Pulse spot welder (above 2kVA 230VAC 50Hz mains input power) - Page 1
Forum section: electronics / projects

author: eneuro
author_rank: Super Contributor
timestamp: September 02, 2014, 02:18:30 pm
content: Hello,Just thinking of improoving my triac based spot welder driver.I have a huge transformer with custom secondary windings (ratio 1:100) to be able output above 1kA in pulse:Secondary winding is connected with spot welder electrodes by pipe wires in a way to get as big connection area as possible:There is my concept prototype circuit of puse driven transformer primary from capacitors battery powered by rectified 230VAC 50Hz mains with soft start & current limit circuit to around 10A-15A 230VAC mains.In this simple simulation it looks good and when discharge mosfets switch (a few in pararell to get RDSON <1 Ohm ) is ON there is quite nice spike on its output close to 2kAPulse frequency will be not higher than 100Hz and synchronized with mains peak voltages to discharge capacitors in detected mains power maximum-trying to keep them at higest voltages possible between spots.My concern is protecting primary transformer switching mosfets when they switch OFF and there still might be quite high voltage available on capacitors, so I've added diode for the moment to prevent this switch from primary transformer spikes like powering inductive load, but what about transformer core saturation and overcurrent protection of this mosfets switch?When pulses width will be short and choosen with transformer primary inductance in mind and frequency not higher than 100Hz there are core saturation issues when spot welding steel (or other metals) with quite low load resistance, so during current drop in transformer primary energy can be absorbed by secondary short circuit?Just play with this circuit in this simulator while waiting for mosfets delivery and try to find any issues which might go wrong andit.I will have optoisolated drivers for 1000Vds rated mosfets in switch and soft start circuits, so I do not expect MCU powered from optoisolated power supply damaged during tests, but this discharge switch is my biggest concern for the moment.

author: mzzj
author_rank: Super Contributor
timestamp: September 02, 2014, 02:43:24 pm
content: I would use igbt instead of mosfets. Or a thyristor.Transformer core magnetisation can be one of the problems in this kind of use, ie the core will be residual magnetism in the core after the first pulse and during the second pulse your core will saturate and kill your mosfets. Since you are using EI core transformer this is probably lesser problem, toroidal transformer would be much worse.

author: robrenz
author_rank: Super Contributor
timestamp: September 02, 2014, 04:06:31 pm
content: This threadmayhave some useful info or links.

author: nctnico
author_rank: Super Contributor
timestamp: September 02, 2014, 04:15:11 pm
content: With a 1:100 ratio 2kA is 20A at the input. I'd forget about the whole DC conversion and use mains directly. The amount of energy can be adjusted by the ignition point during half a mains cycle. In commercial spot welders they use a triac but each weld uses a mains cycle in the oposite direction to avoid core saturation.

author: eneuro
author_rank: Super Contributor
timestamp: September 02, 2014, 08:35:09 pm
content: Thank you for provided links and confirmation that core saturation might be issue in this DC pulse design, but it looks like idea of using huge transformer is not so bad, while thoseCapacitive Discharge (CD)welders using only a few Farad capacitors and trying to pulse a few kA with tens of mosfets, while forgeting about huge di/dt which can easy kill them due to output wires inductance is not a good idea-maybe for small light spot welders or more very professional designs.Quote from: nctnico on September 02, 2014, 04:15:11 pmThe main reason I decided to try this approach with rectyfing 230VAC and using caps on transformer primary is to avoid overloading or interfering home 230VAC grid network, while as I know it is rated to 25Amax.The beauty with rectifier is that when 3 phase rectifier were used this thing could be powered by more powerfull power source: 3 phase AC, but still possible to power this thing from 1 phase 230VAC using only two wires and 2/3 diodes in rectifierWhen calculated RC constant with 10000uF caps and about 3 Ohm resistance (1.2R transformer primary + about 1.5R mosfets) excluding pararell capacitors ESR it is about 0.03s, so when those caps a charged to 300V and after discharge time of 0.01s its voltage drops to about 70%, so should be about 216V, so energy difference is about 200J and about 22kW pulse power.However this estiamtion not include transformer primary inductance, which will drop when saturated.Is it possible to simulate transformer cores inspiceproviding its histeresis, nuber of turns, core cross section area etc.?I'm not sure for the moment how much energy is needed to make spot welded area of a few mm2, but I found welding book and I hope I will be able to estimate this energy to get orders of magnitude.Maybe using a classic mosfets H bridge to drive transformer primary could resolve core saturation issues and make still DC powering available and more powerfull when charged capacitors used on its primary?I'd like to try in near future add ultracapacitors to this DC design and output as much pulse power as possible and it looks like key to do this is avoid core saturation, but not by using directly AC mains switched by triac or AC switch, so any hints are welcomeBTW: This was interesting what can be found in link above with day to day notes of developing CD welderQuote

author: mzzj
author_rank: Super Contributor
timestamp: September 02, 2014, 08:57:48 pm
content: If core magnetizing turns out to be a problem you can quite easily thinker a circuit to de-magnetize or even pre-magnetize the core with opposite flux.Have you tested your transformer with mains supply?how much was the primary current and how long duration did you need? You can probably scale your requirements for the capacitor discharge system if you have some measurement results based on lower power mains connected system.

