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French
this page,  physics - electromagnetism

 Coils, electromagnetic field.   Electromagnetic power generators. - Electromechanical applications - Current converters. - Gigantism.


translation French/ English : sens= direction, bobine=coil, courant=current

Solenoid: it is a succession of electrical windings (turns).The real coil below is an example and we can see, in yellow, the beginning of a winding (one turn). The number of turns and layers can be very large (for example, 300 revolutions per cm length). The copper wire is insulated with a varnish - and not a rubber sheat - to limit any additional thickness. The wire diameter can range from one hundredth of a millimeter to several millimeters.

Any coil powered by a direct current creates two opposite magnetic poles + and -

Any coil powered by an alternating current creates alternating opposite magnetic poles + then - that are changing of polarity ( - then + ) etc.

With the 115/230 volt alternating electric current, this appears 50 times per second.


This coil carrying an electric current creates a strong electric field that will get the iron rod "swallowed"  in his hole, presented here completely removed to clarify the explanation.  With a current direction according to red arrow, the North pole of the coil is created by the magnetic flux directed as the large white arrow. 

Theses two movements match the progress of a corkscrew introduced at right hand (blue wire side).

This created pole N induces a pole south in the close iron rod  ("the" s "white),
Nota : a spring take the rod back when the the power is cut (here, 24 volts).

When the iron rod enter the coil, the calculated magnetic field tesla T that existed in the air (vacuum hole) is multiplied by the permeability μ, of the ferrous metal and becomes more important.

There are no magnetic poles in a closed magnetic circuit, but a continuous oriented field that can be used in the vacuum center.



Fields produced by coils : some idea on what is acting :

- the number of turns of copper wire,

- the intensity of the current flowing in the coils.

- the diameter and length of the coil with the following consideration: we only consider the length of a long coil (ten times its radius, for example), and only the diameter for a coil short ! (not always obvious). Finally, a factor is applied.

Any variable magnetic flux induces unwanted currents  in the metal frame that counter the initial variation.

They fight against this drawback by stacking sheets of isolated metal instead of using solid one, that breaks the circuit of "bad" current. This gives a striated metal (right hand transformer picture).

hibis Note. For precise calculations and complex cases, search in Internet works and mathematical proofs published on this subject.)

 

- Air Gap. It is an air space in a magnetic circuit that creates two opposite poles, and consequently an magnetic airf field, as a horseshoe magnet.

This contactor is composed of a magnetic coil  (yellowish "bobine") with its central round iron bar (noyau).  The coil is surrounded by a metallic circuit   open by a an hinged piece; an air gap, highlighted in yellow, is open without electric power; this hinged piece is attracted when the power is on, an push then an electric contact, or acting on another mechanic gear.

Magnetic circuit. This is usually the piece of metal or equivalent permeable to the flux, it captures and contains the magnetic flux rather than the surrounding air
A coil of wire is usually wrapped around by metallic pieces (converter, electro-magnets, inductors, etc.) This gap must be reduce because the magnetic fields that fancy for escapades in the open air.
Above contactors are used to switch on and off a remote circuit that need short high diameter cupper wire : it is the one you shut when turning the ignition key of your car, by a thin wire remote control. This contactor put then the power on the electric starter gear.




Some application of electromagnetism.
There has been the first great invention to communicate over long distances, immediately and out of sight : the telegraph.  At the departure station, a controller can swich on and off a current, which is received at the end of the wire on an electromagnet ; a stylus to draw dots and dashes (Morse method) ; for a variant still valid, sounds are received through a headphone.

Engines and generators have revolutionized the society (lighting, industrial, domestic) but also the measuring devices, contactors, circuit breakers, electromagnetic relays (factories, power plants, power supply cabinets, previous generations of automatic telephony, servomechanisms, transformers. The phone headset, speakerphone and radio HI FI; the atria of our players and MP3 players are always based on the principle of the electromagnet which makes vibrate a membrane. There were timers in staircase : a pendulum was launched by a rod "sucked" in the coil of an electromagnet when pressing the button, then swaying a few minutes; light extinction coming at its stopping.
The telephone ring or station ring stem also from the electromagnet, as gongs and chimes door did. Switches for railways, Industrial trucks and high speed trains with linear motor (Germany, Japan);  capture (petanque balls, screws, needles on the ground, steel particles in oil car), sorting industrial metals (I know a who has always him on a small magnet on his door keys); cranes for clutching scrap metal, industrial machinery, closing control lock, remote control for model aircraft or car doors locking, and in the field of ultra precision electronics, a whole generation of cassette players, CD and DVD; disk drives, video recorders. Let us not forget the scientific field (EMF) and medical too (magnetic resonance imaging (MRI). These are just a few examples.  

