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Onan Magniciter Theory FAQ


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  #51  
Old 12-13-2015, 06:06:55 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

Keith or someone else, please correct me if I am wrong, but I would think the mag amps had to operate within a narrow frequency range, so if that is true, how do you explain that the EMDs were throttled which implies their speed and thus frequency would also vary? I was under the impression that at least the early-on diesel electrics were strictly DC machines.

On another matter, I would like to amplify my early on comments in post # 46 to say that the 3-phase 277/480 volt, 4-wire magneciters employed a separate AC winding to power the exciter. For example, see the Onan wiring diagram 611C807. That was not the case for the single phase 3-wire 120/240 volt magneciters, see 611C853.
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  #52  
Old 12-13-2015, 06:40:42 PM
Newoldstock Newoldstock is offline
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Default Re: Onan Magniciter Theory FAQ

There are limits too how high the frequency can be.
How high is a question I can not answer, but I think a practical limit would be eddy currents in the core of the transformer.
I would guess at 20 to 20k, and I come by this number because audio amps with transformers operate in these ranges without serious problem.

The traction motors of most locos are DC.
It's only in the past 20 years that power electronics have come down enough in price and high enough reliability to allow for AC traction motors.
There is still new equipment made that runs on DC because its hard to beat a DC series traction for torque.

I have not looked at the exicter but if I see it and I time I will comment on it.
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  #53  
Old 12-13-2015, 08:53:59 PM
Vanman Vanman is offline
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Default Re: Onan Magniciter Theory FAQ

Quote:
Originally Posted by Leon N. View Post
Keith or someone else, please correct me if I am wrong, but I would think the mag amps had to operate within a narrow frequency range, so if that is true, how do you explain that the EMDs were throttled which implies their speed and thus frequency would also vary? I was under the impression that at least the early-on diesel electrics were strictly DC machines.
Leon, that sounds like an excellent point! Those engines operated from around ~ 300 - 900 rpm. When Newoldstock mentioned magnetic amplifiers being used in locomotives, I only guessed that one must have been responsible load control. Would LOVE to see a schematic of how they really worked. Maybe a reasonably stable AC source was derived from a rotary inverter, like on aircraft from the good era?

I believe they had three generators, driven directly off the engine.

An auxiliary DC generator that charged the battery, powered the lights and controls as well as the cab heaters. Oldies were 32 volt, but most I saw were 75 volt (64 volt battery).

An auxiliary AC generator that powered the traction motor cooling fans and the radiator fans. In this manner the speed of these fans varied with the speed (and thus the load) of the engine. I recall that the radiator fans were also thermostatically controlled. If the engine was working hard (high rpm) you could distinctly hear them turn on and off. The motor fans probably ran all of the time, except maybe in idle... I remember that the forward one was down in the nose with the bathroom lol. I don't recall if it was running when the engine was idling. I'd bet it was noisy in there at full rpm!

And of course the traction generator.

Up until around the 60's the traction generators were DC. I remember looking in on one in a small ~1200 hp switch engine. That was a BIG commutator! After that they were AC with solid state rectification.

Keith
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  #54  
Old 12-13-2015, 09:16:15 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

When I was a kid, some time in the fifties, I was very interested in trains. I had the opppoortunity to ride in the cab of a switcher and I clearly recall seeing and hearing those magnetic contractors clunk in when the switcher moved forward. Around that time I use to marvel at the New Haven RR multiple EMD diesels trying to start a long freight train. The trick the engineer used to minimize wheel slippage was to first back up just enough to take advantage of the play in all the knuckle couplers as he then reversed direction and started to move forward. Was quite impressive, belching smoke and applying sand to gain traction. I am sure those traction motors were 100% series wound DC motors!
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  #55  
Old 12-13-2015, 10:11:57 PM
Newoldstock Newoldstock is offline
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Default Re: Onan Magniciter Theory FAQ

I found a nice a picture that really simplifies the mag amp.

G is the power source and it is AC, these amps use AC to drive the load L.

The signal however is DC, and this point is important.
Most iron cores in transformers operate around a max of 100k lines of magnetic flux per inch of steel.
Bellow this is the linear region where increase in current is proportional to increase in flux and it is linear ( this means straight line on a graph ).

