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Onan Magniciter Theory FAQ


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  #61  
Old 01-20-2017, 11:54:20 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

JLB just noticed your comments. I am confused by your term DC saturation of the core? Seems to me the revolving field is essentially four coils of wire wound around four pole pieces and the amount of magnetic flux is proportional to the amount of DC current flowing in the coils. Unless my mind is slipping, there is no saturation involved. Saturation has to do with, for example, coupling or transformer action which obviously is not the case here. BTW, Onan states that on the 705 JB magneciter the normal full load DC magnetizing current for the rotor is 40 volts @ 400 watts and (here is the kicker) a transient response of 1400 watts! So from that one could reason that the full load rotor current of 400/40 or 10 amps could increase to 1400/40 or 35 amps during transient responses! That information is stated on the Onan 705JB spec sheet.
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  #62  
Old 01-23-2017, 12:58:22 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

To clarify a bit what I said about core saturation of the revolving field in post #61. Yes an iron core can become saturated magnetically speaking but I do not think that is the case here simply because the transient response of the exciter is 3.5 times the normal full load rating. Also, I do recall that the Magneciter output 60 hertz waveform has two spikes, one in the positive and one in the negative part of the cycle. This second harmonic does not appear to affect modern day appliances, best I can tell.
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  #63  
Old 01-30-2017, 09:34:12 PM
BergmanJ BergmanJ is offline
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Default Re: Onan Magniciter Theory FAQ

Leon,

Sorry if delayed: Dial-up & phone line was out most of last week!

Possible "saturation" to which I was referring was in toroid core if used as proposed "inside" field driver rectifier bridge (DC side), rather than "as-is" with two torroids "outside" (AC side) the bridge. If the DC could be controlled "inside", it would save one toroid application. Just "thinking outloud -- outside the box". Your (or anyone else's) thoughts???

Regards, JLB
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  #64  
Old 03-31-2017, 01:41:50 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

For those folks who from time to time want to add low speed, low frequency protection to their magneciter design voltage regulator I just want to add some information that they may want to review. Looking at a typical wiring diagram for the magneciters, one should identify the two terminals labeled E1 & E2 on the exciter circuit board. E1 & E2 are the generator 120 volt leads typically from the stator with E2 usually the neutral or ground lead. E1 & E2 monitor the generator output voltage and provide power to the magneciters exciter board and thus the mag amp toroid's.

Terminals F1 & F2 are the regulated DC output leads from the exciter board and are connected to the slip rings which power the 4-pole revolving field of the rotor.

One can find the magneciter connection diagrams for a specific plant/factory build spec/exciter p/n referenced at the bottom of page 69 in the Onan JB/JC Major Service manual, 967-500.

I would not recommend fusing the E1 AC power lead in an attempt to protect the mag amp coils from over heating. Doing so may provide some protection to the magneciter if operated for a undetermined time period at low RPM/frequency, but adding a fuse to E1, may prematurely interrupt the transient response of the generator.
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  #65  
Old 03-31-2017, 03:05:28 PM
Vanman Vanman is offline
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Default Re: Onan Magniciter Theory FAQ

Yep, a fast blow fuse would undoubtedly limit the machine's short time overload capacity. This is why you would select the proper dual element, time delay fuse, or a circuit breaker with the proper time current trip curve.

Indeed, if the proper overcurrent device were selected for the Magneciter supply, this would serve VERY well as protection for the output of the generator itself as well.
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  #66  
Old 03-31-2017, 03:50:34 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

Vanman technically you have a point but the $64 question is: How are you going to determine the proper size/time delay fuse or circuit breaker and where or how would you physically mount it?

As for me, a better approach is to keep the prime mover in good operating condition. And - add a generator shunt trip circuit that cuts the ignition should the frequency drop below, lets say 55 Hz.

I have installed such a device on my JB which I could easily be modified to not only disconnect the generator load but also shut off the JB ignition.
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  #67  
Old 03-31-2017, 05:18:00 PM
Vanman Vanman is offline
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Default Re: Onan Magniciter Theory FAQ

Your under-frequency shut down device sounds like a pretty slick mod! Does it also inhibit the controller from attempting to restart the engine after the ignition is cut?

