US3350609A

US3350609A - Electromagnetic control means

Info

Publication number: US3350609A
Application number: US422739A
Authority: US
Inventors: Owen S Steele
Original assignee: AO Smith Corp
Current assignee: Commander Electrical Equipment Inc; AO Smith Corp
Priority date: 1964-12-31
Filing date: 1964-12-31
Publication date: 1967-10-31
Anticipated expiration: 1984-10-31

Description

Oct. 31, 1967 Filed Dec. 31, 1964 9 L4 z] La.4 IE KL E o2 Ll 87 Z Emma? INVENTOR. OWEN 5. STEELE Jim nus f 514)"? Afforzgvs United States Patent O 3,350,609 ELECTROMAGNETIC CONTROL MEANS Owen S. Steele, Toronto, Ontario, Canada, assignor, by mesne assignments, to A. O. Smith Corporation, Milwaukee, Wis., a corporation of New York Filed Dec. 31, 1964, Ser. No. 422,739 13 Claims. (Cl. 317-123) This invention relates to electromagnetic control means and particularly to an energizing and de-energizing control for an electromagnetic means such as a lifting magnet, power solenoid or other similar device having a substantially high inherent self-inductance.

Lifting magnets of the electromagnetic variety and similar devices of a highly inductive characteristic will produce a high voltage upon disconnection of the coil or winding from the source of energizing current. The high voltages are capable of causing severe arcing and related problems in the circuit of the magnet as well as the life of the winding insulation. Circuits have therefore been developed to compensate or reduce the effects of the voltages. Such circuits preferably also provide for a rapid discharge of the stored energy in the coil to permit a rapid drop of the material from the lift mechanism and also permit opposite momentary energization of the lifting magnet such that it is substantially devoid of magnetic flux at reset.

' The present invention provides a control for a lifting magnet or the like in a relatively simple and highly reliable controller.

Generally, in accordance with the present invention, a discharge circuit is permanently connected across the magnet and is operative immediately upon opening of the lift contacts. Generally, the discharge circuit includes a discharge bridge having nonlinear resistors in selected branches. The output of the bridge circuit operates a dropping relay having contacts operable to rapidly reduce the current to zero and then reverse the current and pass a demagnetizing current through the device. After a selected demagnetizing current period, the relay automatically opens to return the system to standby.

During the initial discharging period, the nonlinear resistor automatically adjusts the resistance so that the voltage across the magnet is kept to a reasonably low peak value. As the voltage decreases, the resistance value of the nonlinear resistor increases with a related increase in the voltage differential applied to the dropping relay to provide the previously described sequence function. The nonlinear and other resistors of the bridge circuit in the present invention are preferably selected such that only very minimal current passes through any portion of the bridge circuit during normal lifting excitation. Further, the circuit is polarized to protect the nonlinear resistors of the discharge circuit. Thus, if the input power is connected with reverse polarity, the circuit will not be energized.

The present invention thus provides for a very rapid controlled discharge of the stored energy with an automatic two-step sequencing, which maintains the voltage at the lowest minimum value consistent with a high speed operation. The discharge circuit is permanently connected across the inductive device and thus there is no danger of malfunctioning or opening of the circuit without the presence of the discharge circuit. This arrangement also isolates the discharge voltage from the power supply system. Further, a two wire control system can be applied from a make-break controller of other switching means.

The drawing furnished herewith clearly illustrates an embodiment of the invention incorporating all of the aforementioned features and advantages as well as others which will be clear from the following description.

In the drawing:

FIG. 1 is a schematic circuit diagram of a lift magnet system incorporating the subject matter of the present invention;

FIG. 2 shows a portion of the schematic circuit diagram shown in FIG. 1 with the circuit energized to eifect a lifting operation and with the current paths schematically shown by arrows;

FIG. 3 is a view similar to FIG. 2 illustrating the paths in the discharge circuit immediately following the opening of the power circuit to the magnet; and

FIG. 4 is a view similar to FIGS. 2 and 3 showing the circuit at the moment the discharge or drop contacts are closed.

