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Model Forum / General / Models / October 2006Question relating to power factors and old welder
I have been wondering about this for some time since the thread on electricity usage a short while ago. I have a Oxford 180 Amp arc welder. The tapping's on the primary allow from approx 210 to 440 Volt if memory serves at all. The power factor on the unit will be horrific being mostly a reactive load so was considering trying to fit a power correction capacitor to save a bit of current in the supply cable the question I have to the group is how would the cap or caps be fitted. I only use it on 240 Volt supply so could fit a cap across that tap to neutral. I could fit a cap across the 440 volt tap to neutral and still feed on the 240 volt tap or I could have two caps one from 440 volt tap to 240 volt tap and another across 240 to neutral and feed across this. It has been a long time since I did any phase diagrammes or problems of this sort so assistance is always appreciated. The use of one of the cheap Maplin power meters should give a good idea of power factor on idle current and how the output tapping's affect it. so I guess it will always be a compromise no matter what value I fit. Adrian > I have been wondering about this for some time since the thread on > electricity usage a short while ago. [quoted text clipped - 26 lines] > > Adrian MMM isn't it that you are only using power when you are striking an arc. if so i would not worry some welders make a buzzing noise all the time they are on without striking an arc ...would that be an oxford one ...i would be worried if i had one of these . there again i can be wrong on most things electrical all the best.mark > MMM isn't it that you are only using power when you are striking an > arc. [quoted text clipped - 8 lines] > > all the best.mark It may be that I have to work out power factors etc when stricking an arc yes, but the question at present is how would the cap be fitted. My best guess at this time of night is across the full winding of the transormer! There is one very good thing about Oxfords, they just keep on going, are cheep and generally do not get nicked especially when it takes two to lift. Hang on that was three things. It is fine for the type of basic welding I do when the cost of purchasing an inverter style would be cost prohibative. Adrian > > MMM isn't it that you are only using power when you are striking an > > arc. [quoted text clipped - 23 lines] > > Adrian Some guy told me a story about oxfords This how it went Apparently ...what happens ...is as they heat up and cool down condensation happens in them . The water droplets then go to the bottom because oil floats on oil . After a number of years this either causes a short that buggers them up or rots the bottom out .. The more on-off sessions you have the more condensate will be likely. He said changing the oil every couple of years should stop this . all the best.mark > He said changing the oil every couple of years should stop this . > > all the best.mark Thats right, need to drain it and check it, do it about every 30 years! Adrian <snip> > Some guy told me a story about oxfords > [quoted text clipped - 13 lines] > > all the best.mark I've heard this story too. Funnily enough, the guy who told me the story was trying to sell me a £50 service and oil change on my Oxford. Other people have told me that there isn't a great deal of truth in the rumour. Anyone know for sure? Best wishes, Chris ><snip> > [quoted text clipped - 20 lines] >people have told me that there isn't a great deal of truth in the >rumour. Anyone know for sure? Yes it does happen. I believe it's known as 'breathing'. 'Every couple of years' would normally be grossly excessive. If you're worried, take the transformer out of the tank and pump out a little of what's at the bottom. If it's brown & watery, keep pumping until reasonably clean oil starts to come up. Also if there's been a lot of water, the bottom of the laminations will be covered with brown crud. The oil should only really need changing if it's very old or been left to get really bad. Tim Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service >Yes it does happen. I believe it's known as 'breathing'. >'Every couple of years' would normally be grossly excessive. [quoted text clipped - 7 lines] > >Tim We still have some free tranny oil, collection from Luton... Peter -- Peter A Forbes Prepair Ltd, Luton, UK prepair@easynet.co.uk http://www.prepair.co.uk >><snip> >> [quoted text clipped - 30 lines] > The oil should only really need changing if it's very old or been left > to get really bad. Mine was checked over by the welding equipment supplier I bought it from 10 years ago. Whether they changed the oil then I don't know. It hasn't seen heavy use since then but has lived in a fairly damp environment. Maybe I should open it up, but I'm reluctant. Best