electromagnet

[diàn cí tiě]

Physical device

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

An electromagnet is a device that generates electromagnetism when it is energized. Wrap a conductive winding matching its power on the outside of the iron core coil Like a magnet, it is also called an electromagnet. We usually make it into a strip or hoof shape to make Iron core Easier magnetization 。 In addition, in order to demagnetize the electromagnet immediately after powering off, we often use soft iron or silicon steel materials with fast demagnetization. Such electromagnet has magnetism when it is powered on, and the magnetism will disappear after the power is cut off. Electromagnet is widely used in our daily life. Its invention has also greatly improved the power of the generator.

Chinese name: electromagnet
Foreign name: electromagnet
Discipline: Physics, electromagnetics

Principle: Electromagnetic force generated after power on
Classification: 1. AC electromagnet 2. DC electromagnet
Related figures: Oster

catalog

fourteen Magnetic energy source
fifteen Loss of excitation hazard
sixteen Comparison with permanent magnet

brief introduction

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principle

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electromagnet

When Energized solenoid Internal insertion Iron core After that, the iron core is magnetic field Magnetization. magnetization The rear iron core also becomes a magnet, so as the two magnetic fields overlap each other solenoid The magnetism of is greatly enhanced. In order to make the electromagnet more magnetic, the iron core is usually made Hoof shape 。 But pay attention to the coil The winding direction of must be opposite. One side must be clockwise, and the other side must be counterclockwise. If the winding direction is the same, the magnetization of the two coils on the iron core will cancel each other, so that the iron core is not magnetic. In addition, the iron core of the electromagnet is made of soft iron instead of steel. Otherwise, once the steel is magnetization Later, it will remain magnetic for a long time without demagnetization, so the strength of its magnetism cannot be controlled by the size of the current, and the advantages of the electromagnet will be lost.

Electromagnet is a non permanent device that can generate magnetic force by passing current magnet It can be easily magnetically started or eliminated. For example: large Crane Use the electromagnet to lift the abandoned vehicle.

When the current passes through the wire, a magnetic field will be generated around the wire. Applying this property, the current is passed through solenoid Then, a uniform magnetic field will be made in the solenoid. Assume to place it in the center of solenoid Ferromagnetism The ferromagnetic material will be magnetized and the magnetic field will be greatly enhanced.

Generally speaking, the magnetic field and current generated by the electromagnet coil The number of turns is related to the center ferromagnet. When designing electromagnets, we will pay attention to the distribution of coils and the selection of ferromagnets, and use the current to control the magnetic field. Because the coil material has resistance, this limits the size of the magnetic field that can be generated by the electromagnet Superconductor The discovery and application of will have the opportunity to surpass the existing limitations.

classification

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By current

one alternating current magnet

two DC electromagnet

By use

1. Brake electromagnet: It is used as the mechanical brake of the motor in the electrical transmission device to achieve the purpose of accurate and rapid shutdown. The common models are MZD1 (single-phase), MZS1 (three-phase) series.

two Lifting electromagnet : Used as a lifting device to lift steel, iron sand, etc permeability magnetic material , or used as electromagnetic manipulator to hold steel and other magnetic materials.

3. Electromagnet for valve: use magnetic force to push the magnetic valve, so as to achieve the purpose of opening, closing or reversing the valve port.

4. Traction electromagnet: It is mainly used to implement automatic control tasks with traction mechanical devices.

Direction judgment

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The magnetic field direction of the electromagnet can be Ampere rule To judge.^[1]

The ampere rule is the rule that represents the relationship between the current and the direction of the magnetic induction line of the current excited magnetic field, also called Right hand spiral rule 。

(1) The ampere rule (ampere rule I) in the electrified straight wire: hold the electrified straight wire with your right hand thumb Point to the current direction, then the four fingers point to the surrounding direction of the magnetic induction line.

