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Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Hi. I am brainstorming ways to ballast my pig and i think a saturable reactor will work best. I was thinking of taking this three phase reactor and replacing the center winding with a control winding and having the 2 outer windings, one for each 120v line kinda like this.
I have looked through this thread and it gave me some info but i still need to know if my transformer would work. It seams to me rated for a nice amount of amperage so tell me what you think and what i would have to do. Now getting into the control winding, would it be better for me to have the same amount of turns as the ballast windings or if lets say if i have twice as many control windings will i have to increase the voltage but divide the needed current in half? I am also assuming that the current on the dc winding is proportional to the current on the ballast windings?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I'm 90% sure that when the reactor starts saturating, it will "chop" the mains waveform much like a SCR/TRIAC angle controller does (it'll be a bit softer but still)
The result will be large short circuit current, poor power output and non-optimal (higher) coil heating.
Things would get better if you could connect a ballast inductor in series with the pig which would set the maximum current (without the reactor connected).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Use a small ballast choke in series with a SCR controller, then. Same result with less iron, since the ballast only needs the same inductance that the saturable reactor has when saturated. (dr. kV: this is not zero, it's greater than or equal to the inductance of the coils with the core removed)
I'm a big fan of the solid-state variac thing, I've never managed to blow a single SCR with it. Fuses and breakers always seem to pop before the SCRs do. The only drawback I'm aware of is DC offset from uneven firing.
The OP's original diagram is bass ackwards: the AC current must be such as to magnetize the core, and the way he has it, the flux will cancel. The control winding should be two windings out of phase such that the induced EMF from the AC winding cancels. This induced EMF would otherwise be thousands of volts for a reactor with a useful amount of current gain, since this implies more turns on the control windings than the load windings.
By the same token, since all three windings have the same turn count, a reactor made from an old three-phase line reactor will have a current gain of about 2: if you want 40A out, you'll need to put 20A DC at a few volts through the control winding. That's if it works: these line reactors have an airgap to prevent saturation, which is the last thing you want!
If it was me, I'd remove the I part of the core, connect all three coils in series, and just hook it up as a fixed ballast.
Registered Member #1628
Joined: Wed Aug 06 2008, 08:48PM
Location: Huntsville, AL USA
Posts: 95
If your really interested in a saturable reactor, check this out. Pretty infoormative and it explains how the current is controlled and what kind of turns ratio you need to be looking for.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Bamacoiler wrote ...
If your really interested in a saturable reactor, check this out. Pretty infoormative and it explains how the current is controlled and what kind of turns ratio you need to be looking for.
Yes, the page you reference was made by Carl Litton of the Memphis HV Group, who posted this idea after I suggested he try it and then decided not to credit me when he discovered that it worked Needless to say, I have a rather low opinion of him and his group! The picture of the "naked" pole pig at the bottom of that page is also an uncredited picture of one of my pole pigs!
Anyway, this schematic does work, but it IS dangerous; if you look at what you're actually doing, you're feeding low-voltage DC into the HV windings of MOTs under power, and you're counting on their high voltages to cancel out. This will never quite happen, so your control needs to be designed with this in mind. For example, isolation on the AC line feeding the control is a good idea, as is transient voltage suppression on the control. And don't forget a long plastic handle on that variac!
Since proposing this idea I haven't tried scaling it up, but the third schematic in the following image is where I'd intended to go: http://silicon-arcana.com/MMSR.gif
Note that the pig neutral is connected to the AC line neutral, and the 120V legs are controlled by separate banks of MOTs which are subject to the same control source. One advantage I see with this arrangement is that the even distribution of voltage across the transformer's LV windings would (I think) serve to somewhat limit the degree to which the "error" in the MOT HV windings would accumulate, hopefully limiting the amount of AC seen at the control. Plus, it just gave me a warm, fuzzy feeling inside to think that I could actually connect the neutral instead of letting it float, but that also could have been a premonition of being electrocuted
If you try this, BE EXTREEEEEMELY CAREFUL!! You MUST regard the control side as dangerous and take precautions.
Cheers, Aaron, N7OE
EDIT: I should add, I suppose, that the reason why I never proceeded to implement a large-scale version of this is that I decided other methods were easier and safer. Slide chokes, for example, are easy to build. And, despite their lossiness, I am actually a big fan of resistor ballasts, having built one with little more than a garbage can full of water and a few PVC and copper pipes. Sure, it heats up, but man is it smoooooth. Slide chokes can be almost as easy to build. Just put a few hundred turns of #6 on some six-inch PVC and throw a few bundles of ceiling wire in there. Remove or add bundles as needed for more/less current. Lots of people agonize over what to use for the core material until they ultimately give up. Just toss some wire or welding rods or whatever in there. It'll heat up a little, but who cares; you're not going to run the thing 24x7!
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
J. Aaron Holmes wrote ...
EDIT: I should add, I suppose, that the reason why I never proceeded to implement a large-scale version of this is that I decided other methods were easier and safer. Slide chokes, for example, are easy to build. And, despite their lossiness, I am actually a big fan of resistor ballasts, having built one with little more than a garbage can full of water and a few PVC and copper pipes. Sure, it heats up, but man is it smoooooth. Slide chokes can be almost as easy to build. Just put a few hundred turns of #6 on some six-inch PVC and throw a few bundles of ceiling wire in there. Remove or add bundles as needed for more/less current. Lots of people agonize over what to use for the core material until they ultimately give up. Just toss some wire or welding rods or whatever in there. It'll heat up a little, but who cares; you're not going to run the thing 24x7!
well seeing how difficult it may be to make the saturable reactor from that particular reactor i am starting to shift my ideas to a water ballast or a slide choke ballast. can you elaborate on how you made the water ballast? size of trash can, size of electrodes, or any other thing that i may need to know. as a more permanent solution when i can get my hands on some #6 wire i will probably make a slide choke ballast but for right now just drawing arcs, i think a water ballast will do.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
teslacoolguy wrote ...
can you elaborate on how you made the water ballast?
Certainly. I wish I had some good photos of mine to share. It's been in pieces since I moved last year, but I'll try to whip up a diagram of it and post it here later tonight. It's really very, very simple. Nothing is too critical; I just made it all up as I went. There are two copper pipes held in parallel and immersed in a plastic garbage can full of water. One pipe has a PVC sleeve over it that slides up and down, allowing more or less of that particular pipe to be exposed to open water. My garbage can was ~40gal, I think, but I'll check when I get home. Anyway, after building the copper pipe assembly and filling the can with water, I just added baking soda until the maximum current was what I wanted it to be: About 40A at 240V. Since conduction begins at the bottom of the assembly when the PVC sleeve is raised, convection currents make very efficient use of the total water volume in the can, heating it uniformly. Thus, for longer runs, the design should scale well (i.e., get a bigger can and/or use more water). I was actually thinking that the next one of these I build will use multiple pipe pairs wired in parallel so that the heat production is more distributed, and also so that less doping of the water (with baking soda) is required.
Eventually, the copper oxidizes a bit and the pipes and water become greenish in color. This didn't seem to impact performance too much. In fact, I didn't discard the water until things started growing in it
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
I've always seen the slide choke as probably the easiest to control the current. I've seen some ugly ones around (big and bulky), but I reckon with a bit of creativity it shouldn't be too hard to make it into a practical design.
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saturable reactor (now water ballast)
Needless to say, I have a rather low opinion of him and his group! The picture of the "naked" pole pig at the bottom of that page is also an uncredited picture of one of my pole pigs! 