That would probably come in handy in the event that your A/C unit gets short cycled. When the compressor failed to start the Onan will invariably drop down below 55 cycles due to the sustained, heavy overload. At which point your unit would shut everything down, preventing the condition from persisting for more than a few seconds. Pretty cool.

For determining fuse or circuit breaker size one would have to take some amp readings, probably ideally with an oscilloscope, to see what the peak Magneciter input current is when the set is delivering it's peak overload.

A steady state reading with the machine at full rated load would be a good start though. A dual element "slow blow" fuse rated to carry that much current continuously would likely do the trick as regards handling momentary overloads while reliably opening under sustained overloads.

For mounting of the fuse holder, I'd probably think "Where would Onan have mounted this fuse holder?". Perhaps in the controller? I'd likely get a panel mount fuse holder, so it could be readily inspected or changed as necessary.

Keith
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  #68  
Old 03-31-2017, 08:00:48 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

Vanman to answer your first question: I was not clear. My under frequency controller merely activates the generator power contactors to remove power from the connected loads if the magneciter frequency drops below 55 Hz. It has a built in time delay to handle startup and momentary drops in frequency while running.

I could easily have it also shut down the JB by adding another relay connected in series with the ignition circuits. Have not got around to doing that. Attached is a picture of my under frequency controller.

BTW,the magnetizing current to the rotor is not where one would install the circuit protection. Recall we are talking about the increase in AC current due to the drop in inductive reactance of the mag amp toroid's. Not easy to translate the AC current increase to the dc current being supplied to the rotor.

I do know from my original Onan JB magneciter spec, the exciter is rated 400 watts@40 volts dc with a peak transient response of 3x. I do not know what that translates to on the 120 VAC side or input to the exciter via terminal E1.
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  #69  
Old 04-01-2017, 01:07:18 AM
Vanman Vanman is offline
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Default Re: Onan Magniciter Theory FAQ

Is that the under frequency controller up on top of the cabinet? I see the contactors down below. Looks pretty neat! How does it work? Do you have a schematic for the controller?

Keith
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  #70  
Old 04-01-2017, 09:35:12 AM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

Vanman yes that is the under frequency controller on top of the cabinet. The under frequency controller is essentially a series R-L-C circuit tuned to 60 HZ and connected to a Schmitt trigger circuit which in turn operates a relay which is connected to the two main contractors in the cabinet. Yes, I have the schematic and parts list if interested.
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  #71  
Old 04-03-2017, 12:38:39 PM
Vanman Vanman is offline
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Default Re: Onan Magniciter Theory FAQ

Hi Leon, yes I'd love to have a schematic! I'll add it to the one I got from you for your battery charger. Looks like your generator shed would be a fun place to visit. Wish I lived closer.

I had to Google Schmitt Trigger. Interesting.

Keith
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  #72  
Old 04-04-2017, 09:44:45 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

Keith, I will write myself a note to send that off to you tomorrow. Oh, send me a PM with your email address.

Leon
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  #73  
Old 11-06-2017, 09:43:31 AM
JoeE. JoeE. is offline
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Default Re: Onan Magniciter Theory FAQ

Out of curiosity, looking for something to replace a magnaciter with, I read through this thread and found the discussion about Powertronics and their UVR500 and SE350...

After looking through their site, I find those two items on their "Discontinued Products" list...
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  #74  
Old 11-09-2017, 08:42:18 PM
zuhnc zuhnc is offline
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Default Re: Onan Magniciter Theory FAQ

Can any of the Magnecitor Guru's decipher these waveforms? One is no load, one is 50% load. 6MDJB-3R/2268R - 0368024947. I recently rewound the stator, and replaced all the rectifiers on the Magnecitor. With straight DC applied to the stator (battery only), the AC waveform looks normal, and indistinguishable from the commercial power waveform. The top waveform is AC output, as measured at E1-E2 on the Magnecitor. E1/E2 and T1/T2 are directly connected together at the end of the windings. I have the unit connected for straight 120VAC - T1/T3 connected as the hot, and T2/T4 connected to the chassis with a solid connection. The bottom waveform is DC at the F1 and F2 terminals on the Magnecitor.
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  #75  
Old 11-09-2017, 09:10:38 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

Yes, I can relate to your observations noted in post 74. Back in 1970 when I refurbished my 7.5 JB equipped with the magneciter back end I was aware of possible TV interference when running on generator power. So Onan sent me a copy of their tech bulletin T-023 dated August 1964.