wishes, Chris > >><snip> > >> [quoted text clipped - 39 lines] > > Chris My Oxford copy is dead easy to open Chris, nothing too complicated. Pop it under a block and tackle, attach to the handles, undo the bolts and lift away. Let it drip for an hour or two. Change oil for that kindly supplied by Peter Forbes and reverse the sequence ! AWEM >I have been wondering about this for some time since the thread on > electricity usage a short while ago. [quoted text clipped - 27 lines] > > Adrian Dad had one of those for donkeys years - made the electric meter run backwards. The Electric board turned out to investigate odd meter readings, found the cause was an honest one and left it at that. Steve >>I have been wondering about this for some time since the thread on >>electricity usage a short while ago. [quoted text clipped - 33 lines] > > Steve Now that really is weird. I'm puzzled there! Chris >I have been wondering about this for some time since the thread on >electricity usage a short while ago. [quoted text clipped - 26 lines] > >Adrian Power factor can be corrected by capacitance connected to either or both of the 240v & 440v taps in any convenient combination. Corrective effect is proportional to voltage squared so 10 uF at 440v is equivalent to 33.6 uF at 240v. Power factor correction at idle is OK because the required value of capacitance is almost independent of load. Power factor correction has neglible effect on a domestic electricity bill. However it is kinder to the electricity supply company and slightly reduces the voltage drop on any long supply lines. Jim  SignaturePosted via a free Usenet account from http://www.teranews.com On Thu, 26 Oct 2006 12:26:49 -0700, pentagrid wrote: > Power factor can be corrected by capacitance > connected to either or both of the 240v & 440v taps in any [quoted text clipped - 11 lines] > > Jim Hi Jim So if I go off your response feeding at a lower tap at 240 Volts and having the cap across the 440 point will be OK in fact will result in a lower capacitor value. I am trying to reduce the dipping effect in the house is possible by reducing the current drawm by what ever I can achieve, it may also not take out the 20 amp breaker as often on switch on although I suspect this will not be changed and may even be worse with a cap across the transformer. I guess see what values I have and try it and see. Adrian >On Thu, 26 Oct 2006 12:26:49 -0700, pentagrid wrote: > [quoted text clipped - 29 lines] > >Adrian Swap the (probably) class B MCB for a class C, or even class D. That will stop the breaker popping when you turn it on. Unfortunately, power factor correction capacitors are likely to make the inrush problem worse :-(. I'm thinking of doing similar with my little Oxford Bantam welder. Mark Rand RTFM >>Adrian > [quoted text clipped - 6 lines] > Mark Rand > RTFM Hi Mark; The daft thing about my garage supply is that i have a current breaker in the garage fed from another breaker in the house! Then the ELCB protects the whole lot. I also pondered about asking Peter Forbes if he had any circuits with zero point switching scr's that would help, but I never asked after thinking about the voltage spikes that could be generated by the switch off points and the field collapsing within the transformer. I will see if I can blow my Maplin meter to bits in the next few days. Adrian > Swap the (probably) class B MCB for a class C, or even class D. That will stop > the breaker popping when you turn it on. Judging by past experience I'll probably get flamed for daring to point out anything to do with safety on this group, but I will anyway as others may not realise the implications of changing the class of an MCB to avoid nuisance tripping. Changing from a standard domestic type B to a type C or even D has serious safety implications, you need to check the prospective fault current (PFC) is still high enough to ensure the MCB will trip fast enough to prevent electric shock by indirect contact, i.e. a live wire touching an exposed metal part of a machine. Typically you will have to thicken the cable in the circuit to increase the PFC in order to trip the MCB fast enough, otherwise you could be hanging on to a live bit of your lathe, unable to let go, for long enough to kill you. If you still can't achieve the required PFC, possibly due to the impedance of the supply to the premises, then an RCD can be fitted in order to provide supplementary protection which will trip fast enough to protect you, but the preferred solution is to thicken the cable rather than relying on an RCD. The bottom line is, if you don't understand this and have no equipment to measure PFC then you shouldn't be changing the type of an MCB without professional help. Greg >> Swap the (probably) class B MCB for a class C, or even class D. That will >stop [quoted text clipped - 24 lines] > >Greg Not a flame, a comment:- If a class B breaker is big enough to avoid