The mag amp operates a little differently.
Once we get above the 100k line of flux things saturate ( remember that word ) and the increase in current does not increase the flux as much at all.
The straight line climbing up on the graph flattens out.

The small signal DC we use is helping us drive the core into saturation and once we do this there is a large increase in current across L powered by the energy suplied by G.
R and B are DC components.
A small increase in DC current will skew the flux in the core so that it saturates and the current on the AC increases..

Sometimes there are other inputs added to bias the core and fine tune its operation.
My explanation is not the best but it is important you understand the idea of core saturation and how this effect the AC current on the load side.
Otherwise its just a black box full of magic smoke..

More pictures.
The flattening out of the BH curve shows us where the large increase in current for a small increase in flux density happens above the saturation point of the steel.


I posted a video that is better than any I have seen for explaining this.
But also I found the author has a web site that goes into greater detail.
Much better than I can explain.

http://sparkbangbuzz.com/mag-amp/mag-amp.htm

Way back I worked on some electric trolleys that had a mag amp to detect the slip between the front and rear trucks.
A taco generator would create a dc voltage and as long as the sum of the two remained zero on the mag amp no error voltage would be amplified and created.
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  #56  
Old 12-17-2015, 03:20:36 PM
BergmanJ BergmanJ is offline
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Default Re: Onan Magniciter Theory FAQ

Thanks Newoldstock, this should help some more folks understand the "good-ole" Magneciter.

Regards, JLB
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  #57  
Old 06-20-2016, 02:14:54 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

I would like to add some additional information about the transient response capacity of the magneciters that was deleted when Onan switched to the brushless designs employing electronic voltage regulation.

I have a copy of the original Onan rating table for the 7500 watt JB magneciter equipped machine which states the exciter nominal capacity as 400 watts at 40 volts with a capacity during transient period of 1400 watts.

That tells me that the exciter is capable of providing ~ 3 times or better than the nominal power to the 4-pole rotor during transient over loads or to look at it another way, 3 times or better than the rated full load amperage to help get motors spinning.

I cannot locate a similar rating table for the early on JCs, for example, the 12 DJC, but I did spot under the Stak Vintage Literature similar magneciter performance figures for the 10 and 15 KW Onan MJC marine application magneciters. The numbers shown there I believe most likely apply to the Onan 10 and 15 KW J-series plants. The corresponding numbers for the 10KW exciter equipped plant are 600 watts at 40 volts and a capacity during transient period of 2100 watts. Once again that is a factor of ~3 times or better than the nominal rated amperage for the 10 and 15KW magneciters.
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  #58  
Old 09-27-2016, 12:32:20 PM
armandh armandh is offline
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Default Re: Onan Magniciter Theory FAQ

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Originally Posted by Ted_Cool View Post
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...
the control reactor would [IMHO] smooth out any spikes in the AC power without adding any lag to the response.

Last edited by armandh; 09-27-2016 at 11:43:57 PM.
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  #59  
Old 10-10-2016, 07:30:11 PM
Leon N. Leon N. is offline
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Default Re: Onan Magniciter Theory FAQ

Armando introducing capacitance into the otherwise inductive mag amp circuits may result in a resonant condition which could be detrimental to the mag amp response. The Magneciter output 60 HZ waveform has a notch in both halves of the cycle which obviously introduces a second harmonic. However, I have never heard of any detrimental reports by users. And the design specification is a 3% tolerance on the output voltage. What more could one want especially when considering the ruggedness of the system.
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  #60  
Old 01-05-2017, 04:36:00 PM
BergmanJ BergmanJ is offline
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Default Re: Onan Magniciter Theory FAQ

Armandh,

I really don't think that a capacitor of any size (great, medium, or small) will do any particular good in that the rotating field inductance is already high enough to "smooth" any of the 120 Hz "spikes", or "ripple" IN the field current or resulting magnetic field itself, as it's time constant is likely many times longer than any of that 120 Hz coming from the magneciter drive circuit. It may not either create any hazard, except for Leon's previous mention of potential resonance being created. My personal response is: "Don't mess with a 'proven' thing."

I have also "messed", in my mind, with why Onan didn't put a single mag-amp "inside" the rectified control circuit, rather than using two of them "outside" the driver bridge?

Leon, any thoughts about "that" idea? Perhaps problems with DC. saturation of the core??

Regards, JLB
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