US3723825A

US3723825A - Magnet controller

Info

Publication number: US3723825A
Application number: US00219020A
Authority: US
Inventors: Viney T De
Original assignee: Square D Co
Current assignee: Schneider Electric USA Inc
Priority date: 1972-01-19
Filing date: 1972-01-19
Publication date: 1973-03-27
Anticipated expiration: 1990-03-27
Also published as: AU463330B2; CA965832A; GB1384001A; JPS53512B2; JPS4882356A; AU5118773A

Abstract

A control system for a lifting magnet measures the lift current of the magnet and causes the drop-out current to reach a predetermined percentage of the lift current at interruption so that clean drops are provided, without adjustment, regardless of the condition of the circuit elements, the temperature of the lifting magnet, or the size of the magnet.

Description

United States Patent 1 111 3,723,825 De Viney [4 1 Mar. 27, 1973 [54] MAGNET CONTROLLER 3,368,119 2/1968 Littwin ..317 157.5

75 l t T E.D V ,S H", l 1 men or i g 6 "my even x S Primary Examiner.l. D. Miller Assistant Examiner-Harry E. Moose, Jr. [73] Assignee: Square D Company, Park Ridge, Ill. Atm e H rld J. Rathbun er a],

2 d: 1 1 72 l 2] 9 57 ABSTRACT 21 A l. N 219,020 1 pp 0 A control system for a lifting magnet measures the lift current of the magnet and causes the drop-out current [52] U.S. Cl 317/123, 317/157.5 to reach a predetermined percentage of the lift ur- [51] Int. Cl ..H0lf 13/00 rent at interruption so that clean drops are provided, Field of Search DIG. without adjustment, regardless of the condition of the 295 circuit elements, the temperature of the lifting magnet, or the size of the magnet. [56] References Cited 12 Claims, 2 Drawing Figures UNITED STATES PATENTS 3,579,053 /1971 Littwin ..3l7/l57.5 X

m: 34 4 44,56 74 80 a2 59 l flu? 47 7o 79 I L8; 87 a 57 55 43 46 6.9

32 37 39 I58 50 5M 54 467 a5 9/ I061, /.97

. 94 /0/ /04 S 6/ 64 76 I24 5 7 40 WW /7 62 \96 m2 3/ 67 550/ l [22 [25 /2a 552 m5 1 36 f30 l MAGNET CONTROLLER This invention relates to an improved system for, and a method of, controlling inductive devices, and more particularly to an improved control system and method for automatically effecting total demagnetization of inductive devices such as lifting magnets in-a short time.

It is common practice to magnetize a lifting magnet by connecting it to a source of constant unidirectional voltage and to demagnetize the magnet by reversing the connections to the source through a fixed value of resistance. The reverse current is interrupted automatically when the magnet flux is approximately zero.

To provide for a clean drop of a load of scrap iron or similar material upon demagnetization of the magnet without leaving stickers and dribblers, it is necessary that the reverse current, upon its interruption, be of a value which does not leave any residual magnetism in the magnet. This magnitude of reverse current is dependent upon several variables. Among these are the size of the magnet, the temperature of the magnet, variations in line voltage, and the condition of circuit components such as contact surface condition.

Prior art devices utilize various means to fix the value of reverse current at which the drop cycle is terminated. Some use timing circuits and start a fixed time period either from the beginning of the drop cycle or from the instant of current reversal and terminate the drop cycle at the end of the fixed time period. Other devices measure the voltage across, or the current through, the magnet during the drop cycle to determine when the proper level of reverse current has been reached. Each of these devices terminates the drop cycle after the reverse current has reached a preselected value independent of variations in circuit conditions.

However, when the temperature of the magnet or other circuit condition changes, the magnetization curve for the magnet is altered; for example, a value of reverse current which will fully demagnetize a cold magnet will leave residual magnetism in a hot magnet. For this reason, prior art circuits have included adjustments to compensate, in a hunt-and-peck manner, for such variations. Unfortunately, since circuit conditions are constantly changing, frequent adjustments are required and a significant amount of time is lost making these adjustments. Often, magnet operators prefer not to make the adjustment and either try to drop stickers and dribblers by shaking the magnet or just simply ignore them. This consumes even more time and greatly decreases the efficiency of the operation.