tripping when an Oxford arc welder is turned on at voltage maximum, it can be replaced by a smaller class C and get better protection. e.g. A Bantam needs 40A class B or 30A class C Mark Rand RTFM Brain fart, too early in the morning:- >If a class B breaker is big enough to avoid tripping when an Oxford arc welder >is turned on at voltage maximum, it can be replaced by a smaller class C and >get better protection. I meant voltage minimum. Very inductive loads have the greatest inrush current when turned on at zero volts. This is relevant to the earlier comment about SCR/TIAC switching (i.e. don't do it) Mark Rand RTFM > I meant voltage minimum. Very inductive loads have the greatest inrush current > when turned on at zero volts. This is relevant to the earlier comment about > SCR/TIAC switching (i.e. don't do it) Yes that's right, though it's totally counter intuitive the worst inrush occurs when switched on near zero crossing which almost inevitably causes core saturation. Greg "Mark Rand" <randm@internettie.co.uk> wrote in message > Not a flame, a comment:- > > If a class B breaker is big enough to avoid tripping when an Oxford arc welder > is turned on at voltage maximum, it can be replaced by a smaller class C and > get better protection. Yes I should think that's quite possible, you have to measure (or calculate) the PFC and check it against the curve for the particular MCB to ensure it trips fast enough. > e.g. A Bantam needs 40A class B or 30A class C I'll take your word for it Mark as I don't know the welder. Greg "Greg" <news@SPAM123voyager2.nildram.co.uk> wrote in message > Yes I should think that's quite possible, you have to measure (or calculate) > the PFC and check it against the curve for the particular MCB to ensure it > trips fast enough. Just to clarify, there are graphs of tripping curves for each combination of class (B, C, D) and the current rating, so you check the PFC against the exact one you wish to use. Greg >>> Swap the (probably) class B MCB for a class C, or even class D. That will >>stop [quoted text clipped - 32 lines] > >e.g. A Bantam needs 40A class B or 30A class C My 225A Oxford has never tripped the 40A ClassB through which the supply passes, nor did the 250A set which I used briefly. However my 250A TIG set (transformer/thyristor type) will trip it on switch-on. That *appears* to have some built-in power factor correction, across the 440V terminals of the transformer. I say 'appears' because there's a capacitor symbol across the relevant terminals, with a pair of wires disappearing into the depths of the machine, none of which is shown on the makers' published circuit diagram. Despite that, the power factor quoted on the rating plate is awful. Cheers Tim Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service >>Typically you will have to thicken the cable in the circuit to increase the >>PFC in order to trip the MCB fast enough, otherwise you could be hanging on >>to a live bit of your lathe, unable to let go, for long enough to kill you. I thought that was (one of) the point(s) of using AC, that you could let go? besides, if you've got a metal machine which isn't earthed, you're asking for trouble anyway - and if a live wire goes astray and hits en earthed metal cabinet then it should trip PDQ. Of course, if you're running a machine with a high-voltage DC motor by using a sod-off rectifier, then what you say above would apply. But I rather doubt many people are. SignatureAustin Shackles. www.ddol-las.net my opinions are just that If all be true that I do think, There are five reasons we should drink; Good wine, a friend, or being dry, Or lest we should be by and by; Or any other reason why. - Henry Aldrich (1647 - 1710) >>>Typically you will have to thicken the cable in the circuit to increase the >>>PFC in order to trip the MCB fast enough, otherwise you could be hanging on [quoted text clipped - 9 lines] >a sod-off rectifier, then what you say above would apply. But I rather >doubt many people are. 'twas actually Greg who spake thussly:-) Mark Rand RTFM >>>>Typically you will have to thicken the cable in the circuit to increase the >>>>PFC in order to trip the MCB fast enough, otherwise you could be hanging on [quoted text clipped - 11 lines] > >'twas actually Greg who spake thussly:-) yer quite right. I happened to notice it in your followup though. SignatureAustin Shackles. www.ddol-las.net my opinions are just that If all be true that I do think, There are five reasons we should drink; Good wine, a friend, or being dry, Or lest we should be by and by; Or any other reason why. - Henry Aldrich (1647 - 1710) "Austin Shackles" <austinNOSPAM@ddol-las.net> wrote in message > I thought that was (one of) the point(s) of using AC, that you could let go? No, that's an old wives tale, AC will contract your muscles just as well as DC so if you are grasping a handle for example you won't be able to let go. > besides, if