US2472526A - Underspeed motor protective system - Google Patents

Description

A. c. FR-AZEE June 7, 1949.

' UNDERSPEED MOTOR PROTECTIVE SYSTEM Filed Jan. 26, 1945 Us 2 Q Patented June 7, 1949 [1N] T E D S TAT E S OF F il C UNDERSPEED MOTOR PROTECTIVE Albert C. Frazee, Wilnrcnding, Pa.,- assignor to- The.

Westinghonse Air Brake Company,..Wilnier.ding, Pa, a corporation of Pennsylvania,"

Application January 26,.19455Seria1N0. 574;7801a.

Mympresent invention is devised particularly as aksolntioniorthe problem of undesired stoppage-oLconveyorbelts inindustrial plants, such as foundries, where the location of the conveyor belt-,e- .oruextraneous. noise,

atingacondition ofv the conveyor belt.

For example, in foundries, the sand shaken out,.of. the :molds. at the time the castings are removed from their molds drops through a horizontalivibrating shake-out" screen onto an endprevents. workmen fromsdetecting-an abnormal or unwanted oper- 2 Claims. (Cl. 318e-4611 lessvbeltlconveyorwhich transports the sand,

eithermdir'ectly orthroug'h the medium of additiona1-.'endless .belt. conveyors, to other. portions of theLfoundry where the sandis reclaimed and reconditionedior further use. Ordinarily, the endless. conveyor belts are located in sub-floor compartments or rooms and are not visible to the'JWorkman the main foundry room. It sometimes happens. that the sand load on the endless belt conveyor conducting the sand away from the shake-out Screen becomes excessive and the endless .belt of the conveyor slips on the drivingdrum, which may be driven by an electriomotor. In such case, therefore, the endless belt' conveyor stops or slows down to an abnormally low speed and the sand from the shakeout-screenpiles upon the belt and eventually stopsitJlB'y the time the workmen in the main foundryroom. at the shake-out screen become aware of the piling up of the sand on the endless beltconveyor beneath the shake-out screen, the-amount oi the sand accumulated is such as to requir'eacomplete shut-down of operations in the foundry until such time as the sand can be .dugawayfrom around the endless belt conveyon to permit the normal operation thereof. ThisiI-fresults .in inefiicient operation and lost times:

In..order,. therefore, to detect the undesired stoppage oi the endless belt conveyor in thesituati'onMabove described I have devised a simple electrical'apparatus for detecting the abnormal slowing-down or-stoppage of the endless belt. conveyor .-.-and .for efiecting a suitable control operationor. signal indicationof sucha nature as Lib to immediately apprise the workmen in the main paratu-sr. comprises a-lsingle-pole double-throw switchloperated by :a rotary .camthat is, in turn, driven according .itothe movement of the-end-- less-belt as bytconnection to-anidler-drum associated with the endless belt, ior the purposeot alternately..chargingwand, discharging :a pair .of

electrical? condensers in a, suchsequence that onecondenser is abeing acharged. whilev.the .other. .ris being. discharged, the chargingacurrenaof the condensers -being-.: e1iective mto Voperatively ener glze thawinding. of. a relaytso -1ong-asthe con-:

denser. charging-,current exceeds .a certain value. If the speedof movement .ohthe. endless con veyor asi reflected inathe rotary speedof the cam for. operating. the single-pole. double-throw" switch, .reduces .rto- .ana abnormally low value, or if the conveyor/and the. cam stop. completely, 7 the charging current for one condenser will have anopportun'itnto .reducewsufficiently before the:

charging. current forrtheothercondenserstarts to flowlthrough'the. windinguof. the relay that the relay lwillube at least momentarily dropped-out,-

thatis restoredlto itsI-deenergized position.