Attempts have been made to provide automatic compensation for individual factors which change the required reverse demagnetization current. One of these comprises using a thermocouple embedded within the magnet so that the magnet controller can provide automatic temperature compensation. However, no automatically adjusting system has been able to compensate for the several varying conditions responsive for changes in magnetization.

l have discovered that the variations in magnetization of the magnet for which compensation is required are directly reflected in changes in the lift current through the magnet caused by these differing conditions and that complete demagnetization may be effected, through the application of the correct magnitude of reverse current for each drop, by measuring the magnitude of the lift current at the termination of the lift cycle and removing the reverse current when it reaches a predetermined percentage of the lift current at interruption of the lift circuit.

Accordingly, it is an object of this invention to provide an improved magnet controller and more particularly to provide a magnet controller having means for measuring the lift current at the termination of the lift cycle and terminating the drop cycle when the reverse current through the magnet reaches a predetermined percentage of the lift current.

These and other objects and advantages of the present invention will become apparent from the following description in which reference is made to the drawings wherein:

FIG. 1 is a schematic wiring diagram of a magnet control system in accordance with this invention; and,

FIG. 2 is a schematic wiring diagram of the magnet control system of FIG. 1 illustrating a preferred static discharge control unit therefor.

In general, the magnet demagnetizing method of this invention includes the steps of measuring the lift current through the magnet at the termination of the application of lift voltage, storing a first voltage proportional to lift current, applying reverse voltage to the magnet to produce a reverse current in the magnet in a direction opposite that of the lift current, comparing a second voltage proportional to the reverse current with the first voltage, and disconnecting the reverse voltage when the second or reverse current proportional voltage bears a predetermined relationship to the first or lift current proportional voltage.

Referring now to FIG. 1, a magnet control circuit in accordance with this invention is illustrated as connected to control the energization of a magnet 11 from a direct current source.

A positive terminal 12P of the direct current source is connected by a conductor 14 through a normally open contact 15a of a lift contactor 15, a junction 16 and a normally open contact 17a of a drop contactor 17 to a junction 19, and by a conductor 20 through a resistor 21, a normally open drop contact 17b, a junction 22, a normally open lift contact 15b and a junction 24 to a negative terminal 12N of the source. The mag net 1 l is connected between the junctions l6 and 22 by conductors 25a and 25b. Connected between the

junctions

19 and 24 is a voltage divider comprising

seriesconnected resistors

26 and 27 having a junction 29 therebetween.

A conductor 30 connects the positive terminal 12? of the direct current source through a junction 31, a junction 32 and anormally open auxiliary contact of the drop contactor 17 to a junction 34.

A master switch 35 has a normally open contact 36 and a normally closed contact 37. The master switch may be constructed to provide overlapping operation for the

contacts

36 and 37 when the master switch 35 is moved from its LIFT position to its DROP position so that the contact 37 closes before the contact 36 opens during this operation. As can be seen by the schematic representation of the master switch 35, the contact 36 is open in the DROP position and closed in theLlFT position of the master switch 35 while the contact 37 is closed in the DROP position and open in the LIFT position of the master switch.

Electrical connection of the master switch 35 is provided by a conductor 39 connected from the junction 32 through the contact 37, a normally open auxiliary contact 15c of the lift contactor l and an operating winding 17w of the drop contactor 17 to the junction 29, and by a conductor 40 connected from the junction 31 through the master switch contact 36 and an operating winding w of the lift contactor 15 to a conductor 41. A resistor 43 is connected in parallel with the winding 15w, and the conductor 41 is connected between the junction 24 and a static discharge control unit 42.

The static discharge control unit 42 is connected by a conductor 44 through a resistor 45 to the junction 34, by a conductor 46 through a resistor 47 to the junction 34, by a conductor 49 to the junction 19, by a conductor 50 to the conductor 39 adjacent the positive side of the drop contactor winding 17w and by a conductor 51 to the conductor 39 adjacent the negative side of the drop contactor winding 17w.

Operation of the magnet control circuit of FIG. 1 will now be described. Although, for the purposes of this description, the overlapping master switch 35 is used, it should be understood that a push button, or other type, magnet controller could be substituted because the required contact overlap is provided by the connection of the resistor 43 across the lift contactor winding 15w to delay opening of the lift contacts 15a, b, 0 until after closure of the drop contacts 17a, b, c.