(2) Ampere rule in the energized solenoid (Ampere rule 2): hold the energized solenoid with your right hand to make the four fingers bend in the same direction as the current, then the end pointed by your thumb is the energized solenoid N-pole 。

advantage

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Electromagnets have many advantages: whether the magnetism of electromagnets can be controlled by switching on and off current; The size of magnetism can be controlled by the strength of current or the number of turns of the coil; The magnetic value can also be controlled by changing the resistance to control the current; Its magnetic pole can be controlled by changing the direction of current, etc. That is, the strength of magnetism can be changed, the existence of magnetism can be controlled, the direction of magnetic pole can be changed, and magnetism can disappear due to the disappearance of current.

The electromagnet is Current magnetic effect （ Electrogenerated magnetism ）Is closely related to life, such as electromagnetic relay 、 Electromagnetic crane 、 Magnetically Levitated Train 、 Electronic door lock Intelligent channel turns, electromagnetic flowmeter, etc.

classification

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Electromagnets can be divided into DC electromagnet and alternating current There are two types of magnets. If electromagnets are classified according to their purposes, they can be divided into the following five categories: (1) Traction electromagnets are mainly used to pull mechanical devices, open or close various valves, and perform automatic control tasks. （2） Lifting electromagnet - Used as a lifting device for lifting Ingot , steel, iron sand, etc magnetic material 。 (3) Brake electromagnet - mainly used for Motor Brake to achieve accurate parking. (4) Automatic electrical Electromagnetic system - such as electromagnetic relay and Contactor Electromagnetic system, automatic switching electromagnetic overload release And operating electromagnet, etc. (5) Electromagnets for other purposes - such as grinding machines Electromagnetic chuck as well as Electromagnetic vibrator Etc.

history

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As early as the spring of 1820, Denmark Oster discovered this principle by accident. 1822, French physicist Arago and Lussac It was discovered that when the current passes through the winding line with iron blocks in it, it can magnetize the iron blocks in the winding line. This is actually the first discovery of the principle of electromagnet. In 1823, Sturgeon He also did a similar experiment: he wound 18 coils of bare copper wire on a U-shaped iron bar that was not a magnet bar Voltaic battery When connected, the copper coil wound on the U-shaped iron bar generates a dense magnetic field, which turns the U-shaped iron bar into an "electromagnet". The magnetic energy on this kind of electromagnet is magnified many times than the permanent magnetic energy. It can pick up iron blocks 20 times heavier than it. When the power is cut off, the U-shaped iron bar can't hold any iron blocks and becomes an ordinary iron bar again.

Sturgeon's invention of electromagnet made people see electric energy The bright prospect of transforming into magnetic energy soon spread in Britain, the United States and some coastal countries in Western Europe.

In 1829, Henry, an American electrician, made some innovations to the Sturgeon electromagnet device, which was insulated by magnetoelectricity wireway It replaces bare copper wire, so there is no need to worry about being too close to the copper wire and short circuit. Because the wires have an insulating layer, they can be tightly wound round and round. The denser the coils are, the stronger the magnetic field is generated, which greatly improves the ability to convert electric energy into magnetic energy. In 1831, Henry trial produced a new electromagnet. Although its size was not large, it could lift a ton of iron.

stay Oster Inspired by the current magnetic effect experiment and a series of other experiments, ampere Recognizing that the essence of magnetic phenomenon is current, the interactions involving current and magnet are attributed to the interactions between currents, and the search for Current element The basic problem of interaction law. In order to overcome the difficulty that isolated current elements cannot be measured directly, Ampere carefully designed four zero indicating experiments, accompanied by careful theoretical analysis, and obtained the results. However, due to the concept of over distance action of ampere on electromagnetic action, the assumption that the force between two current elements is along the line was imposed in theoretical analysis, and it is expected to comply with Newton's Third Law , so that the conclusion is wrong. The above formula is the result of discarding the wrong assumption that the force is along the line and correcting it. It should be understood from the perspective of close action that the current element generates a magnetic field, and the magnetic field exerts a force on another current element.

nature

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The ampere rule of linear current is also applicable to a small section of linear current. Toroidal current It can be seen as composed of many small segments of linear current Ampere rule Determine the direction of magnetic induction intensity on the central axis of the annular current. The direction of the magnetic induction line on the central axis of the annular current is obtained by superposition. The ampere rule of linear current is basic, and the ampere rule of ring current can be derived from the ampere rule of linear current positive charge The direction of motion is the same as that of the negative charge.

significance

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Ampere's law And Coulomb's law It is equivalent to the basic experimental law of magnetic interaction, which determines the nature of the magnetic field and provides a way to calculate the current interaction.

be careful

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WEISTRON electromagnet^[1]

Electromagnet: a device that uses the magnetic effect of current to make soft iron (core shaft inside the electromagnet coil, which can be quickly magnetized and demagnetized) magnetic.

