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Onan Magniciter Theory FAQ


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  #1  
Old 03-07-2008, 01:51:17 PM
Ted_Cool Ted_Cool is offline
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Default Onan Magniciter Theory FAQ

Thanks to several members for sending the Magneciter diagrams and schematics to me. Very Helpful!

After pondering the information on saturable core reactors here, and carefully looking over the schematic, I think I have the basic idea: ( 04SX and 06SX Magneciters )

1: The gate reactors limit the current supplied to the field. There are two, one for each half of the AC cycle. Each one only receives current in one direction. The magnetic flux in each reactor is always in the same direction.

2: The control windings of the gate reactors are wound to oppose the flux of the gate windings. Increased DC voltage on the control windings reduces the current flow through the gate windings by de-saturating the core.

3: The control windings are powered by the generator output. As the generator voltage increases, the control voltage increases.

4: The control reactor seems only to reduce the voltage/limit the current to the control rectifier. It is not magnetically coupled to the gate reactors. Just a big inductor.

5: The compound winding seems to be for core flux bias. It saturates the cores to move the saturation into the 'knee' of the saturation curve, where they will be most sensitive to control inputs. These are also wound to oppose the gate flux.

6: The calibrated shunt ( hand noted on my drawing as being for 06SX only ) seems to be used to set the current in the bias winding very low. The cores only need a little bias , not full field voltage.

Magneciters are pretty cool. Hard to break them, simple to trouble shoot.

Has anyone experimented with changing the values of the damping resistor to try and tighten it up? I'm thinking of replacing it with a 500 ohm rheostat and moving it around a little to see what happens. It's in parallel with the field, so it seems like there's little risk.

It seems like the optional voltage regulator filter would be a large capacitor placed at the output of the control rectifier to smooth out the control voltage. It must be pretty choppy there. Has anyone seen anything like that? The other alternative is to have the big cap right at the feed to the field. Maybe both places could use a little smoothing. Yes, I know about bleeder resistors. I don't want to weld screwdrivers or take a farad across my heart...
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  #2  
Old 03-07-2008, 07:35:29 PM
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Talking Re: Magneciter Theory...

Ted, does this mean you are now going to be the "GO TO GUY" on Magniciter questions?
I am sure Jim and Gunny would like to have someone else field the Magniciter questions.
We could even get Harry to make a "Magniciter FAQ" just for you. The rest of us think
Magniciters, are just MAGIC.
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  #3  
Old 03-07-2008, 08:16:34 PM
Ted_Cool Ted_Cool is offline
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Default Re: Magneciter Theory...

Hmmm. There's not much that can go wrong with a magneciter, other than letting the very expensive blue smoke out of the reactor coils, or the much cheaper smoke out of the diodes. I'm always glad to help out!

Along that note - upon seeing the prices that Onan wants to replace the magneciter, I've decided that I will be adding fuses in line to both sides of the magneciter AC input. 10 amps should be about right. That should help to keep the magic smoke in where it belongs...
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Old 03-07-2008, 11:05:21 PM
Donald Randall Donald Randall is offline
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Default Re: Magneciter Theory...

Ted,
Wouldn't the field windings act as a choke and provide adequate filtering for the DC pulses, or is its inductance, (inductive reactance actually), too small to have much influence? If you have an operating unit, would it be a good idea to take, and make notes of, the voltages and currents of the: field windings, control windings, and each reactor winding separately, while the generator is running, unloaded or with a specific load,for future troubleshooting reference? Or does Onan supply some baseline values? I find very few specifics on voltage, current, and component values, or their functioning, in any of their manuals in my posession. If you figured out the operation of this unit only by studying the schematics, I am impressed. Don
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Old 03-08-2008, 12:09:50 AM
Donald Randall Donald Randall is offline
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Default Re: Magneciter Theory...

Did you notice that the reactor coils are phased oppositely? (Note the dots). I understand that they induce some voltage into the control windings, and being out of phase with each other, cancel out the effects of the induced voltage on the control voltage. Which I believe, is also why they each use only their particular half of the ac cycle. One half of the ac cycle has more influence than the other, so they use the one with the least influence on each coil. Don
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Old 03-08-2008, 02:52:56 PM
Ted_Cool Ted_Cool is offline
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Default Re: Magneciter Theory...

I haven't gone through and measured things yet, except the field voltage, around 40 volts at 30% load ( 10 KW single phase gen end ).

The reactor coils are phased oppositely, and the control windings are arranged so the control flux is always opposite the main ( gate ) flux - this provides the negative feedback mechanism.

