Electromagnetic field

 

The electromagnetic field is a physical field that corresponds to the degree of electromagnetic force in space and consists of a magnetic field and an electric field, which are interconnected. The electric current creates a magnetic field in that area of ​​the charged body, which affects the movement of bodies around the charged body. An electric voltage is induced in a conductor that moves in a magnetic field and thus the conductor (current flows through it) can move in a magnetic field = a certain force acts on it.

The electric field cannot be separated from the magnetic field, because they are related to each other and thus we speak of the electromagnetic field.

Maxwell's equations (they were published in 1873 by the theoretical physicist James Clerk Maxwell) are the basis of the description of the electromagnetic field, they are natural laws that do not need to be derived from other laws, and other equations of the electromagnetic field can be derived from them.

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The magnetic field of the toroid

 

A toroid is a coil, the same turns of which are wound on a ring, and we can think of it as a solenoid, wound into the shape of a ring. It is a special case of a coil without magnetic poles. Its induction lines run only inside the ring as concentric circles and the created magnetic field is almost homogeneous.

 

Fig.1Application of Ampere's law for a toroid

 

Electromagnetism

Electric charge is the most basic object of electromagnetism. Electric charge (if we take it as the basic term for describing electromagnetism) is so fundamental that we can only talk about its properties.

The basic propertiesof electric chargeinclude the following statements:

• in nature, there aretwo types of electric charge, positive (marked witha + sign ) and negative (marked with a – sign),
• electric charges act on each other repulsively (charges with the same sign repel each other ) or attractively (charges with the opposite sign attract ),
• the law of conservation of electric charge applies (it means that the charge of the universe is constant),
• electric charges occurring in nature  are integer multiples of the basic quantum of electric charge with an absolute value of e = 1.602 176 462. 10-19 C.

Electromagnetism , as a part of physics that studies the connections between electrical and magnetic phenomena, did not exist. Until the end of the 18th century, electric and magnetic energy were studied  separately , because no mutual effects were observed between the phenomena. Various important mathematicians participated in building the theory of electromagnetism , but it was James Clerk Maxwell who created the pioneering theory of electromagnetism and formulated the four laws of electromagnetism = Maxwell's equations . 

 

Electromagnetic induction

 

Michael Faraday  (1791 – 1867) devoted himself to electromagnetism and from the knowledge of Hans Christian Oersted and André Marie Ampére , who proved with their experiments that the presence of a magnetic field causes the flow of an electric current (directed movement of an electric charge), he logically assumed that this analogy also applies the other way around, i.e. that the source of electric current can be a magnetic field . The reasoning was the basis of many attempts.

Fig. 2 Generation of induced electric current

(coil connected to galvanometer)

 

Electromagnetic induction  is a phenomenon in which an induced electromotive voltage and an induced current occur in a conductor as a result of a time change in the magnetic induction flux.

A change in the magnetic flux induction can occur: 

• by changing the magnetic induction,
• by changing the content of the area,
• by changing the angle between the surface normal and the induction lines.

 

Electromagnet

 

An electromagnet  is an electromechanical device that changes electrical energy into work = mechanical energy, i.e. it outputs a certain force. It is part of all switching, fuse and protection devices. If frequent switching and small strokes are involved, the electromagnet is suitable for controlling these switching machines.

 

Use of electromagnets

 

The use of the electromagnet  is mainly for the development of smaller forces (it is related to the expansion in the contactors) but also if it concerns applications in heavy industry or electrical machines . The lifting time of the electromagnet armature is small, thus the amount of work performed is also small, and therefore the amount of energy is not essential, but its pulling force , which acts during the lifting time.

The static pulling characteristic  is the dependence of the pulling force on the armature lift, which expresses the properties of  DC  or AC electromagnets. Anchor tightening time is also an important parameter. When supplying the electromagnet with direct current, the current does not change in the steady state, nor does the voltage of the coil, and one electrical parameter of the excitation winding always remains constant. From the shape of the electromagnets, the size of the air gap and the type of power supply = the shape of the static pull characteristic . 

 

Distribution of electromagnets

 

Electromagnets can be divided into:

According to current:

• direct current electromagnets,
• electromagnets for alternating single-phase current,
• electromagnets for alternating three-phase current.

According to use:

• motion electromagnets – mechanical force is produced by the movement of the armature (switches, brakes, valves...),,
• holding electromagnets – they hold ferromagnetic material (clamping, sorting cylindrical, load...),,
• special electromagnets (electromagnetic couplings...). 