This is a brief 4 page document discussing defective television reception, RFI, input waveform distortion comparing commercial power versus asynchronous operation. I never noticed any problems but Onan does discuss asynchronous generator operating equipment employing magnetic amplifiers and some battery chargers which produce a notched wave form.

I do know the magneciter produces a notched waveform very similar to what you show.

Back in 1970 I had an oscilloscope and conformed the fact that the magneciters do not produce a clean waveform but one that is rich in the second harmonic. There is a notch on both the positive as well as the negative portions of the 60 cycle waveform. Your pictures do not seem to show that fact.

That's about all I can say. If you want a copy of T-023 send me a pm with your email address and I will send you a copy.
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  #76  
Old 11-09-2017, 09:55:26 PM
zuhnc zuhnc is offline
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Default Re: Onan Magniciter Theory FAQ

There is a decided notch in the negative portion of the AC waveform where the DC on the field is lower voltage. There is also a slight notch in the positive portion of the AC waveform, where the DC on the field is higher voltage. I had to increase the time constant of the scope to observe it. My question, and concern, is the imbalance in the DC drive to the field. I would assume it would be similar on both halves of the AC waveform. I do have a copy of the T-023 document, thanks to the fine individuals whom have made all the documentation available on the twinslan site. I do not know enough about magnetic amplifiers, in spite of reading all the documentation available, to investigate a specific area for faults. zuhnc
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  #77  
Old 11-09-2017, 11:15:44 PM
Vanman Vanman is offline
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Default Re: Onan Magniciter Theory FAQ

Is it possible that you have a bad diode, and your field is being supplied only with half wave rectification?
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  #78  
Old 11-10-2017, 04:49:05 PM
zuhnc zuhnc is offline
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Default Re: Onan Magniciter Theory FAQ

Yes, I had thought of that, as well as the "bias" wire going the wrong way through one or both of the gate reactors. So, I checked all of the rectifiers (approximately 0.52v drop) and bridge diodes (approximately 0.47v drop); they all check good. Reversing the direction of the bias wire through either or both of the gate reactors resulted in output voltage drop; no change in waveforms. If someone else has a scope, pictures of the waveforms from a known, good, machine would be enlightening. On my Fluke dual-trace scope, the upper waveform is the AC output, the lower is the field input, F1 to the scope lead, F2 to scope ground. Would one think that having the same ground for both scope inputs would give these waveforms? I will have to disconnect and disable the "B" input and remeasure. I will post results here; give me a couple of days, life (SWMBO) tends to slow down the Green Disease zuhnc
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  #79  
Old 11-20-2017, 10:55:37 PM
zuhnc zuhnc is offline
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Default Re: Onan Magniciter Theory FAQ

I have figured out where the anomaly in the DC field waveform is coming from. It turns out that the Fluke Scopemeter 123 I am using has the grounds tied together internally. So, one can not measure both the AC output waveform, and the DC field waveform simultaneously - no common ground point. The Magneciter is completely isolated from the generator chassis ground. The internal meter common ground was playing games with both waveforms. Here is the actual DC field waveform, isolated. The 26V is no load, the 30V is 1/2 load. You will notice the frequency indicated is twice the actual AC output frequency; the unit is running at 60-62Hz.Click image for larger version

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  #80  
Old 11-20-2017, 11:10:21 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

Re your post 79 pictures. I presume you are showing the DC voltage applied to the rotor with pulsating ripple which corresponds to the second harmonic associated with the knotches that distort the generator output AC waveform. Onan does spec the full load rotor voltage as 40 volts@400 watts.
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