http://www.samsvojmajstor.com/portal/forums/uradi-sam/bravarija/mala-skola-zavarivanja Evo, po naredbi tenzora sinoc na chatu, i moj doprinos ovoj temi Princip rada transformatora i transformatorskih aparata za zavarivanje.

Transformator je elektrini ureaj koji transformie energiju iz jednog kola u drugo posredstvom magnetne sprege, bez ikakvih pokretnih delova. Transformator se sastoji od dva (ili vie) spregnuta namotaja ili jednog namotaja sa vie izvoda i, u veini sluajeva, magnetnog jezgra koje koncentrie magnetni fluks. Naizmenina struja u jednom namotaju e indukovati struju u drugim namotajima.

Transformatori se koriste da sputaju ili diu napon, da menjaju impedansu i da obezbede elektrinu izolaciju izmeu kola.

Najprostiji transformator se sastoji iz dva namotaja - primara i sekundara.

Odnos napona na primaru i sekundaru je direktno proporcionalan (srazmjeran) odnosu broja namotaja:

U1/U2=N1/N2Gdje je:U1 - napon na primaruU2 - napon na sekundaruN1 - broj namotaja na primaruN2 - broj namotaja na sekundaru

Dok su struje na primaru i sekundaru obrnuto proporcionalne (srazmjerne) broju namotaja

I1/I2=N2/N1I1 - struja kroz primarI2 - struja kroz sekundar

Da bi se omogucilo topljenje elektrode, kroz elektrodu mora proticati "jaka" struja (za 2,5mm elektrode 90-100A)Da bi se stvorio elektricni luk (koji topi elektrodu) potreban je odredjen napon (sto veci, to je jaci luk), napon opasan za covjeka je 50V, zato transformator u aparatima za zavarivanje obezbjedjuje smanjenje mreznog napona (220V) na 48V u praznom hodu (kada aparat miruje, sa pocetkom procesa zavarivanja ovaj napon se smanjuje).

Koristeci gore navedene formule, sad cemo izracunati odnose struja i napona na transformatoru u aparatu za zavarivanje.

U1/U2=N1/N2I1/I2=N2/N1Iz ovoga sljedi > U1/U2=I2/I1

U1 je mrezni napon 220VU2 mora biti 48VI2 cemo uzeti da mora biti 100A, znaci da samo fali da odredimo struju I1 koju transformator "povlaci" iz mreze.

220V/48V=I2/100A4,583=I2/100AI2=100A/4,583I2=21,81Naravno ovi proracuni se odnose na idealan transformator, a u praksi se uzimaju u obzir i razni gubitci.

Kako su za razlicite debljine elektroda, a i materijala koji se zavaruje potrebne razlicite struje, mora se napraviti neka vrsta regulacije.Dva su osnovna tipa:1. Regulacija pomicnim jezgrom (jedan dio jezgra transformatora se pomjera i time mjenja karakteristike jezgra, proracun je malo komplikovan i mislim da bi bilo previse da se tome posvecue previse paznje ovdje)

2. Regulacija izborom broja namotaja na primaru.Ovaj vid regulacije se izvodi tako sto se pri motanju transformatora na primaru ostave razni izvodi, pa se njihovim preklapanjem mjenja broj namotaja N1, sto dalje utice na odnos N1/N2, a samim tim i na odnose struja i napona.

I jos da se malo dotaknemo same prakticne konstrukcije.S obzirom da kroz sekundar teku struje reda 100A, zica kojom se mota sekundar mora biti odgovarajuce debljine.Profesionalni aparati za zavarivanje se UVIJEK motaju lakiranom bakarnom zicom, dok se za amaterske aparate sve vise upotrebljava aluminijumska zica u izolacionom plastu.I jedan i drugi tip namotaja imaju svoje prednosti i mane:

-Prednosti bakra su: Bolji provodnik (manji pad napona, pa samim tim i manje zagrijavanje), Bolje odvodi toplotu-Mane su mu: Veca cijena, veca masa.

-Prednosti aluminijuma su: Manja cijena, i manja masa-Dok su mane: Vise se zagrijava, slabije odvodi toplotu (zbog plasta kojim je obavijen.

