US2564767A

US2564767A - Control system

Info

Publication number: US2564767A
Application number: US772990A
Authority: US
Inventors: Rene P Piperoux; David B Hertz; Ferdinand P Diemer
Original assignee: Celanese Corp
Current assignee: Celanese Corp
Priority date: 1947-09-09
Filing date: 1947-09-09
Publication date: 1951-08-21
Anticipated expiration: 1968-08-21

Description

g- 1951 R. P. PIPEROUX ET AL CONTROL SYSTEM Filed Sept. 9. 1947 20 LLO INVENTORS. RENE PIPEROUX. mwvo a. HERTZ. By FERDINAND P. DIEMER.

ATTORNEYS Patented Aug. 21, T951 CONTROL SYSTEM Rene P. Piperoux, Radburn, N. J., and David B. Hertz, New York, and Ferdinand P. Diemer, Woodhaven, N. Y., assignors to Celanese Corporatlon of America, a corporation of Delaware Application September 9, 1947, Serial No. 772,990

23 Claims.

This invention relates to a control system, and relates more particularly to electronic control devices. The invention is particularly adapted for application to electrically driven pulverizing equipment.

In the operation of electrically driven pulverizing devices, material is frequently supplied to the pulverizing mill by means of feed screws. Often overloading of the pulverizing mill occurs causing the stalling thereof. This overloading may result in dust explosions due to theexcessive heat generated by the frictional energy developed in the pulverizing mill chamber by the mill hammers.

Electrically operated and electronic control devices .for preventing overloading in pulverizing mills have heretofore been employed. However, such prior devices did not provide the degree of flexibility or the sensitivity desired between the normal and overload conditions. For example if the overload relay formerly employed were ad- Justed to operate on the stalled current for the pulverizing mill motor, the service would not be restored automatically when the load on the millv returned to normal, thus requiring manual restarting. Other disadvantages of prior overload control devices were that they changed in sensitivity with changes in input signal and they had an inherent time lag.

It is an important object of this invention to provide a simple, inexpensive and efllcient electronic control device for effecting complete supervisory control of the operation of a pulverizing mill, which is sensitive over a wide range of input signals.

Another object of this invention is the provision of a pulverizing mill control device wherein the set points of certain of its component elements may be readily and continuously changed over a wide range of operative conditions without altering the sensitivity thereof.

A further object of this invention is the provision in a pulverizing control device of means comprising an induction coil for supplying the input signal.

Still another object of this invention is to provide in a pulverizing mill control device a nonintegrating timing circuit whereby the control device will automatically reset when the input signal is restored to a value corresponding to normal operating conditions.

The foregoing and other objects of this invention will be readily understood from an embodiment of this invention hereinafter described in connection with the accompanying drawing wherein,

The single figure is a circuit diagram exemplifying the invention.

In' its broadest aspect, this invention comprises a pulverizing mill control device basically consisting of a pick-up or sensing unit, a voltage difler- .vacuum tubes.

ence measuring circuit, timing device and a relay and/or current proportioning element. These elements perform the functions of shutting oil the feed screw motor after an overload persists on the hammer mill motor for a period of time adjustable from 1 to 10 seconds, turning on the feed screw motor when overload conditions on the hammer mill clear in a predetermined interval of time and turning oil the hammer mill motor if the overload has not cleared within a period of time adjustable from 20 to seconds.

The pick-up or sensing element comprises a current actuated electrical circuit wherein a change in current feeding the hammer mill motor is converted into a suitably related voltage signal. The voltage signal is generated in a coil inductively coupled to the wire or wires supplying current to the hammer mill motor from a suitable source of power. Since the current feedingthe hammer mill motor provides the control signal, the current feed is dependent upon the mechanical disposition of the load in said hammer mill. This expedient for obta'ning a signal voltage has desirable features in that a negligible amount of power is dissipated in the pick-up element, the control signal requires at most two conductors be tween the control point and the controller, and the sensing unit may be readily removed whenever desired if a split type of coil is employed.

