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Onan Mystique-Why so much respect?


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  #81  
Old 09-30-2015, 03:08:48 PM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

Leon ... I have seen car engines make very low power , run very rough, and low rpm (struggling to idle). But it only occurred in exstream cases. Water condensation was in distributor cap after washing hot engine, so you would either get firing of wrong plug or low spark voltage due to leakage to ground.

Don't think JB has distributor, but if high voltage is reduced then plugs may randomly not fire and rpm can go down. But I suspect your temp-stable dry shed will not allow the engine to sweat. The silcone rubber spark wires and boots of modern cars ( > ~ 1975) do a wonderfull job of keeping out sweat. Cars start even when plugs a wet with sweat here in the condensing two weeks of spring.

Once had a guy "tune up " a car we sold him, (previously I had tuned it up perfectly). He brought the car complaining of engine running very rough, not making much power. I found he replaced distributor cap with wrong one (internal gaps were too large, lost 5 KV), and timing and dwell were way off in left field. I fixed all and it ran perfect again. Engine could move car at sort of a normal speed , but it labored heavily to do so.

To protect Magniciter might design in a time delay fuse or two somewhere. So exciter can supply the transients of motor starting but not let the amp-time factor be too high to heat windings much. Thank you for your past comments about amp-time curves, that motivated me to learn more about them.

Last edited by len k; 09-30-2015 at 06:59:59 PM. Reason: spelling
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  #82  
Old 09-30-2015, 04:31:23 PM
Vanman Vanman is offline
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Default Re: Onan Mystique-Why so much respect?

Leon,

I agree with you about the genius that is the Magneciter. I had never heard of such a thing until I came to this forum, but after studying the schematic I think it's a fantastic design. What an awesome idea. The only thing I would add is a fuse to protect the magneciter in the event of a sustained overload on it, ie engine running too slow for too long. A bit surprised they didn't come that way from the factory.

I have also tried unsuccessfully to start my central AC on my 6-1/2 kw Kohler set, but not since adding a start relay and capacitor. Will have to try again now.

I have a large collection of Century repulsion start motors, including a 10 hp one, with a massive rotor. I had thought to use that one the same way you use yours. I was going to parallel capacitors with it to supply the reactive current taken by the motor. I never would have thought that a 1/2 hp would be enough! That's really cool! More experimentation is on the horizon

I would say that it isn't back emf like a DC motor that helps the generator start the A/C, but rather the added inertia. When the generator loses speed, the speed of the rotating field in the motor drops below that of the rotor, and it becomes an induction generator, adding power to the line, supplied by it's inertia. Splitting hairs I suppose

Motorhead,

I agree that the exciting current supplied by the AVR will be more under load, but I would say that as the set slows down it still could never be higher than when the AVR essentially connects it's supply directly to the field, and that that must happen every time the set is shut down, with or without load.

My mom's old motorhome, was built in '92, so presumably the Onan has an AVR? It has happened many, many times while driving all day long with the A/C on that the fuel level would drop below a 1/4 tank and we'd notice it getting warm. That set has also sat for several years, only to be started and run without any trouble at all. I had never heard anything about cleaning collector rings back then. My friend owns it now, and it still works fine. Maybe some of them are better than others?

Len k,

I read that Flight Systems post about cleaning the rings, but I'm just not buying it. It flies in the face of all the old engineering books I've read that say that the dark color the copper takes on lubricates the brush / copper interface and makes both last longer. I still say that, if companies are going to be bent on using AVRs, then they need to be designing them to work with the inherent characteristics of brush to ring contact.

Keith
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  #83  
Old 09-30-2015, 05:11:50 PM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

Leon....I haven't looked at JB schematic but at 950 rpm centrifugal switch sounds like a way to confirm engine has started to lock out starter.

For what it's worth , the electronic gov on my Onan 7NHM (similar basic gen as NHE) will shut down engine if < 1760 rpm for 30 seconds. Also kills engine instantly if > 2700 rpm.



Keith.... I think killing a NHE type reg by high ohm slip rings and shut down with load is real, too many people have reported it. In past I was going to buy a NHE in a RV that made power then sat for 3 years, but seller went to start it and reg was dead. Sounds like the problem might be in design of windings (resistance and inductance.). Otherwise I expect Flight Systems would have already designed the problem out of their AC voltage regs.

Last edited by len k; 09-30-2015 at 05:25:27 PM.
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  #84  
Old 09-30-2015, 06:02:44 PM
Leon N. Leon N. is offline
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Default Re: Onan Mystique-Why so much respect?

len k your comments and observations are well noted, Thank you. But, the way I see it, sure one could add additional protection to protect the Magneciter. However, my philosophy is to install , operate and maintain the JB in top condition as suggested by Onan. Yes, I have circuit protection on the generator output but I have not modified the basic design. If something breaks, I will have to take my chances. But note, I have been operating this Magneciter equipped JB since 1970 and the only major problem I have had was the starter shift solenoid contact failure. I replaced the solenoid shifter and all continues to operate as designed. Oh, I do recall from my early experiences, I had a mice infestation problem. I first became aware of it when the JB would start but ran very rough and indeed would not come up to speed. What I found was a electrocuted mouse wrapped around one of the spark plugs! I kid you not.