(1) Insert the soft iron bar into a spiral coil. When the coil is connected with current, the magnetic field inside the coil magnetizes the soft iron bar into a temporary magnet, but when the current is cut off, the magnetism of the coil and the soft iron bar will disappear.

(2) The magnetic field generated by magnetization of soft iron bar, together with the magnetic field in the original coil, greatly increases the total magnetic field strength, so the magnetic force of electromagnet is greater than that of natural magnet.

(3) The larger the current of the spiral coil, the more coils, and the stronger the magnetic field of the electromagnet.

Application of electromagnet

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（1） Crane : It is a powerful electromagnet for industrial use, which can be used to lift steel plates, containers, scrap iron, etc. with high current.

(2) Telephone: Introduction in the next section.

(3) Ammeter, voltmeter, galvanometer

(4) Bell and so on.

（5） Automatic control equipment

(6) Industrial automation control, office automation.

(7) Packaging machinery, medical equipment, food machinery, textile machinery, etc.

（8） electromagnetic relay

(9) Maglev train

Manufacturing principle

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1. The circular coil leads to the magnetic field formed by the current

(1) The direction of the magnetic field at the center of the coil can regard a small section of wire on the coil as a straight line Right-handed rule Judge.

(2) The magnetic field generated by each small segment of current on the circular coil with current is pointing in the same direction in the coil, so the magnetic field in the coil is stronger than that generated by the current of the straight wire.

(3) When the circular conductor is connected with current, the magnetic field outside the coil is not in the same direction as the magnetic field generated by each small segment of current, so the resultant magnetic field generated is weaker than the magnetic field inside the coil.

(4) The larger the current and the smaller the radius of a circular coil, the greater the magnetic field intensity at the center of the coil.

(5) The magnetic force lines of circular coil and disc thin magnet are similar in shape.

2. Magnetic field of spiral coil current

(1) Winding a long wire coil into a spiral shape with a long wire is equivalent to connecting many circular coils in series. The magnetic field created by each circular wire at the center is in the same direction, which can enhance the effect. Therefore, the magnetic field at the center of the coil is stronger than that of a single turn circular coil.

(2) The magnetic force lines inside the coil form a straight line in the same direction, and the magnetic force lines at about both ends of the coil gradually bend outward.

(3) The magnetic line of force of spiral coil is similar to that of bar magnet, and the magnetic line of force inside the coil is just opposite to that outside the coil.

(4) The strength of the magnetic field in the coil is proportional to the current on the coil and the number of coils per unit length.

3. The magnetic field direction in the spiral coil current Right hand spiral rule (Ampere theorem): Hold the coil with your right hand, point four fingers to the current direction, and the direction pointed by your thumb is the direction of the magnetic force line in the coil.

Cause of loss of excitation

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The generator is not used for a long time, resulting in the loss of residual magnetism contained in the iron core before leaving the factory, and the excitation coil cannot establish the proper magnetic field. At this time, the engine runs normally but cannot generate electricity. This phenomenon is a new machine. Or there are many units that have not been used for a long time.

Treatment method: 1) press the excitation button if there is an excitation button, 2) use the battery to magnetize the button if there is no excitation button, 3) carry a light bulb load and run for several seconds at overspeed.

Magnetic energy source

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Low axial resistance alternator Although only about 50% of negative torque magnetic energy can be converted into positive torque magnetic energy in principle design, the generated positive torque is enough to offset the negative torque (because it is impossible to convert all negative torque magnetic energy into positive torque magnetic energy in fact).