The voltage difference measuring circuit, which functions to interpret electrically the meaning of the changes in the input signal, consists of two sections or parts, namely, a voltage reference source and a voltage-change-sensitive amplifier. The voltage reference source comprises a transformer secondary winding and a potentiometer connected across, or a variable resistor in series with the winding so that the reference voltage may be varied to fit the desired control conditions, means being provided to vary both the phase and the magnitude reference voltage. The voltage-change-sensitive amplifier consists of a balanced transformer input circuit in which both the primary and secondary windings of the transformer are center tapped. Between one side of the primary winding and the center tap is connected the signal voltage source; between other primary terminals and the center tap is connected the reference voltage source. When the desired hammer mill motor operating conditions obtain, the output voltage across the secondary windin of the transformer is zero because the net flux in the transformer core at any moment is zero. Similarly, two secondary winding terminals are connected to the grids of a pair of triode vacuum tubes, or to the grids of two triode sections of a single vacuum tube and the center tap thereof is connected to the cathode circuit of said The input circuits on either or both sides of the transformer may or may not be grounded as determined by the desired control conditions. The two triode vacuum tubes are operated as degenerative, biased amplifiers with an alternating current supply voltage feeding.

their plate circuits. In order to accommodate a large range of operating conditions, both the magnitude and phase of the degeneration and grid bias voltage are made variable. The plate load of each of these tubes may consist of a relay coil shunted by a series combination of resistance and capacitance, or of a relay coil shunted by a capacitor. The purpose of these latter elements is to provide filterin for the rectified plate current from the tubes thus facilitating more stable relay operation.

The timing device which interposes a variable and calibrated delay or series of delays between successive control functions in order to initiate a predetermined program of events, which depend on the magnitude, direction and frequency of signal voltage variation, comprises two variable period electronic timers connected in cascade. In one of said electronic timers, a vacuum tube is maintained at plate-current cut-off by applying a very large magnitude of negative bias voltage on its controlled grid. In the grid cathode circuit of this vacuum tube is connected a parallel combination of resistance and capacitance. To initiate the timing interval, the biasing voltage source is disconnected from the grid circuit, thus permitting the capacitor to discharge through the timing resistance. Since the time interval required to effect the condenser discharge from full applied voltage to a given voltage is a constant for a given value of resistance and capacitance, the time interval between the disconnecting of the grid bias voltage and the closing of a suitable relay in the plate circuit of the said vacuum tube is of fixed value. The duration of the time interval may be varied by adjusting the value of the timing circuit resistance. In the present application where the control system is applied to a pulverizing mill, a short period timer, wherein the variable delay is approximately 1 to seconds, is employed to obtain a time delay between the start of a material-overload in the hammer mill and the shuttin off of the screw conveyor .drive which feeds material to the said hammer mill. This inter-period timing device is initiated by the current control tube in the voltage difference measuring circuit above referred to. After the predetermined time interval is completed, the relay in the plate circuit is made to operate. The other vacuum tube of the timing device, the operation of which is initiated simultaneously with the operation of the control tube of the voltage difference measuring circuit, is adapted to have a relatively longer adjustable time constant, namely, of 20 to 50 seconds duration. When this latter time interval has elapsed, a relay in the plate circuit of this latter vacuum tube is employed to initiate the control function.

The relay and/or current proportioning element functions to halt the operation of the entire apparatus.

While the novel control device of this invention is adapted for application to various machines and mechanisms employing one or more motors for driving one Or more elements, an important application thereof is for controlling the operation of a hammer mill and feed screw of a pulverizing mechanism, and will be described in con-' nection therewith.

Referring now to the drawing for a detailed description of the control device, reference nu- 4 meral i indicates a switch shown in open position. When switch i is closed, the control circuit is energized and motor Ml which drives a hammer mill (not shown), and motor M2, which drives the screw means (not shown) for feedin material to the hammer mill, are caused to operate through suitable standard interlocking devices by closing push button switch Si. When the hammer mill is operating at a preset value of hammer mill motor current, the pulverizing mechanism operates normally and the control system of the present invention does not function operatively. When, however, the hammer mill motor current increases due to excessive hardness of the material being ground, or because of the presence of foreign substances in said material, or for any other reason, the control device will function as follows:

