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Post by Albertabuck on Jul 12, 2021 23:14:06 GMT -6
KenMB...or well anyone really, figured Ken might know about this given his experience with electrical.
I have an older Hobart diesel powered portable welder, DCC350P, its a military model, was used by both US and Can.
350amp, I've had it for 20 plus years, butter smooth, love that machine.
Problem is, occasionally it loses its residual magnetism. There is a system on it, where it is supposed to polarize it each time you start it, but it didn't want to work for me today. Spend a couple hours with the manual and multimeter and more, and did finally get it to spark up.
I'll take the time to get into more specific details if Ken or someone does know something and we can talk about how the hell it all works. Apparently it is a problem with different welders, particularly the older ones. My memory is fuzzy, but as I recall, this big Hobart of mine actually runs an alternator, as opposed to a generator as Lincoln ect. They're kind of a different fish, but one hell of good welder.
If anyone knows about this issue, beam me back and I'll get into details. Thanks
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Post by kenmb on Jul 14, 2021 12:33:14 GMT -6
If you have a schematic to take a picture of and post that would help AB I case I am off on a tangent.
Speaking generally right now and probably repeating most of what you know, but always good to start with basics and build from there:
Yes, rotor should have a residual magnetism. It can loose this after sitting for a long time or when a generator came to our shop for rebuild but no diesel engine so no chance to run up in shop. With a residual magnetism, once you start spinning the rotor inside a stator that is how voltage is produced. It may be a very small amount but all it has to do is have enough magnetism to start creating a voltage, if it does that, then the voltage in the stator will build up a bit which feeds voltage into the voltage regulator which feeds voltage back into the rotor via the field winding which now has a greater charge in the rotor interacting with the stator to build more stator voltage and the loop continues.
If you think you have lost residual magnetism the process is actually simple.
What you need to do is take any kind of a battery, usually it is the 12v battery from starting system but even a 6v lantern battery would work, and apply that battery voltage to the field winding for a few seconds. What a guy does is put a volt meter on the stator output and watch that while applying dc voltage to the field winding. As soon as you see the stator build voltage (let's say 20 v ac or whatever) you can remove that battery voltage applied to the field. My experience is we are talking a few seconds needed.
Basically, you are inserting an external voltage into that loop I previously mentioned. And this will work the same whether it is an ac or dc generator because the field (either the rotor or the field supplying the rotor) always works with DC voltage.
If you have, and can see, the slip rings/brushes on the rotor and are easy to get to, you could apply dc right there too. I haven't done that since I am always working with larger units and can identify and access the rotor field winding and they are usually clearly identified as + and - coming off the voltage regulator. Dont need to disconnect anything, if you find the voltage regulator output leads to the field you can apply the battery voltage to those terminals.
If you have done all this and get the unit to produce voltage, but only while you have your dc battery attached, then you need to look at other possibilities such as a bad voltage regulator or grounded field winding. This process of applying battery voltage to field (or rotor) is therefore an actual troubleshooting method. Depending on how the stator voltage responds with a separate dc supplied will aid in figuring things out.
Generally a unit does not loose magnetism. I've seen it once, but most likely something else isn't working. For example, if you do have resisdual magnetism but a voltage regulator is fried, then it can't feed voltage back into field to continue building voltage. In such a case, as soon as you remove your battery voltage, then the stator voltage falls off. Or you have a grounded field where it does seem like it lost magnetism, and you get it working, but the generator voltage falls off when hit with a load. Welders are harder to work on, I have only messed with one, it had an ac output for running an ac load like a baseboard heater so could troubleshoot it more confidently.
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Post by Albertabuck on Jul 14, 2021 13:07:40 GMT -6
Ken, thanks thats awesome you know about this stuff! I need to go baling but I will get back later on or maybe a day or two with details of what was going on and what I did ect. ect.
I got some pics and I can def scan the schematics as well so you can see the system that is supposed to keep things magnetized that is wired into the start circuit. Loss of magnetism is actually mentioned in the manual, its a common thing to this particular unit apparently, have had it happen in the past but never have much issue getting it to spark up after a restart. I appreciate your reply, I just didn't want to take the time to get all technical and explain everything first if no one knew what I was talking about lol.
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Post by Beerwiser on Jul 14, 2021 14:07:07 GMT -6
Ken, I am not sure if you have ever seen the guys convert alternators over to small portable welders or not. I am just wondering if that is the same idea that you mentioned of them running 12v to the stator to get it going? Sorry for the off topic, sort of.
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Post by kenmb on Jul 14, 2021 20:35:54 GMT -6
No, never heard of that beerwiser but I guess it could be done. A welder is a current source, which is a little different than a 3 Ph genset or alternator which is best thought of as a voltage source so some of my thinking has to change when looking at a welding generator.
AB,those welders are a different thing, I really need to put some thought into it. A 3 Ph ac generator I understand pretty well and can likely walk a guy through troubleshooting over the internet. A welder is different and have only had my eyes on one before so I have to learn a bunch of theory as to how the are supposed to work.
But if the manufacturer has built a circuit to overcome loss of residual magnetism then what I posted initially is still valid. We would call it "flashing the field" because often it is literally just momentarily touching the + and - wires from a battery to the field. Ie a flash. A guy might be able to search that phrase on the internet.
If it is simply the auto remagnetization scheme not working then we can probably work through it with some schematics and get it working. And as I mentioned, flashing the field is a valid troubleshooting step since you may need to prove what happens when you do this as there could be something else that is the issue.
Anyway, things aren't too busy here so I can look over documents and drawings. I was doing a little reading today and realizing how much I have forgotten over the last 5 to 10 years so no promises but learnt a lot more complicated stuff in the past. With good documentation and time a guy can figure most stuff out.
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