After further research and analysis on the structure and working principle of conventional generators, we finally found a breakthrough, which is to use the "energy buffer transfer method" on the basis of conventional power generation principle structure to achieve the above purpose; That is, part of the induced current in a fixed direction is temporarily stored and then released within the lag time. The released energy can not only continue to be output to supply the load, but also the additional magnetic energy generated in the armature freewheeling winding can also do positive work (generate positive torque) to the rotor. This is the source of positive torque magnetic energy of low shaft resistance generator.

Loss of excitation hazard

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alternator Loss of excitation fault means that the excitation of the generator suddenly disappears completely or partially. The causes of loss of excitation are: rotor winding fault, exciter fault, false tripping of automatic de excitation switch, damage of some elements in semiconductor excitation system or circuit fault and misoperation.

Due to asynchronous operation, the mechanical speed of the generator rotor is greater than the synchronous speed. Due to the slip, the stator winding current increases, and the rotor winding generates induced current, causing additional heating of the stator and rotor windings. The analysis shows that the loss of excitation of the generator will cause different degrees of harm to the power system and the generator itself, which can be summarized in the following aspects.

Hazards to the generator:

(1) After the loss of excitation of the generator, the magnetic leakage at the end of the stator increases, which makes the end parts and the end iron core overheat.

(2) After asynchronous operation, the equivalent reactance of the generator decreases. As a result, the reactive power absorbed from the system increases, causing the stator winding to overheat.

(3) The differential frequency current in the generator rotor winding will cause additional loss in the rotor winding, which will cause the rotor winding to heat up.

(4) For large direct cooling type turbogenerator The maximum value of the average asynchronous torque is small, the inertia constant is relatively reduced, and the rotor is obviously asymmetric in vertical and horizontal axes. For these reasons, the torque and active power of the de excitation generator will swing violently under heavy load. This impact on Hydro generator More serious.

Hazards to power system:

（1） alternator After the loss of excitation, due to the swing of active power and the decrease of system voltage, it may lead to the loss of synchronization between adjacent normal generators and the system, resulting in system oscillation.

(2) The loss of excitation of the generator causes the lack of a large number of reactive power in the system. When the reactive power reserve in the system is insufficient, the voltage will drop. In serious cases, it will cause voltage collapse and system collapse.

(3) Loss of excitation of one generator causes voltage drop, and other generators in the system are automatically regulating Excitation equipment Under the action, its reactive output will be increased. So that some generators, transformers and transmission lines are overcurrent, Backup protection The fault range may be expanded due to overcurrent action.

Comparison with permanent magnet

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Permanent magnets and electromagnets can be manufactured to produce different forms of magnetic fields. When choosing a magnetic circuit, the first thing to consider is the work you need the magnet to do. Permanent magnets are dominant in situations where electricity is inconvenient, power is often cut off, or there is no need to adjust the magnetic force. Electromagnets are useful for applications requiring magnetic force changes or remote control. Magnets can only be used in the way originally intended. If the wrong type of magnet is used for a special purpose, it may be extremely dangerous or even fatal.

Many machining operations are carried out on heavy block materials, and these applications require permanent magnets. Many machine factory users believe that the greatest advantage of these magnets is that they do not need electrical connections.

The permanent magnet features a lifting capacity of 330~10000 pounds, and the magnetic circuit can be connected or disconnected only by rotating one handle. The magnet is generally equipped with a safety lock to ensure that the magnet will not be accidentally disconnected during lifting. The magnet group can be used for long loads that are heavier and cannot be handled by a single magnet.

Also, in many cases, the parts to be machined are very thin (0.25 inch or more), and must be extracted from a pile of similar parts. Permanent magnets are not suitable for lifting only one piece from a pile of parts at a time. Although the permanent magnet is extremely reliable when used correctly, it cannot change the magnetic force. In this respect, the electromagnet enables the operator to control the magnetic field strength through the variable voltage control device, and can select one from the stacked parts. Self contained electromagnet is the most cost-effective magnet according to unit lifting capacity, and its lifting capacity can be extended to 10500 pounds.

Magnets powered by batteries are useful and they are self-contained Gel battery Increase lifting capacity, and can handle flat, round and component shaped products. The magnet powered by the battery can repeat the lifting action, providing considerable lifting capacity without external power supply.

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