 

Principle of operation of electromagnets

 

The electric current, which is the source of the magnetic field, passes through the excitation coil and windings, where it creates a magnetic voltage , which pushes the magnetic flux through the magnetic circuit = a magnetic field with a certain magnetic intensity and a certain magnetic flux is created. 

The magnetic flux  is directly proportional to the current flowing, which means that the greater the current or the greater the number of turns of the excitation coil, the stronger the magnetic field will be. A magnetic field acts on the armature of the electromagnet and develops a certain attractive force acting on the armature (ferromagnetic material). If the current of the magnetic field is interrupted and the force exerted by it is interrupted, the armature will fall back to its original position.

 

Construction of electromagnets

 

The electromagnet has a simple construction consisting of an excitation coil , a movable armature and a fixed ferromagnetic core .

Fig. 3 Structural parts of an electromagnet

 

Exciting coil  - can have a single-layer or multi-layer winding. The number and shape of the air gaps depends on the shape of the magnetic circuit and the coil. The armature , located inside the coil, can be attracted to the landing surface or the electromagnet can attract it to the excitation coil. The magnetic circuit is realized with regard to the economy of operation with acceptable dimensions and weight so that the magnetic flux mostly (or completely) passes through the ferromagnet, which has a very high magnetic conductivity. Individual types of electromagnets (trying to ensure that the air flow is only in the working gap of the electromagnet) have different static tensile characteristics, on the basis of which the correct type of electromagnet is then selected for the given application. 

The core  and  armature  are mostly formed in DC electromagnets (the composition of the magnetic circuit is also made of several sheets) of gaseous ferromagnetic material, in AC electromagnets – insulated transformer sheets of ferromagnetic material (to reduce eddy current losses during alternating magnetization). Cast iron and steel (magnetically hard materials) are not recommended because of their residual magnetism, which persists even after the electrical current is interrupted.

The work of the electromagnet is affected by eddy currents , which reduce the speed of its attraction.   

 

DC electromagnets

 

A DC electromagnet (marked DC ) is made up of a magnetic circuit made of ferromagnetic material (although they have high conductivity and allow a strong magnetic field to be created, but permeability is dependent on magnetic induction), a moving armature and an excitation coil , which is powered by direct current. 

The core of  a DC electromagnet is usually made of solid material.

 

Advantages of DC electromagnet

• simple construction,
• silent operation
• in terms of current ratios and their effects on power ratios, dimensions and use of the magnetic circuit,
• the anchor does not need to reach the end position,
• the switching density is limited only by the armature pull-up and drop-out speed.

Disadvantages of DC electromagnet

• slower pull
• slower pull
• lower pulling force compared to alternating electromagnets,
• permanent (maximum) current during the entire time of attraction.

 

Alternating electromagnets

 

The excitation coil  of alternating electromagnets (designation AC ) is powered by an alternating current source. The current is determined by the resistance and the inductance of the coil, while the inductance is dependent on the position of the armature. 

The switching density  is limited by the density of the armature attraction and dropout and also by the maximum allowed warming. Their  power ratios  also depend on the degree of core saturation.

Advantages of alternating electromagnet:

• faster anchor pull.

Disadvantages of an alternating electromagnet:

• a magnetic circuit that consists of electrotechnical sheets,
• the anchor must be placed in the end position so that no vibrations occur,
• as a result of the vibration of the plates, there is a hum,
• when the armature is pulled, shocks occur due to the fact that the attracted current is many times greater than the rated current.

 

Used literature:

1. Adamovic Andrej, Bc., Analýza a optimalizácia vlastností elektromagnetu, Žilinská univerzita v Žiline, Elektrotechnická fakulta
2. Varga Radovan, Bc., Prípravok pre demonštrovanie účinkov elektromagnetickej indukcie, Technická univerzita v Košiciach, Fakulta elektrotechniky a informatiky
3. https://sk.wikipedia.org/wiki/Elektromagnetick%C3%A1_indukcia
4. https://sk.wikipedia.org/wiki/Elektromagnet
5. https://moodle.uiam.sk/pluginfile.php/7056/mod_resource/content/0/Kapitola_6/Kap6fA4str85-101.pdf
6. http://domes.spssbrno.cz/web/DUMy/ELE/VY_32_INOVACE_44-14.pdf
7. https://moodle.uiam.sk/pluginfile.php/7063/mod_resource/content/0/Kapitola_7/Kap7fA4str102-109.pdf