Electromagnet

   An electromagnet is a device that is energized to generate electromagnetism. A conductive winding that matches its power is wound on the outside of the iron core. This current-carrying coil is magnetic like a magnet, and it is also called an electromagnet. We usually make it into a bar or hoof shape to make the core easier to magnetize. In addition, in order to demagnetize the electromagnet immediately after power off, we often use soft iron or silicon steel material that demagnetizes faster. Such electromagnets are magnetic when energized, and disappear when the power is turned off. The electromagnet is widely used in our daily life, and the power of the generator has been greatly improved due to its invention.

Overview

When the iron core is inserted inside the energized solenoid, the iron core is magnetized by the magnetic field of the energized solenoid. The magnetized iron core also becomes a magnet, so that the magnetism of the solenoid is greatly enhanced due to the superposition of the two magnetic fields. In order to make the magnetism of the electromagnet stronger, the iron core is usually made into a hoof shape. However, it should be noted that the winding direction of the coil on the shoe-shaped iron core is opposite, one side must be clockwise, and the other side must be counterclockwise. If the winding directions are the same, the magnetization of the two coils on the iron core will cancel each other out, so that the iron core is not magnetic. In addition, the core of the electromagnet is made of soft iron, not steel. Otherwise, once the steel is magnetized, it will remain magnetic for a long time and cannot be demagnetized, so the strength of its magnetic properties cannot be controlled by the magnitude of the current, and the advantages of electromagnets will be lost.

An electromagnet is a device that can generate a magnetic force by passing an electric current. It is a non-permanent magnet and can be easily activated or deactivated. Example: Large cranes use electromagnets to lift abandoned vehicles.

When current flows through a wire, a magnetic field is created around the wire. Using this property, when an electric current is passed through a solenoid, a uniform magnetic field is created within the solenoid. Assuming that a ferromagnetic substance is placed in the center of the solenoid, the ferromagnetic substance will be magnetized and the magnetic field will be greatly enhanced.

Generally speaking, the magnetic field produced by an electromagnet is related to the magnitude of the current, the number of coil turns, and the ferromagnet in the center. When designing electromagnets, attention is paid to the distribution of coils and the selection of ferromagnets, and the magnitude of the current is used to control the magnetic field. The electrical resistance of the coil material limits the magnitude of the magnetic field that an electromagnet can generate, but with the discovery and application of superconductors, there will be opportunities to go beyond existing limits.

Direction Judgment

The direction of the magnetic field of an electromagnet can be determined using Ampere's law.

Ampere's law is a law that expresses the relationship between the direction of the magnetic field lines of the current and the magnetic field excited by the current, also known as the right-hand spiral law.

(1) Ampere's rule in the energized straight wire (Ampere's rule 1): Hold the energized straight wire with the right hand, with the thumb pointing in the direction of the current, and the four fingers in the direction of the magnetic field lines around the energized straight wire.

(2) Ampere's rule in the energized solenoid (Ampere's rule 2): Hold the energized solenoid with the right hand, so that the four fingers are bent in the same direction as the current, then the end pointed by the thumb is the energized solenoid the N pole.

Advantage 

    The electromagnet has many advantages: the magnetism of the electromagnet can be controlled by on and off the current; the magnitude of the magnetism can be controlled by the strength of the current or the number of turns of the coil; the magnitude of the magnetism can also be controlled by changing the resistance to control the magnitude of the current. ; its poles can be controlled by changing the direction of the current, etc. That is, the strength of magnetism can be changed, the presence or absence of magnetism can be controlled, the direction of magnetic poles can be changed, and magnetism can disappear due to the disappearance of current.

Electromagnet is an application of current magnetic effect (electricity-generated magnetism), which is closely related to life, such as electromagnetic relays, electromagnetic cranes, maglev trains, electronic door locks, intelligent channel turns, electromagnetic flow meters, etc.

Classification

    Electromagnets can be divided into two types: DC electromagnets and AC electromagnets. If the electromagnets are divided according to their purposes, they can be mainly divided into the following five types: (1) Traction electromagnets—mainly used to pull mechanical devices, open or close various valves, and perform automatic control tasks. (2) Lifting electromagnets - used as lifting devices to lift ferromagnetic materials such as steel ingots, steel materials, and iron sand. (3) Braking electromagnet—mainly used to brake the motor to achieve the purpose of accurate parking. (4) Electromagnetic systems of automatic electrical appliances - such as electromagnetic systems of electromagnetic relays and contactors, electromagnetic trippers of automatic switches and operating electromagnets, etc. (5) Electromagnets for other purposes - such as electromagnetic chucks of grinders and electromagnetic vibrators.