author: eneuro
author_rank: Super Contributor
timestamp: September 03, 2014, 09:49:54 am
content: Quote from: mzzj on September 02, 2014, 08:57:48 pmThis circuit can be awiki: H bridgeQuoteIn the case of 3 phase AC rectified I will have constant input power and switching frequency limited by transformer iron core.In the case of 230VAC mains current detection system will help to choose right moment to fire spot welder pulse depending on amount of capacitors used, while I do not like to stress them too much during discharge pulses. it could be also good idea to measure input voltage on those caps to ensure there is right input power needed between pulses.So, it looks like I need 3 optoisolated MCU pins to control this H bridge in the case of spot welder transformer primary driving at quite low frequencies ( 1 pin to choose one of the bottom switches and 2 for upper ones).Toggling bottom switches between pulses with upper switched OFF willwiki: Charge pumpupper N-channel mosfets gates power supply which is required by some mosfets drivers.Mains input filter and soft start circuits will be independent with current (input power) limit set by 3rd potentiometer I guess.It is time toit and redesign existing H bridge circuit for this project

author: johansen
author_rank: Super Contributor
timestamp: September 03, 2014, 11:19:34 pm
content: 2kamp at the output isn't really anything interesting.a transformer coupled capacitor discharge spot welder might be interesting if you were trying to hit 20,000 amps on the output, and you were switching the transformer from an H bridge made from 300 amp IGBTs, and you did the switching at say 120hz or something like that.you can pull 40 amps for a second through your 15 amp mains at a 10% duty cycle and you won't blow the breaker anyways. so that should be enough to get some serious spot welding done.those long half inch copper pipes you've got there are eating up most of your power.realistically you should try to go for a 1 or 2 turn secondary and use a transformer at about 1 volt per turn.and the secondary should be at least 2cm^2 solid copper.

author: eneuro
author_rank: Super Contributor
timestamp: September 04, 2014, 08:49:28 am
content: Quote from: johansen on September 03, 2014, 11:19:34 pmWith those 4.7mF 400V 24mESREPCOS B43456A9478Mpower caps according to its datasheet at 77mm diameter they are capable to provide close to 50A ripple current, so when a few of them (2 or 3) were put in pararell it looks like significant current supported by rectified mains voltage maximum could be taken from this filtered power supply in short pulse on transformer primaryI've found those caps are in price range around 111$ per 4.7mF@400V, but three of them in pararell gives 14mF at 325V has energy close to 750J equivalent to rise 75kg weight man 1m above the ground, and still doesn't cost fortuneSo, maybe its is worth to try to put those huge caps and synchronize with current sensed in main to get a huge pulse on transformer primary?Unsure, why are you talking just about 120Hz @ 10% PWM-is it any difference when 100Hz is used?It gives only about 1ms pulses -isn't too short?Quote from: johansen on September 03, 2014, 11:19:34 pmThis is 200 mm2, which gives about 16mm in diameter, but is solid copper really better than pipe with the same area?In the case of transients skin effects might apply so pipe copper conductors should perform better-isn't it?

author: johansen
author_rank: Super Contributor
timestamp: September 07, 2014, 06:41:02 pm
content: skin effect isn't a problem here.what i meant by 10% duty cycle is pulling power out of your grid, not the transformer.running the spot welder transformer at 120hz or something like that will allow you to double the power density of the transformer, so you can use 1 turn on the output of the coil instead of 2. this will reduce the resistance in half doubling the maximum current, however you could be limited by leakage inductance and a higher switching frequency may not help you out.that said, your 100:1 transformer shows 4 or 5 turns on the secondary, this means you have 400 or 500 turns on the primary.if that transformer is as big as it looks, then that is or was a 480 volt transformer and you should run it as such.the thought occurs to me that the solid arms of spot welders are for strength and thermal capacity, not resistance.

author: robrenz
author_rank: Super Contributor
timestamp: September 07, 2014, 10:37:34 pm
content: Quote from: johansen on September 07, 2014, 06:41:02 pmThese specs of my 110V input spot welder tell a different story. Notice the direct effect of the change in throat (= tong length = resistance) on the secondary amperage.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 07, 2014, 11:34:37 pm
content: If the high power is only needed for a few cycles, you can go over the nominal rating without tripping the breaker. It is no different than starting a big motor.

