US2181539A - Magnet control - Google Patents

Description

Nov. 28, 1939.

R. O. WERTZ MAGNET CONTROL Original Filed March 9, 1935 l l i I I I I I l l I 1 I l I 1 I I 3nventor Q0y O. MfRTZ (Ittorueg Patented Nov. 28, 1939 MAGNET CONTROL Roy 0. Wertz, to The Ohio Cleveland Heights, Ohio, assignor Electric Mfg. 00., Cleveland, Ohio,

a corporation of Ohio Application March 9, 1935, Serial No. 10,243

Renewed October 15, 1938 10 Claims.

This invention relates to control means for electromagnets and more particularly relates to means for automatically eliminating the residual magnetism in electro-magnets.

One of the most common uses for electromagnets is in and about steel mills and foundries where they are usually operated in connection with traveling or "Gantry cranes and the like. The crane operator controls the magnet operation as well as the movement of the crane about the plant. Obviously in controlling the many movements and functions of the crane there must be provided numerous controls. Any simplification of the controls that can be made without losing their efiectiveness is highly desirable from the standpoint of the operator.

In the operation of electric lifting magnets direct current of various potentials is usually supplied for energizing the same. A magnetically operated main circuit breaker, controlled by manual master switch at a remote point, makes and breaks the circuit to the magnet.

It is also important to have the circuit and switches so arranged that a reverse current at a lower potential may be supplied to the magnet to reverse the polarity thereof for the quick elimination of the residual magnetism that ordinarily remains in the magnet when the main current is discontinued. Failure to dispose of this residual magnetism prevents the load from being dropped quickly and in some instances permits portions of the load to adhere to the magnet. If the magnet should be moved away from the point where the load was dropped with some part of the load adhering thereto, the adhering piece might finally fall and cause damage.

In the conventional installation, the master switch operates the main contactor in the circuit breaker to connect and disconnect the magnet from the current source and to reverse the current to demagnetize the magnet, being provided with three positions, namely on, oii, and reversed. The operator moves the switch to "on a and the magnet is energized. When he desires to release the material, he moves it to reverse and the current, usually at reduced potential, is directed through the windings in the opposite direction to demagnetize the magnet, permitting the load to be quickly released. The switch must be held in the reverse position until the load is released, and then it may be returned to o Should the control be held in reverse too long the polarity would again be built up in the oppoa site direction to the main polarity; therefore it is necessary that the magnet should not be held in the reverse position too long.

The present invention contemplates automatically applying the reversin g current to the magnet ior a sufficient period of time to eliminate the residual magnetism and then to discontinue the application of the reversing current. By providing for automatic application of the reversing current, several important advantages are obtained. The load is discharged quickly when the control is moved to an off position. The operator does not need to judge or guess that surficient reverse current has to be applied. There is a positive assurance that the exact amount of reverse current to eliminate the residual magnetism will be applied. The Work of the operator is simplified in that he has less controls to manipulate;

he need not see the load to determine if it is dropped and because thecontrols are simplified he can devote more of his time to the remaining controls speeding up the other operations.

Referring to the drawing,

I have shown a pair of main conductors B connected to a source of direct current A, the polarity being indicated by the usual signs. The conductors are connected to contacts C of a main circuit breaker or contactor through blow out coils J adjacent the contactor. The circuit to the magnet is adapted to be completed through the blades D which blades are operated in unison by a current applied to an operating coil E. The current to the operating coil is controlled by a manually operable master switch H having a lead F connected to the main positive conductor which,

when thrown to the on position, completes the circuit to the negative conductor through the conductor G, and

operating coil E, the end of which is connected to the other main line near to contact C. When the master switch H is thrown to the off position this circuit is broken releases the armatures D, cuit.

and the operating coil opening the main cir- It is now necessary to eliminate the residual magnetism in the reversal, but which in m accomplished as follows.

magnet M which, as previously stated, was heretofore accomplished by a manual y improved circuit is The winding of the magnet M has considerable inductance. Breaking the circuit causes a large voltage jump due to the self-inductance which may cause the potential across the terminals of the magnet to increase from 220 volts to 1000 volts or higher. The voltage jump of course varies with the main applied potential and the flux of the magnet. This voltage jump or surge is utilized to put into operation a, relay K, which relay controls the reverse current.