********************

Video kurs za zavarivanje:

http://www.fulldls.com/torrent-others-884943.htmlknjiga:

http://www.fulldls.com/download-ebooks-1853779-Soldering%2C+Brazing+and+WeldingA+Manual+of+Techniques++Derek+Pritchard.torrent

***********Konkretno o impedansi (koja se cesto naziva i indukcija jer je uglavnom i cini kalem tj namotaji koje serviseri zovu prigusnica).

Impedansa ima zadatak da uspori rast struje kod "zavarivanja kratkim spojem" kao i da pruzi tzv akumuliranje struje za ponovno uspostavljanje luka posle posle kratkog spoja.

Uprosceno:- pri zavarivanju u rezimu kratkog spoja, zica dodirne osnovni materijal i nastane kratak spoj izmedju zice i materijala. Tada je napon nula a struja krene da raste do neke velike vrednosti. Porast struje je pracen toplotom koja rastapa i malo materijala ali i vrh zice koji dira taj materijal. U jednom momentu se rastopljeni vrh zice odvoji u vidu kapljice i padne u rastopljeno kupatilo, sto je praceno nekim prstanjem (udari kap u kupatilo) i rasprskavanjem (moze sama kap da se rasprsne) a stvori se prostor tj luk izmedju vrha zice i materijala. Posto dodavac gura zicu, a struja luka je nedovoljna da je istopi, vrh zice ponovo udara u materijal i pravi kratak spoj... I tako nekih 80-120 kratkih spojeva u sekundi...Impedansa ima veze sa onim delom "kada zica dodirne materijal, napon pada na 0 a struja raste do neke vrednosti...". Postavlja se pitanje za koliko vremena ce struja porasti do te vrednosti, dok se kapljica sa vrha zice ne odvoji... To regulise impedansa tj prigusnica. Ako je impedansa veca, to vreme ce biti vece tj kapljica ce se odvojiti kasnije nego kada je impedansa mala.Rezultat, kada se doda impedansa, je sledeci:- Kapljica se odvaja glatkije, sa manjim prstanjem (kao recimo razlika izmedju kaplice koja bucne na neku povrsinu vode pa izazove rasprskavanje i recimo da pazljivo pipetom kap iz pipete prenesemo na povrsinu te vode),- Broj kratkih spojeva je manji (manji broj izvora kratkog spoja znaci manji broj izvora prstanja pa samim tim i ukupno prstanje),- Vreme trajanja luka je duze pa je kupatilo fluidnije, tj majstori bi razlivanje je vece,- Samim tim posto ima vise luka i lice sava je ravnije i glatkije...- Ima izuzetan znacaj pri zavarivanju cistim CO2 gasom. Ako aparat ima dobro odmerenu impedansu, u krakom spoju se sasvim lepo i bez prstanja moze zavarivati u cistom CO2 gasu.- Obrnuto, ako ima mogucnost podesavanja tako da impedansa bude vrlo mala, dobija se kupatilo koje se brzo steze... Ovo je narocito bitno kod tankih limova (auto limarija)... Recimo da se zicom 1.0 mm moze povuci 50 mm punog sava na limu 1.0 mm bez progorevanja.- kada se radi sa zicama 0.6 mm i 0.8 mm velika imedansa nije potrebna jer vec same zice zbog malog precnika imaju veliku otpornost tj povecavaju impedansu.- povecanjem impedanse cini se kao da je povecana struja, jer je razlivanje vece a sve je to rezultat da je duze vreme kada gori luk u odnosu na vreme kratkog spoja kada je luk ugasen...

Kada se radi u spreju, gubi se smisao impedanse tj ima smisla samo pri paljenju luka, tj prvom i jedinom kratkom spoju, pa je tada treba staviti na minimum.Jedna od tajni rada sa tankim limovima od prohroma je staviti sto je moguce vecu ili barem prilicno veliku impedansu... Pa majstor komentarise tada "luk se tesko pali ali se zato lepo razliva..."