The pick-up or sensing element, generally indicated by reference numeral 2, transmits a voltage signal through its pick-up coil 3 to the transformer 4 of the control device. The pick-up coil 3 preferably consists of a toroidal winding on a laminated iron core through which is passed one of the conductors 5 which supplies current to the hammer mill motor Ml. In accordance with the principles of electro-magnetic induction, the open circuit voltage of coil 3 may be made directly proportional to the current linking the coil. In this manner, then, a signal proportional to the magnitude of the hammer mill motor current may be obtained. It is this induced voltage which is balanced against a reference voltage which is supplied by secondary winding 6 of a transformer generally indicated by reference numeral 1, when the reference voltage from secondary windings 6 is pre-set by variable resistor 8. When resistor 8 is adjusted to supply a reference voltage in accordance with a predetermined value of current and the signal voltage from pickup element 2 equals this reference voltage in magnitude and is in phase opposition, then the net flux in transformer l is zero. The pick-up or sensing element, however, may be made so that the output voltage is proportional to a rate of change in current and/or predetermined value of current. Aside from indicating the magnitude of current, a suitably connected set of these coils, similar to pick-up coil 3, may be used to obtain a signal proportional to the unbalance of current in a polyphase power system should that condition be desirable.

The reference numerals 9 and II, and reference numerals i2 and I3 indicate respectively the center tapped primary and secondary windings of transformer 4. When the signal voltage is greater than the reference voltage, there is a net flux produced in the core 4' of transformer l. Under this condition. the instantaneous value of the grid voltage of the upper section Haof vacuum tube ll increases relative tothe instantaneous value of the plate voltage supplied by the secondary winding H of transformer 1. If this increase in voltage is sumciently large, the plate current from the section Ma of tube II will cause a relay, generally indicated by reference numeral IE, to operate. The magnitude of grid voltage at which this condition will occur is determined by the value of resistors l6 and I1. Resistor l8, in series with the grid circuit of tube section Ma, is provided to act as a current limiting circuit element, preventing an excessive flow of rectified grid current. When a large degree of unbalance exists between the signal voltage and the reference voltage. this circuit expedient stabilizes the network because we voltage drop across the grid resistance tends. to increase the negative bias on theigrid, counteracting any further increase in arid current. Resistors i5 and I1 determine the degree of degeneration in the amplifier circuit as well as the grid bias for tube section l4a. Resistor ll functions, as the internal resistance of the relay coil 2|. Capacitor 22 and resistor 23 are connected in series and across the relay coil 2| for the purpose of providing a higher degree of filtering for the plate current of tube l4, thus facilitating more stable relay operation. When the signal voltage from pick-up or sensing element 2 is less than the reference voltage supplied by pick-up coil 3, the following operating conditions obtain: A voltage is induced in the secondary. winding l3 of transformer 4 of such phase and magnitude that the grid 24 of the lower section Nb of tube 14 becomes positive with respect to the cathode. Under this condition. a

. relay, generally indicated by reference numeral v25, is made to operate because of an increase in plate current of this section of the tube. Reslstors 25 and I1 determine the operating point of tube section l4b, that is, the grid voltage, at which the relay 25 operates. Resistor 21 functions in the same manner as resistor l8, namely, as a current limiting circuit element. Capacitor 23, which is connected across the coil 29 of relay 25,.functions in a similar manner to capacitor 22, namely, to provide a higher degree of filtering of plate current.

When relay I5 is operated, contacts 3| and 32 of this relay open the negative potential supply line in the grid circuits of sections 33a and 33b, respectively, of vacuum tube 33. Section 33a of vacuum tube 33 constitutes the control means for the short period electronic timer, hereinafter described, and section 33?) of vacuum tube 33 is the control means for the long period electronic timer, hereinafter described. Vacuum tube 34 in conjunction with resistor 35 and secondary winding 39 of transformer 1 constitutes an electronic current rectifying system which supplies a high, constant value of negative volt- 3;

age to the grid circuit of section 33a of tube 33.

Referring to, the short period timer, namely section 33a of tube 33, when relay I5 is operated,

contact 32 of said relay is opened. Capacitor 31 is then allowed to d scharge through resistances 35 and 39. The section 33a of vacuum tube 33 is permitted to pass sufficient plate current to operate a relay, generally indicated by reference numeral 4!, aft r a predetermined time interval.

This is accompished by the discharging of the :r

capacitor 31. Capacitor 31 which is connected in parallel to resistances 38 and 39 forms the timing circuit for tube section 33a. Since the timerequired for a givenvoltage to fall a definite amount for a given parallel combination of resistance and capacitance is constant, it is possible to have the:

plate current of tube section 33a increase--sufii ciently to cause relay 4| to operate for various va'ues of time delay. Resistance 39 is made variable to allow for the adjustment of? the timing interval desired. Capacitor 42 connecting across relay coil 43 of relay 4| serves to alter the time constant of relay 4| in accordance with desired operating conditions. Resistor 44 in the cathode circuitof vacuum tube section 33a'serves to fix the operating point on the tubes characteristics for which the required plate current will flow.