The relay K is provided with a core having three windings L, N and O; L and N are so wound on the core as to be of the same polarity, and O is wound in the opposite direction to provide opposite polarity. Winding L, in series with a valve P, is connected across the terminals of the magnet. The valve P is preferably a material having the property of decreasing its ohmic resistance when the potential across it increases, one such material I have found particularly suitable is known to the industry under the trade name of Thyrite, which has the property of decreasing its ohmic resistance twelve times for every doubling of the voltage. For instance, at a potential of volts its ohmic resistance may be 50,000 ohms while at 10,000 volts the resistance is only one-half ohm. The operating potential of 229 volts across the magnet is not great enough under ordinary circumstances to cause the valve P and coil L to pass suihcient current to actuate the relay, but when the main circuit is broken the potential across the magnet raises and since the ohmic resistance of the valve P decreases proportionately to the voltage raise it permits the surge current to be short circuited through the coil L, energizing the same.

Energizing of the coil L moves the relay armature Q, which armature makes contact with the contacts R and S. Contact R is connected to the positive side of the main line. The armature therefore controls current flow from the main line through a bucking coil 0 and resistance U to the magnet. The other side of the magnet is connected to the negative side of the main line through the resistance T. Hence we see that a reverse current at a lower potential is supplied to the magnet. The polarity of the magnet is changed as soon as the impedance is overcome. The contact S connects the positive side of the main line to the negative side of the main line through the holding coil N. The holding coil N maintains the relay closed until overcome by the bucking coil. A resistance V may be provided in series with the holding coil N, the purpose being to permit a smaller and more compact winding for that coil. It is however quite apparent that this is merely for economy in manufacture, and the resistance could be eliminated if the resistance of the coil itself were increased.

Means is provided to cut oil the reversing current when the polarity of the magnet is reversed through the medium of the coil 0 which, is of opposite polarity to the coil L and N. Being of opposite polarity, as soon as the current in the magnet builds up to a certain value, the coil 0 neutralizes the effect of the holding coil N and permits the spring to return the armature Q to its original position, opening the contacts R and S, and de-energizing the holding coil and magnet. A blow-out coil J is provided adjacent the contact R.

If it should be desirable to provide a manually controlled reverse current for the magnet, this can be done by another tap on the master switch connected into the resistance V, by a connector W which holds the reverse contactor closed. This additional control is however optional, not being otherwise necessary to the operation of the circuit.

Having thus described my invention and embodiment thereof, I am aware that numerous and extensive departures may be made therefrom without departing from the spirit of the invention.

I claim:

1. In a control circuit of the class described, an electro-magnet, a circuit breaker interposed between a current source and the magnet for controlling current flow to the magnet, said magnet being adapted to be energized upon application of current thereto, means to eliminate residual magnetism in the magnet upon breaking of the main circuit, comprising a subsidiary circuit for temporarily reversing the polarity of the magnet, a shunt circuit coupled to the magnet for controlling the subsidiary circuit, and a resistance material connected in the shunt circuit operative upon increased potential across the magnet terminals to increase the current flow in the shunt circuit.

2. In a control circuit for an electro-magnet, a magnet, a circuit breaker for controlling cur" rent flow to the magnet from a current source, an auxiliary circuit for reversing the current flow to the magnet to depolarize the same, a relay for controlling the auxiliary circuit, means for controlling the relay comprising a shunt circuit coupled to the magnet, and a resistance element in terposed in said circuit and operable upon increased voltage in the magnet to permit increased current to flow into said relay for energizing the relay and coupling the auxiliary circuit to the magnet.

3. In a controller for electro-magnets, a magnet, a main switch operable to connect a source of current to the magnet, a subsidiary circuit coupledto the source of direct current for reversing current flow in the magnet, a relay for connecting the subsidiary circuit and magnet together and having a field coil, and a shunt circuit including a variable resistance in series with the field coil and the magnet, said resistance having the property of automatically decreasing its resistance upon an increase in voltage to pass an increase in voltage, due to breaking the main circuit into the field coil of the relay to energize the relay and connect the subsidiary circuit to the magnet.

4. In a control circuit for an electro-magnet, a magnet, a main circuit for connecting the magnet to a source of current supply, a circuit breaker in the main circuit for making and breaking the circuit to the magnet, a subsidiary circuit for reversing current flow through, said magnet, a circuit breaker for said subsidiary circuit to make and break the circuit to the magnet, and a shunt circuit connected to the magnet operable to control the circuit breaker of the subsidiary circuit including an inductance adapted to actuate the circuit breaker when energized and a resistance in series with the inductance operable to resist current flow at normal voltage but permit current flow at abnormal voltages.