I have reconsidered the purpose of the compound winding that loops through each of the reactor coils. I don't think it's a bias winding - those are only used when the control windings are fed AC. The magneciter control is DC. I also couldn't see anything like a calibrated shunt on my set, even though I have an 06SX magneciter. If I can't find it, I'll add it in to see if it provides tighter regulation. The current it would provide would be fairly miniscule. The name Onan gives it must be meaningful, but I haven't yet figured it out. as one reactor coil's field collapses, a current is induced, and transfers to the other reactor as it is building it's field. I haven't figured out yet whether it increases or lowers the flux of the opposite reactor coil, or what purpose it serves.
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Old 03-08-2008, 04:08:00 PM
Gunny Gunny is offline
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Default RE: Magniciter Theory FAQ

Short sweet and easy repair course on Magneciters.

If the coils are good, replace the diodes and/or SCR as needed.

If the coils are bad (smoke got out) call Power Tronics and get a replacement regulator that replaces the whole thing. Way far cheaper.

Magneciters, thats as close to smoke and mirrors as anything I've ever encountered. Whoever dreamed up that idea was either a genius or had some very good pharmaceuticals. Same goes for the folks who designed some of those Woodward governors.

Ted cool Comment:
I agree with Gunny - If the reactor coils go bad, it's done.

Last edited by BTPost; 05-02-2009 at 01:18:54 PM. Reason: Cleaning up FAQ's
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Old 03-10-2008, 08:39:49 AM
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Default RE: Magniciter Theory FAQ

OK, after pondering the Magneciter troubleshooting guide that describes the symptoms of trouble with the compound winding, I believe I have figured it out:

The compound winding is a positive feedback loop driven by the energy of the field collapse of the reactor that just went 'out of phase'. As the voltage goes to zero, the current is reaching its peak. This transfers energy to the other reactor coil as positive feedback, countering the action of the control windings of the now 'in phase' reactor during the first half of its cycle.

This acts as a 'ratchet' of sorts in response to demand.

1: Load increases

2: Voltage drops

3: Control voltage/current drops ( less negative feedback )

4: Field current increases

5: Compound winding current increases

6: Voltage increases

7: Control voltage increases ( more negative feedback ). Counteracted in part by increased compound winding current.

8: Voltage stabilizes at higher field current.

That 1/2 cycle delay in action seems to be crucial!


1: Load decreases

2: Voltage rises

3: Control voltage/current rises ( more negative feedback )

4: Field current decreases

5: Compound winding current decreases

6: Voltage decreases

7: Control voltage decreases ( less negative feedback ). Less counteraction by decreased compound winding current.

8: Voltage stabilizes at lower field current.

I'm still not sure about the calibrated shunts that connect the compound windings. Since it's in series with the DC to the field, it acts as sort of an ammeter shunt - more field current means more current in the compound winding circuit. I'm not sure, but I think it is set up as more positive feedback. This might be to trim the action of the compounding circuit, as it is only one turn around the coil, and a second turn might be too much.

The Magneciter is an interesting magnetic balancing act. It's amazing to me that it works as well as it does.

BTPost's Comment: Like I said...... MAGIC.......

Last edited by BTPost; 05-02-2009 at 01:20:36 PM. Reason: Cleaning up FAQ's
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  #9  
Old 04-12-2008, 11:09:57 PM
Jim Rankin Jim Rankin is offline
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Default RE: Magniciter Theory FAQ

You have to go down a ways, but there's a description of the saturable reactor/magnetic amplifier at this link
http://www.allaboutcircuits.com/vol_2/chpt_9/7.html

Of course this is just part of the magneciter system, but it shows how part of it is done and why there are two reactors and why the phasing is important.

Robert S's Comment:
you will impress me more if you can tell me if it will cook my hotdog to proper temperature!!!!!

Last edited by BTPost; 05-02-2009 at 01:22:20 PM. Reason: Cleaning up FAQ's
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Old 04-13-2008, 11:30:12 AM
VaughnSimon VaughnSimon is offline
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Default RE: Magniciter Theory FAQ

I had almost forgotten about this thread, and it is too important to be lost. Is it possible that it could be incorporated into the FAQ?

Ted Cool Comment:
Hi Vaughn,
You are certainly welcome to incorporate this into the FAQ in any way you find helpful. Thanks for doing this, and I hope it is of use to others. It does seem that the bottom line is to take care of it ( fuses/breakers ) , replace the diodes as necessary, and scrap it if anything else goes wrong.

BTPost's Comment:
Absolutely, that is what the FAQ is all about. Anyone can add a Post to the FAQ, and if anyone finds information in the FAQ, that is suspect or Wrong, they can get ahold of one of this forums Moderators, and we will fix it, or get a consensus of the "Resident SmartGuys" about the the item, and then update it.

Last edited by BTPost; 05-02-2009 at 01:25:03 PM. Reason: Cleaning up FAQ's
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