Referring to the drawings and particular-1y to FIG. 1, a lift magnet 1 is diagrammatically shown forming a part of a drum-cable unit 2 which is adapted to lower and raise the lift magnet 1 for movement of any suitable load, not shown. The lift magnet 1 includes an electromagnetic coil 3 connected through a power circuit 4 to a set of incoming power lines 5 and 6.

In the illustrated embodiment of the invention, the schematic circuit is shown as an across-the-line type diagram with the power lines 5 and 6 shown in a vertical position and with the horizontal branch lines numbered Ll through L-4, respectively, for purposes of simplicity and clarity of explanation. Power lines 5 and 6 are connected to any suitable direct current source; for example, a combination transformer and full wave rectifier unit.

A control circuit 7 is connected in lines L3 and L-4 and controls a lift contactor 8 and a drop contactor 9 for controlling the supplying of power to circuit 4. The lift contactor 8 is connected in line L3 in series with a control switch 10 and controls a plurality of associated contacts 8-1 through 8-4. Contacts 8-1 are connected in branch line L-l immediately between the power circuit 4 and the power line 5. Contacts 8-2 similarly connect branch line L-2 to the opposite power line 6 and thus provide a circuit path from line 5 via line L-1 through circuit 4, line L-2 to line 6.

The drop contactor 9 is connected in line L-4 in series with a set of normally closed contacts 8-4 of lift contactor and a set of voltage sensitive relay contacts 11-1 of a relay 11 in circuit 4 for timed actuation, as hereinafter described. The contactor 9 actuates associated contacts 9-1 and 9-2 which are connected respectively in lines 1-2 and lines L-l to provide a reverse connection of the power circuit 4 from line 5, line L-2, circuit 4, line L-l to line 6. Contactor 9 also actuates the normally closed contacts 9-3 in line L-3 to prevent simultaneous energization of the contactors 8 and 9.

In the illustrated operation, a voltage sensitive bridge circuit 12 is provided with the input connected between lines L-1 and L-2, generally in parallel with the coil 3 and with voltage sensitive relay 11 connected across the output.

Before proceeding with a more detailed description of circuit 4, the operation of the device is briefly described. The switch 10 is closed to complete the circuit to the contactor 8 which closes its related contacts 8-1 and 8-2 to supply current to the coil 3. Only a minimal current flows through the bridge circuit 12 for reasons fully developed hereinafter and the magnet coil 3 is provided with maximum energization for lifting of a suitable load. When the load is to be dropped, the switch 10 is opened to deenergize the contactor 8 and open contacts 8-1 and 8-2. At that moment, the inductive characteristic of the coil 3 establishes a relatively high voltage opposing the decrease in current. This high voltage is applied to the voltage sensitive bridge circuit 12 but is isolated from the supply lines 5 and 6 as the contacts of both contactors 8 and 9 are opened. The inductive voltage discharges through the circuit 12 until a selected voltage level is obtained. At that time, the voltage sensitive relay 11 is energized to close its related contacts 11-1 in line L4 and completes the operating circuit to the drop contactor 9 which closes the related contacts 91 and 92. As a result, a reverse voltage is impressed across the coil 3, causing the current therethrough to rapidly drop to zero and thereafter to momentarily maintain a demagnetizing current. After a selected time period, the differential voltage across the bridge 12 drops below the holding voltage of the voltage sensitive relay 11 and the contacts 11-1 open. As a result thereof, contactor 9 is de-energized and contacts 9-1 and 9-2 open, returning the circuit to the standby position shown in FIG. 1.

More particularly, referring to FIG. 1, the discharge or bridge circuit 12 for controlling the voltage sensitive relay 11 is connected in the form of a wheatstone bridge and includes a pair of

fixed resistors

13 and 14 forming two legs connected in series between lines L1 and L-2. The central junction 15 of

resistors

13 and 14 constitutes an output terminal which is connected to one side of the voltage sensitive relay 11 in series by a pair of normally closed contacts 8-3 of the lift contactor 8. The contacts 8-3 open during the lifting operation to positively assure that the voltage relay 11 will not be energized during the lifting function. The contacts 8-3 may be eliminated if the characteristic of the other components hereinafter developed are properly selected.

