Saturable Reactors as Ballast Discussion
from The Tesla List
Updated: 07-29-06 & 5-21-11
Date: Wed, 21 Dec 2005 15:05:03 -0700 - Subject: Saturable Reactors as Ballast
Original poster: "Carl Litton"
Does anyone on this list have any experience building saturable reactors for ballasting purposes? I found some older notes in the archive suggesting that a second winding on an iron core inductor can be used to introduce a variable DC voltage that will give full range control of the inductance as the core approaches saturation.
I have been able to demonstrate the effect on a small scale with a simple step down transformer by putting the primary in series with a 120 VAC ciruit and connecting the the secondary to the rectified out put of a Variac. A 2.7 Henry inductor was reduced to a little less than 1 Henry with 140 VDC in the control winding, allowing a small light bulb just enough current to give off some visible light (measured current 0.188 Amp with no DC control and no light to 0.42 Amp with 140VDC control and visible soft glow from bulb).
However, all attempts to do this on any large (20 to 250 lbs.) inductors controlling a 240 volt circuit in the 30 to 150 Amp range have been not only fruitless but have almost instantly slagged the 25 Amp bridge rectifier connected to the control winding.
I need to understand what I am missing here. Any theory or especially winding diagrams of working reactors would be greatly appreciated. I did find one article that suggested 2 AC power windings in series and in phase have to be used with 2 DC windings in series and "out of phase" with each other in order to cancel the effect of induced AC in the control winding. But here again, no practical application, turns ratios, winding configurations, etc.
Any thoughts? Thank you, Carl Litton, Memphis HV Group
Date: Wed, 21 Dec 2005 17:28:27 -0700 - Subject: Re: Saturable Reactors as Ballast
Original poster: "Mike" <induction
About the PRS (Primary Saturable Reactor) control systems, we use that often at work with induction heaters. Three usual modes of power level control are used, SCR, Thyratron and PSR.
On the PSR's three ways to effect control, one is a mag amp that takes pot input or instrument input (pyrometers, etc), the other is a simple SCR zero to ~ 90 volts, up to 25 amps available to control windings. Both of the prior are 115 volt input, 0 to 90 VDC out. The third and less costly is a simple variac and rectifier as you said.
For three phase, there is one saturable reactor per leg going to the plate transformer primary. While the three PSR's are isolated phase to phase, the DC control loop is in series with each DC winding of the 3 PSR's. Also, each usually has a 100 Ohm resistor across both ends of each DC or control winding to equalize. Typical current for a 10 through 50 kW machine, through all 3 control windings, is in the area of 4 to 5 amps. The greater current, the more "pass through". Most PSR control systems limit voltage to 90 volts figuring a group of 3 reactors will be in the loop. Expect a ~ 10 volt drop so set the XMFR taps one step below your line voltage.
It is important that you understand a few things about PSR control, first, the things are very load dependant. The plate transformer may park at minimum of ~5 to 10 percent power, even when the control is saying minimum power, with the oscillator tube as the minimum load, so PSR's rarely go all the way off. Also, they are a tad slow, it may take 2 or 3 seconds to flatten out where you set the power control pot. Fine for long term jobs but if somebody is looking for on, heat, off cycles in a .5 or 1 second window, this is not the control system to use but rather SCR or thyratrons.
If you remove the minimum load, where the plate kV meter may say 1000 volts, the plate transformer will go right up to full voltage. If I have a Tech Support call saying HV overloads, I have them remove the Osc tube filament connection so the tube won't conduct. Any current reading past that is Cap leakage, or other items like blocker caps, etc. As soon as the Filament is off the tube, the voltage goes pretty much all the way up so it's a self contained hipot.
The inductance, the transformer impedance, the load current range minimum to maximum, need to be known to get the right swing of minimum to maximum control and stay within the control range of the pot for linearity.
About the windings, most of the ones we use in rebuilding machines have a Burndey connector jumper setup so we can set 240 or 480 volt range per leg. This leaves two wires, one per leg in, one to transformer, out. The two small wires go to the DC control loop and the equalizing 100 ohm resistor across each DC winding. The input is of course on a contactor, the output connects to each plate transformer input. So, you need to know expected load current swings at least to get a PSR in the right area, else you will not have good control on it. You can often get these in the KVA range you are working at by finding an old tube induction heater with PSR in it. Some even had an outside, roll around PSR mini frame you could connect to the machine. Auctions, Ebay, you name it. Decent PSR's are about 1800 Bux each, not cheap. Slow but very smooth control, also helps wash out line spikes from getting your diode stacks. Hope this helps.
Date: Wed, 21 Dec 2005 17:28:44 -0700 - Subject: Re: Saturable Reactors as Ballast
Original poster: m.j.watts
Wouldn't the control winding have to be energized with AC to control an AC waveform?