The long period timer, comprising section 33b 'of vacuum tube 33, incudes resistors 45 and 45, capacitors 41 and 43, relay 49, relay coil 5i,

capacitor s: and resistor is, all 01' which elements '56 and 51 passing to and through terminals :1 and b in the right terminal strip and by lead lines 58 and 59 in series with the holding coil 5| of motor starter 62 and power source 53. When the ham mer mil. motor current exceeds its predetermined or pre-set value, due to a condition obtaining in the hammer mill or for any other reason, both the short period and long period timers will be set into operation, that is, the capacitor 31 will be permitted to discharge through resistors 33 and 39, and capacitors 41 and 48 through resisters 45 and 45. If the-duration of the overload exceeds the time interval for which the short period timerhas been set by adjustment of resistor 39, then relay 4| and specificaly contacts 54 and will be operated. Operation of the relay causes the opening of contacts 54 and 55, thus opening the circuit to holding coil 6| which in turn releases contacts 63, 64 and 65. Thus, the power to feed motor M2 is shut oil and no additional material is fed thereby to the hammer mill, The initiation of the long period timer described above is arranged to occur when contact 3i of relay I5 is opened, thus allowing capacitors 41 and 48 to discharge through resistors 45 and 46. If the aforementioned increase in hammer mill motor current continues after the feed screw has been shut off by the short period timer and the hammer mill motor current exceeds its predetermined or pre-set value for a duration greater than the time interval pre-set in the long period timer, relay 49 is caused to operate, thus causing the hammer mill motor Ml to be shut oil. Th s is accomplished in a manner similar to the shutting off of feed motor M2. i. e. through contacts 66 and 51, lead lines 63 and 69 and the terminals 0 and d on the left terminal strip through lead lines 1| and 12 which connect holding coil 13 of contactor 14 in series with power source 63. When holding coil 13 is deenergized, contacts 14', 15 and 15 are released shutting oil the power to the hammer m ll motor M I.

If the duration of excessive hammer mill motor current is less than the time interval of the long period timer, then power will be fed to feed motor M2 bv means of the system which includes t e underload control tube section l4b, its associated relay 25 and contacts 11 and 18 of relav 25, the

operation of which is described above. The variable resistor 25 is adjusted in such a manner that relay 25 wil not operate until the hammer mill motor current has reached its no load value,

'- which value of current is less than the value of

lines

83 and 84 to pick-up coil 3.

7 and the grid circuit of the short period timer will bereconnected through contact 32 to the negative high voltage rectifler system vacuum tube 34, resistor 35 and secondary transformer winding 36. Connections 9 and h on left terminal strip are connected to power source 63 feeding current to primary winding 82 of transformer I. Terminal k on left terminal strip and terminal n on right terminal strip are connected by lead Secondary winding 85 of transformer I supplies filament power for vacuum tubes ll, 33 and 34.

It is to be understood that the foregoing detailed description is given merely by way of illustration and that many variations may be made therein without departing from the spirit of our invention. I

Having described our invention, what we desire to secure by Letters Patent is:

1. In a machine comprising at least one electrically driven element and electrical conductors for supplying current to said element, a device for controlling the operation of .said driven element including a pick-up element operatively connected to at least one of said conductors for picking up a voltage signal which is a function of the current flowing through said conductor, means comprising a voltage difference measuring circuit connected to said pick-up element for selectively receiving said voltage signal. and timing devices selectively operated by said signal receiving means when said voltage signal has a value outside a predetermined range, said timing devices being electrically connected to said driven element for controlling the operation of said driven element.

controlling the operation of said driven element, including a pick-up element operatively connected to said conductor for picking up a voltage signal which is a. function of the current flowing through said conductor, means, comprising a voltage dlflerence measuring circuit connected to said pick-up element and having a predetermined reference voltage applied thereto, for selectively receiving said voltage signal in accordance with the magnitude of said voltage signal, and timing devices selectively operated by said voltage signal receiving means when said voltage signal has a value outside a predetermined range, said timing devices being electrically connected to said driven element for controlling the operation of said driven element.