The other two legs of the bridge are formed by a nonlinear resistor 16 connected in series with a diode 17 and a fixed resistor 18 between the lines L-1 and L-2. The junction 19 between the nonlinear resistor 16 and the diode 17 constitutes the other output terminal connected to the opposite side of the voltage sensitive relay 11. The diode 17 is biased with respect to the connection between lines L-l and L-2 to prevent the flow of current from line L-l through the nonlinear resistor 16. A resistor 20 is connected in line L-2 between the legs of the bridge 12 and is operably inserted in the opposite legs as hereinafter described.

A second nonlinear resistor 21 is connected in parallel with the nonlinear resistor 16 and the diode 17 as well as the series connection of the resistor 18 and the coil 3 to provide a discharge path upon de-energization of the coil 3.

The illustrated control circuit 7 for controlling the energization and de-energization of the magnet 3 includes the contactor 8 connected in series circuit with the control switch 10 and a polarizing diode 22. The diode 22 is desirable to protect the nonlinear resistors. Thus, if the polarity of the lines and 6 is reversed for any reason, current flow through the control circuit is prevented.

The contactor 9 in line L-4 is automatically controlled through the set of normally closed interlocking lift contacts 84 of contactor 8 and the normally open voltage sensitive relay contacts 11-1 of the voltage sensitive relay 11. Contacts 8-4 positively prevent energization of contactor 9 unless contactor 8 has been de-energized to open the related lift contacts 81 and 8-2.

The voltage sensitive relay contacts 11-1 provide an automatic energizing of the contactor 9 to close its related contacts 91 and 9-2 for a timed period during which the magnet coil 3 is de-energized and subsequently demagnetized as a result of the following circuit action.

Referring particularly to FIG. 2, the condition of the circuit is illustrated as a result of the closing of switch in branch line L-3 and the resulting energization of the contactor 8. Contacts 8-1 and 8-2 are closed and set up a pair of current circuits with the line 23 indicating the main current path through the coil 3. A small current shown by the dotted line 24 may also pass through the

resistors

13, 14 and in parallel with the electromagnetic coil 3. Rectifier diode 17 blocks current through the associated paths. As a result, maximum power is supplied to the coil 3 to provide the desired lift energization thereof.

To drop the load, the switch 10 is opened to break the circuit to the contactor 8 and open contacts 8-1 and 82. This will then establish the circuit shown in FIG. 3. The coil 3 and the related power circuit are isolated from the lines 5 and 6 by the open lift contacts 8-1 and 8-2 and the open drop contacts 9-1 and 9-2. Immediately upon opening of the contacts 81 and S-2, a current which may be of the order of 100 amperes will be maintained for a short period as a result of the self-inductance of the coil 3 but will decrease at an exponential rate. The voltage which is generated is a direct function of the resistance of the nonlinear resistance 16 and may for example be limited to 650 or 750 volts. The current from the magnet coil 3 primarily flows through the nonlinear resistor 16 and diode 17 as shown by full line 25 and also through the circuit of fixed

resistor

13, 14 and 20 as shown by the dotted line 26. This will establish an unbalance across the four legs of the bridge circuit 12 with the resulting voltage differential between the

junctions

15 and 19. The four legs consist respectively of resistor 13, resistor 14 in series with resistor 20, resistor 16, and resistor 18 in series with diode 17. However, the voltage generated between

junctions

15 and 19 is below the voltage at which the relay 11 will pick up. For example, the voltage sensitive relay 11 may be selected to pick up at approximately 30 volts whereas during the initial discharging of the coil 3 only 10 volts are produced across the voltage sensitive relay 11. The circuit will therefore continue to discharge the energy in the coil 3 as shown in FIG. 3. This provides a rapid dissipation of the inductive energy and in approximately one second, the current will have dropped to a relatively low value. The resistance of the nonlinear resistor 16 increases as the current decreases and the voltage distribution shifts and increases the voltage differential between

junctions

15 and 19. The voltage across the relay 11 will therefore increase to the pickup level. and actuate the voltage sensitive relay 11 to close contacts 11-1 in line L-4 and energize contactor 9. This results in closing of contacts 91 in line L-2 and 9-2 in line L-1 to produce the circuit condition of FIG. 4.