Ugradjena prigusnica i uopste tzv "nagib" tj elektricne karakteristike aparata su ono po cemu se neki aparati isticu medju drugima. Jedni su utrosili silne pare na ispitivanja da bi u 1 sekundi napravili najbolji broj kratkih spojeva (malo ne valja a mnogo ne valja), ugradili prigusnicu koja daje najglatkije odvajanje kapljice sa sto je moguce manje prstanja u cistom CO2 gasu za razne precnike zice i razne zice, dosli do najbolje vrednosti struje kratkog spoja kada treba da se rastopljena kapljica otkine sa vrha zice bez da sama kapljica explodira, odredili najbolju velicinu kapljice koja se odvaja, pri kojoj najboljoj brzini zice da se to desava, i sve to za sve precnike zice kao i za razne zice od niskolegiranih do visokolegiranih itd itd. Ovo mogu da rade samo ljudi inzenjeri, strucnjaci za elektromagnetiku i plazmu...Zbog toga ja ne shvatam ozbiljno "hajde da sam napravim aparat jer imam vremena" ... ili kada neko kopira drugog ne shvatajuci sta je onaj original u stvari.Kome treba perfektno zavarivanje, kupice aparat koji je plod razvoja i znanja i koji na probi daje perfektno zavarivanje. Ja licno uvek probam po nekoliko aparata sa raznim precnicima zica i raznim legurama pa donosim sud.Inace, u Srbiji retko ko prodaje aparate zna sta je impedansa.Obicno vole da kazu: "ako okrenes ovamo tu ti je mek luk" Kada ga pitate da li je suprotno "tvrd" luk, i cemu konkretno sluzi mek a cemu tvrd luk kod ovog precnika ove zice ili onog precnika one zice... pogube se ljudi...

*****************

How-to: Build your own spotwelderposted Jun 23rd 2009 8:30am by Steve Watkinsfiled under: how-to, misc hacks, tool hacks

Spot welders are used in the fabrication of automobiles, PC cases, power supplies, microwave ovens, electrical junction boxes, Faraday cages, and various electronics. A spot welder is used because it produces a highly defined point of contact weld. The materials are welded without excessive heating, so working pieces are handled easily. The weld is also highly controlled and repeatable. In this how-to we cover the basics of a spot welder, and then show you how to build one from a microwave oven transformer.

A spot welders electrodes serve at least three functions. They transfer electrical energy to the material while also holding it together; this also controls resistance. The greater the pinch force the less the resistance, which results in decreased resistive heating. A lesser pinch force results in increased resistive heating. The electrodes also conduct heat away from the material, while in the off cycles, helping to cool and temper the weld. A resistive spot weld is commonly referred to as a nugget. Spot welders are generally confined to ferrous materials which somewhat limits their application range. Most produce a weld with low voltage and high current. The welder in this How-to operates from a secondary of 3vac. The primary is 120vac line voltage that should be treated with respect. The low voltage secondary makes the welder very safe, so the electrical shock hazard from the electrode is virtually non-existent. There is however the risk of burn due to the high temperatures as with any welder.

This particular welder is not intended to weld a body panel on your 1966 Jeep; it will not work well on material heavier than 20gauge sheet metal. The intended use is for small projects, for it is not capable of continuous operation. Possible uses are as follows: Welding electrode material for electrolysis cells. Working with the fine components of a vacuum tube. Building a light weight frame for a small robotic platform. Most of us have enough parts laying around to build a spot welder. If you have a microwave oven transformer (MOT) laying around, then you are half way there. On a related note, we had covered a microwave oven arc welder in 2006.

We also needed some heavy gauge copper wire. We used about four feet of 4AWG wire to build the spot welder in the photo. Other materials included scrap 26, 22, two copper screw type lugs, two copper welding cable lugs, two MIG welder tips, two 4 x 3/4 zinc plated corner braces, drywall screws, and three washers.

Pictured above is a working MOT. The first thing we had to do was remove the secondary coils. Namely the high voltage winding, and the low voltage winding. We used an angle grinder with cut off wheel while being careful not to cut the primary winding.

We cut the secondary flush with the MOT laminate core. Both sides of the MOT should be cut. Inspect the MOT for signs that the laminate has been welded. We have found that welded MOTs can handle a little more abuse than their sealed only counter parts. If possible try to keep the core insulation intact, where the secondary will be wound. Though it is not a show stopper if the insulation becomes damaged. The insulation makes it a little easier to wrap the heavy gauge secondary.