Initially, with the master switch 35 in its DROP position, the contact 36 is open so that the lift contactor winding 15w is not energized and the lift contacts 15a and 15b are open. Although master switch contact 37 is closed, auxiliary lift contact 15c is open and the drop contactor winding 17w is therefore not energized so that the drop contacts 17a and 17b are open. Thus, initially, the magnet 11 is disconnected from the direct current source. 7 7

When the master switch 35 is moved to its LIFT position, the master switch contact 36 closes and the master switch contact 37 opens. The closing of the contact 36 completes a circuit from the positive terminal 121 through the conductor 30, the junction 31, the contact 36, the conductor 40, the lift contactor winding 15w and the conductor 41 to the negative terminal 12N. This circuit energizes the lift contactor winding which causes closure of the contacts 15a, 15b and 15p. The closing of contacts 15a and 15b connects the magnet 11 directly to the direct current source so that current flows through the magnet 11 in the direction of an arrow 52 to magnetize the magnet 11 in a well known manner. The closing of the contact 150 does not affect LIFT operation since the master switch contact 37, serially connected with the contact 15c, is open.

Demagnetization of the magnet 11 is accomplished by supplying a reverse current of suitable value to the magnet. When the master switch 35 is moved to the DROP position, the master switch contact 37 closes before the contact 36 opens. The closing of the contact 37 energizes the winding 17w through a circuit from the positive voltage terminal 12? through the conductor 30, the junction 32, the contact 37, the conductor 39, the still closed lift contact 15c, the winding 17w and the resistor 27 tothe negative voltage terminal 12N. The drop contactor winding 17w causes closure of

contacts

17a, 17b and 17c. Closing of the contacts 17a and 17b places the resistor 21 and the serial combination of

resistors

26 and 27 each in parallel with the magnet 11. Closure of contact 170 provides a power supply input for the static discharge control unit 42 from the junction 32 through the resistor 45 and conductor 44 and completes a holding circuit for the drop contactor winding 17w through the resistor 47, conductor 46, static discharge control unit 42 and the conductor 50 as will hereinafter be explained. I

When the master switch contact 36 opens, the lift contactor winding 15w is de-energized and

contacts

15a, 15b and 15c open. Now voltage is applied to the magnet 11 through a circuit from the positive voltage terminal 121 through the resistor 21, the conductor 20, the contact 17b, the conductor 25b, the magnet 11, the conductor 25a, the contact 17a, the resistor 26, and the resistor 27 to the negative voltage terminal 12N. Opening of the contact leaves the winding 17w energized solely through the static discharge control unit 42 so that termination of reverse current through the magnet l 1 can be controlled thereby.

Although the polarity of the voltage applied to the magnet 11 is reversed by the opening of the contacts 15a and 15b, the current throughthe magnet 11 after the opening of the contacts 15a and 15b is generally equal to, and in the same direction as, the lift current just before the drop contacts 17a and 17b closed becauseof the highly inductive nature of the magnet 11. Accordingly, there is a voltage drop across the resistor 27, with the junction 24 positive with respect to the junction 29, the voltage drop having a magnitude directly proportional to the lift current at the instant the contacts 15a and 15b open. This voltage is transmitted by the conductor 41 and

conductors

39 and 51 to the static discharge control unit 42 which includes means for storing this voltage in a manner to be 1 described.

As application of reverse voltage continues, the current through the magnet 11 diminishes and eventually begins to flow in a direction opposite that indicated by the arrow 52. Thereupon, a voltage is placed across the resistor 26 with the junction 19 positive with respect to the junction 29. This voltage is of a magnitude proportional to that of the reverse current through the magnet and is transmitted through the conductor 49 and through the serially connected

conductor

39 and 51 to the static discharge control unit 42. The voltage across the resistor 26 is compared by the static discharge control unit 42 with the previously stored voltage from across the resistor 27. When the relative magnitudes of these voltages indicate that the reverse current has reached a predetermined percentage of the terminal 1 value of lift current, the static discharge control unit 42 de-energizes the drop contactor winding 17 w. This may be accomplished by completing a circuit by-passing the winding 17w through the resistor 47, the conductor 46 and the conductor 51.

De-energization of the winding 17w opens the

contacts

17a, 17b and 17c. Opening of the contact 17c removes power from the static discharge control unit 42. Opening of the contacts 17a and 17b disconnects the magnet 11 from the direct current source so that the magnet control circuit is turned off and the magnet 1 1 is completely de-energized.

