Modifications to change polarity on Cebora 883

[jfurman]

I was recently given a Cebora 833 Mig welder. It came with a regulator, but no gas tank. It came with a reel of flux core wire, too. Since this welder was intended for gas shielded use only and its low cost, there is no provision to reverse the polarity to DCEN for flux core, no gas welding. I took this as a challenge. I traced out the circuit and I discovered the trigger switch connects to the electrode lead in the gun, rather than using a pair of wires to the switch independent of the electrode lead. This seems to be a cost saving measure which complicates the polarity reversal modification. The motor speed control circuit needs raw DC in order to function since it has an SCR to switch. I bit the bullet and added a bridge rectifier so that regardless of the work-electrode polarity, the motor controller always sees the same polarity. The bridge rectifier is readily available at low cost; a wide range of specs (400 volts or more and 10 amps or more) is usable. The particular one I used, 12 Amps and 1000 volts, cost me $0.50 new at a local electronics surplus store. A full price bridge may cost $3 to $5. I fixed the +, -, and work lead terminals to a piece of insulating glass fiber-epoxy sheet. This is printed circuit board base material without copper. The fourth terminal, the electrode lead, I left flying, connected directly to the opposite polarity post from the work lead. The three terminals on this board support the flying leads from the transformer and choke where they are subject to bending otherwise when changing polarity. Notice the inboard side of the choke is strapped for polarity; the work lead proper remains connected to the other end of the choke. The motor controller then always sees the raw DC from the power supply, rather than the arc voltage. You need to add a few wires and move some connections in order to make these changes. I've attached a jpg of the original and changed schematics. When I sketched the changes, they were proposed, but, now I have actually installed them and I can reverse the electrode polarity without any problems with the wirefeed motor control.
Considering the economy of construction of this welder, this change may be overkill, however, it's offered as an idea for others to use as they see fit.
I don't have a photo of the modification to attach at this time.

Jeff Furman

 

[rtbcomp]

I bit the bullet and added a bridge rectifier so that regardless of the work-electrode polarity.......The motor controller then always sees the raw DC from the power supply, rather than the arc voltage.

What about connecting the input to the new bridge rectifier to the DC arc voltage? It's output will always be the same polarity but will vary with the arc voltage.

Welcome to the forum BTW!

 

[rikrobson]

i looked on the cebora site and searched for a833 this came back with a 190 compact which was 3 phase. is this the one.

if its like the 130/snap-on/boc one. with the triger and gas valve in the hangle of the torch then reversing the polartiy may be a problem due to the way the trigger for the wire feed and contactor works off the main power feed in the torch. You will need to check how the contactor works.

there are quite a few threads on this subject. when i converted my 130 to euro torch these threads helped.

http://www.mig-welding.co.uk/forum/showthread.php?t=15027
http://www.mig-welding.co.uk/forum/showthread.php?t=969

 

[jfurman]

Thanks for the greeting; I joined to share this information. I know the frustration of not being able to exercise some extended functionality I expect. Reversing the polarity is an example of a simple idea complicated by economic restrictions. For me, the expense of the modifications is cheaper than buying a gas tank, etc.

I considered leaving the choke permanently connected to one side of the power supply and switching the two leads with the bridge connected essentially across the arc, but, I realized the choke tends to isolate the arc's voltage changes especially if short circuit transfer is active. Since the motor controller uses an SCR with phase control, the only way to reset the SCR is to drop the supply voltage until the SCR lets go. The fact the motor is turning causes its back emf (running briefly as a generator) to help reverse bias the SCR. This definitely requires cyclic power interruption as in the unfiltered DC from the power rectifiers and the transformer secondary, but before the choke. The choke tries to maintain constant current by varying its voltage. The arc side of the choke has an undefined voltage waveform that depends on the welding conditions; I don't know the details of this, however I see the original circuit does this so I did the same. If the trigger switch were isolated from the electrode lead, I could have simply left the motor circuit connected in its proper polarity even when the two welding leads are reversed. Other welders, the ones with reversible polarity often do this. I decided not to discover there might not be a free contact on the trigger switch, and thus be frustrated in an effort to thread an additional wire in the cable. In keeping with the economy of the construction of the wirefeed mechanism (I'm trying to be nice here) I visualized a very limited rendition of a combined electrical switch and gas valve. I must admit I have more confidence in my ability to rewire stuff than rebuild mechanisms. When I need to repair the gun, I will discover what is really in there, but I need to develop the habit of leaving what works alone. I am happy with my modification, but the wirefeed driver is noisier than I expect...

The cited polarity mods for the Snapmig 130 Turbo show these two issues: The simplicity of the two-wire trigger switch arrangement, and the necessity for the raw DC in order to control the motor speed. The second example adds a capacitor that reduces the DC ripple considerably, at least enough so that the SCR remains conducting constantly, and the motor only runs at top speed. The fix for that is to use a parallel power supply bridge without a capacitor, that has the unfiltered DC that allows the SCR to shut off every half powerline cycle. This restores the phase control to the motor drive.