Tlie..re1ay... may',be .employed for any..desired purpose suchas the operation of a signal, either audible or visible, orfltheuoontrol .of oneor more motors .of. a.conveyor.system; such asthe motoroperating. the shake-outscreen .andthe .motordriving the endless .belt conveyor: immediately effected .in. the. .foundry .vsystem previously described( 'Iheelectrical apparatus which I have devised" for detecting the abnormal slowing-down or stoppage of a belt. .conveyoris particularly advantageouswin that itrequires only .the installation of a smarll .cam-roperated switch in immediate-association with the conveyor; Due to pol-t lutionof the-rain by fumes and-gases and due alsowto. theusualdack of space around the conveyors in a "foundry," which render installation: of apparatus difficult, 1the-.-electrical apparatusa :which I have devised-his particularly "advantageousin .that it -maycbein'stalled in a relato the switch through electrical wires.

Itis an object. or any. invention,-therefore; to provide a novel apparatus-oi the electrical type fora indicatinga: the; abnorniailcislowing down or stoppage .ofi-za movingzelemenu suclr as an endless belti conmeyonl- It is a further object of my invention to provide apparatus of the type indicated in the foregoing object, and characterized by simplicity and low cost together with ease of installation and minimum space requirements.

The above objects, and other objects of my invention which will be made apparent hereinafter, are attained by means of apparatus subsequently to be described in detail and shown in the accompanying single figure of the drawing which depicts in simplified form a motor-driven endless belt conveyor and my novel electrical apparatus associated therewith.

Referring to the single figure of the drawings, the endless belt conveyor l I is shown in simplified form as comprising a drivingdrum I2, an idler drum [3, an endless belt it carried on the drums l2 and I3, and an electrical motor l5 for driving the drum l2 as through an endless belt drive in the manner shown. For simplicity, the journals for mounting the drums I2 and it as well as the guiding frame work and support for the endless belt I4 have been omitted from the drawing but it will be understood that the conveyor ll is of standard construction.

The electrical apparatus provided according to my present invention comprises a single-pole double-throw switch It, of the telephone type, operated by a cam I'I fixed on a shaft I3 attached to and rotating with the idler drum I3, a pair of electrical condensers I9 and 2b, a resistor 2|, an electric relay 22 and a source of directcurrent voltage indicated as a potentiometer resistor 23 having suitable taps for providing a supply voltage of relatively low value, such as volts, from a higher line voltage, such as 230 volts; and a control switch such as a double-pole singlethrow knife switch 24.

The switch I6 may comprise a pair of flexible contact fingers Mia and ltb secured in insulated relation in a supporting base or block IBc with a movable or flexible contact arm Ifid interposed therebetween in such a manner as to engage the two contact fingers Ifia and I61) alternately upon oscillatory movement of the contact arm l6d in the plane of the contact fingers.

The cam I! is associated with the contact arm I 601 in such a manner as to cause the contact arm I6d to engage the contact finger Mia and the contact finger I617 twice for each revolution of the cam I'I. Obviously cam ll may have any other design for causing any other frequency of oscillation of the contact arm ltd of the switch I6 if the characteristics of the electrical condensers, the resistor 2| and the winding of the relay 22 are designed in accordance therewith.

The switch It is so connected in circuit relation with the condensers I 9 and 2t and the winding of the relay 22 to the voltage source as to cause a flow of charging current to condenser 26 through the resistor M and the winding of the relay 22 when the contact arm ltd of the switch engages the contact finger Mia, the condenser l9 beingat the same time discharged through the resistor 2| connected directly thereacross. The circuit connections of the switch It are also such that when the contact arm Hid engages the contact finger Ifib, current flows to charge condenser I 9 through the resistor 2! and the winding of the relay 22, while at the same time the condenser discharges locally through the resistor 2| connected directly thereacross.