5. In a machine comprising an electrically driven element and an electrical conductor for supplying current to said element, a device forcontrolling the operation of said driven element, including a pick-up element operatively connected to said conductor for picking up a voltage signal which is'a function of the current flowing through said conductor. means, comprising a voltage difference measuring circuit connected to said pickup element for selectively receiving said voltage signal in accordance with the magnitude of said voltage signal, means including a transformer and a variable resistor for applying a predetermined reference voltage to said voltage d'iflerence measuring circuit, and timing devices selectively operated by said voltage signal receiving means when said voltage signal has a value outside a 2. In a machine comprising a plurality of. electrically driven elements and electrical conductors for supplying current to said elements, a device for controlling the operation of at least one of said elements, said device including a pickup element operatively connected to at least one of said conductors and to another of said elements for picking up a voltage signal which is a function of the current flowing through said conductor, means connected to said pick-up element for selectively receiving said voltage signal and non-integrating timing devices selectively predetermined range, said timing devices being electrically connected to said driven element for controlling the operation of said driven-element.

6. In a machine comprising an electrically driven element and an electrical conductor for supplying current to said element, a device for controlling the operation of said driven element. including a pick-up element operatively connected to said conductor for picking up a voltage signal which is a function of the current flowing through said conductor, means, comprising a voltage difference measuring circuit connected to said pick-up element for selectively receiving said voltage signal in accordance with operated by said signal receiving means when said voltage signal has a value outside a predetermined range, said timing devices being electrically connected to at least said controlled element for controlling the operation of said controlled element.

3. In a machine comprising a plurality of electrically driven elements and electrical conductors for supplying current to said elements, a device for controlling the operation of at least two of said elements, said device including a pick-up element operatively connected to at least one of said conductors and to a controlled element for picking up a voltage signal which is a function of the current flowing through said conductor, means comprising a voltage difference measuring circuit connected to said pick-up element for selectively receiving said voltage signal and timing devices selectively operated by said signal receiving means when said voltage signal has a value outside a predetermined range, said timing devices being electrically connected to at least two of said driven elements for controlling the operation of said driven elements.

4. In a machine comprising an electrically driven element and an electrical conductor for the magnitude of said voltage signal, means including a transformer and a variable resistor for applying a predetermined reference voltage to said voltage difierence measuring circuit, timing devices selectively operated by said voltage signal receiving means when said voltage signal has a. value outside a predetermined range, said timing devices being electrically connected to said drivenelement for controlling the operation of said driven element, and means adjusting the time interval delay at which said timing devices operate.

7. In a machine comprising an electrically driven element and an electrical conductor for supplying current to said element, a device for controlling the operation of said driven element, including a pick-up element operatively connected to said conductor for picking up a voltage signal which is a function of the current flowing through said conductor, means, comprising a voltage difference measuring circuit connected to said pick-upelement and having a predetermined reference voltage applied thereto, for selectively receiving said voltage signal, a relay adapted to be'operated by said voltage receiving means when the signal voltage is greater than the re ference voltage, and timing devices operated by said relay, said timing devices being electrically con- 9 nected to said driven element for controlling the operation of said driven element.

8. In amachine comprising an electrically driven element and an electrical conductor for supplying current to said element, a device for con trolling the operation of said driven element, including a pick-up element operatively connected to said conductor for picking up a voltage signal which is a function of the current flowing through said conductor, means, comprising a voltage difference measuring circuit connected to said pick-up element and'having a predetermined reference voltage applied thereto, for selectively receiving said voltage signal, a relay adapted to be operated by said voltage receiving means when the signal voltage is greater than the reference voltage, and a timing device operated by said relay, said timing device comprising a plurality of timers each adapted to operate at a different time interval delay, said timeis being operatively connected to said driven element for controlling the operation of said driven element.

9. In a machine comprising a motor for driving a pulverizer, a second motor for driving a feed screw for feeding material to said pulverizer, and electrical conductors for supplying current to said motors, a device for controlling the operation of said motors, said device including a pick-up element operatively connected to the pulverizer motor conductor for picking up a voltage signal which is a function of the current flowing through said conductor. means connected to said pick-up element for selectively receiving said voltage signal, and non-integrating timing devices selectively operated by said signal receiving means when said voltage signal has a value outside a' predetermined range, said timing devices being electrically connected to said feed motor for controlling the operation of said feed motor.