As can be seen from the previous discussion, the static discharge control unit 42 may be any circuit which will store a voltage proportional to the final value of lift current, compare this voltage to a second voltage proportional to reverse current, and de-energize the drop contactor winding when the reverse current reaches a predetermined percentage of the final value of lift current. A preferred embodiment of such a static discharge control unit 42 is illustrated in FIG. 2 and is shown connected as part of the magnet control system of FIG. 1. Although the'representations of circuit components have been rearranged to clarify description, the magnet control system as illustrated in FIG. 2 is electrically the same as the representation of FIG. 1. Accordingly, the numbers used in FIG. 1 have been retained in the description of FIG. 2. The conductor 41 from the junction 24 is connected to a terminal 41T. The conductor 44 is connected to a terminal 44T.

Conductors

46 and 50 are respectively connected to terminals 46T and 50T which have a diode 54 connected therebetween. Terminating the conductor 49 from the junction 19 is a terminal 49T while the conductor 51, also connected to the conductor 39 leading from the junction 29, is connected to a terminal 51T which is connected to serve as the system common or ground. The terminal SIT is also connected to a common conductor 55. v

Connected between the terminal 44T and the conductor 55 are a diode 56 and a Zener diode S7 serially connected by a conductor 59. A capacitor 60 is connected in parallel with'the Zener diode 57. The diode 56, Zener diode 57 and capacitor 60 combine to function as a power supply for the static discharge control unit 42.

A diode 61 is connected between the terminal 50T and a branch 55a of the common conductor 55 while a conductor 62 connects a thyristor 64 between the terminal 46T and the common conductor branch 55a. The gate of the thyristor 64 is connected through a resistor 65 to a junction 66 which is in turn connected by a resistor 67 to the common conductor branch 55a.

The conductor 59, between the diode 56 and the Zener diode 57, is connected by a conductor 69 through a resistor 70 and a unijunction transistor 71 to the junction 66. A conductor 72 connects the conductor 59 to the common conductor branch 55a through a of an NPN transistor 86. The junction 84 is connected to the base of the transistor 80. Conductors 87 and 89 connect the conductor 59 to the collectors of

NPN transistors

90 and 91, respectively. The emitter of the transistor 91 is connected to the base of the transistor 90 and the emitter of the transistor 90 is connected by a conductor 92 through a junction 94 and a diode 95 to the emitter ofthe transistor 86. Connected between the junction 94 and the common conductor 55 is a resistor 96.

The base of the transistor 91 is connected by a conductor 97 through a diode-99 and a diode 100 to the common conductor 55. A capacitor 101 is connected between the conductor 97, adjacent the transistor 91, and the common conductor 55 by a conductor 102. The conductor 97 is also connected, at a point between the

diodes

99 and 100, by a conductor 104 through a resistor 105 to the common conductor 55 and by a conductor 106 through a resitor 107 to the terminal 41T.

Serially connected between the terminals 49T and 51T by a conductor 109 are a diode 110, a resistor 111, a junction 112 and a resistor 114. A conductor 115 connects the junction 112 through a threshold means such as a silicon unilateral type switch (SUS) 116 to the junction 66. A capacitor 117 is connected between the conductor 115 and the common conductor 55.

The terminal 49T is connected by a conductor 119 through a resistor 120, a junction 121, and a resistor 122 to ajunction 124. The junction 124 is connected to the base of the transistor 86 and is connected through a resistor 125 and, by a parallel path, through a diode 126 to the common conductor 55.

Joining the junction 121 to the common conductor 55 is a conductor 127 which serially connects a diode 129, ajunction 130, a resistor 131, ajunction 132 and a resistor 134. A thyristor 135 is connected between the junction and the common conductor 55 by a conductor 136 while a conductor 137 connects the gate of the thyristor to the junction 132 through an SUS 139.

Operation of the magnet control circuit utilizing the static discharge control unit circuit illustrated in FIG. 2 will now be described. When the master switch is moved to its LIFT position, the magnet is energized through a circuit from the positive voltage-terminal 12P through the conductor 14, the contact 15a, the conductor 25a, the magnet 11, the conductor 25b, the conductor 20 and the contact 15b to the negative voltage terminal 12N. Voltage is not applied across either the resistor 26 or the resistor 27 and both the master switch contact 37 and the contact are open so that no input voltage is applied to the static discharge control unit 42.

