US2195017A - Lifting magnet controller - Google Patents

Description

March 26, 1940.

A. L. WARD LIFTING MAGNET CONTROLLER Filed Nov. 10, 1937 ARTHUR L. WARD HIS ATTORNEY.

Patented Mar. 26, 1940 UNITED STATES PATENT OFFICE to The Electric Controller 8 Mann! will! Company, Cleveland, Ohio, a corporation of Ohio Application November 10. 1031, Serial m. 11am iiOlaims.

This invention relates to a control system for inductive devices, and more particularly to a protective control system for lifting magnets, power solenoids and the like.

Unless a voltage limiting protective device is used, the inherent self-inductance of a highly inductive winding will cause a high voltage to be induced consequent upon disconnection of the winding from the source of supply. This large induced voltage-results in destructive arcing at the switching contacts and also causes a high voltage strain on the insulation of the winding itself and the conductors connected thereto.

In some applications of inductive windings it is common practice to connect a resistance across the terminals of the winding at the time of interruption. The use of such a resistance limits the discharge voltage and prevents destructive arcing or high voltage strains. However, the use of a resistance in this manner results in aslow decay of magnetic flux which is very undesirable in lifting magnet operation because of the resultant delayed dropping of the load.

To reduce the flux quickly and still prevent damaging induced voltages, it has been found expedient to apply reverse voltage to the winding after it has been disconnected from the source of supply. Even if this is done, it is also necessary in order to prevent the induction of destructive voltages, to provide a discharge circuit at the instant of the removal of the energization voltage. A further improvement in the control of such inductive devices is disclosed in United States Reissue Patent No. 20,274, issued on May 10, 1938, to D. C. Wright, wherein is described a control system providing for the connection of a lifting magnet to a reverse power source prior to the removal of energizing power connections and which permits the magnet to discharge into the supply source through protective resistance, which resistance also serves to prevent a short circuit across the supply circuit during the switching operation. 7

Amongvarious prior control systems with which the present invention may be used is one such as described in the Wright patent which operates very satisfactorily and has eliminated nearly all of the defects of the earlier prior art devices. But one of the most common troubles experienced in the use oi any lifting magnet controller has not been eliminated. This trouble is the failure of the discharge resistors and resultant open-circuit condition. One of the reasons for prevalence of this trouble is that the discharge resistors are subjected to sudden impulses of very high currents which heat them to high temperaturessothat theyhavetobeplacedin anexposed position in order to permit cooling. Furthermore, they are usually mounted upon thecrane which is handling the magnet and are thus subiected to very severe vibrations. This alternate heating and cooling, exposure to the elements, and vibration make it impossible to design practical resistors which will not break after a certain length of time. Another common reason for the failure of discharge resistors to limit the discharge voltage is that the conductors required to connect the controller itself to the resistors, which must be mounted in an exposed place, are often necessarily long and thus exposed to mechanical damage.

Heretofore, if a discharge resistor became opencircuited for any reason whatsoever, it was possible to continue using the magnet, with consequent deleterious arcing and possibility of damage to the magnet itself. In such cases, if the operator would refrain from reporting the damage because it would tend to delay him in completing his task, or for other reasons, or if the contactors were enclosed in a cabinet preventing the trouble from being quickly detected, the contacts on the switch supplying power to the magnet would be badly damaged or destroyed, or the insulation of the magnet would be punctured and the windings short-circuited. Although signal lights or alarm bells could be used to inform those in authority that such a break had occurred, the impracticability and additional expense of such complications in the crowded cabs of cranes using lifting magnets is readily apparent.

It is therefore an object of this invention to provide a lifting magnet control system employing resistors and wherein energization of the magnet is impossible when a discharge resistor is open-circuited.

It is a further object of this invention to provide an improved protective lifting magnet control system of this character which requires no extra equipment to render impossible the energization of the magnet as long as a discharge resistor is open-circuited.

A more specific object is to provide such a system which employs switching means for closing the circuit to the magnet and which means are so series connected with the resistors that the switching means are rendered inoperative to close the circuit when the resistors are open-circuited.