Date: Wed, 21 Dec 2005 17:29:27 -0700 - Subject: Re: Saturable Reactors as Ballast
Original poster: BillEaver
<http://www.tpub.com/neets/book8/32m.htm>Methods of changing inductance /mag amps
Just tried this link and it works good. Best info on what you want to do with reactors.
Denis Despins, KC6TRW
Date: Thu, 22 Dec 2005 12:14:55 -0700 - Subject: Re: Saturable Reactors as Ballast
Original poster: Ed Phillips
Tesla list wrote:
>Original poster: BillEaver
>Methods of changing inductance /mag amps
>Just tried this link and it works good. Best info on what you want to do with reactors.
>Denis Despins, KC6TRW
Warning! There are problems with both Figures 2-32 and 2-33. OK in principle but in practice those circuits would result in AC current flowing in the DC control winding. More common practice would be to use a 3-leg magnetic circuit. The power winding would be the center one. Outer two windings would carry the control current and be connected in series such that the magnetic field the produce would be in the same direction in the center leg but that the induced voltages from the power winding would cancel out..
Date: Thu, 22 Dec 2005 12:15:29 -0700 - Subject: Re: Saturable Reactors as Ballast
>Original poster: email@example.com
>Wouldn't the control winding have to be energized with AC to control an AC waveform?
Yes, I have used a 12 Amp DC control current to control 240 VAC at 30 Amps. It was wound around the center I section of an E-I core (used pole xmfr core).
Date: Fri, 23 Dec 2005 12:41:15 -0700 - Subject: Re: Saturable Reactors as Ballast
Original poster: "Mike" <induction
I already made reply to your post, however in support of your information search on saturable reactor control, today at work I located and copied 4 prints of working commercial versions used in induction heaters. I then scanned them when I got back here at the lab. Please find 4 simple prints in one zip file, 4 megabytes. Each file is a JPG format picture of a 8 X 11 paper. Access path is : www.hot-streamer.com/mike2004/reactor.zip You will note that most can be wired 240 or 480, also note on a couple prints, they use "Sa" and "Fa", "Sb" and "Fb" at the terminals on the print. This format is also used on the DC winding markings. It means Start A, Finish A, etc. This lets you be certain that you are in phase when jumping them for series and 480 or parallel for 240 regarding the two AC windings. Also, the DC control winding is marked in this format for cases that have 3 saturable reactors, the control windings each have references of winding position Vs polarity. Please note typical per reactor DC control voltages are ~ 25, with three in series and each shunted with 100 Ohm resistor ~ 100 watt, the control loop voltage is usually set to max out at ~ 90 volts and 4 or 5 amps. A bit more amps on the real big machines. Next week or two, if I can cut loose from other stuff at work, I will try and use the LRC meter to get you some L values on the AC windings as well as the control windings.
Date: Fri, 23 Dec 2005 12:41:48 -0700 - Subject: Re: Saturable Reactors as Ballast
Original poster: FIFTYGUY
In a message dated 12/21/05 5:06:32 PM Eastern Standard Time,
>I need to understand what I am missing here. Any theory or especially winding diagrams of working reactors would be greatly appreciated. I did find one article that suggested 2 AC power windings in series and in phase have to be used with 2 DC windings in series and "out of phase" with each other in order to cancel the effect of induced AC in the control winding. But here again, no practical application, turns ratios, winding configurations, etc.
I have no experience with Saturable Reactors or Magnetic Amplifiers. But I have glanced at the theory before, and after your question I figured I'd take a closer look. It appears that the "out-of-phase" two-winding connection for the DC control is mandatory to prevent AC from being induced in the control side, as you have found. Moreover, doing so only suppresses the fundamental frequency and odd harmonics thereof. Therefore, the flow of even-harmonic currents in the control winding is a design issue. In fact, there's a distinction between "free-flow even-harmonic current" and "suppressed even-harmonic current" designs. To suppress the even harmonics, the DC control circuits impedance should be high relative to the induced currents. I've seen mention of filtering chokes in the DC control circuit. I guess the aim would be to achieve DC control with the lowest DC current, most control wire turns possible? The correct operation point on the magnetization curve is also important, as has already been mentioned. As to two AC ("gate") windings required, I don't see why this would be necessary. One explanation is that the texts consider a Saturable Reactor as a single DC winding with a single AC winding. Magnetic Amplifiers are built using one or more SR's, so a MA built with two out-of-phase DC windings requires two SR's. This gives the necessary configuration on the DC side, but you end up with two AC windings. There are also some distinctions between whether the AC windings are parallel or series connected. In addition, by putting a half-wave rectifier in series with each AC winding, but both connected to the load (the two rectifiers in opposite direction), AC is still run through the load, but the evil even-harmonic currents are suppressed. Each winding conducts through half a cycle.