History

    As early as the spring of 1820, Oersted in Denmark discovered this principle by chance. In 1822, French physicists Arago and Lussac discovered that when an electric current passes through a winding with iron in it, it can magnetize the iron in the winding. This was actually the original discovery of the electromagnet principle. In 1823, Sturgeon also made a similar experiment: he wound 18 turns of bare copper wire on a U-shaped iron rod that was not a magnet rod. The copper coil on the iron bar generates a dense magnetic field, which turns the U-shaped iron bar into an "electromagnet". The magnetic energy on this electromagnet is many times larger than that of the permanent magnet, and it can absorb iron blocks 20 times heavier than it. When the power supply is cut off, the U-shaped iron rod can not absorb any iron blocks, and becomes a new one. An ordinary iron rod.

Sturgeon's invention of the electromagnet made people see a bright prospect of converting electrical energy into magnetic energy, and this invention soon spread in the United Kingdom, the United States and some coastal countries in Western Europe.

In 1829, the American electrician Henry made some innovations to the Sturgeon electromagnet device, replacing the bare copper wire with a magnetoelectrically insulated wire, so there was no need to worry about being too close to the copper wire and short-circuiting. Since the wires have an insulating layer, they can be tightly wound together in circles. Since the denser the coils, the stronger the magnetic field generated, which greatly improves the ability to convert electrical energy into magnetic energy. In 1831, Henry trial-produced a newer electromagnet, which, although not very large, could pick up a 1-ton iron block.

Inspired by the Oersted current magnetic effect experiment and a series of other experiments, Ampere realized that the essence of magnetic phenomena is current, and attributed various interactions involving current and magnets to the interaction between currents, and proposed the search for current Fundamental questions about the law of meta-interaction. In order to overcome the difficulty that the isolated current element cannot be directly measured, Ampere carefully designed four zero-display experiments accompanied by meticulous theoretical analysis, and obtained the results. However, due to Ampere's concept of action at a distance for electromagnetic action, he imposed the assumption that the force between two current elements is along the connection line in the theoretical analysis, expecting to obey Newton's third law, which made the conclusion wrong. The above formula is the revised result after discarding the assumption that the wrong force is along the connecting line. It should be understood from the point of view of proximate action that a current element generates a magnetic field, and the magnetic field exerts a force on another current element in it.

Nature

    Ampere's law for straight-line currents also applies to small sections of straight-line current. The annular current can be regarded as the composition of many small linear currents, and the direction of the magnetic induction intensity on the central axis of the annular current is determined by the Ampere's law of the linear current for each small linear current. The superposition gives the direction of the magnetic field lines on the central axis of the annular current. Ampere's law of linear current is basic. Ampere's law of toroidal current can be derived from Ampere's law of linear current. The ampere's law of linear current is also applicable to the magnetic field generated by the linear motion of electric charge. At this time, the direction of current is the same as the direction of movement of positive charge. The same, with the opposite direction of negative charge movement.

Application of electromagnet

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

(2) Telephone: introduced in the next section.

(3) ammeter, voltmeter, galvanometer

(4) Bells, etc.

(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

Ampere force

The force df12 of the current element I1dι on another current element I2dι at a distance of γ12 is:

μ0 I1I2dι2 × (dι1 × γ12)

df12 = ── ────────────

4π γ123

In the formula, the direction of dι1.dι2 is the direction of current; γ12 is the radial vector from I1dι to I2dι. Ampère's law can be divided into two parts. One is that the magnetic field generated by the current element Idι (that is, the above-mentioned I1dι) at γ (that is, the above-mentioned γ12) is:

μ0 Idι × γ

dB = ── ──────

4π γ3

This is Pissar's Law. The second is the force df (that is, the above-mentioned df12) that the current element Id1 (that is, the above-mentioned I2d12) receives in the magnetic field B is:

df = Idι × B

Principle

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 be regarded as a straight line with a small section of wire on the coil, which is determined by Ampere's right-hand rule.

(2) The magnetic field generated by each small 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 the magnetic field generated by the straight wire current.

(3) When the circular wire passes through the current, the magnetic field outside the coil is not in the same direction due to the direction of the magnetic field generated by each small current, so the resulting combined magnetic field is weaker than the magnetic field inside the coil.

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

(5) The magnetic field lines of the circular coil and the disc-shaped thin magnet are similar in shape.

2. Magnetic field of helical coil current

(1) A long wire is wound into a long spiral coil, which is equivalent to a series of many circular coils. The magnetic field established by each circular wire at the center is in the same direction, which can enhance the effect, so the coil The magnetic field at the center is stronger than a single-turn circular coil.