Other objects and advantages will become epparent from the following specification, wherein reference is made to the drawing, in which:

Fig. 1 is a full diagrammatic diagram of a' A magnet winding 7 of an electromagnet I has terminals 9 and N which are connected to the conductors 4 and 5 respectively when the switch 8 is in a closed position. An electro-magnetic switch having an operating coil llw, normally-open contacts Ho and llb, and a biasing spring Is, is arranged to provide reverse voltage connections for the magnet winding 1 by connecting terminals 9 and ID to conductors 5 and 4, respectively, through conductors l5 and I4 and resistors 35 and 34, respectively.

A master switch 18 controls the operation of the switches i and H. The master switch 18 is preferably of the snap action type and is shown as having a toggle action spring Its, having one end connected to a movable part of the switch l8 and the other end stationary so that when the switch 18 is moved past a dead center position, the spring lBs will cause the switch to rapidly move to one of its closed positions. The switch H has a lift button I2, 9. drop button II, and stationary contacts I9, 20, 2| and 22. When the drop button i3 is operated, a connector |Ib electrically connects the contacts 2| and 22, and when the lift button |2 is operated, a connector |8a electrically connects the contacts I! and 2|, and the connector i8?) is removed from the contacts 2| and 22. The connectors Ila. and llb move simultaneously so that only one of them is completing a circuit when the master switch it is in one 01 its closed positions.

The contacts I! and 2| are connected to the conductor H at a point 24, and the contact 20 is connected to one terminal of the coil he of the electromagnetic switch I. A conductor 21 leads from the other terminal of the coil Iw to the conductor II at a point 26. The contact 22 is connected through the contacts 6c of the switch I to a terminal 01' the coil llw; the other terminal of the coil ||w is connected through the contacts Id of the switch I to the conductor 5. Another connection from the terminal of coil llw which is connected to the contacts d is through a resistor 4| to the conductor II at the point 2|. A resistor 4| shunts the contacts 6c and completes a circuit from the contact 22 to the terminal of the coil ||w which is connected to the contacts 40. The purpose of the resistors 4I and 4| is to reduce to a safe value the voltage applied to the coil llw when the magnet I is discharging.

To provide an adjustment of the operating characteristics of the switch I, an adjustable resistor 31 having an adjustable connector II is connected in parallel with the coil an. A fixed resistor 28 is connected in series with the adjustable resistor 31.

The control system operates as follows:

If the knife switch I is closed so as to apply power from the source and 2 to the conductors 4 and 5, and if the lift button I2 is operated connect contacts l8 and 2| through the connector Its, an operative circuit is set up for the coil 410 from the conductor 4 through the resistor 34 to the point 24 on the conductor I4, through the master switch II by means of connector lIa to the coil B10, and by means of the conductor 21 to the point 2! on the conductor II, and thence through the resistor II to the conductor 6. Energizatlon of the coil Iw operates the switch I to its closed position and power is supplied to the magnet winding 1 through a circuit which extends from conductor 4, the contact Ia, the terminal 9, the winding I, the terminal II, and the contact 5b to the conductor I.

When it is desired to deenergise the magnet winding 1, the drop button II is operated to connect contacts 2| and 22 by means of connector IIb, and simultaneously connector IIa is removed from the contacts II and 2I. The coil Ilw of the switch II is then energized through a circuit which extends from the conductor 4, through the resistor 24 to the point 24 on the conductor 4, the connector |Ib, through the now closed contacts Io of switch I, the coil Hu: and through the now closed contacts Id to conductor 5. g