you've got a metal machine which isn't earthed, you're asking > for trouble anyway Another old wives tale is that if something is earthed you can't get a shock from it, in fact you will see typically 2/3 of the mains voltage on an earthed metal part if a phase touches it. It's obvious if you think about it, the metal is earthed through a thinner wire than the phase conductor (usually) but one just as long, the supply voltage at the sub station is applied across a potential divided made up of the two wires so you get a high potential at the metal part. The only place this won't happen is right at the sub-station where the earth and neutral are bonded together and bonded to an earth grid burried under it, but most people are a distace away. > - and if a live wire goes astray and hits en earthed > metal cabinet then it should trip PDQ. Yes that's the whole point, it should trip quickly so the shock you get doesn't last long enough to do you much harm, but if you don't match the MCB to the prospective short circuit current it may not trip quickly at all. > Of course, if you're running a machine with a high-voltage DC motor by using > a sod-off rectifier, then what you say above would apply. But I rather > doubt many people are. What I said does apply, it's fundamental stuff that any electrician has to understand. Greg >"Austin Shackles" <austinNOSPAM@ddol-las.net> wrote in message > [quoted text clipped - 34 lines] > >Greg Hey! We agree on things :-). Although I would like to see no more than 50-80V on a chassis in an event. That takes specific design action, such as my computer rooms that have distributed Earthing so that the earth impedance is significantly lower than the supply impedance. I've spent the last couple of weeks sending emails back and fourth with a 'large' 'bluish' Computer manufacturer over whether their use of a particular Burndy multi-pole connector for mains electricity was legal in the UK on the grounds that I felt that even my fingers (let alone an IEC-950 finger test probe) could touch live parts (Electricity at Work Regs). They say that they've installed the kit in multiple paces all over the UK and Europe. I say that if they have, they've installed illegal kit in multiple places. And I don't normally worry too much about safety :-| Mark Rand RTFM > Hey! We agree on things :-). 8-) >Although I would like to see no more than 50-80V > on a chassis in an event. Yes you would think there should be some limit in the regs but I can't remember seeing one, the fact that twin and earth has a thinner CPC seems to imply they don't care!. > I've spent the last couple of weeks sending emails back and fourth with a > 'large' 'bluish' Computer manufacturer over whether their use of a particular [quoted text clipped - 4 lines] > that if they have, they've installed illegal kit in multiple places. And I > don't normally worry too much about safety :-| I find the test pin to be the real trouble with enclosures rather than the test finger, though not sure how that applies to connectors. I assume you're talking about a US company, we're in 'negotiations' with the US supplier of a climatic test chamber for our factory that has no interlocks or even a padlock hasp on the door, you can open it at 100 degrees and 100% humidity and scald people within a 10 foot range!, their attitude is "so what, that's fine in the US". For the country that exported their litigation culture to us they're remarkably lax about safety. Greg > "Austin Shackles" <austinNOSPAM@ddol-las.net> wrote in message > [quoted text clipped - 5 lines] > DC so if you are grasping a handle for example you won't be able to let > go. So its not true then that positive going cycles contract the muscles and negative going ones relax? Bugger. >> besides, if you've got a metal machine which isn't earthed, you're asking >> for trouble anyway Serious question. My mill and lathe are both earthed via the 13amp plug. Both machines are fed from a 60Amp ELCB (All the garage sockets are fed from this). However I have a 6 foot copper stake burind in the ground on the wrong side of the garage wall - is there any point in tying both machines back to this earthing point? Both machines are within 4metres of it. TIA Steve >So its not true then that positive going cycles contract the muscles and >negative going ones relax? > >Bugger. I understood it was the presence of a zero in the middle that was the relevant bit. Mind you, with 3-phase you don't get a zero. SignatureAustin Shackles. www.ddol-las.net my opinions are just that "Ask yourself whether you are happy, and you cease to be so." John Stuart Mill (1806 - 1873) > So its not true then that positive going cycles contract the muscles and > negative going ones relax? No, not at all, I used to know someone who got himself attached to a live machine and he remembers thinking "if I don't let go soon I'll be dead", fortunately he managed to prise himself off it in