Question 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
Reply to
Adrian Hodgson
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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
Reply to
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
Reply to
Adrian Hodgson
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
Reply to
mark
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
Reply to
Steve W
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
Reply to
Christopher Tidy
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
Reply to
Tim Leech
We still have some free tranny oil, collection from Luton...
Peter -- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk
formatting link
Reply to
Prepair Ltd
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
Reply to
pentagrid
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
Reply to
Adrian Hodgson
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
Reply to
Mark Rand
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
Reply to
Adrian Hodgson
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
Reply to
Christopher Tidy
pumping
environment.
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
Reply to
Andrew Mawson
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
Reply to
Greg
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
Reply to
pentagrid
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
Reply to
Mark Rand
Brain fart, too early in the morning:-
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
Reply to
Mark Rand

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