By employing suitable constants for the condensers I9 and 20, the resistor 2i and the winding of the relay 22, the drop-out of the relay 22 may be effected in response to the reduction in the rotational speed of the cam I! below any desired speed. For example, when I have employed condensers I9 and 20 of 500 microfarad capacity, a resistor 2! having 15 ohms resistance, and a relay 22 having a winding of 700 ohms resistance, the relay 22 is maintained picked-up by the charging current to the condensers I9 and 20 so long as the cam ll rotates at a speed of 15 R. P. M. or higher. If the rotational speed of the cam I'I reduces substantially below a speed of 15 R P. M., the charging current to either of the condensers I9 and 20 will be reduced or attenuated sulficiently in the interval of time elapsingfrom the instant the contact arm ltd of the switch It first engages the contact finger Ita (or Itib) to the instant it first engages the contact finger I61) (or Ilia) so that the relay contact will be restored to its dropped-out position.

It will be seen, therefore, that if the rotational speed of the idler drum I3 of the endless belt conveyor II is normally substantially in excess of 15 R. P. M. and for any reason such as previously pointed out, as for example the excessive loading of sand on the endless belt I4, the rotational speed of the idler drum l3 and consequently of the cam ll reduces below 15 R. P. M., then the contact of the relay 22 will be alternately dropped-out and picked-up. If the idler drum I3 and cam ll stop altogether, the relay 22 will be deenergized and the contact thereof restored to its dropped-out position and remain therein.

The relay 22 may of course be employed in any suitable manner for control or indicating purposes. For purposes of illustration, I have shown in simplified form a suitable circuit arrangement whereby the drop-out of the relay 22 will automatically cause stoppage of the motor I5 and operation of a signal device such as a signal lamp 26. In actual practice I have employed the drop-out of the relay 22 to stop not only the driving motor l5 but also other motors in a conveyor system of which the endless conveyor I! is only one, as well as the motor (not shown) which operates the vibrating shake-out screen above the endless belt I4. For purposes of my present invention, however, it is immaterial what function the relay 22 performs because relay 22 may be utilized in well known manner for any desired control or indicating purpose.

As shown in the drawing, the motor I5 is supplied with power from a main three-phase 440 volt alternating-current source by three wires 21 under the control of a main control switch 28, the terminals of the motor l5 beingconnected to the wires 2? by corresponding branch wires 22 under the control of a contactor or relay 30 which is, in turn, under the control of the relay 22 as will be hereinafter explained.

As will be apparent in the drawing, the relay (-39 may comprise a suitable Winding 30a and five contacts or switches designated 3%, 3G0, 30d, 3lle and 30 respectively. The contacts 361), 39c, 30d, and 386 are front-contacts, that is, they are normally in open position and are actuated to closed position in response to energization of the winding 30a of the relay til. The contact 36 is a back-contact, that is, it is normally in a closed position when the winding of the relay 3D is deenergized and is actuated to open position in response to energization of the winding of the relay.

The three contacts 30c, 30d and 30c are interposed respectively in the three branch wires 29 7 ated from a dropped-out or open position to a picked-up or closed position and maintained therein only so long as the current energizing the winding exceeds a certain value, said condensers being connected in series relation with each other across said source of voltage with the winding of said relay interposed in series relation between the outer terminal of one of said condensers and one terminal of the source of voltage, a singlepole double-throw switch having two spaced stationary contacts and a movable contact shi-ftable alternately into engagement with the stationary contacts, a resistor having one end connected to the movable contact of said switch and the other end connected to the common terminal of connection between said two series-connected condensers, one of the stationary contacts of said switch being connected to the terminal of one condenser opposite said common terminal and the other of said stationary contacts of said switch being connected to the terminal of the other of said condensers opposite said common terminal, the arrangement being such that when the movable contact of said switch engages said one of the stationary contacts it establishes a circuit through which current flows from said source to charge the other of said condensers and also simultaneously establishes a local circuit including said resistor through which said one condenser is discharged, the movable contact of said switch being effective when it engages the said other of the stationary contacts to establish a circuit through which current flows to charge said one condenser and also simultaneously establishes a local circuit including said resistor through which said other condenser is discharged, rotary cam means associated with the movable contact of said switch and operative in accordance with the linear speed of travel of the conveyer belt to effect the operative engagement of the movable contact of said switch alternately with the two stationary contacts at a frequency corresponding to the linear speed of travel of the conveyer belt, thereby producing a pulsating uni-directional current that energizes the winding of said relay, the electrical characteristics of said condensers and said resistor being such that the condenser charging current energizing the winding of said relay is maintained above said certain value only so long as the conveyer belt travels at a speed exceeding a certain speed, with the result that the contact of said relay is restored to its dropped-out or open position when the condenser charging current energizing the winding of the relay reduces below said certain value consequent upon reduction of the convey-er belt to a speed of travel less than said certain speed, and electro-responsive control means controlled by the contact of said relay in such a manner as to interrupt the power supplied to the driving motor when said contact is restored to its dropped-out or open position.

