at a value slightly greater than the drop-out value. Under these conditions, the impedance of the energizing circuit is maintained substantially constant throughout slight decreases in voltage, that slightly and to such an extent that the current and flux fall below the drop-out values, the armatures l2 starts to open and the impedance of the energizing circuit increases considerably due to the decrease in the inductive reactance caused by the introduction of an air gap in the magnetic circuit. The resulting decrease in current throuigh the winding it permits the armature I! to drop unhesitatingly to its open position.

I claim:

1. In combination with an electromagnetic device having a winding, a member movable to change the inductance of said winding, an alternating current energizing circuit for said winding, and means in said energizing circuit for causing the impedance of said energizing circuit to increase as said member moves to decrease the inductance of said winding.

2. In combination with an electromagnetic device arranged to be operated by alternating current-and having an operating winding, 9. mem

ber movable'to change the inductance oi said winding, an energizing circuit for supplying alternating current to said winding, and capacitive re-' movable to change the inductance of said'winding, an energizing circuit for supplying alternating current to said winding, and capacitive reactive means connected in said energizing circuit in series with said winding for causing an increase in the impedance of said energizing circuit as said member moves to decrease the inductance oi said from a sourceofialternating current .of variable voltage andhaving an armature movable consequent upon said voltage reaching a predetermined reduced value, o! a= capacitance connected in serieswith said winding and said source of current andhavingaresctanee greaterthanthereact-- anosoisaidwindingwhensaidarmaturdisinits 5. In combination with an'electromagnetic device having a' magnetic circuit and a winding, a member movable to change the air gap 01' said magneticcircuit, an alternating current energizing circuit for said winding, and a capacitance connected in said energizing circuit in series with said winding, said capacitance being so proportioned in relation to the inductance of said winding when the air gap of said magnetic circuit is substantially zero that the impedance of said en-' ergizing circuit increases as said member moves to increase the air gap of said magnetic circuit.

6, The combination with an electromagnetic device having a movable armature and a windingarranged to be energized from a source of alternating current, of a capacitance connected in series with said winding and said source 01' current and having a reactance approximately twice the reactance of said winding when said armature is in its closed position.

7. The combination with an alternating current electromagnetic switch having an operating winding, a condenser, means for connecting said wind-- ing to a source of alternating current independently of said condenser until said switch is closed, and means operable upon closure of said switch to interpose said condenser in a circuit between member movable to change the inductance of said winding, an energizing circuit for supplying alternating current to said winding, a saturableinductv ance and a capacitance connected in said enersaid winding, said capacitance being so proportioned in relation to the combined inductance of said saturable inductance and said winding when said magnetic circuit is closed that the impedance of said energizing circuit increases as said member moves to decrease the inductance of said 9. In combination with an electromagnetic device having a magnetic circuit and a winding, a

member movable to change the inductance of said winding, an alternating current energizing circuit for said winding, and a capacitance and a resistor connected in said energizing circuit in series with each other and said winding, said capacitance being so. proportioned in relation to closed position with said voltsse at a-vaiue slight- A.

the inductance of said winding when said magnetic circuit in closed that the impedance of said energizing circuit increases as said member moves to decrease the inductance or said winding.

* 10. The combination with an electromagnetic switch of the type having a movable armature adapted to be held in a cl position by. magnetic flux produced by a winding energized with alternating current, the holding force of said flux being in opposition to an opening force which suddenly decreases during and before completion of an opening cycle initiated upon reduction oi said flux below a predetermined value, of a capacita'nce connected. in series with said winding and so proportioned relative to the inductance of said winding that said flux decreases from its reduceiivaiue as a result of movement oi said armature away from its closed position.

WARDLBMI'I'H.

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