5. In a control circuit for an electro-magnet, an electro-magnet, a main circuit for connecting said magnet to a source of current, a circuit breaker interposed in the maincircuit for making and breaking the circuit to the magnet, a subsidiary circuit for reversing current flow in said magnet to demagnetize said magnet, a circuit breaker interposed in said subsidiary circuit for making and breaking the circuit to said magnet, a shunt circuit connected to the magnet and comprising an inductance for controlling the circuit breaker in the subsidiary circuit and a resistance in series with said inductance, said resistance having the property of decreasing its ohmic resistance as the potential across it increases.

6. In a control circuit of the class described, an electro-magnet having an inductive winding, a main circuit for connecting said magnet to a source of current, a circuit breaker disposed in the main circuit and interposed between the magnet and the source of current, a subsidiary circuit for reversing the polarity of the magnet to demagnetize the same including a second circuit breaker adapted to connect said magnet to the current source and disconnect said magnet from the current source, means to control the second circuit breaker comprising an inductance shunted across said magnet and a resistance in series with said inductance, said resistance having the property of decreasing its ohmic resistance as the potential across it increases, a holding inductance of the same polarity as the shunt inductance and adapted to be connected into the main circuit to hold the reversing circuit closed after the shunt circuit has operated, and a bucking inductance of opposite polarity in series with the source of current and the magnet and adapted to oppose the holding inductance when the current flowing in the subsidiary circuit builds up to a predetermined value to cause the subsidiary circuit to be opened.

7. In a control circuit of the class described, an electro-magnet having an inductive winding, a main circuit for connecting said magnet to a source of current, a circuit breaker disposed in the main circuit and interposed between the magnet and the source of current, a subsidiary circuit for reversing the polarity of the magnet to demagnetize the same including a second circuit breaker adapted to connect said magnet to the current source and disconnect said magnet from the current source, means to control the second circuit breaker comprising an inductance shunted across said magnet and a resistance in series with said inductance, said resistance having the property of decreasing its ohmic resistance as the potential across it increases, a holding inductance of the same polarity as the shunt inductance and adapted to be connected into the main circuit to hold the reversing circuit closed after the shunt circuit has operated, and a bucking inductance of opposite polarity in series with the source of current and the magnet and adapted to oppose the holding inductance when the current flowing in the subsidiary circuit builds up to a predetermined value to cause the subsidiary circuit to be opened, a manually controlled switch electrically connected to the circuit breaker for operating the same.

8. In a control circuit for an electro-magnet including main and auxiliary circuits connected to the magnet, a shunt circuit operatively connected with the auxiliary circuit for controlling the same, a resistance means unresponsive to low voltage but responsive to increased voltage connected in shunt to the main circuit and operatively connected with the shunt circuit for controlling the same.

9. In a control circuit for an electro-magnet, a main circuit and a main circuit breaker in the main circuit for controlling current flow to the magnet from a current source, a reversing circuit for reversing the current fioW from said source to the magnet to depolarize the same, means to close the reversing circuit connected in shunt with said magnet including an auxiliary circuit having a resistance operative upon increased potential across the magnet terminals to increase the current flow in the shunt circuit, means including a. circuit to hold said reversing circuit closed, and means including a circuit operated in opposition to the holding circuit by a predetermined increase in reversing current through said magnet to open said reversing circuit.

10. In a control circuit for an electro-magnet, a main circuit breaker for controlling current flow to the magnet from a current source, a. reversing circuit for reversing the current flow from said source to the magnet to depolarize the same, a relay for controlling current flow in the reversing circuit, means to dispose of the inductive voltage in said magnet upon breaking of the main circuit and to initiate the operation of the reversing circuit including an auxiliary circuit having a resistance and said relay connected in series with each other and connected in shunt with said magnet, said resistance being operative upon increased potential across the magnet terminals to increase the current flow in the shunt circuit, and circuit means in said relay to hold said reversing circuit in electrical connection with said magnet after the relay contacts close, and a second circuit means in said relay connected to the magnet and operable by a predetermined increase in reversing current through said magnet to oppose said holding means to release said relay and open said reversing circuit.

ROY O. WERTZ.

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