 

Electric transformer/converter
It only works with alternating current, for the magnetic fiels must to change quickly . What would we do without it?  there are everywhere, including radios, ancient and modern as it is often necessary to adapt the diffrent powers or voltage. Well  known are tranformers/converters we connect to an outlet to lit an halogen lamp or reload all kinds of cell phones, players, radios, etc.. There are throughout the house. Some of them are dedicated to safety, in wet areas or in the bathrooms for electric razors. In these cases, it does not alter the current (ratio 1), but serves to isolate the main current from the socket with its dangerous connexion with the earth.
The largest are those of the electrical distribution system. The internal insulation of the windings in this case is provided by fluid fat (oil), which also serve to cool. The first oil used has been banned because of its harmfulness.

The best transformer is basically composed of a metal ring closed (called torus, left hand picture), around which are wrapped two types of wires:

- A coil of cupper wire C1 (very long and thin) called primary, is connected to the power tension (eg 227 volts) ; its electric current  produces then a magnetic flux in the metal circuit.

 - This magnetic field induces a current - eg 12 volts - in another coil of cupper wire C2 ( relatively short and thick, called secondary. The power consumed is linked to the magnetic field produced, i.e to the primary number of turns n1 as well as the intensity of the primary current  i1.

The power produced is linked to the magnetic field received from the primay coil (n1, i1), according to the secondary  (n2, i2)

If n1 is ten times n2, i2 is ten times i1

The torus transformer (ring) is the best because it reduces the loss of flux (who do not like corners ..). But it is expensive to produce and use its power to the towers, is reserved for amplifiers HI FI superior (less than magnetic losses, less noise). Current transformers are all like "angular" ie rectangular, with two coils stacked plants (right hand picture).

Whether the transformer is a toris or a basic one, nothing is perfect, but the out power is less with the toris. Best of them can limit the loss at 2 or 3 percent, i.e a restitution ratio of 97/98 %.


Electromagnetic power generators
Like batteries, alternators and dynamos produce electricity. Their size varies greatly   from the miniature record passing through the eye of a sewing needle (which works!) Than electric dams, the principle remains the same, and there is little easier:

While approaching a magnetized needle close to a wire carrying a current, and seeing her turning, the physicist œrsted discovered that an electric current creates a magnetic field.
Practical application ; they can measure the intensity passing through a 220 volts AC wire with a clip shaped like two half coils that are tightened around the wire. The magnetic field of the wire induces a current in the coil which can be measured.

Conversely, if you pass the tip of a magnet in front of an electric wire, a tension (voltage) is created. ;  Better could be a coil. but you hardly could make a bulb light ! the most sensitive would be a meter or a LED.
Our bikes dynamos are alternators Alas of poor quality. A rotor magnet is actuated by the toothed wheel that rubs against the tire.
The rotor comprises here a set of 4 magnets (N and S) distributed around of which the magnetic flux "sweep" the wires of the armature coil, also called stator as opposed to the rotor. This spinning magnetic flux creates a current in the armature (inducing.
In reality, the armature coil is placed in a metallic part which forms 8 magnetic poles, (4 magnets) so that the flux received by the rotating magnetic armature is stronger. Indeed, a coil in the air does not concentrate a magnetic flux passing through it, while a metallic mass or better, a ferrite composed of agglomerated powder highly permeable to magnetism, concentrates highly this flux.
They put some of these ferrites at the ends of power cables connecting electronic devices (PC, players etc..), in the aim to protect them against unwanted electrical currents that would disrupt the cicuits. We see clearly a strong cylindrical bulge near the connectors).

 Dynamos and alternators require a mechanical power just over the electrical power they deliver (because of losses due to friction and heat). They are the only ones able to produce very high power.
As a matter of fact, nuclear plants dont produce electrity but heat !! they are steam plants !