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

(3) The characteristics of the magnetic field lines of the helical coil are similar to those of the bar magnet, and the magnetic field lines in the coil are opposite to the outer direction of the coil.

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

3. The right-handed spiral rule for the direction of the magnetic field in the current of the helical coil (Ampere's theorem): Hold the coil with the right hand, with the four fingers pointing in the direction of the current, and the direction pointed by the thumb is the direction of the magnetic field lines in the coil.

Cause of loss of magnetism

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

Treatment methods: 1) Press the excitation button if there is an excitation button, 2) If there is no excitation button, use the battery to magnetize it, 3) Load a light bulb and run it at overspeed for a few seconds.

Magnetic energy source

 Although the low shaft resistance generator can only convert about 50% of the negative torque magnetic energy into positive torque magnetic energy in principle design, the positive torque generated is also enough to offset the negative torque (because it is actually impossible. Convert all negative torque magnetic energy into positive torque magnetic energy).

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 the conventional power generation principle structure to achieve the above purpose; After the current is temporarily stored, and then released within the lag time, the released energy can not only continue to output and supply the load, but also the additional magnetic energy generated in the armature freewheeling winding can also do positive work on the rotor (generating positive rotation). moment). This is where the positive torque magnetic energy of the low shaft resistance generator comes from.

Demagnetization Hazard

 Generator loss-of-excitation fault refers to the sudden disappearance or partial disappearance of the excitation of the generator. The causes of loss of field are: rotor winding failure, exciter failure, automatic de-excitation switch erroneous trip, damage to some components in the semiconductor excitation system or circuit failure and misoperation.

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

Hazards to the generator itself:

(1) After the generator loses its magnetism, the magnetic flux leakage at the end of the stator increases, which makes the end components and end iron cores overheated.

(2) After asynchronous operation, the equivalent reactance of the generator is reduced, from to . As a result, the reactive power absorbed from the system increases, overheating the stator windings.

(3) The difference frequency current that appears in the rotor winding of the generator produces additional losses in the rotor winding, causing the rotor winding to heat up.

(4) For large direct-cooled turbo-generators, the maximum value of the average asynchronous torque is small, the inertia constant is relatively low, and the rotor is obviously asymmetrical in the longitudinal and transverse axes. Due to these reasons, the torque and active power of the demagnetized generator will swing violently under heavy load. This effect is more serious for hydro-generators.

Hazards to the power system:

(1) After the generator loses its magnetism, due to the swing of active power and the reduction of the system voltage, the synchronization between the adjacent normal running generators and the system may be lost, causing the system to oscillate.

(2) The loss of magnetism of the generator causes a large amount of reactive power shortage in the system. When the reactive power reserve in the system is insufficient, the voltage will drop. In severe cases, the voltage collapses and the system collapses.

(3) A generator loses its excitation and causes the voltage to drop, and other generators in the system will increase their reactive power output under the action of the automatic adjustment excitation device. As a result, some generators, transformers, and transmission lines are overcurrent, and the backup protection may act due to overcurrent, expanding the fault range.

Compared with permanent magnets

  Both 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 job you need the magnet to do. Permanent magnets are dominant in situations where electricity is inconvenient, where power outages are frequent, or where it is not necessary to adjust the magnetic force. Electromagnets are useful for applications that require a change in magnetic force or remote control. Magnets can only be used in the way they were originally intended, and applying the wrong type of magnet for a particular purpose can be extremely dangerous or even fatal.

Many machining operations are performed on heavy, bulk materials and these uses require permanent magnets. Many users of machine shops believe that the greatest advantage of these magnets is that no electrical connection is required.

Permanent magnets feature 330 to 10,000 lbs of lift capacity and can be turned on or off with a single rotation of a handle. Magnets are generally fitted with safety locks to ensure that the magnets cannot be accidentally disconnected when lifted. Magnets can be used for long loads that are heavy and cannot be handled by a single magnet.

Also, many times the parts to be machined are very thin (0.25 inch or finer) and are drawn from a stack of similar parts. Permanent magnets are not suitable for jobs where you only pick one piece out of a pile of parts at a time. Permanent magnets, although extremely reliable when used correctly, cannot change the magnitude of the magnetic force. In this regard, the electromagnets allow the operator to control the strength of the magnetic field through a variable voltage control and to select one piece from a stack of parts. Self-contained electromagnets are the most cost-effective magnets per unit of lift capacity, which can extend up to 10,500 lbs.

Battery powered magnets are useful, they use self-contained gel batteries for increased lift capacity, and can handle flat, round, and member-shaped products. Battery powered magnets can repeat the lifting action, providing considerable lifting capacity without an external power source.