************

My AC compressor is equipped with a low voltage so called hard start kit. The name plate says it will operate between 208 and 240 VAC. Maybe that helps the JB somewhat.
My Century Electric motor is rated as 1/2 HP with @1750 RPM. So the slip frequency is just under 5%. I do know the JB frequency and votage does momentarily drop below the spec values and I suspect the heavy flywheel spinning electric motor is pumping energy into the lines during the transient. All I know is that this added spinning motor inertia added to that of the furnace fan motor does lighten the load on the JB during compressor start up.
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  #85  
Old 09-30-2015, 06:20:36 PM
Vanman Vanman is offline
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Default Re: Onan Mystique-Why so much respect?

I agree, there are too many threads on here about failed regulators for it not to be true. There's got to be a way to design them to be immune to those two types of incidents though.

Keith
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  #86  
Old 09-30-2015, 07:05:05 PM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

I have a different philosophy, if something seems wrong I redesign it and test that I'm right. I did new product development.

Got to find the source of the failures first. So far as NHE type regs the clue is the reg's one and only power output transistor (IGBT) is dead. It happened to Magnetite's reg and mine looks to be the same.

Fundamentally the Q winding power is rectified and stored in a DC cap, which is connected to rotor via one IGBT power transistor.
Control circuits tell/drive the IGBT gate when to turn on thru a 100K resistor, so I don't think any shorting currents are coming thru such a high ohm resistor. I just don't know enough about EE to understand the failure mode if it's electrical in nature VS thermal.

Found a junk large plasma TV that has a bunch of IGBT's in it that look close enough. Someday when I get a chance I can replace the IGBT and run it with dirty rings and put a scope on it and see what's going on. Got a free supply of $10 IGBTs if it dies.

Last edited by len k; 09-30-2015 at 09:40:03 PM.
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  #87  
Old 09-30-2015, 09:26:52 PM
Vanman Vanman is offline
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Default Re: Onan Mystique-Why so much respect?

Semiconductors are at the fringes of my knowledge. Probably part of why I prefer simpler solutions lol. They can do wonderful things, but I feel that they are over applied.

Some of those devices are sensitive to excess voltage at the gate, aren't they? But you said there's a 100k resistor there, so.... IDK.

But maybe the idea of the highly inductive field winding putting a high voltage across something that shouldn't have high voltage on it when the voltage supply to the field is cut off is the problem? I have no idea

I like the idea of a diode across the brushes. That'll put a stop to that lol. Or maybe that IGBT simply needs to be a bigger one? Or both?

I still like my idea of building a saturable core reactor voltage regulator, using the core of a microwave oven transformer, and the correct number of turns on one winding in series with the field rectifier (just few enough to get it into saturation, and drop the required voltage at no load) and the correct number of turns (probably very few) of another winding in series with the load (such that the load drives the core further into saturation, thus decreasing the voltage drop to the field).

It may not work as well as a Magneciter, but it will be simple to make, and the only semiconductors will be a rectifier. Try breaking that with dirty rings or running out of gas under load lol. Then one can choose to kiss that electronic AVR goodbye!

I'm going to try it, at least. I've already harvested the guts from a dumpster microwave oven, and have plenty of rectifiers laying around

Keith
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  #88  
Old 09-30-2015, 09:37:13 PM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

Think I read magnetic amp (magneciter) used a special lamination magnetic material. Believe is has a sharp knee in it's magnetic B-H curve. Likley makes for a more sharp change in current vs bias voltage.

I thought about designing a magneciter for my NHM, but I realized it would burn up a lot of time tweaking it to get the details right. I can see that by Onan using a short piece of different gauge wire sizes as a resistor over different specs

Last edited by len k; 09-30-2015 at 09:56:04 PM.
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  #89  
Old 09-30-2015, 11:05:57 PM
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EricWood EricWood is offline
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Default Re: Onan Mystique-Why so much respect?

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Originally Posted by len k View Post
....... Someday when I get a chance I can replace the IGBT and run it with dirty rings and put a scope on it and see what's going on. Got a free supply of $10 IGBTs if it dies.
Len, with regards to the IGBT's, do you think low gate energy may be a problem, as the generator comes to a stop? Gated devices can potentially fire weak enough, to cause the current to pass through the junction plane near the gate, rather than spreading across the entire junction, and create a burn-out in the gated area. Don't know all the engineering aspects of it, but comes up quite often in failure analysis discussions.

I feel the subject of regulators failing at shutdown is limited to the smaller unit's regulators, and those of antiquity. With today's modern regulators, the V/Hz function is pretty much standard, and with those regulators, output will back off as a function of reduced hertz, and pretty much collapse by the time it gets to idle speed.
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Old 09-30-2015, 11:40:11 PM
sdowney717 sdowney717 is offline
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Default Re: Onan Mystique-Why so much respect?

It would be high gate voltages that would burn the device. Are the IGBT's burning out?