2. Protective apparatus for use in connection with a motor-driven conveyer belt, said apparatus comprising a source of direct-current voltage, two electrical condensers, an electrical relay having a winding and a contact adapted to be actuated from a dropped-out or open position to a picked-up or closed position and maintained therein only 50 long as the current energizing the winding exceeds a certain value, said condensers being connected in series relation with each other across said source Of voltage with the winding of said relay interposed in series relation between the outer terminal of one of said condensers and one terminal of the source of voltage, a singleg pole double-throw switch having two spaced stationary contacts and a movable contact shiftable alternately into engagement with the stationary contacts, a resistor having one end connected to the movable contact of said switch and the other end connected to the common terminal of connection between said two series-connected condensers, one of the stationary contacts of said switch being connected to the terminal of one condenser opposite said common terminal and the other or" said stationary contacts of said switch being connect-2d to terminal of the other of said condensers opposite said common terminal, the arrangement being such that when the movable contact or said switch engages said one of the stationar' contacts it establishes a circuit through which current flows from said source to charge the other of said condensers and also simultaneously establishes a local circuit including said resistor through which said one condenser is discharged, the movable contact of said switch being efiective when it engages the said other of the stationary contacts to establish a circuit through which current flows to charge said one condenser establishes a local circuit including said resistor through which said other condenser is discharged, rotary carn means associated with the movable contact of said switch and operative in accordance with the linear speed of travel of the conveyor belt to effect the operative engagement of the movable contact of said switch alternately with the two stationary contacts at a frequency corresponding to the linear speed of travel of the conveyor belt, thereby producing a pulsating uni-directional current that energizes the winding of said relay, the electrical characteristics of said condenser said resistor being such that the condensers charging current energizing the winding of said relay is maintained above said certain value only so long as the conveyor belt travels at a speed cxceedirr a certain speed, with the result that the contact of said relay is restored to its dropped-out or open position when the condenser charging current energizing the winding of the relay reduces below said certain value consequent upon reduction of the conveyer belt to a speed of travel iess than said certain speed, an electro-responsive control device having a winding and contacts efieotive to establish a power supply circuit for the motor when the winding is energized and to interrupt the power supply circuit for the motor when the winding is deenergized, said control device also having an additional contact that is actuated from an open position to a closed position responsively to the energization of the winding of said control device, a manually operated starting switch having two normally open contacts, one of which is connected in parallel to the contact of said relay and the other of which is connected in parallel to said additional contact of said control device, the contacts of said manually operated starting switch bein effective when actuated to their closed position to initially establish the energizing circuit for the winding of said control device, and the contact of said relay and the additional contact of said control device cooperating to maintain the energizing circuit for said control device following restoration of said contacts of said manually operated switch to their open positions, the arrangement being such as to preclude establishment of the power supply circuit to the motor following deenergization of the winding of the control device responsively to drop out of the and also simultaneously contact of said relay unless the contacts of the manual starting switch are first operated to their closed positions.

ALBERT C. FRAZEE.

REFERENCES CITED The following references are of record in the file of this patent:

Number UNITED STATES PATENTS Name Date Owens Dec. 17, 1912 Fogwell et a1 Aug, 16, 1932 Crout et a1. Nov. 28, 1933 Hunt May 31, 1938 Number 10 Number OTHER REFERENCES Ser. No. 367,329, Kronenberger (A'. P. C.) pub. May 25, 1943.

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