In FIG. 4, a main path 27 is cornpleted from power line 5 through contacts 9-1, resistor 20, coil 3, contacts 9-2 to line 6. This forces the current to rapidly drop to zero and reverse to pass a demagnetizing current. Durmg the initial portion of this sequence, the bridge circuit 12 is energized as follows. A voltage is applied across fixed resistor 13 in series with fixed resistor 14, as shown by the dotted line 28. A voltage also appears across resistor 20 in series with the magnet coil 3. Thus, the condition of the bridge circuit 12 is such that the resistor 20 is no longer in the leg including resistor 14 but is now in the leg including the magnet winding or coil 3. At this point, the nonlinear resistor 16 may not be carrying current upon the characteristic of the particular resistor shown. In any event, a sufiicient voltage differential is maintained across the voltage sensitive relay 11 to hold it in the energized position. As a result, contactor 9' is maintained energized for a selected period to maintain the reverse voltage application to the coil 3, produce the rapid drop to zero and actual momentary reversal of current.

A very short period after, perhaps one-half second, the current in magnet coil 3 has increased to the level where the voltage between junctions 15 and 19a becomes less than the holding voltage of relay 11, which drops out and returns the circuit to standby. The nonlinear resistor 21 functions at this time to discharge the energy in coil 3 resulting from the reverse current and limits the voltage.

In practice the nonlinear resistor 16 which functions to limit the voltage at initial de-energization of coil 3 is a relatively thin element having a large diameter in order to d1scharge the substantial amount of energy in coil 3.

Rectifier 17 'isolates and thereby protects the resistor 16 an acceptable level.

" The circuit therefore functions to rapidly discharge the energy stored in the magnet coil without creation of dangerous voltage peaks or application thereof to the power supply system. The discharge circuit is permanently connected to provide instantaneous response and produces a positive voltage limitation. The two wire control system of the present invention permits complete control by a simple push button with automatic discharge action upon release thereof. Relays and contactors presently used in magnetic lift controls can be used in the present invention and thus the cost of the system is minimized.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. A circuit means for rapid discharge and reset of a magnetic load energized from a direct current source, comprising power means for applying oppositely polarized currents to the load and having load terminals adapted to be connected to the load,

' a discharge circuit connected in parallel with said load terminals and including a plurality of current paths at least one of which includes nonlinear impedance means and constructed to impress self-induced load voltages upon the nonlinear impedance, said discharge circuit including a pair of output terminals in different current paths, and

a control means for said power means connected to the output terminals and responsive to a selected voltage differential therebetween for momentarily applying a reverse current to the inductive load and the discharge circuit, said nonlinear impedance means serving to initially discharge a portion of the energy in the load at a selected maximum volt-age peak and then changing the voltage distribution in the associated current paths to establish the selected voltage differential.

2. The circuit means of claim 1 having means in the discharge circuit to essentially isolate incoming power from the nonlinear impedance means.

3. The circuit means of claim 1 including control means for connecting of power to the terminals having polarized means whereby power of a selected polarity if operative and of an opposite polarity is inoperative.

4. The circuit means of claim 3 wherein said polarized means is a rectifying means.

5. A circuit means for rapid discharge and reset of an inductive load energized from a direct current source, comprising a pair of load terminals,

a discharge circuit connected in parallel with said load terminals and including a plurality of current paths arranged in a bridge circuit with output control terminals one each in a different current path, at least one of said paths having a nonlinear impedance element connected therein and arranged to impress the self-induced load voltage upon the nonlinear impedance,

power means for applying oppositely directed currents through the load and the discharge circuit, and

a voltage responsive switch means for operatively conmeeting said power means to the load terminals and having operating means connected to the output terminals and responsive to a selected voltage differential to momentarily apply a reverse current to the load and the discharge circuit, said nonlinear impedance serving to intially discharge a portion of the energy in the load at a selected maximum voltage peak and then changing the voltage distribution in the associated current paths to establish the selected voltage differential.

