choke or capacitors

[bigtip]

hi to all, newbie here.

i am looking to buy a 200a mig very soon and i have settled between the portamig/mta and the oxford migmaker.

but i am trying to make my final decision based on a couple of technical matters:

it seems that the mta/portamig has a large coil or choke next to the transformer which i think smoothes the output. the oxford says it has cap+ which i guess is a bank of capacitors to do the smoothing. i have seen a picture of the inside of the portamig (thanks to weldequip) and that seems to show a big choke but no caps. i have not seen a pic of the inside of the oxfords. something shenion said about caps and chokes doing a totally different thing has made me want to know what the difference is.

also, i have read that the mta portamig has a good wirefeed motor but only 2 rollers and i have read that the oxford has a 4 rolller mechanism but i can't be sure this applies to the 200a machine. does anyone know if this is true? and how important is it really?

much appreciated for any useful info,

 

[shenion]

The caps help maintain some output between the rectified AC cycles. It is not really needed on true 3-phase machines as the rectified 6, half cycles overlap. Note, this is not just 415V but uses all 3 phases.

The thing with caps is it increases the surge when you pull the trigger. I added a bank to my Miller and it did smooth the arc but it makes the output vary more. The output RMS (average) varies with load more. I find lower currents are more sensitive to torch/work length. Set mine to 18V and the meter will read 26V until you strike an arc. Then drops to 18V.

Both machines will have a choke on the output.

Caps help, but they are worked hard and will need to be replaced over time.

As for 2 vs 4-roller. Id put more in the quality of the drive. Many use a bearing as an idler. So only one half is driven. Better feeders will have both side driven (usually you can see a gear on the idler.)

My feeder is a 2-roller setup with both driven. I had a torch with a bad kink in it and it had no problem feeding it (found the problem when using the torch on a single-driven feeder.)

My feeder has no problem with a 15' (5m) torch. You normally see 4-roller on 25' torches or robotic setups. 4-roller will help with aluminum wire but I'd go for a spool gun as it is hard to feed on a long torch.

The inductor on the output is needed for short-arc (dip) mode. Without it, when the wire touches the weld pool, it the current surges way up and chunks of wire melt off and the weld pool splashes out. Then the arc forms but the current drops significantly. So there is no "wetting" of the weld pool. With the choke, current builds up until the end of the wire melts and it pinches off a droplet. Then an arc starts and the energy stored in the choke maintains the current to "wet" out the weld pool.

 

[bigtip]

thanks for replying,

i couldn't see any caps in the photo that i have seen on this site of the innards of the portamig. only electrical components (other than switchgear) are: the transformer, a similar but smaller item which i assume is the choke and two rectifiers. i can't find the picture again right now with a quick search but there was no cap that i could see. but what does the choke do?

have you any idea what it looks like inside an oxford migmaker 200? i really would like to see a pic of the oxford guts..

regards,

 

[shenion]

I can't find the picture again right now with a quick search but there was no cap that i could see. but what does the choke do?

As I said in my post:

The inductor on the output is needed for short-arc (dip) mode. Without it, when the wire touches the weld pool, it the current surges way up and chunks of wire melt off and the weld pool splashes out. Then the arc forms but the current drops significantly. So there is no "wetting" of the weld pool. With the choke, current builds up until the end of the wire melts and it pinches off a droplet. Then an arc starts and the energy stored in the choke maintains the current to "wet" out the weld pool.

It will also help maintain some current through the drops in the rectified AC.

have you any idea what it looks like inside an oxford migmaker 200? i really would like to see a pic of the oxford guts..

regards,

Have no idea what is in them.

 

[bigtip]

sorry for not re-reading your explanation in the previous post. my mind was elsewhere trying to remember what you said on another thread (which i can't find anymore) and hoping you would further explain that. it was replying to someone and correcting their understanding of how caps and chokes effect current and voltage but i've spent so much time searching about welders that i can't read properly anymore...

i just can't see any caps in the portamig and i'm trying to work out how important they are...

 

[shenion]

Caps are important if it was designed to use them.

The PortaMIG probably has several small caps to reduce noise being radiated back on the input power lines.

Noone on this forum has complained about the arc stability of the PortaMIGs. So it is a non-issue if it uses caps.