10. In a machine comprising a motor for driving a pulverizer, a motor for driving a feed screw for feeding material to said pulverizer and electrical conductors for supplying current to said motors, a device for controlling the operation of said motors, said device including a. pick-up element operatively connected to the pulverizer motor conductor for picking up a voltage signal which is a function of the current flowing through said conductor, a voltage difference measuring circuit connected to said pick-up element for selectively receiving said voltage signal, and timing devices selectively operated by said circuit when said voltage signal has a value outside a predetermined range, said timing devices ,being electrically connected to said feed motor for controlling the operation of said feed motor.

11. In a machine comprising a motor for driving a pulverizer, a motor for driving a feed screw for feeding material to said pulverizer and electrical conductors for supplying current to said motors, a device for controlling the operation of said motors, said device including a pick-up element operatively connected to the pulverizer motor conductor for picking up a voltage signal which is a function of the current flowing through said conductor, an electronic device for measuring voltage difference connected to said pick-up element for selectively receiving said voltage signal in accordance with the magnitude of said voltage signal, means including a transformer and variable resistor for applying a predetermined reference voltage to said electronic device, and timing devices selectively operated by said electronic device when said voltage signal has a value outside a predetermined range, said timing devices 10 being electrically connected to said feed motor for controlling the operation of said feed motor.

12. In a machine comprising a motor for driving a pulverizer, a motor for driving a feed screw for feeding material to said pulverizer and electrical conductors for supplying current to said motors, a device for controlling the operation of said motors, said device including a pick-up element operativel connected to the pulverizer motor conductor for picking up a voltage signal which is a function of the current flowing through said conductor, an electronic device for measuring voltage difference connected to said pick-up element for selectively receiving said voltage signal in accordance with the magnitude of said voltage signal, means including a transformer and variable resistor for applying a predetermined reference voltage to said electronic device, timing devices selectively operated by said electronic device when said voltage signal has a value outside a predetermined range, said timing device being electrically connected to said feed motor for controlling the operation of said feed motor and means for adjusting the time interval delay at which said timing devices operate.

13. In a machine comprising a motor for driving a pulverizer, a motor for driving a feed screw for feeding material to said pulverizer and electrical conductors for supplying current to said motors, a device for controlling the operation of said motors, said device including a pick-up element operatively connected to the pulverizer motor conductor for picking up a voltage signal which is a function of the current flowing through said conductor, an electronic device for measuring voltage difference connected to said pick-up element for selectively receiving said voltage signal in accordance with the magnitude of said voltage signal, means including a transformer and variable resistor for applying a predetermined reference voltage to said electronic device, and a timing device selectively operated by said electronic device when said voltage signal has a value outside a predetermined range comprising two timers electrically connected to said motors for controlling the operation thereof.

14. In a machine comprising at least two electrically driven elements and electrical conductors for supplying current to said elements, a device for controlling the operation of said elements including a Dick-up element operatively connected to at least One of said conductors for picking up a signal, means selectively controlled by said signal for halting the operation of one of said driven elements when said signal has a value outside a predetermined range, means for automatically restarting the operation of said driven element when said signal returns to said predetermined range and means to halt the operation of said driven elements when said signal has a value outside said predetermined range for a predetermined time interval.

15. In a machine comprising at least two electrically driven elements and electrical conductors for supplying current to said elements, a device for controlling the operation of said elements including a pick-up element operatively connected to at least one of said conductors for picking up a signal, means selectively controlled by said signal for initiating a timing cycle for halting the operation of one of said driven elements when said signal has a value outside a predetermined range, means for restarting the operation of said driven element when said signal returns to said predetermined range and means for initiating a 11 timing cycle for halting the operation or said driven elements when said signal has a value outside said predetermined ranseior apredetermined time interval.

16. In a machine comprising a motor for driving a pulverizer, a second motor for driving a feed screw'ior feeding material to said pulverizer and electrical conductors for supplying current to said motors, a device for controlling the operation of said motors, said device including a pick-up element operatively connected to the pulverizer motor conductor for picking up a signal, means selectively controlled by said signal for halting the operation of said feed motor when said signal exceeds a predetermined value, means for automatically restarting the operation of said feed motor when said signal falls below said predetermined value and means for halting the operation of both said feed and pulverizer motors when and signal exceeds said predetermined value for a predetermined time interval.