author: johansen
author_rank: Super Contributor
timestamp: September 08, 2014, 03:12:14 pm
content: Quote from: robrenz on September 07, 2014, 10:37:34 pmi am fairly sure the difference is inductance limited, not resistance.also notice the open circuit volts is 1.6, yet it can pull 5000 amps is there a big capacitor on the primary side?

author: robrenz
author_rank: Super Contributor
timestamp: September 08, 2014, 03:32:32 pm
content: No capacitor, just a manual trigger switch on the primary. At 120 Hz and a straight (non coiled) conductor I would think resistance would dominate.

author: eneuro
author_rank: Super Contributor
timestamp: September 08, 2014, 03:34:02 pm
content: Quote from: johansen on September 07, 2014, 06:41:02 pmResistance will be lower too, but higher inductance, however thermal capacity with planned additional aluminium heatsinks on those copper pipes on secondary designed to disipate more heat from welding electrodes-also to fit larger parts together one might need longer output wires, so these about 0.5 long could be helpfull.I can easy change them for higher diameter and lower its resistance if needed.Quote from: robrenz on September 07, 2014, 10:37:34 pmThank you for those specs.While there is 1.6V/110V it looks like 1:69 transformer ratio, while 2.5V/230V gives in my case about 1:92 (close to 1:100) ratio in current setup but it can be changed for lower number of turns.It is interesting this 50% duty cycle @ 50Hz/60Hz-it looks like that maybe they fire triac in voltage maximum and keep it conductiong to the end of half wave period than the same in 2nd 10ms half period, so by setting total pulses length in 20ms periods of time in the case of 50Hz supply we can adjust amount of energy in pulse.I have 2 pots for pulse setting: first to adjust each pulse power and second its total length, so lets say 1st pot adjusts each pulse length by choosing phase from 90*-180* will allow select each energy pulse.I will test AC mosfets switch tomorow when PCB prototype is ready and try diffrent settings of phase shift as well as total pulses length on 2nd pot.AC mosfets switch should limit current to about 54A when wired at 325Vmax for 230VAC while RDSON of used mosfets is typical 3 Ohm each in series, but I do not like to do 50% duty cycle of 100Hz pulses (10ms), but 1ms-2ms and see if home main fuse (25A i guess) will not fire during this spot welding tests.I have 1000V 25A KBPC25 rectifier capable according to datasheet to 300A pulses up to 8ms, but I haven't got H bridge finished yet, so AC mosfets switch for the moment without dealing with DC powered transformer and I will see how it performs

author: robrenz
author_rank: Super Contributor
timestamp: September 08, 2014, 03:55:51 pm
content: There are no electronics on this spot welder. You hold the mechanical trigger switch long enough to get the weld you are after. With 18" tongs and two pieces of .062" material it takes a few seconds of on time for a weld. If you are welding something only .01" thick you have to just tap the switch for a very short pulse to not vaporize the material.

author: Yago
author_rank: Frequent Contributor
timestamp: September 08, 2014, 04:08:33 pm
content: Hi all, been reading this thread as I am interested in making a small welder as a future project.In my net "travels", I found this guy and his project that seems very close to what the OP is seeking and also far down a development path too.Some great info in the blog and also he sells PCBs and processors etc.http://frikkieg.blogspot.co.uk/

author: eneuro
author_rank: Super Contributor
timestamp: September 08, 2014, 05:38:52 pm
content: Quote from: robrenz on September 08, 2014, 03:55:51 pmIn the XXI century MCUs are cheap, so it is easy to make even automatic detection of pressed top welder arm and.... wait lets say 5s to fire desired pulses.There is something else. If you will try make spot welded batteries than this puse (dual pulse method is often used) has to be short, but powerfull, so there is not too much time to disipate heat and overheat spot welded batteries. Probably I haven't got <10ms reaction timeThat is why I want have to pots as mentioned before: to controll each pulse energy and total pulses lengthI saw many comercial spot welders too, any most of them had only adjustable number of 20ms periods of welding in the case of 50Hz 230VAC, but transformer was used not CD welding method.Quote from: Yago on September 08, 2014, 04:08:33 pmThis from their web page:QuoteDo they use transformer or crappy CD method with tens of mosfets? With this transformer which I have a few chip mosfets will do the job.I had triac and MOC3041 before but looking for more power-600W in linked project is... 3A on 230VAC-I'm interested in >30A, so this CD welding without transformer I guess is useless.I saw many projects where people were trying to switch in CDs welders with ultra capacitors a few kA with... burned IGBTs.From linked web page:QuoteTry to download any docs herehttp://frikkieg.blogspot.com/2011/06/ver-25-avialable.htmlI'd like to see this CD welder in the size of 2 car starter batteries uncovered inside only to see if it has powerfull transformer or notUpdate:Yeah, there are photos of a few such CD welders inside, but those small transformers here are used to charge this big power cap I guesshttp://frikkieg.blogspot.com/p/welder-photos.html