 

[jfurman]

The 833 is a very low end 110v. mig welder. It has a leaf spring to adjust the wirefeed roller pressure; the entire wirefeed side of the welder is a one piece plastic molding.
I will post some pictures of this 'hobby welder' and the wiring mod in the next day or so.

 

[R Kraft]

SCR type ? Harbor freight and some others use those.
Is it the same speed control as this? :http://www.mig-welding.co.uk/forum/album.php?albumid=918&pictureid=4389
Or is it the one with the live tip ?:http://www.mig-welding.co.uk/forum/album.php?albumid=918&pictureid=4403
Robert

 

[jfurman]

this is a hot tip, no relays, contactors, solenoids, etc
the schematic shares features of the two you show.

 

[R Kraft]

Interesting, it has no phase shifting capacitor across the pot resistance.
Can it be turned to a very slow speed, or is it one of the ones that only goes down to 60 or so amps ?
Robert

 

[jfurman]

It looks to me that the transistor comes out of saturation to let gate current through the diode. More base drive means higher collector current needed to escape saturation when the collector voltage starts to rise; since the half sinewave supply voltage gradually increases the collector current starting at zero, increasing the base drive has the effect of delaying the phase of the start of conduction. The transistor base drive is an adjustable fraction of the motor voltage. The smaller this fraction is, the higher the motor voltage must be, thus higher speed. It seems to be a stable closed loop with negative feedback. (reduced speed due to increased mechanical load -> reduced voltage -> reduced base drive -> lower collector current needed to escape saturation -> earlier start of conduction or longer conduction angle -> more motor drive -> increased speed -> ...) This is just an informal analysis, I haven't done any detailed analysis or simulation (yet.)

I'm too new at mig welding to judge the current implications of wire feed rate. The wire seems to be able to feed at maybe an inch in a few seconds or so at the slowest. The drive capstan isn't knurled, so there is just barely enough force available to push the wire. The setting for the presser roller leaves something to be desired...

 

[jfurman]

pictures of modified Cebora 883

here are some photos of the Cebora 833 with modifications to allow reversing electrode polarity. The wirefeed is visible in some of these. The support for the polarity strapping terminals is shown bolted to the rectifier plate for support and the + connection. The added bridge rectifier is bolted to the front panel below the motor speed control assembly. I'm not crazy about the compression connections of aluminum wire, but, that's what there is.

 

[jfurman]

I need to apologize regarding the model number of this Cebora welder: it's obvious from the photos it's a model 883, and I made a special trip to my garage to check this, but my old brain must have its own dyslexic ideas. (It doesn't help that the form of the digits 8 and 3 are similar.)

Definitely, if anyone is trying to find info on their own 883 model, this misnomer will not help.

 

[Wozzaaah]

I've edited the titles of the posts accordingly.

 

[Don Dunick]

I am just going thru the Chang Procedure you have put up I don't seem to have the Bridge Rectifier going I have not connected it as is I guess /I will turn it to DC as well ,Do you have a clearer Drg
Thanks Don Dunick Brisbane

 

[Cobbler]

Hi Don, the original poster has not been on here since 2010, so I'm not sure he will see your enquiry welcome to the forum by the way.

 

[Don Dunick]

I bit the bullet and added a bridge rectifier so that regardless of the work-electrode polarity.......The motor controller then always sees the raw DC from the power supply, rather than the arc voltage.

What about connecting the input to the new bridge rectifier to the DC arc voltage? It's output will always be the same polarity but will vary with the arc voltage.

Welcome to the forum BTW!

Hi Don, the original poster has not been on here since 2010, so I'm not sure he will see your enquiry welcome to the forum by the way.

It looks to me that the transistor comes out of saturation to let gate current through the diode. More base drive means higher collector current needed to escape saturation when the collector voltage starts to rise; since the half sinewave supply voltage gradually increases the collector current starting at zero, increasing the base drive has the effect of delaying the phase of the start of conduction. The transistor base drive is an adjustable fraction of the motor voltage. The smaller this fraction is, the higher the motor voltage must be, thus higher speed. It seems to be a stable closed loop with negative feedback. (reduced speed due to increased mechanical load -> reduced voltage -> reduced base drive -> lower collector current needed to escape saturation -> earlier start of conduction or longer conduction angle -> more motor drive -> increased speed -> ...) This is just an informal analysis, I haven't done any detailed analysis or simulation (yet.)

I'm too new at mig welding to judge the current implications of wire feed rate. The wire seems to be able to feed at maybe an inch in a few seconds or so at the slowest. The drive capstan isn't knurled, so there is just barely enough force available to push the wire. The setting for the presser roller leaves something to be desired...