6. A discharge and reset circuit means for a highly inductive coil such as a lifting magnet,

a direct current power circuit including said coil and a discharge bridge circuit connected in parallel,

a pair of power lines,

first contactor means having contacts connecting the power circuit to the pair of power lines to supply lifting power to the coil,

a second contactor means having contacts connecting the power circuit to the pair of power lines to supply dropping power to the coil,

a diode,

an on-otf switch connecting said first contactor to the power lines in series with said diode,

said bridge circuit including a plurality of operative legs connected between input terminal means and output terminal means and including circuit impedances at least one of which is a nonlinear impedance to provide automatic current responsive voltage distribution within the operative legs and to opposite sides of an output terminal,

electroresponsive means connected to the output terminals and having switch means connecting said second contactor to the power lines, and

means connecting said coil to the input terminals to discharge the energy thereof into the bridge circuit upon opening of the first contactor with an intitial discharge period during which the nonlinear impedance adjusts its impedance value to hold the voltage across the magnet to minimum value consistent with rapid operation and increasing inimpedance as the discharging current decreases to create an operative voltage across the output terminal to automatically energize the electroresponsive means and thereby actuate the second contactor to reverse the voltage applied to the coil and rapidly reduce the current through the coil to zero and create an oppositely directed demagnetizing current which after a time period creates a voltage differential below the operative voltage of the electroresponsive means.

7. A discharge and reset circuit means for a highly inductive coil such as a lifting magnet,

a power circuit including said coil and a discharge bridge circuit,

control means selectively connecting the power circuit to supply lifting current to the coil and to supply reset current to the coil,

said bridge circuit including a closed circuit loop having four operative legs each of which includes circut resistors with output terminal means at one set of opposite junctions connected to operate said control means to supply reset current and having input terminal means at the other set of opposed junctions connected in parallel with said coil, one of said legs including a nonlinear impedance to provide automatic current responsive voltage distribution within the four operative legs and the adjacent leg forming an output terminal including a rectifying means to block lifting current through said nonlinear impedance, and

said coil being connected to discharge into the bridge circuit upon removal of the lifting current with an initial period during which the nonlinear impedance adjusts its impedance value to hold the voltage across the coil to maximum peak value consistent with rapid operation and increasing in impedance as the dis- 8.- linear impedance connected in parallel with the 7 charging current decreases to create an operative voltage impressed upon said control means to automatically establish the reset current applied to the coil and thereby rapidly reduce the current to zero and create a demagnetizing current which creates a. differential across the output terminals of the bridge circuit actuating after a selected period which releases said control means.

The circuit means of claim 7 including a second nonfirst named nonlinear impedance and said rectifying means.

A discharge and reset circuit means for a highly inductive coil such as a lifting magnet,

switch means having a first position said bridge circuit including a four legged power circuit including said coil and a discharge bridge circuit,

connecting the power circuit to supply lifting power to the coil and a second position connecting the power circuit to supply dropping power to the coil,

control relay for actuating said switch means to the second position,

closed loop having four operative legs including circuit impedances with output terminals at one set of opposed junctions connected to said relay with input terminals at the other set of opposed junctions connected in parallel with said coil, one of said legs including a nonlinear impedance to provide automatic current responsive voltage distribution within the four operative legs, said bridge circuit including rectifying means to block current through said nonlinear impedance with the switch means in the first position, and

means to actuate the switch means to and from said first position for selectively energizing the coil, said coil discharging into the bridge circuit upon movement of the switch means from the first position with an initial period during which the nonlinear impedance adjusts its impedance value to hold the voltage across the magnet to minimum value consistent with rapid operation and increasing in impedance as the discharging current decreases to create an operative voltage impressed upon said relay to automatically establish the second position of the switch means and reverse the voltage applied to the coil to rapidly reduce the current to zero and create a demagnetizing current which creates a voltage differential below the holding voltage of the relay.