After removal of the secondary we had something that resembles the above photo. If the magnetic shunt material falls out be sure to replace it as it was before. The shunt keeps the core from transferring too much power to the secondary. A magnetic ballast if you will. The shunt acts to control the saturation of the core. A brute force project like this relies on such a shunt for proper operation.

Rewinding a MOT with 4AWG is no walk in the park. If youve damaged the core insulators, we suggest wrapping a layer of electrical tape in their place. This will help to avoid damaging the insulation on the wire as it is pulled through the core. Our experience is that 3-4 windings is plenty. After all, this spot welder relies on high current and marginal resistance. Not high voltage.

We were careful to ensure that the secondary coil was wrapped in a helical manner to complete the secondary.

We mounted the MOT and 22 to the 26 base. This particular build used 12 26 with two 7 22. These dimensions may or may not work depending on the physical size of your MOT. The only critical part here is keeping the wire length as short as possible.

After the lower jaw was mounted, we also attached the corner braces. It was found that a spare piece of 22 as a shim worked well to align the upper and lower jaw. After the upper jaw was aligned we attached it to the corner braces with screws. This formed the hinged portion of the jaw.

The picture above shows the MIG welder tip and the screw type copper lug. This is an improvement from an earlier model we had built. Initially, we used copper tubing with a hole and a piece of 6AWG grounding wire serving as the welding electrode. The grounding wire was held in place by a screw that threaded inside the copper tube perpendicular to the electrode. It was very crude, but it worked. This new method is much more practical.

Here are the two electrodes ready to be fixed to the lower and upper jaws. We double checked the MIG electrodes to make sure they were tight. A loose connection will take heat away from the weld nugget.

Evenly aligning the welding electrodes, we were careful to keep the upper jaw in the natural position where it was mounted. This maintained a flat contact area for the welding electrodes. After we were sure that the electrodes had been properly aligned, the jaws were marked. We then drilled a small hole. Since we mounted with the grain of the 22 the holes helped to protect from splitting the 22.

With the electrodes mounted, we cut the wire to proper length. We never cut the exact amount we need. We always cut more than we need. This rule of thumb should apply to all electrical wiring. After all it is much easier to cut off excess than wrap a new secondary.

We bent the wires in to the approximate positions in which they were to be assembled and stripped the wire in preparation for the crimp type welding lugs. It is a good idea to strip more than is needed here as well. Simply cut off excess after sizing up the lugs depth. Never crimp insulation with the lug. This will create a potential problem area due to the loss of conduction.

Using a good non insulation crimp tool to secure the wire. We inspected the crimp and gave it the tug test. Simply tug on the wire if it is loose it will pull out. It if doesnt pull out then an adequate crimp suitable for high current has been made.

The crimped wires were attached to the welding electrodes with screws. We were careful not to over tighten the screws. If a drywall screw had stripped out of the wood, we would have had to use a larger wood screw in its place. After both welding electrodes were fixed to the jaws, we aligned the electrodes. Using pliers we bent the electrodes so that they contacted each other evenly. The electrodes should be fairly close already since they were aligned before drilling.

We opened the jaws and wired the primary to an electrical cord and then tested the secondary. If the breaker trips, check for the following:

1. The secondary is shorted (the jaws are closed)

2. The magnetic shunts are missing or not properly reinstalled

3. Faulty line wiring to primary or shorted primary

4. Too much load on the circuit of test or undersized breaker

We observed proper electrical wiring practices. It is also stressed that this is a welder and it should have a dedicated circuit as any other welder would have.

With the power physically disconnected we verified the welding electrode alignment with the material we intended to work on. Before connecting the power and performing an initial weld, we observed a few safety guidelines. This is a welder and will produce very high temperatures. Keep fingers away from the welding electrodes. Allow the material to cool prior to handling. Always wear eye protection. You may be interested in reading about spot welder parameters. Theres also the problem of combustible materials

This Compaq used very thin aluminum to support the screen and connect the hinges. The metal broke and destroyed most of the lower plastic. We were able to make new supports from 22AWG stainless steel sheet metal. All the welds were made using the spot welder with a special power controller. The power controller will be covered in another how-to.