When the master switch is moved to the DROP position, energizing the winding 17w of the drop contactor while thewin ding 15w of the lift contactor remains energized, the drop contacts 17a and 17b close placing the serial combination of

resistors

26 and 27 and the resistor 21 in parallel with the magnet 11. This places a voltage drop across the serial combination of

resistors

26 and 27 equal to the voltage of the direct current source. If the ohmic values of

resistors

26 and 27 are equal, this places a voltage drop across each of the

resistors

26 and 27 which is one-half of the line voltage with the junction 19 positive with respect to the junction 29 and the junction 24 negative with respect to the junction 29. It should be kept in mind at this time that the junction 29 is connected through

conductors

39 and 51 to the common conductor 55 of the static discharge control unit 42 at the terminal 51T so that the voltage drop across the

resistors

26 and 27 are reflected by the voltage at the

junctions

19 and 24, respectively. The junctions l9 and 24 are connected through the

conductors

49 and 41, respectively, to the corresponding input terminals of the static discharge control unit 42 so that these voltages will be applied thereto.

Energization of the winding 17w causes closure of the contact 17c which, through the resistor 45 and the conductor 44, energizes the power supply circuit comprising the terminal44T, the diode 56, the conductor 59 and the parallel-connected combination of the capacitor 60 and the Zener diode 57. Closure of the contact 17c also completes a holding circuit for the winding 17w through the resistor 47, the conductor 46, the terminal 46T, the diode 54 and the terminal SOT to the conductor 50 and winding 17w.

The negative voltage at the junction 24 is applied through the conductor 41 to the terminal 411 of the static discharge control unit 42 and applied through the resistor 107 to the conductor 97. This voltage is then applied to the common conductor 55 through the diode 100 and is blocked from the capacitor 101 and the base of the transistor 91 by the diode 99. Thus, the voltage drop across the resistor 27 has no effect during this portion of circuit operation. The voltage across the resistor 26 is applied through the conductor 49 to the terminal 49T. The voltage at the terminal 49T is applied across the series combination of the diode 110, resistor 111 and resistor 114. The resistor 111 and 114 form a voltage divider which places a predetermined voltage at the junction 112 when the voltage drop across the resistor 26 is about one-half of line voltage. The capacitor 117 provides a time delay before the voltage at the junction 112 causes the SUS 116 to break over into conduction. However, the period of overlap of the master switch contacts 36 and 37', during which this magnitude of voltage is applied to the terminal 49T, is shorter than the time delay provided by the capacitor 117. Therefore, this portion of the circuit does not affect normal drop operation.

The voltage at the terminal 49T is also applied, through the resistor 120,-to the junction 121. This voltage is placed, through the diode 129, across the voltage divider comprising the

resistors

131 and 134. During this period of lift and drop contact overlap, the voltage sistors 122 and 125 so that the voltage at the junction 124, and accordingly at the base of the transistor 86, is too small to cause the transistor 86 to conduct. Thus, operation of the magnet control circuit of FIG. 1 is generally unaffected by the static discharge control unit -42 during the period of overlap of the

master switch contacts

36 and 37.

After the contact overlap period, the contact 36 opens de-energizing the winding 15w. The resistor 43, which is connected across the winding 15w, further delays opening of the lift contactor to ensure the closing of the drop contacts 17a and 17b before the lift contacts 15a and 15b open so that arcing of the lift contacts may be prevented, as is well known to those skilled in the art.

As has been previously indicated, after the opening of the lift contacts 15a and 15b, the magnet 11 is energized by a circuit from the positive voltage terminal 12? through the resistor 21, contact 17b, conductor 25b, magnet 11, conductor 25a, contact 17a, resistor 26 and resistor 27 to the negative voltage terminal 12N.

dicated by the arrow 52 and is approximately equal to the magnitude of the current through the magnet 11 just before the closing of the DROP contacts. Because of this, there is a voltage drop across the .resistor 26 with the junction 19 negative with respect to the ju'nction 29 and a voltage drop across the resistor 27 with junction 24 positive with respective to the junction 29.

The negative voltage at the junction 19 is applied through the conductor 49 to the terminal 49T of the static discharge control unit 42 and then applied through the conductor 119 and the

resistors

120 and 122 to the diode 126 which is poled to transmit this voltage and prevent the biasing into conduction of the transistor 86 during this portion of the drop cycle. The diodes and 129 are poled to block this voltage. Thus, the voltage across the resistor 26 does not affect this portion of the drop cycle.