The BIGGEST electromagnets. In September 2008, the largest particle accelerator in the world, the Large Hadron Collider or "LHC" is started at CERN* , the French-Swiss border. Located at l00 feet underground and 27 km long, the LHC is the largest particle accelerator in the world. It has two tubes in a high vacuum in which are sent particle beams. The beams are guided by a powerful electromagnetic field generated by some 1600 magnets. Aligned with a precision of tenths of a millimeter, each of the 1232 main magnets is 15 meters long and weighs 35 tons! Rarity among accelerators, these magnets are superconducting, ie their windings conduct electricity without resistance, up to 13,000 amperes, or loss of energy. This property appears only at very low temperatures. The magnets are placed in an enclosure in which circulates superfluid helium *, allowing them to cool to - 271.3 ° C. It takes two months to cool one of eight sectors. Equipment ultra complex are reminiscent of science fiction interplanetary: would it really? the exorbitant cost him, is real.
CERN is the European Organization for Nuclear Research. "Research in October 2009."

Although less numerous, but 10 times heavier!

ITER is an abbreviation for "International Thermonuclear Experimental Reactor" (in French). For a temperature of 150 million degrees in the reactor core, containing a plasma of deuterium and tritium, and allow the merger to occur, superconducting electromagnets 18 to 360 tons each, produce an electromagnetic field completely insane. "Research" No. 437 in January 2010. The billions of euros fly, 10, then 20, then .. it does not even know what material to resist enough to ensure sealed.

Giant Alsthom alternators for hydraulic dam. The rotor (inductor) pictured below left, has coils powered by electric current  (Permanent magnets are inadequate for very high power).
The current for transportation and industry requires four or three wires : for example, three wires "active" so-called "phase" and another more called "neutral" linked to the ground and which take back the current to the generator. Coils are constructed accordingly.

If the principle of current production is simple, everything is unfortunately complicated in the wires arrangement that is knitting mesh ; that is for different reasons, learned from  experience and calculs. It is particularly important that each driver bath in the flux producted between each pole inductor, and no other parts of the wire cable are induced by a reverse currents, they are to the fight against noisy currents induced, fight against distortion fields generated by magnetic fields opposed by the wires "induced" themselves.


Giant alternator driven by hydraulic turbines. Here the "armature" (inductive), which produces the electricity under the effect of a "scanning" magnetic fields produced by the rotor (inductor).

 Note: theses view are distorted by perspectives The rotor  (inductor) rotates inside the armature. Composed of magnetic poles) (solenoids), it must be supplied with electricity.  The rotor then emits electric fields that rotate with it, sweeping the wires of the armature.
La Recherche. Alsthom (Montbéliard is the world's Number 1 hydropower.  Its turbines and generators have been saved from the purchase by Siemens.

The static part around the rotor core, generally said armature (inductive), is which that produces the current.
The current is "induced" because its copper wires are swept by the magnetic fields of the rotor, said inductor, which is spinning inside. We can see here the ends of copper wires that look like much more rods than usual wires. Everything is huge here : of which 160 kilometers of copper rods perfectly arranged and firmly bond.

right,hand a turbine for the Vietnam Hall of hydraulic turbines and generators of the Three Gorges Dam (China). 15 out of 26 are produced by Alsthom. Embedded in the dam, a whole unit is as tall as a building of 10 storeys. and meters 21 in diameter. At the bottom of each well is placed a water turbine (with wings), which is operated by the torrent of water from the dam. Above its axis make spinning the alternator rotor, surrounded by its armature (see left for armatures and inductors).

Unfortunately, half are only running (February 2010) due to the shortage of water in the Yangzi !  and some problems arose also for the bank.

An alternator is perfect to produce both industrial and domestic alternating current (380 and 220 volts), transported under high and medium voltage (respectively hundreds of thousands and then tens of thousands of volts). Transformers reduce tensions in fairly good conditions (relative simplicity, good performance).

Alternators have been fitted out for long our cars. Their size is about twenty cm. Formerly, they were dynamos, weaklings and fragile, with delicate settings. The advent of alternators has been an enormous progress : the alternator can charge high, even at idle. So, they could increase the electrical equipment and replace some mechanical assistance or depression by electrical ones (including gear assistance).

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