 

[stormin]

Shenions reply seems to be based on his experience with fitting Caps to a three phase machine, which as he has already said is naturally rectified due to the phasing of the input.

It's a different matter with a single phase were caps will be a lot more useful in getting something approaching 3 phase arc stability.

 

[shenion]

I fitted caps to a single-phase machine. I said it made the arc more stable but the voltage varied more with load.

Edit: I have put 4,700uf on another welder. Does not change the voltage much but does add some stability.

The Miller, i had added 30,000uf.

 

[stormin]

Not sure were you get the information that the Oxford is a 4 wheel wire feed, certainly not for the Migmaker range of lower amp machines. The web-site brochure seems to suggest it is an upgrade option on the larger S migs with seperate wire feed units.

I sent an e-mail to technical arc earlier this month asking for more information on the differences in the MTA and Portamig machines and was told it was simply the colour and the minimum amps, not much else. Not a great reply I didn't think.

 

[tigler]

The fundamental difference between chokes and caps is that chokes will pass DC, caps only pass AC, they block DC.
Although a choke will pass DC it damps sudden changes in the current passing through it. They don't hold a charge like a cap. Chokes, or inductors to to give them the proper name, are used in many electronic and electrical applications, in a welder it seems they are used to prevent surges of the arc.

 

[bigtip]

i asked a salesman how come the portamig and the mta had different minimums, perhaps a tap on the transformer in a different place? he answered my other questions but i d'idnt get an actual answer on that one so i'm still interested to know.

i think i must have read something vague about the oxford wire feed which led me to believe it was a 4x4. that's why i asked about it as i was also doubtful that it would be on the little ones. it does say it's got a brake to stop wire overrun though...is that something special or is that feature just taken for granted on a machine of that size?

as regards the inductor/capacitor question, i would have to imagine the 'cap+' technology that the oxford mentions would be an improvement on the portamig/mta's fairly invisible caps. but then again, it could be just sales bulls**t.

 

[keith19]

Bigtip, have you thought about approaching it from another angle, like asking Oxford if they can give you details of satisfied customers who use their Oxford machines on a daily basis?

There are plenty of folks on here who use Portamigs and seem well satisfied with them, although mostly I think on a home workshop basis.

The Oxfords may be aimed at the commercial workshop rather than enthusiastic home and restorer use, but because of this their amperage might not go so low.

 

[stormin]

I've actually e-mailed Technical Arc today and suggested they get one of their tech guys to register on the forum to answer questions such as this and others about their machines that crop up on a regular basis.

 

[bigtip]

Keith, you're right: contacting oxford is exactly what i should do, but it's not just the satisfied customers i want to hear about. sounds a bit twisted but i want to hear 'bad' stories as well that include the inevitable problems that all machines are subject to.


i'm going to put out a call for all/any Migmaker users to share their experiences or, even better, a direct comparison with the portamig from those who have used both. there's loads of portamig reviews on here but not much regarding the oxford. i remember reading a 'weld-off' some time ago which may or may not have been on this site and may or may not have included either of those two welders but, as usual, i can't find it anymore.

 

[bigtip]

Stormin, that will hopefully be a great benefit to many.

It's quite intriguing that the oxford and tecarc/mta/portamig are all actually made in the same factory (so i have read). wonder why they offer so many machines that kind of compete with each other. it's a bit like chainsaws; electrolux actually own husqvarna, jonsered, mcculloch and many others and their factories sometimes produce the same machines but some are red ones which are badged jonsered, the orange ones husky and the yellow ones mcculloch.
i'm sure there is some infallible marketing logic behind it all.

 

[Silver Surfer]

this like almost all range cookers being made in the same factory in Italy. I found this out whilst trying to order bits

My thought is that certain factories are tooled up to make certian products. The factory will then vary the component quality to meet a brands (clients) price point or market percieved quality

 

[Wozzaaah]

I've actually e-mailed Technical Arc today and suggested they get one of their tech guys to register on the forum to answer questions such as this and others about their machines that crop up on a regular basis.

One of their tech guys has been a forum member for several years. He doesn't seem to post much but has proved to be very helpful in answering queries and sorting members out with parts etc.