' The positivevoltage at the junction 24 is appliedthrough the magnet 11 just before initiation of the drop cycle. The voltage stored by the capacitor 101 biases the

transistors

91 and 90, which are connected to operate as a Darlington transistor emitter follower, into conduction to place a voltage substantially equal to that stored by the capacitor 101 across the resistor 96.

The high gain of the emitter follower circuit maintains the voltage level at the junction 94 without discharging the capacitor 101 during the time between the opening of the lift contacts 15a and 15b and the buildup to the proper level of reverse current through the magnet l 1.

Continued application of voltage to the magnet 11 with the junction 22 positive relative to the junction 16 decreases the current which is flowing through the magnet 11 in the direction of the arrow 52. This, however, has no effect on the voltage stored by the capacitor 101 in the static discharge control unit 42 due to the blocking action of the diode 99. The current through the magnet 11 decreases to zero and then reverses and starts to build up in a directionopposite to that of the arrow 52. This causes a voltage drop across the

resistors

26 and 27 with the junction 19 positive with respect to the junction 29 and the junction 24 negative with respect to the junction 29.

The negative voltage at the junction 24 is applied through the conductor 41, the terminal 41T, the resistor 107 and the diode 100 to the common conductor 55. The voltage is blocked by the diode 99 so that it does not affect the voltage stored by the capacitor 101. The positive voltage at the junction 19 is applied through the conductor 49, the terminal 49T, the through the resistors and 122 to the junction 124 at the base of the transistor 86. This voltage is blocked by the diode 126 so that the junction 124 is at a voltage which is proportional to the magnitude of reverse current through the magnet 11. Although the positive voltage at the input terminal 49T is not blocked by the diode 110 or the diode 129, the voltage levels established at the

junctions

112 and 132 are too small to trigger either the SUS 116 or the SUS 139, respectively.

When the voltage at the junction 124, which is proportional to the reverse current through the magnet 11, bears a predetermined ratio to the voltage at the junction 94, which is proportional to the lift current, the base-emitter voltage of the transistor 86 reaches a magnitude sufficient to bias the transistor 86 into conduction. Thereupon, current flows from the conductor 59, in the power supply circuit, through the resistor 82, the resistor 85, the transistor 86, the diode 95 and the resistor 96 to the common conductor 55 and establishes a voltage at the junction 84 which causes the transistor 80 to conduct. This completes a charging circuit for the capacitor 76 through the transistor 80 and the resistor 79 and rapidly charges the capacitor 76 to the trigger voltage of the unijunction transistor 71. The voltage thereby produced at the junction 66 upon conduction of the transistor 71 generates a pulse that is fed through the resistor 65 to the gate of the thyristor 64 causing it to conduct.

The conduction of the thyristor 64 completes a circuit from the input terminal 46T through the thyristor 64, conductor 62 and the common conductors 55a and 55 to the input terminal SlT so that the holding circuit for the drop contactor winding 17w is by-passed and current flows from the conductor 46 through the static discharge control unit 42 to the conductor 51 instead of the conductor 50. The winding 17w is de-energized and the

contacts

17a, 17b and 17c consequently open to terminate the drop cycle. The free wheeling diode 61 provides a current path for the induced voltage in the winding 17w when the thyristor 64 is conducting.

Because the reverse current through the magnet 11 at the end of the drop cycle always reaches the proper magnitude determined by the lift current to completely demagnetize the magnet, a clean drop is always provided for the magnet without any adjustment being required.

It is desirable to provide circuitry which terminates the drop cycle after a fixed period of time so that the drop cycle will be terminated even if the time required for the reverse current buildup in the magnet is greater, due to circuit malfunction, than the fixed time period or if the controller is cycled without a magnet. For this purpose, the resistor 74 is connected between the power supply conductor 59 and the junction 75 to provide an independent charging circuit for the capacitor 76. With the proper selection of capacitors 76 and resistor 74, the time required to charge the capacitor 76 to the trigger voltage of the unijunction transistor 71 can be preset to be just greater than the longest time constant of any magnet which will be operated by the control system. When the drop contact 17c closes at the beginning of the drop cycle, thereby energizing the power supply portion of the static discharge control unit 42, the capacitor 76 is charged slowly through the resistor 74. If the buildup of reverse current is not rapid enough to turn on the transistor 86 during the charging period of the capacitor 76, the capacitor 76 reaches the triggering voltage of the unijunction transistor 71 to gate the thyristor 64 on and terminate the drop cycle as has been previously described.

