Overclockers Australia Forums
OCAU News - Wiki - QuickLinks - Pix - Sponsors  

Go Back   Overclockers Australia Forums > Specific Hardware Topics > Electronics & Electrics

Notices


Sign up for a free OCAU account and this ad will go away!
Search our forums with Google:
Reply
 
Thread Tools
Old 18th November 2012, 6:16 PM   #1
RussellK Thread Starter
Member
 
Join Date: Jun 2003
Posts: 395
Default 3 phase AC welders - how work?

Back when I did a welding course at the local TAFE, the machines were massive old 3-phase machines, which we were told provided a smoother arc than the single phase AC machines we'd use at home, due to the combined phases not having the same zero-crossing extinguishing the arc 100 times a second.

So I assume these machines have a 3-phase to single phase transformer in them - does anyone know of a diagram for one of these, and what the output waveform looks like?
RussellK is offline   Reply With Quote

Join OCAU to remove this ad!
Old 18th November 2012, 6:21 PM   #2
desertstalker
Member
 
Join Date: Feb 2005
Location: Melbourne
Posts: 309
Default

Welders are DC aren't they?

If so they likely use this http://en.wikipedia.org/wiki/Rectifi...ase_rectifiers

The resultant DC is smoother with more phases, as the waveforms will overlap. I doubt they bother to do much smoothing on welder supplies (the caps would need to be pretty massive).
desertstalker is offline   Reply With Quote
Old 18th November 2012, 6:43 PM   #3
Symon
(Plugging your Socket)
 
Symon's Avatar
 
Join Date: Apr 2002
Location: Brisbane QLD
Posts: 3,473
Default

Quote:
Originally Posted by desertstalker View Post
Welders are DC aren't they?
Not all of them.

The older 3-phase welders were just a single phase welder with a 415V primary (only used two of the three phases). The more expensive ones had a rectifier that gave you DC put still was only single phase on the primary.

Later on the DC units had a full 3-phase rectifier to give an unregulated DC output, but the ones these days have a full active front end which regulates the DC output voltage.
__________________
Founder of the bakasan Technical College -- My Web Server in a Box -- www.phased.com.au - electricians online community

"The biggest problem in Australia is the tall poppy syndrome. Without tall poppies you have only weeds" - Lindsay Fox
Symon is offline   Reply With Quote
Old 18th November 2012, 6:50 PM   #4
simmo2302
Member
 
Join Date: Dec 2011
Location: QLD, 4655
Posts: 207
Default

Quote:
Originally Posted by desertstalker View Post
Welders are DC aren't they?

welding mild steel, stainless steel and titanium are DC negative i think. electrode is the negative pole, the earth clamp is positive pole.

aluminium welding is high frequency AC.
simmo2302 is offline   Reply With Quote
Old 18th November 2012, 7:19 PM   #5
TERRA Operative
Member
 
TERRA Operative's Avatar
 
Join Date: Jul 2005
Location: Niraikanai
Posts: 6,907
Default

Smoothing is used on many welders, using big capacitors almost the size of a coffee cup (I have a few sitting here).

The output is smoother on a three phase machine due to the DC from the rectifier being smoother as mentioned above.
However, with inverter technology, it's all pretty much the same. The thee phase units just kick out more current.


For MIG, mild steel uses a positive electrode, as does stainless and aluminium.
However, when migging gasless/cored mild steel wire, you go for negative electrode.
For TIG, steels use negative electrode and aluminium uses AC.
TERRA Operative is offline   Reply With Quote
Old 18th November 2012, 7:28 PM   #6
Symon
(Plugging your Socket)
 
Symon's Avatar
 
Join Date: Apr 2002
Location: Brisbane QLD
Posts: 3,473
Default

I always thought that for consumable electrode it was positive to the electrode, and for non consumable electrode it was negative to the electrode?
__________________
Founder of the bakasan Technical College -- My Web Server in a Box -- www.phased.com.au - electricians online community

"The biggest problem in Australia is the tall poppy syndrome. Without tall poppies you have only weeds" - Lindsay Fox
Symon is offline   Reply With Quote
Old 18th November 2012, 7:34 PM   #7
TERRA Operative
Member
 
TERRA Operative's Avatar
 
Join Date: Jul 2005
Location: Niraikanai
Posts: 6,907
Default

Not for cored or gasless wires in MIG machines. They need reverse polarity (negative electrode).

Other than that, what you say is pretty much true.
TERRA Operative is offline   Reply With Quote
Old 18th November 2012, 7:41 PM   #8
RussellK Thread Starter
Member
 
Join Date: Jun 2003
Posts: 395
Default

To clarify, these were AC output units used for stick welding. You can still buy cheap-arsed versions of such things, often referred to as buzz boxes.

If it is the case that these so-called 3-phase units were in fact just using 2 phases, the claimed difference in zero-crossing compared to a single-phase unit is a load of crap.

With regard to polarity, in stick welding the general principle is that running the electrode negative (workpiece positive) puts more heat in the work - this is often called straight polarity. Conversely with the work negative and the electrode positive, more heat goes into the electrode, and can help with welding thinner sections - it's also called reverse polarity.