time. In fact AC is the really dangerous one, if you get more than a few mA through your heart it makes the muscles pulse rappidly and even when the shock is removed the heart is stuck in this new rhythm, but the valves can't work that fast so no blood gets punped and you die about 4 minutes later. The only way to stop this is to apply a DC shock, which stops the heart and lets it re-start in a more normal rhythm, that's what a defibrillator does, CPR almost never stops ventircular fibrillation although it can keep someone alive until the equipment arrives. > Serious question. My mill and lathe are both earthed via the 13amp plug. > Both machines are fed from a 60Amp ELCB (All the garage sockets are fed from > this). I have an RCD (the new name for an ELCB) on my garage distribution panel for this reason, although it's technically mandatory on any downstairs ring anyway in case it gets used for garden machinery. > However I have a 6 foot copper stake burind in the ground on the wrong side > of the garage wall - is there any point in tying both machines back to this > earthing point? Both machines are within 4metres of it. Well it won't hurt, and may lower the loop impedance a bit which is a good thing, it's little different from bonding the supply earth to the water and gas incomers. It's normal to wire the rod to the earth bar in the consumer unit rather than to the machines directly. What people don't realise is that an earth rod is not necessarily at earth potential!, it can have tens of ohms impedance to earth depending on the ground conditions, and if you think about it 240V/10R is only 24A which will never, ever operate the 32A MCB for the ring. This is why any property not supplied with an earth by the electricity board, thus using their own rod, MUST fit an RCD to the whole installation or they will not have protection against shock by indirect contact, i.e. a wire touching a metal case. But back to your question, it won't hurt but it may not make a huge improvement as the supply should already be down to a very few ohms, and your RCD should operate faster than anything. Greg >> So its not true then that positive going cycles contract the muscles and >> negative going ones relax? > >No, not at all, I used to know someone who got himself attached to a live >machine and he remembers thinking "if I don't let go soon I'll be dead", >fortunately he managed to prise himself off it in time. Snip From personal experiece. If you get a bad handlocked AC shock and can't let go - walk backwards to drag yourself free. Jim SignaturePosted via a free Usenet account from http://www.teranews.com > From personal experiece. If you get a bad handlocked AC shock > and can't let go - walk backwards to drag yourself free. Useful advice... which I hope I never have to try out 8-) Greg >>> So its not true then that positive going cycles contract the muscles and >>> negative going ones relax? [quoted text clipped - 6 lines] > From personal experiece. If you get a bad handlocked AC shock > and can't let go - walk backwards to drag yourself free. Interesting - my experience is that I only have a vague recollection of what legs are for at that point! Steve >On Thu, 26 Oct 2006 12:26:49 -0700, pentagrid wrote: > [quoted text clipped - 29 lines] > >Adrian If you are running of 240v, then peak inrush current will be much lower with capacitors connected to the 440v tap because the peak current will be limited by the residual resistance and leakage inductance ofthe welder windings. If capacitor peak current is still a problem put a power thermistor in series with your capacitor. Jim SignaturePosted via a free Usenet account from http://www.teranews.com > If you are running of 240v, then peak inrush current > will be much lower with capacitors connected to the 440v tap [quoted text clipped - 3 lines] > If capacitor peak current is still a problem put a > power thermistor in series with your capacitor. It's worth pointing out that a run capacitor connected to a 440V tap has to have nearly double the voltage rating of one connected across the supply which may affect the economics of it. Greg >>I have been wondering about this for some time since the thread on >>electricity usage a short while ago. [quoted text clipped - 7 lines] >>of current in the supply cable the question I have to the group is how >>would the cap or caps be fitted. Snipped aload..... > Power factor correction has neglible effect on a > domestic electricity bill. However it is kinder to the > electricity supply company and slightly reduces the voltage drop > on any long supply lines. > > Jim A long long time ago I worked on the street lighting gang at the local council - I was pretty keen on electronics at the time and asked the electrician why we weren't fitting the PF caps to the street lights. "The light comes on without them so why spend the money" Steve |
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