It is a well-known fact that the contacts of an electromagnetic switch close more quickly upon energlzation of its operating coll than the contacts of a similar switch open upon deenergization of its operating coil. Thus the switch closes before the switch I opens and a dmcharge circuit is then provided through the power lines before the magnet is disconnected therefrom. If the natural time 0! opening oi the switch I is not longer than the natural closing time of the switch any of. several well-known arrangements may be used either to speed up the closing of the switch H or to slow down the opening of switch I, or both. For instance, a short-circuited winding may surround the core of the switch I to delay its demagnetization, or the coil llw might be designed to provide an extremely large closing force. The discharge circuit extends from the terminal II of the magnet winding I, through the contacts MD, the conductor I4, the resistor 24, the conductor 4, the knife switch I, the supply source and 2, the knife switch I, the conductor I, the resistor II, the conductor II, and the contacts Ila to the terminal I 'of the winding 1. when the switch I opens, the dis-' charge voltage of the magnet is opposed to the supply line voltage and when the discharge voltage, which may be twice or three times as much as the supply voltage at the beginning of the discharge, has reduced to a value equal and opposed to the applied voltage, the current in the magnet winding 1 is zero. Current now begins to flow in the reverse direction through the magnet winding 1, over a circuit which extends from the conductor 4, the resistor 24, the conductor l4, the contacts llb, the terminal II, the winding 1, the terminal I, the contacts Ila, the conductor II, and the resistor II to the conductor I. The current now begins to build up in the reverse direction in the magnet winding 1 and tends to cause the remaining flux in the magnet to decrease to a zero value.

The original circuit to the coil llw through the contacts Ic and 6d of switch I was interrupted when the switch I was opened, but the coil ||w remained energised due to the discharge voltage of the magnet over a circuit which extends from the terminal I, the contacts Ila, the

' conductor lithe point 20, the resistor 40, the

coil llw, the resistor 4|, the connector lib, the point 24, the conductor l4, and the contacts ||b, to the terminal l0. when the discharge voltage reaches a predetermined value due to the reverse current, the operating coil to will be sufliciently deenergized to permit the switch II to open, thereby cutting 08 the current at the moment the magnet 8 is de-magnetized to the required degree.

The adjustable resistance 31 is provided in the circuit of the coil ||w to permit control of the value of reverse current which will allow operation of the switch II to its open position. By use of the adjustable resistance 31 the reverse current can be set to give the best drop on any material which the magnet 8 might be lifting and can also be adjusted for changes in line voltage. The purpose of the fixed resistance 39 is to prevent too great a current inrush through th auxiliary contacts 6c and 6d of the switch 6 upon the closure of the switch II in case the adjustable resistance 31 should be all removed from the circuit by means of the adjustable connector 38.

When the magnet 8 is discharged and the drop button I3 01 the master switch l8 closed, 9. current from the source I and 2 flows through the coil Ilw over a circuit which extends from the conductor 4, through the rmistor 34 to the point 24 on the conductor H, the connector lBb, the resistor 4|, the coil Hw, the resistor 40, the point 25 on the conductor l5, and the resistor 35 to the conductor 5. The current in this circuit is not suflicient to close the switch due to the high ohmic value of the resistors 40 and 4| in relation to the voltage of the source and 2. Due to the fact that the resistors 40 and 4| are also included in the operative circuit previously traced for the coil ||w when the magnet l is discharging, it is obvious that an open circuit in either of the resistors 40 or 4| would prevent proper operation of the switch II and consequent destructive arcing or other damage, as is true of prior art controllers which employ resistors for a similar purpose. However, it is to be noted that no current flows through the resistors 40 and 4| when the switch 6 is closed, thus greatly reducing the heating thereof and rendering them less likely to break and cause an open circuit.

If the lift switch 6 was permitted to close, as was the case heretofore, even though the discharge resistor 34 or 35 might be burned out, the magnet winding 1 would be energized. In such case, the closing of the drop switch would be of no effect, and upon the opening of the lift switch 6 a voltage of some ten times the line voltage would be induced in the winding I to cause a high voltage strain on the insulation and a destructive arc to be formed at the contacts of the switch 8.

By virtue of the connection of the operating coil 8w of the switch 6 to the conductor H at the point 24, and to the conductor l5 at the point 25, no current can flow through the coil 6w from the source and 2, and the switch 8 cannot be operated to energize the magnet winding 1, unless both resistors 34 and 38 are in good condition. In event that either or both of these resistors should burn out or become disconnected for any reason, no current can be supplied to the operating coil 610 and consequently no power can be applied to the magnet winding 1. Thus it is impossible for the operator of the magnet to use the magnet unless the discharge resistors which limit the discharge current to a safe value are in condition to perform their proper function.