author: mzzj
author_rank: Super Contributor
timestamp: September 09, 2014, 07:43:22 am
content: Quote from: eneuro on September 08, 2014, 05:38:52 pm600Ws vs. 600W600Ws CD spot welder is easily in the 100kW ballpark

author: eneuro
author_rank: Super Contributor
timestamp: September 09, 2014, 04:53:10 pm
content: Quote from: mzzj on September 09, 2014, 07:43:22 amDon't noticed thisunits there, but I decided not to go into CD welders, but try to reuse this huge transformer with such AC mosfets switch in first tests, designed to be able easy change any failed mosfets if any and put many of them in pararell for lower RDSON if needed.It is 2inch2 PCB and critical power tracks (those 4mm-5mm width) will be covered with copper wire and soldered before assembling. Its gate driver is not shown here-just considering a few options includingwiki: GDTs-of course optocoupled from control MCUs pins and galvanic isolated gate power supply.

author: eneuro
author_rank: Super Contributor
timestamp: September 15, 2014, 06:18:10 pm
content: This spot welder upgrade project doesn't have highest priority, however in spare time a few additional modules completed including custom photocoupled 0.5A gate driver for experimental AC mosfets switch on the bottom controlled by 230VAC mains current Hall sensor with DIP8 MCUReused microwave oven AC 230VAC mains filter with fuse for the moment, while its inductor windings looks quite good, but unsure if for higher currents maybe it is worth to replace this filter inductor with something more powerfull, howerver It looks much stronger than from MiniATX PC power supply 300W?Any good references to 230VAC 50Hz mains filter (inductor) design for surge currents close to 25A (maybe even higher )in 1ms-5ms 100Hz pulses not longer than 1s total time?Update: Just successfully tested this AC mosfets switch using 2 galvanic isolated power supplies for gate driver & 230VAC switch power with 40W light bulb.While RDSOn of used mosfets is typical 3 Ohms there is 200mW power loss in this AC mosfets switch- 0.1W each of two mosfets used, so after disconecting this thing from 230VAC and checking its temperature this switch is of course air cold

author: eneuro
author_rank: Super Contributor
timestamp: October 06, 2014, 10:46:59 am
content: Finally decided for the moment to use two 25A SCRs in pararell similar to this AC switching in this videoHowever, while I need galvanic isolation from control MCU, SCRs can be trigered by pulse transformers or other optoisolated gate drivers, but GDTs should be fine since thyristor typical gate triger current IGT is only 9mA.It will be safer use SCRs here while at a few kW fast turning OFF AC mosfets switch in wrong time (230VAC AC mains line is not perfect 50Hz sinus) could cause huge HV spikes, while SCRs will turn off when load current will drop below typical 18mA holding current in the case of thyristors used.Hall based current sensor is used to monitor 230VAC mains and auto detect when welder electrodes presses spot welded metals, so automatic mode available without pressing any fire buttons to start welding based on set power and time by two potentiometers

author: SeanB
author_rank: Super Contributor
timestamp: October 06, 2014, 06:51:53 pm
content: I think this transformer will do, the primary is from when it was the mains input transformer in a 2kVA ferroresonant UPS, so I took off the secondary welding rods ( I used them as copper welding rods, for which they worked quite well using a gas torch) and replaced with the 3 turn secondary there is there now. Just got to get some phosphor bronze to make tips out of, and make a spot welder out of it. Cable is happy till 200C.

author: eneuro
author_rank: Super Contributor
timestamp: October 07, 2014, 08:53:17 pm
content: Quote from: SeanB on October 06, 2014, 06:51:53 pmLooks beefyJust finished manual drilling of SCRs PCB.I forgot about it for a while and it spend too much time in etching, but who cares about prototype PCB while current flow have no idea how it looks like if power tracks are strong.I think that double 1.5mm2 copper wires soldered inside 5mm width power tracks will be fine since we get 3mm2 of copper, so equivalent of 0.6mm thickness on 5mm width track.Just adding only solder on tracks will not help too much and no way to get 3mm2 copper equivalent, I guess.Thyristors gates will be switched by mosfets put in series with gate diodes, so no need for AC mosfets switches, but still I should be able triger those SCRs using very small charge on mosfets gates (total gate charge 60nC with 1000VDS and 3 ohm RDSON), so power requirements of gate transformer driver will be very low and easy to design, while thyristors will be trigered with current around 1Amax limited by 360 ohm thyristors gate resistorsIn pararell to snubber I will put low power 230VAC halogen lamp, so I hope it will help keep alive those SCRs during spot welding pulsesAnother halogen anyway to highlight welding area and sense 230VAC 50Hz mains input current before SCRs switch with Hall current monitoring sensorSo far nothing