All that said, you'll find plenty of welders who swear by one polarity or the other.
RussellK is offline   Reply With Quote
Old 18th November 2012, 7:51 PM   #9
TERRA Operative
Member
 
TERRA Operative's Avatar
 
Join Date: Jul 2005
Location: Niraikanai
Posts: 6,907
Default

Quote:
Originally Posted by RussellK View Post
If it is the case that these so-called 3-phase units were in fact just using 2 phases, the claimed difference in zero-crossing compared to a single-phase unit is a load of crap.
That depends if they are actually three phase, or 415v single phase.
TERRA Operative is offline   Reply With Quote
Old 18th November 2012, 8:11 PM   #10
RussellK Thread Starter
Member
 
Join Date: Jun 2003
Posts: 395
Default

Quote:
Originally Posted by TERRA Operative View Post
That depends if they are actually three phase, or 415v single phase.
Well, that was my question - if they are indeed using all 3 phases to produce a single phase AC output, how is the transformer configured, and what does the output wave look like?
RussellK is offline   Reply With Quote
Old 18th November 2012, 8:13 PM   #11
bobbavet
Member
 
bobbavet's Avatar
 
Join Date: Apr 2006
Location: East Maitland
Posts: 1,289
Default

A single phase is the same frequency wave as the input power 50Hz. I should imagine a 3phase being three evenly superimposed waves of 50 Hz. Effectivelly creating a 150Hz power , hence a more stable arc and easier arc starting.

As far as I know a step down iron core transformer the output wave characteristics remain the same, It is the Voltage that is lowered and Amperage adjustable.

Wahla!





Some of the early "switch type" 415 AC welders also had a High frequency unit built in to bump the power to the 1000's of Hz so they could be used for Aluminium Tig welding.
__________________
Please "Like" and "Share" THE MEMORY WINDOW

My Silent but Deadly Prodigy WC GTX690 buildlog

Last edited by bobbavet; 18th November 2012 at 8:24 PM.
bobbavet is offline   Reply With Quote
Old 18th November 2012, 8:39 PM   #12
RussellK Thread Starter
Member
 
Join Date: Jun 2003
Posts: 395
Default

But wouldn't there be a summing effect, assuming the step-down transformer had a single winding for the output?
RussellK is offline   Reply With Quote
Old 18th November 2012, 9:15 PM   #13
DarkYendor
Member
 
DarkYendor's Avatar
 
Join Date: Feb 2008
Location: Perth
Posts: 2,907
Default

Rectified single-phase (lots of zero crossing):

Click to view full size!

Rectified 3 Phase (never gets near zero):

Click to view full size!
__________________
RIG: |i5 2500k @4.5 | Asus P8P67 Pro | GTX460 SLI | 8GB DDR3-1600 | Crucial M550 256GB | 2x Dell U2311H|
C&C: | LianLi PC-A71B | Swiftech MCP655 | Swiftech MCR320-QP, MCR240-QP | Heatkiller 3.0 |
TABLET: | Samsung Series 7 Slate (XE700T1A-A01US) | - PHONE: | iPhone 6 64GB |
SERVER: | i3 3220 | 8GB DDR3 | 16TB + SSD | - BACKUP SERVER: | Intel D945GCLF2 | 1GB DDR2 | 8TB |
Successful OCAU trades / New blog & buildlogs
DarkYendor is offline   Reply With Quote
Old 18th November 2012, 9:33 PM   #14
TERRA Operative
Member
 
TERRA Operative's Avatar
 
Join Date: Jul 2005
Location: Niraikanai
Posts: 6,907
Default

^^^ This.

Of course, with the smoothing capacitors (if fitted) the waveform won't get near zero, but it is a lot smoother with 3 phase still.
TERRA Operative is offline   Reply With Quote
Old 18th November 2012, 10:41 PM   #15
RobRoySyd
Member
 
Join Date: Jan 2008
Location: Sydney
Posts: 4,832
Default

Quote:
Originally Posted by TERRA Operative View Post
^^^ This.

Of course, with the smoothing capacitors (if fitted) the waveform won't get near zero, but it is a lot smoother with 3 phase still.
Yes, but with high current DC supplies, a 3 phase bridge rectifier gives around 3% ripple and at 300Hz. If you want to filter out that last bit of ripple you need way less capacitance than for a single phase supply.

Yeras ago now I built a 110V 50A variable supply. Just a 3 phase Variac feeding a 3 phase tranny and a fan cooled bridge rectifier. From memory I also put 4 pretty big caps on the output, mainly because we had them lying around as spares for our big UPSs.

Anyways none of this is answering the OP's question which comes down to "is it possible to make a transformer that has a single phase secondary and a 3 phase primary that spreads the load over the 3 phases". I think the asnwer is Yes but it's a beast of thing.

This might be it: http://en.wikipedia.org/wiki/High-leg_delta

Pictures of them from one supplier: http://www.polyphaz.com/Three_phase_...ansformers.htm
__________________
"I object to violence because when it appears to do good, the good is only temporary; the evil it does is permanent" Mahatma Gandhi
RobRoySyd is offline   Reply With Quote
Reply

Bookmarks

Sign up for a free OCAU account and this ad will go away!

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time now is 3:07 PM.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd. -
OCAU is not responsible for the content of individual messages posted by others.
Other content copyright Overclockers Australia.
OCAU is hosted by Internode!