Having thus described my invention, I claim:

1. In a control system for a lifting magnet, a source of power, switching means for connecting the winding of the lifting magnet directly to the source of power, resistance means, reverse switching means for connecting the winding to said source through said resistance means for applying reverse power to said winding, electromagnetic operating means for said first mentioned switching means, said electromagnetic operating means being connected to the source of power in series with said resistance means, whereby said electromagnetic means is rendered inoperative when said resistance means is open-circuited.

2. In a control system for a lifting magnet to be connected to and disconnected from a source of power, said system including discharge protective means for preventing the occurrence of high induced voltages upon deenergization of the magnet resulting from disconnection of the magnet from the source of power, electromagnetic switching means selectively operable to connect and disconnect the magnet to and from the source of power, means connecting said discharge protective means to the magnet, magnetic operating means energizable from the source of power for operating the switching means to connect the magnet to the source of power and being connected to the source of power through said discharge protective means, whereby the magnetic operating means cannot operate the switching means to connect the magnet to the source of power when said discharge protective means is open-circuited.

3. In a control system for a lifting magnet having an operating coil to be connected to and disconnected from a source of power, a pair of contacts connected to two opposite terminals of the coil, respectively, and to said source in series with the coil, a resistor connected to one of said terminals of the coil and forming a parallel circuit with the coil and with the one of said contacts which is connected to the other of the said two terminals of the coil, the parallel circuit being in series with the other contact of said pair across the source of power, another resistor connected to said other of said terminals of the coil and forming a parallel circuit with the coil and with the contact which is connected to the said one of said terminals of the coil, the second parallel circuit being in series with the said one of said contacts across the source of power, switching means for opening both of said contacts concurrently, whereby, when the contacts are open, the coil remains connected to said source of power in series with said resistors, thereby reversing the power applied to the coil, and electrically operated means operable when energized for closing said contacts and holding said contacts in closed position and operable when unenergized for opening said contacts, and said electrically operated means being connected to the source of power in series with both of said resistors, whereby the switching means are rendered inoperative for closing the contacts when either one, or both, of said resistors are open-circuited.

4. In a control system for a lifting magnet, switching means for closing a circuit through said magnet from a current supply source, electromagnetic operating means for closing said switching means, means for dissipating the inductive discharge of the magnetic field while the magnet is being deenergized, said dissipating means being connected to said source, and a discharge circuit connecting the magnet to said discharge dissipating means, said electromagnetic operatlng means being connected to said source through a point on said discharge circuit between said discharge dissipating means and said magnet, whereby said switching means cannot be operated if said discharge circuit is open-circuited between said point of connection and said source.

5. In a control system for a lifting magnet, resistance means, switching means to complete a circuit for said magnet exclusive oi said resistance means, switching means to complete a.

circuit for said magnet inclusive of said resist v ance means, said second-named switching means being in said second-named circuit between the magnet and said resistance means, and electro-- magnetic operating means for the first-named switching means, said operating means being connected for energization to a point on said second-named circuit between said second-named switching means and said resistance means.

6. In a control system for a lifting magnet, switching means for closing a circuit through said magnet from a current supply source, electromagnetic operating means or closing said switching means, means for dissipating the inductive discharge of the magnetic fleld while the magnet is being deenergized, said dissipating means being connected to said source, and a discharge circuit connecting the magnet to said discharge dissipating means, said electromagnetic operating means being connected for energization only in response to currents flowing in a circuit completed by said discharge dissipating means, whereby said switching means cannot be operated it said discharge dissipating means is opencircuited.

7. The combination with a control system for a lifting magnet and a source 0! power therefor,

including switching means normally operable to connect the lifting magnet to the source of power. and a discharge circuit connected to the maznet and including a discharge resistor, 01' means to constrain said switching means from said normal operation when said discharge resistor is opencircuited.

8'. The combination with a control system tor m a lifting magnet to be connected to a source 0! power, said system including switching means normally operable to connect the magnet to the source of power, means to disconnect said magnet irom the source oi power, and resistance means connected to the magnet to limit the discharge voltage of the magnet, of means operative to render said switching means inoperative ,to connect the magnetic the source of power when said resistance means is open-circuited.

ARTHUR L. WARD.

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