If, for any reason, the master switch is moved to the LIFT position before the completion of the drop cycle, the drop cycle must be terminated to prevent damage to circuit components. Moving the master switch to the LIFT position closes the master switch contact 36 and energizes the lift contactor winding 15w to close the contacts 15:: and 15b. If the drop cycle has not been completed, the drop contactor winding 17w is still energized and the contacts 17a and 17b are closed so that the serially connected

resistors

26 and 27, as well as the resistor 21, are connected in parallel with the magnet 11 across the source. Thus, the voltage drop across the resistor 26 is about one-half of the source voltage with the junction 19 positive with respect to the junction 29. This voltage is applied through the conductor 49 to the terminal 49T and from there through the resistor to the junction 121. The voltage is not blocked by the diode 129 and causes the thyristor to conduct and prevent this high voltage from being applied to the transistor 86. Because the voltage at the terminal 49T is not blocked by the diode 110, it produces a voltage at the junction 112 sufficient to break over the SUS 116 into conduction. After the time delay provided by the capacitor 117, the SUS 116 conducts producing a voltage at the junction 66 which gates on the thyristor 64 to terminate the drop cycle in the manner previously described. This leaves the magnet connected to the positive voltage terminal 12? and negative voltage terminal 12N of the direct current source through the lift contactors 15a and 15b, respectively.

A magnet control system is thus provided which measures the lift current at the termination of the lift cycle and terminates the drop cycle when the reverse current through the magnet has reached a predetermined percentage of the lift current. The drop cycle also terminates within fixed time period after initiation of the drop cycle or when a lift cycle is started before the completion of the drop cycle. It should be clear that, although the magnet controller of this invention has been described in connection with a lifting magnet, it may be utilized for controlling any electromagnet or for de-energizing any inductive device which would otherwise leave an undesirable residual magnetic field.

Although, in the preferred embodiment, the magnitudes of the lift current and reverse current are determined by measuring the voltage drop across

resistors

26 and 27, it should be understood that other means for providing voltages proportional to these currents may be used without departing from the spirit and scope of this invention. It has been found in practice that, for a lifting magnet, the reverse current should preferably be interrupted when it reaches a magnitude of about 15 percent of the lift current because, although magnet flux varies somewhat with the type of load, such a value provides a clean drop for all types of load.

Iclaim:

1. In a control system for selectively connecting and disconnecting an electromagnet to and from a source of power, said control system comprising switching means for selectively connecting and disconnecting the electromagnet directly to and from the source of power to control the flow of a lift current through the electromagnet, resistance means, and reverse switching means for connecting the electromagnet to the source through the resistance means to cause a reverse current to flow through the electromagnet upon cessation of the lift current, the improvement comprising control means for measuring the lift current, measuring the reverse current, and causing cessation of the reverse current through the electromagnet when the magnitude of the reverse current bears a predetermined relationship to the magnitude of the lift current.

2. A control system as in claim 1 wherein said reverse switching means includes a drop contact which closes upon operation of said reverse switching means and said switching means includes a lift contact which opens after the drop contact closes, and said control means includes means'for measuring the lift current after the lift contact opens.

3. A control system as in claim 1 wherein said control means includes means for causing cessation of the reverse current through the electromagnet when the magnitude of the reverse current reaches a predetermined percentage of the magnitude of the lift current.

4. A control system as in claim 1 wherein said control means includes means responsive to one voltage across the resistance means for measuring said lift current and 'means responsive to an other voltage across the resistance means for measuring said reverse current.

5. A control system as in claim 4 wherein said re sistance means includes a first portion and a second portion, said one voltage responsive means is connected across said first portion and said other voltage responsive means is connected across said second portion.

6. A control system as in claim 4 wherein said means responsive to said one voltage includes storage means for storing a first voltage proportional .to the magnitude of the lift current and said control means includes comparison means for comparing a second voltage proportional to the magnitude of the reverse current with said first ,voltage, means responsive to the comparison means for producing an operating signal when said second voltage bears a predetermined relationship to said first voltage, and terminating means responsive to the operating signal for interrupting the reverse current through the electromagnet.

