Wire speed temperature sensitivity

[malcolm]

I'm baffled. My little welder is one of the type that adjusts wire speed with voltage setting. It used to be that I could set it to about 5.5 on the dial and weld happily all day on any voltage setting.

It still likes to start off at about 5.5, but after about 2 inchs of welding it prefers to be nearer 5 (making it tricky to weld small pipes), and by the time I've welded around a foot (in distance) it wants to be at about 4 where it settles down and is happy.

My thought was resistance in cable joints, but the thing is only 1 year old, and I've had all the joints apart and back together and no difference. Checked resistance all over the place and there doesn't seem to be a high resistance anywhere.

I'm completely baffled! Is there anything electrical that might cause this sort of thing - or any control measures that might be added to get around them? Or should I keep looking for bad connectors or dry solder joints?

 

[shenion]

What unit?

If it is the SIP, then I'm not surprised.

 

[irondarren]

Malcolm, have you tested the wire feed. I mean when you start out at 5.5 run the wire out for 10 seconds, measure the length put out time by 6 to get your Mtrs per min. then latter when it warms up and you have to reset the wire feed do it again and compare the mtrs per min. it may be as the thing warms up there is less resistance in the wire feeder control. it might narrow it down a little, to what the problem is

 

[Hitch]

Our old AGA migs have that problem.... the problem is, there ****ed

 

[malcolm]

It's a Portamig 181 rather than some untrusty SIP. Run about 3kg of wire through from new so unlikely to have worn out. It's got to be the wire speed as it feels like it's on the same voltage setting after I've corrected the wire speed (before then it feels like the wire speed is too fast).

I know it's ****ed - just after suggestions for fixing it - doubt it's anything serious just some little electric fault that needs diagnosing.

 

[shenion]

Maybe brushes on the feed motor?

 

[malcolm]

Would be interesting to measure the voltage into the wire feed motor before and after it's got hot - that would say if it's control rather than the motor.

I guess then voltage into the PCB would say if it's that or a problem in the PCB itself or it's supply. Though I'm not entirely sure which feed in to measure - if it's variable with power setting probably the voltage from the power setting switch?

I'll try that tomorrow. If we can narrow it down I'll be able to buy a new bit and be happy again..

 

[irondarren]

Think thats the answer to eliminate as much as possible, i know my old sealy 180 does the same when running for a while, you can get away with setting it at about 6.5 when cold, but as it warms up it needs to drop to 6.25. I always lived with it it was like it when new, yours is a much bigger change malcolm and it shouldnt be like that. I guess voltage to the motor is the best way to go, if it changes it will be somewhere on the pcb

 

[ozzman]

might be the reason the PCB been changed for these new fangled 1's, i reckon ive ive laid down nearly 3 meters of weld today, and mine would only start fizzing, and stubbing after i laid down about 1/2 meter continous.

 

[rtbcomp]

I would check the speed as above, I wonder whether it's something more obscure like the gas flow might be changing as it warms up and altering the speed compensates?

Are you using CO2? In this cold weather it might stop boiling off in the bottle as quickly? Have you got a flowmeter to check?

 

[snowcat]

the problem is, there ****ed

Thats my favorite tech term.. I fused all the ellectrics on site yesterday...that was the very thing I said to the sparks when he turned up to fix it...Us fabers are like poets

 

[rusty69bus]

don't know if this helps..........my little sip is fine on the wire speed until it starts to get hot then i have to turn the wire speed down abit. once it's cooled it's fine.

try starting it in the middle 5.25

 

[malcolm]

A friend did some welding while I stuck a voltmeter into the insides of the welder (don't try this at home). He managed about 3-4 feet of welding on 2mm plate.

Wire speed cold: 4.5m/min
Wire speed hot: 7.7m/min
(The difference is equivalent to increasing the wire speed by 1 on the dial).

Wire motor feed voltage (while welding) cold: 8.0V
Wire motor feed voltage (while welding) hot: 9.1V

Open circuit voltage (cold) cycling between 23.8 and 25.7V
Open circuit voltage (hot) 19.5V

Power input to the welder and into the circuit board remained at 240V cold and hot under load, though was 245V when the welder was switched off.

What do you guys think? I'm thinking a fault with the PCB or wire feed motor as I would have expected the input to the wire feed to drop as open circuit voltage drops. Suppose I can check the resistance of the motor windings cold and hot though I can't see them going down. So PCB?

I've attached photos of the welder and PCB in case anyone else can suggest something to measure. Maybe trimmer getting dirty? - I could adjust it but not sure if high or low resistance in one of those things would cause the trouble.

Don't want to be ordering new bits until I'm sure (though Steve might well lend me one to try). Otherwise I'll either order a new PCB or rig up an elaborate external wire feed control.

 

[rtbcomp]

Were you using a digital voltmeter? If the supply to the motor isn't pure DC a DVM will appear unstable. An old fashioned analogue meter may be better, or go the other way and use an oscilloscope.

Where was the open-circuit voltage measured?

You really need to check the supply voltage to the PCB - if that is going up & down you probably can't blame the PCB or motor.

 

[rtbcomp]

Perhaps the wire feeds faster when hot becuase the tip will have expanded, the hole in it gets bigger and so less friction for the motor to push against.

The motor voltage increasing when hot suggests it is drawing less current, drawing less current AND feeding faster also suggests less friction.

 

[malcolm]

It was a digital meter - I've got an analogue one - I'll give that a shot. I'm probably getting a bit of ac up the torch as it can give me an electric shock through the torch swan neck insulation (started after I replaced the rectifier, but doesn't seem to affect the welding)

Open circuit was measured between the contact tip and earth clamp. I measured the resistance of the torch and earth lead, the earth lead resistance increased from 0 ohms to about 1 ohm cold to hot, and the torch from 0 to 0.3 ohms.

Good point on motor friction - I'll have a fiddle, though would have to be a heck of a lot of friction to slow that great motor down to half speed.

 

[malcolm]

Can't find any source of friction - the 5kg reel was getting a bit low, but only finger tight on the tensioner. Swapped it out for a 15kg reel of 0.8mm and new tip and that took more of a pull to get it through the torch. The spool tension seems the controlling factor (once the wire rollers are disengaged), and having it as loose as I dared it made no difference to the problem.

Can't turn the feed roller that's connected to the motor, but it's geared so I probably shouldn't expect to. Doesn't rule out a fault in the motor gearbox. Suppose I could try the motor from the SIP.

 

[rtbcomp]

I was thinking more of heat-related friction at the tip, as it gets hot & expands the friction (if any) will decrease.

 

[malcolm]

I was thinking more of heat-related friction at the tip, as it gets hot & expands the friction (if any) will decrease.

No noticeable resistance there to start with, but a new tip with the 0.8 wire made no difference. I normally find the resistance increases when the tip and wire gets hot - can grab if you really heat them up.

 

[Drains]

You could try running the wire feed motor for a few mins on it's own (not feeding wire), and seeing what happens. That could eliminate other factors - if the motor behaviour/voltage changes even though no welding has been going on... etc.

Other things not to assume, though i'm sure you havn't - that your supply is working well - dodgy plug connections etc, damaged extension (although would appear not as you've checked your supply voltage).

And how about getting in touch with the eng dept at Portamig? If you were dealing with Lincoln, Fronius etc, they'd bend over backwards to help. I'm sure portamig will too, especially when they realise so many of their machines are sold through YOUR site. (I mean, come on, use your influence, chap )

And if you don't tell them and they still come up with the goods, that's another genuine positive report to give.