10. A lift and discharge control circuit for an electromagnetic lift having a magnet coil wound on a magnetic core,

comprising a pair of direct current power lines,

pair of branch lines connected between said power lines and each having a lift switch means and a drop switch means to the opposite ends thereof, the switch means in one branch line being reversed with respect to the opposite branch line,

means for connecting the coil between said branch lines for selective energization with a lift current in response to actuation of the lift switch means and with a drop current in response to actuation of the drop switch means,

impedance devices serially connected between said impedance unit being connected in' series with a second output terminal of the bridge circuit at the junction of the nonlinear resistor and the diode, second nonlinear resistor connected in parallel with said first nonlinear resistor and said diode,

a fixed impedance in the branch line between the legs of the bridge including one of said first named pairs of fixed resistors and the diode, and

an electroresponsive means connected between said output terminals and having contacts connected to control said drop switch means, said nonlinear resistors being responsive to the discharging currents of the coil to establish an initial energy discharge period during which the voltage differential is insufficient to actuate electroresponsive means and a second discharge period during which the voltage differential is sufiicient to operate said electroresponsive means, the drop current through said coil and said bridge circuit maintaining an electroresponsive means operative until a selected voltage is created across said coil.

11. The control circuit of claim 10 having a circuit switching means comprising an electroresponsive means switch means and being connected contacts.

12. The control circuit of claim 10 having a safety switch means connected in circuit with said electroresponsive means to positively prevent energization thereof and actuated with the lift switch means.

13. A lift and discharge control circuit for an electromagnetic lift having a magnet coil wound on a magnetic core, comprising a pair of direct current power lines,

a pair of branch lines connected between said power lines and each having a lift switch means and a drop switch means to the opposite ends thereof, the location of the switch means being reversed in the respective branch lines,

a pair of fixed resistors serially connected between said branch lines to form two legs of a bridge circuit and having a first output terminal at the junction of the resistors,

a nonlinear resistor forming a third bridge leg connected in series with a diode and a fixed resistor forming a fourth bridge leg connected between said branch lines with a second output terminal of the bridge circuit ,at the junction of the two legs,

a second nonlinear resist-or connected in parallel with said first nonlinear resistor and said diode,

a fixed resistor in the branch line between the legs of the bridge including one of said first named pairs of fixed resistors and the diode, and

a relay connected between said output terminals and having contacts connected to control said drop switch means, said relay being selected with a pull-in voltage in excess of the voltage differential established with the lift switch means closed and a selected holding voltage, said nonlinear resistors being responsive to the discharging currents of the coil to establish an initial energy discharge period during which the voltage differential is below said pull-in voltage and a second discharge period during which the voltage differential is above said pull-in voltage, the drop current through said coil and said bridge circuit maintaining a holding relay voltage differential until a selected voltage is created across said coil.

for actuating said drop in circuit with said References Cited UNITED STATES PATENTS 2,181,539 11/1939 Wertz 3l7l23 2,648,033 8/1953 Hudson 3l7l23 X MILTON O. HIRSHFIELD, Primary Examinerr 7 L. T. HIX, Assistant Examiner.

Claims

1. A CIRCUIAT MEANS FOR RAPID DISCHARGE AND RESET OF A MAGNETIC LOAD ENERGIZED FROM A DIRECT CURRENT SOURCE, COMPRISING POWER MEANS FOR APPLYING OPPOSITELY POLARIZED CURRENTS TO THE LOAD AND HAVING LOAD TERMINALS ADAPTED TO BE CONNENCTED TO THE LOAD, A DISCHARGE CIRCUIT CONNECTED IN PARALLEL WITH SAID LOAD TERMINALS AND INCLUDING APLURALITY OF CURRENT PATHS AT LEAST ONE OF WHICH INCLUDES NONLINEAR IMPEDANCE MEANS AND CONSTRUCTED TO IMPRESS SELF-INDUCED LOAD VOLTAGES UPON THE NONLINEAR IMPEDANCE, SAID DISCHARGE CIRCUIT INCLUDING A PAIR OF OUTPUT TERMINALS IN DIFFERENT CURRENT PATHS, AND A CONTROL MEANS FOR SAID POWER MEANS CONNECTED TO OUTPUT TERMINALS AND RESPONSIVE TO A SELECTED VOLTAGE DIFFERENTIAL THEREBETWEEN FOR MOMENTARILY APPLYING A REVERSE CURRENT TO THE INDUCTIVE LOAD AND THE DISCHARGE CIRCUIT, SAID NONLINEAR IMPEDANCE MEANS SERVING TO INITIALLY DISCHARGE A PORTION OF THE ENERGY IN