How about using a IGBT device with a higher rating, better heat sinking, a fan?, what is the part number?

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The first-generation IGBTs of the 1980s and early 1990s were prone to failure through such modes as latchup (in which the device will not turn off as long as current is flowing) and secondary breakdown (in which a localized hotspot in the device goes into thermal runaway and burns the device out at high currents). Second-generation devices were much improved. The current third-generation IGBTs are even better, with speed rivaling MOSFETs, and excellent ruggedness and tolerance of overloads.[1]
First gen IGBT burn out, well are these boards using older type IGBT's? I would think so.
Newer ones are improved.
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Old 09-30-2015, 11:57:44 PM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

Magnetite's and my IGBT are dead.

Eric ..... I don't know much about IGBT failure modes or what leads to them, except basic stuff like voltage , amps, junction temp. On Magnetite's replacement the gate breakdown voltage is a low ~ 25V, seems many are this low. But gate has an external zener on it to another IGBT lead to protect it. I'm a ME and understand some EE but the EEs I worked with handled the heavy EE details. That was some time ago and IGBTs weren't around yet.

As I recall Flight Systems NHE regs have the Volt/HZ function and their manual still carry the warning to clean slip rings or warentee is void. So I assume they haven't been able to solve the problem either.


Thermal wattage dispensations calculations seem ok, but got to get mine running and measure actual temp. My initial thoughts were use a IGBT with higher ratings and physically bigger package for better cooling. But a little looking suggests voltage drop increases as other specs increase. So wattage dissipation increases (heating).

Onan's IGBT number seems to be a proprietary one , Magnetite couldn't find any specs on it and I looked a bit too. From way he talked I think he was a EE or an electronics tech. I'll look up the Onan IGBT number tomorrow. It's in Magnetite's thead towards the end http://www.smokstak.com/forum/showth...t=99743&page=6

Last edited by len k; 10-01-2015 at 01:18:17 AM. Reason: basic stuff like voltage , amps, junction temp
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Old 10-01-2015, 12:12:36 AM
Vanman Vanman is offline
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Default Re: Onan Mystique-Why so much respect?

Again, by no means my expertise, but would a MOSFET be a better choice in this circuit? I thought those have very low forward voltage drop... Isn't that what pretty much all high power electronics (like inverters and such) use now a days?

Keith
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Old 10-01-2015, 01:14:27 AM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

I'm not good enough in EE to design that and feel comfortable I got it right, for reliability.

IGBT's are a combination of features of easy driving the inputs for FETS and lower on resistance of bi-polar transistor. I read bi-polars are harder to get the base drive and bias right.

Last edited by len k; 10-01-2015 at 02:48:54 AM. Reason: in EE to design that
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Old 10-01-2015, 01:20:22 AM
Vanman Vanman is offline
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Default Re: Onan Mystique-Why so much respect?

I have heard of IGBTs being used in very high power applications, like inverter drives for AC traction motors on locomotives. I guess they come in gargantuan sizes!

Keith
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Old 10-01-2015, 02:48:21 AM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

Hock puck size, or maybe 6 inch diameter package I think, for DC cross country power lines ( ~ 450 KV wrt ground) . DC to AC converter sub-stations.
Maybe 3+ volt voltage drop X 500 amps, water cooled.
Guessing many IGBT hockey pucks in parallel or series to take the amps, volts.

IGBT's claim to fame is it's easy to design drive to it's gate, and it has low voltage drop. So heating of it's core silicon chip is lower, so cooling requirements are lower.

Last edited by len k; 10-01-2015 at 03:02:28 AM.
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Old 10-01-2015, 05:45:42 AM
sdowney717 sdowney717 is offline
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Default Re: Onan Mystique-Why so much respect?

Most likely even absolutely, a non proprietary IGBT substitution will work. All semiconductors have operating ranges. As long as that range is not exceeded the device will function properly. Maybe do some measuring of voltages involved and just pick one to fit with high amp rating. I have subbed mosfets and rebuilt several inverters using totally different part numbers and they all worked ok. I upgraded the amp ratings.

http://www.ixys.com/documents/appnot...bt_basic_i.pdf

Likely this is the failure, and newer IGBT's are better designed, so less likely to fail
Quote:
There is a possibility of latchup due to the internal PNPN thyristor structure.
Basically they get turned on and dont turn off so burn up due to the current flow.
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Old 10-01-2015, 07:12:05 AM
Kevin K Kevin K is offline
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Default Re: Onan Mystique-Why so much respect?

Quote:
Originally Posted by len k View Post
As I recall Flight Systems NHE regs have the Volt/HZ function and their manual still carry the warning to clean slip rings or warentee is void. So I assume they haven't been able to solve the problem either.
The old Onan VR21 regulator used on the UR generator heads had a Volt/HZ function activated by changing a jumper. It's just a simple RC circuit.

For Flight Systems to sell a drop in replacement voltage regulator, they need to keep the same small package size, which I believe is part of the problem. As hobbyists, we are not constrained in the same way. If one of my regulators were to fail, I plan to fully reverse engineer it and manufacture a larger replacement with a heftier output stage and better heat sinking.
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