7. A control system as in claim 6 wherein said storage means is a capacitor.

8. A control system as in claim 4 wherein said means responsive to said one voltage includes storage means for storing a first voltage proportional to the magnitude of the lift current and said control means includes comparison means for comparing a second voltage proportional to the magnitude of the reverse current with said first voltage and means responsive to the comparison means for producing an operating signal dependent upon the relationship said second voltage bears to said first voltage, and terminating means responsive to the operating signal for interrupting the reverse current through the electromagnet when a predetermined relationship between the first and second voltages is reached.

9. A control system as in claim 8 wherein said storage means is a capacitor.

10. A control system as in claim 8 wherein said controlmeans includes timing means activated to initiate a timing cycle upon connection of the electromagnet to the source through said resistance means by said reverse switching means and operative to produce an operating signal after a time delay, and overlap means activated to initiate a timing cycle when the electromagnet is connected directly to the source of power by said switching means while connected to the source through said resistance means by said reverse switching means and operative to produce an operating signal after a time delay, and wherein said terminating means is responsive to each of the operating signal of the timing means and the operating signal of the overlap means for stopping the flow of reverse current through the electromagnet.

11. The method of demagnetizing an electromagnet connected to a source of power to apply a lift voltage causing a lift current to flow through the electromagnet, said method comprising the steps of terminating application of the lift voltage, applying reverse voltage to the magnet to produce a reverse current in the electromagnet in a direction opposite that of the lift current, and disconnecting the reverse voltage when the reverse current bears a predetermined relationship to the lift current.

12. A method of demagnetizing an electromagnet connected to a source of power to apply a lift voltage causing a lift current to flow through the electromagnet, said method comprising the steps of terminating application of the lift voltage, storing a first voltage proportional to lift current, applying reverse voltage to the electromagnet to produce a reverse current in the magnet in a direction opposite that of the lift current, providing a second voltage proportional to the reverse current, comparing said first and second voltages, and disconnecting the reverse voltage when the firstvoltage bears a predetermined relationship to the second voltage.

Claims

2. A control system as in claim 1 wherein said reverse switching means includes a drop contact which closes upon operation of said Reverse switching means and said switching means includes a lift contact which opens after the drop contact closes, and said control means includes means for measuring the lift current after the lift contact opens.

4. A control system as in claim 1 wherein said control means includes means responsive to one voltage across the resistance means for measuring said lift current and means responsive to an other voltage across the resistance means for measuring said reverse current.

5. A control system as in claim 4 wherein said resistance means includes a first portion and a second portion, said one voltage responsive means is connected across said first portion and said other voltage responsive means is connected across said second portion.

6. A control system as in claim 4 wherein said means responsive to said one voltage includes storage means for storing a first voltage proportional to the magnitude of the lift current and said control means includes comparison means for comparing a second voltage proportional to the magnitude of the reverse current with said first voltage, means responsive to the comparison means for producing an operating signal when said second voltage bears a predetermined relationship to said first voltage, and terminating means responsive to the operating signal for interrupting the reverse current through the electromagnet.

7. A control system as in claim 6 wherein said storage means is a capacitor.

9. A control system as in claim 8 wherein said storage means is a capacitor.

10. A control system as in claim 8 wherein said control means includes timing means activated to initiate a timing cycle upon connection of the electromagnet to the source through said resistance means by said reverse switching means and operative to produce an operating signal after a time delay, and overlap means activated to initiate a timing cycle when the electromagnet is connected directly to the source of power by said switching means while connected to the source through said resistance means by said reverse switching means and operative to produce an operating signal after a time delay, and wherein said terminating means is responsive to each of the operating signal of the timing means and the operating signal of the overlap means for stopping the flow of reverse current through the electromagnet.

12. A method of demagnetizing an electromagnet connected to a source of power to apply a lift voltage causing a lift current to flow through the electromagnet, said method comprising the steps of terminating applicatIon of the lift voltage, storing a first voltage proportional to lift current, applying reverse voltage to the electromagnet to produce a reverse current in the magnet in a direction opposite that of the lift current, providing a second voltage proportional to the reverse current, comparing said first and second voltages, and disconnecting the reverse voltage when the first voltage bears a predetermined relationship to the second voltage.