Now for the part where I go off on wild, inexperienced speculation:
It seems to me that the current through the load from an SR/MA is much like that from an SCR controller, since it's a phase-angle controller. At a controlled point on the AC wave, the SR goes into saturation, and current through the load increases. I suppose the advantages of the SR would be that it could be very simple and very robust, requiring no tubes or semiconductors to fry (could run off a car battery). You've also already got the big chunk of iron because of the ballast. The turn on ramp from the SR might be more gentle than the sudden commutation of an SCR, so EMI, noise, and di/dt stresses would be less. OTOH, SCR controllers are very mature technology, and darn tough. they can also be very simple to install, particularly a single-phase, relatively low-voltage/low current application such as the input to a pig.
Date: Fri, 23 Dec 2005 12:42:27 -0700 - Subject: Re: Saturable Reactors as Ballast
Original poster: FIFTYGUY
Just like to add that my Miller "Dialarc" welder uses the Saturable Reactor principle. Instead of transformer taps or a system to mechanically move a core shunt, a pot on the front adjusts the control circuit. It has a schematic on the inside cover, but unfortunately, I can't get to it right now to copy it. But the Miller website has owner's manuals:
I'm pretty sure this is the one for my welder:
The schematic shows only one DC control winding, but it may really be the two out-of-phase windings shown as one. Very simple arrangement, with the front control dial being a 15W rheostat directly controlling the DC control voltage. Nice to see everything first goes through an isolating transformer (with a separate winding to derive the input to the DC control's rectifier). The Magnetic Amplifier winding interconnections look a little complex, but there's also a high/low output switch thrown in there for welder output ranges. A full-wave bridge on the output for DC welding is obviously of no use to us (unless you were cascading Magnetic Amplifiers, and needed that kind of output for the control input to another Amplifier!). Another thing I thought of: keeping the DC control circuit's impedance high makes it easier to filter (if filtering is desirable?).
Date: Sat, 24 Dec 2005 09:37:08 -0700 - Subject: Re: Saturable Reactors as Ballast
Original poster: Finn Hammer
Pls. notice that few LCR meters can supply enough current to produce a useful measurement on iron cored inductors. By exciting the windings with a typical LCR meter, the iron will be operating in the low "initial permeability" area of the B-H curve, and the readings will turn out to be too small.
Instead, you should connect the windings across a variac, with a Ammeter in series, and calculate inductance according to: u = wLi
W = Omega = 2*pi*frequency = 2 * 3.14 * 60 = 377
Cheers, Finn Hammer
Date: Tue, 27 Dec 2005 12:17:25 -0700 - Subject: RE: Saturable Reactors as Ballast
Original poster: "Carl Litton"
Thanks, Finn. What is 'u' in your equation - the input voltage?
Date: Mon, 23 Jan 2006 07:02:58 -0700 - Subject: RE: Saturable Reactors (fwd)Original poster: "Jim Mora"
We have been having an on going discussion about ballasting pole pigs and the effects of the XC/break rate in the secondary, etc reflected back into the primary ballast XL and resulting in cancellation and higher than expected currents.
I think it would be an excellent project to come to a suggested saturable reactor that would be useful to most of the list applications, Sans arc welders, home depot coils of wire, and MOT strings; if for no other reason than for safety sake. But such a plan would be great!
Excuse the cross post but your recent threads relate to just that.
Date: Tue, 3 Jan 2006 21:19:19 -0500 - Subject: Re: Saturable ReactorsFrom: Bob Paddock
On Friday 23 December 2005 11:18 am, Carl Litton wrote:
> Does anyone (Doc, Peter, Jim, etc.) on this list have any experience
> building saturable reactors for ballasting purposes?
I wrote a couple of papers that touched on these a few years ago for Circuit Cellar Online. Look at the "High Voltage" and "Metglas (Applications of Amorphous Metals)" papers in the table near the bottom of the page at: http://www.designer-iii.com/
Many of the links no longer work as the papers were written a long time ago in Internet Time, but still has some useful SR info.
Date: Fri, 17 Mar 2006 19:31:54 -0700 - Subject: Saturable Reactor, I have ordered one to test
Original poster: "Jim Mora"
I struck a deal with the guy on ebay that has real saturable reactors rated @ 10kVA. The transformer alone weighs 200 lbs. Is very much like a three phase reactor with a very big center control winding. The two outer legs are the controlled single phase. This is rated for 480 use but it looks like there are two coils per leg so hopefully it can be wired for 220v use.
Someone had a theory that controlling the outer windings with the center winding of a three phase transformer could not work along the lines that it would in essence remove itself from the three phase transformer. This would seem not to be the case. There are six more of these available. They are brand new too.
Three phase transformers seem to be in abundance as well. I believe they could be made to work nearly as well.
Take a look. ebay # 7573657637 I'll put it under a 10KVA load, vary it to the extremes, and report back, I'll also monitor the waveform. Mots still are cheaper but they don't come in a NEMA rated case, and I still contend the lash up is inherently more dangerous.
Regards to all, Jim Mora
continued at www.pupman.com in the March 2006 archive
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