17. In a machine comprising a motor for driving a pulverizer, a second motor for driving a feed screw for feeding material to said pulverizer and electrical conductors for supplying current to said motors, a device for controlling the operation of said motors said device including a pick-up element operatively connected to the pulverizer motor conductor for picking up a signal, means selectively controlled by said signal for initiating a timing cycle for halting the operation of said feed motor when said signal exceeds a predetermined value, means for automatically restarting the operation of said feed motor when said signal falls below said predetermined value and means for initiating a timing cycle for halting the operation of both of said feed and pulverizer motors when said signal exceeds said predetermined value for a predetermined time interval.

18. In a machine comprising at least one electrically driven element and electrical conductors for supplying current to said element, a control device for controlling the operation of said element, said device including a pick-up element operatively connected to at'least one of said conductors for picking up a signal which is a function of the current flowing through said conductor and at least one non-integrating timing device connected to said pick-up element and selectively operated by said signal, said timing device initiating its timing cycle at a predetermined value of said signal and returning immediately to its initial state when said signal departs from said predetermined value, said timing device being electrically connected to said driven element for controlling the operation of said driven element.

19. In a machine comprising at least one electrically driven element and electrical conductors for supplying current to said element, a control device for controlling the operation of said element including at least one non-integrating timing device connected to at least one of said elements, and means for initiating the operation of said non-integrating timing device for halting the operation of said element when the current flowing to said element has a value outside a predetermined range for a predetermined time interval.

20. In a machine comprising at least two electrically driven elements and electrical conductors 'for supplying current to said elements, a control device for controlling the operation of at least one of said elements including at least one nonintegrating timing device connected to at least 12 one of said elements, and means for initiatint the operation of said non-integrating timing device ior halting the operation of said element when the current flowing to another of said elements has a value outside a predetermined range for a predetermined time interval.

21. In a machine comprising at least two electrically driven elements and electrical conductors for supplying current to said elements, a control device for controlling the operation of said elements including at least one non-integrating timing device connected to at least one of said elements, means for initiating the operation of said non-integrating timing device for halting the operation of at least one of said elements when the current flowing through at least one of said conductors has a value outside a predetermined range for a predetermined time interval, at least one other non-integrating timing device connected to at least one of said elements and means for initiating the operation of a non-integrating timing device ior halting the operation of said driven elements when the current flowing through at least one of said conductors has a value outside a predetermined range for said predetermined time interval of longer duration than the first predetermined time interval.

22. In a machine comprising at least two electrically driven elements and electrical conductors for supplying current to said elements, a device for controlling the operation of said elements including means for halting the operation of one of said elements when the current flowing to another of said elements has a value outside a predetermined range, means for automatically restarting the operation of said driven element when said current returns to said predetermined range and means for halting the operation of said driven elements when said current has a value outside said predetermined range for a predetermined time interval.

23. In a machine comprising at least two electrically driven elements and electrical conductors for supplying current to said elements, a device for controlling the operation of said elements including means for initiating a timing cycle for halting the operation of one of said elements when the current flowing to another of said elements has a value outside a predetermined range, means for automatically restarting the operation of said driven element when said current returns to said predetermined range and means for initiating a timing cycle for halting the operation of said driven elements when said current'has a value outside said predetermined range for a predetermined time interval.

RENE P. PIPEROUX. DAVID B. I-IERTZ. FERDINAND P. DIEMER.

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

Certificate of Correction Patent No. 2,564,767 August 21, 1951 RENE P. PIPEROUX ET AL.

It is hereby certified that error appears in the above numbered patent requiring correction as follows:

In the grant, line 1, name of co-inventor, for Rene P. Piperous read Rene P. Piperoua"; and in the printed specification, column 11, line 20, for and signal read said signal; column 12, line 21, for of a read of said; line 25, for for said read for a;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 6th day of November, A. D. 1951.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,564,767

RENE P. PIPEROUX ET AL.

August 21, 1951 In the grant, line 1, name of co-inventor, for Rene P. Piperous read Rene P. Piper-owe; and in the printed specification, column 11, line 20, for and signal read said signal; column 12, line 21, for of a read of said; line 25, for for said read for a;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 6th day of November, A. D. 1951.

THOMAS F. MURPHY,

Assistant Oomwm'ssioner of Patents.