ST vs Marathon head

[Craig]

Setting aside cost, what are your learned opinions of using a gen end other than an ST with a 6/1? Do people choose STs so often simply because they are inexpensive, or are there other benefits? Is light flicker a function of the gen end and would a marathon eliminate this problem?  If you were building a 6/1 set with no budget constraints which gen end would you choose?

Thanks guys, the information here is invaluable as I plan a lister generator build.

[emerald]

Hey Craig,


Any single cylinder engine will cause flicker, its not the fault of the alternator. Choosing a more expensive model wont solve that problem. A modern alternator such as a brushless type from Marathon, Leroy Somer, Stamford and so on will all have a cleaner wave form (THD) because they do not have windings which enhance the thrid harmonic. Many blame the AVR for poor performance on the ST, however the stator windings are the root cause and without harmmonics there is no output.

Once the RPM of the alternator slows down, so does the frequency and voltage and the single cylinder engine has less time to keep the RPM of the alternator where it ought to be. Engine governor is another factor but I dont want to go off topic for you.

If money was not holding the wallet strings I would choose a PMG first, brushless second choice.

[Doug]

An ST has a cost advantage.

Machining and quality are not their strength

[ronmar]

As Emerald mentioned, flicker is mainly a function of the single cylinder engine accelerating and decelerating.  Another factor when using a typical ST head, it that the basic ST head does not have a output voltage regulator.  It is harmonically excited so the more current you draw, the more field current it creates so it is self regulating to a certain extent.  This does link the voltage output to RPM, so fluxes in RPM from no load to full load result in a voltage change.  Since a 4 cycle single accelerates for 1/2 revolution, coasts for a whole revolution then decelerates for the last half revolution, the speed, and the voltage vary a bit.  Since it is typically a 3:1 ratio from engine to generator, this RPM variation occurs over 6 generator revolutions.  

A ST with an AVR, has very little flicker as the field is controlled to maintain output voltage over the RPM and load changes.  But using an AVR changes the load dynamics applied to the engine with mechanical loads being applied much more agressively.  This equates to larger frequency deviations with the stock basic mechanical governor.  I have run both, and personaly, I think I like the dynamic of the unregulated ST head over the AVR as the slight voltage drop slows/buffers the mechanical load being applied and gives the engine time to "lean into it".  I notice much less black smoke from the exhaust with the stock harmoically excited head during load changes.  With the AVR, the governer applies full throttle a lot more often to counter the rapid RPM decreases.  I also use almost exclusively compact florescent lights in my home.  It is pretty noticeable in the few incandescents I am still operating.  But with CFL's, setting in my living room watching TV, the only way I can really tell if I am on generator power is if I happen to feel the gentle thump thump thump from the engine...

Besides cost, probably the biggest reason for using an ST is the ready availability of a 1800 RPM 4 pole head.  To turn a 1800 RPM head at rateded RPM to get 60HZ using a 650 RPM engine, you need a roughley 3:1 ratio.  From a near 24" flywheel, that means you need an 8" generator pully.  Most generator manufacturers offer a 3600 RPM head.  To get 3600 RPM you need 6:1 or a 4" generator pully.  When you start getting into smaller pullies, you start loosing belt friction area and run into noise and slip/belt wear issues dealing with the massive diesel power stroke from a slow single.  

The ST head is also dead simple.  There are no real active components to fail.  Most people can't justify keeping a spare $400-$1200 regulator board on hand unless this thing is needed to keep someone alive.  And if it craps some dark and stormy night, you are probably going to loose everything in your freezer and reefer before you can get a replacement delivered.  I have a complete set of spares for my ST head on the shelf.  It consists of a $7 heavy duty bridge rectifier and a $4 set of carbon brushes.  That is it, and I can replace both on that dark and stormy night by hand with a flashlight in my teeth in about 10-15 minutes if needed.

People get all wrapped up around the axle on power stability.  "It has to be dead on 60.0000 HZ and 120.0000VAC"  The truth is, that it really dosn't. My simple ST head delivers power to my home at between 126V no load and 114V full load.  It varies between 62HZ no load and 58HZ full load.  This is quite acceptable power.  I have worked on standby power systems most of my adult life and I have looked at most of those I have worked on with an Oscope at one time or another.  I currently have 9 sites with backup generator that I oversee.  Perhaps my ST head is the exception to the rule, but it's power is not all that bad looking and is on par with the 2 new Onan 12KW LPG sets we installed at two of our sites last year.  The contractor looking over my shoulder was quite surprised when I overlayed a waveform image from my ST head to the generator he was installing(mine had a little more slot noise).  Odds are however that the Onan head was indeed made in China also:)

Mine is the only ST head I have had the opportunity to examine, and with the number of chinese producers of these things, maybe I just got lucky and got a good one.

YMMV

Ron

[Tom]

Great synopsis Ron. We should make a sticky out of this thread for the generators section.

[Craig]

Thank you all for the detailed responses, it seems as though not much is gained by paying the premium price for a Marathon end IF you get a decent ST that is.  Any thoughts on the Marathon LIMA MAC?  It is internally regulated which I suppose could be an even worse problem if something went wrong?  With a Magnaplus etc the regulator is external and can be easily replaced.  I don't know what would be required to fix the LIMA.

Ronmar-  Your point of the 1800 rpm 4 pole head is well taken, I had already decided that slow speed was best which is what got started down the lister road in the first place.  I knew I needed a generator, I knew I wanted diesel, and I knew 3600 rpms will burn up an engine more quickly and is a pain to listen to.  So I did a Google search for "slow speed diesel generator" listeroid.com and rocketboys site with the write up, pictures, and videos hooked me.  The big 'ol flywheels and sound of the engine running were all it took. 

-Craig

[ronmar]

Craig
  And don't forget the ammount of fuel a 3600 RPM gasser consumes...  If you havn't been there yet, you need to visit www.Utterpower.com  There is a great deal of info there on the 6/1-ST5 configuration.

Ron

[Doug]

The amount of fuel consumed has little to do with the speed of the Alternator.

Infact a 3600 rpm alternator statisticaly is more efficient than a 1800 rpm generator.

Next point a ST-5 is a little larger than needed for a 6/1. A 2.5 or 3 would be a better fit but many people feel the extra size and rotating massgives them an advantage in surge power ( starting a heavy load ). But since no one has actualty run a test its hard to say if the extra mass and potential increase in surge justifies the extra fuel burned ( no a hugely significant amount ) or added cost.

[ronmar]

The 3600RPM generator may indeed be more electrically efficient, but I was refering to the fuel consumption of a 3600RPM gasoline engine compared to a 650 RPM listeroid diesel... 

Ron

[gpkull]

posts gone wild  2 pole@ 3600 rpm = 60 hz  4 pole @ 1800 rpm = 60 hz  6 pole @ 1200 rpm = 60hz  ect ect ect . rpm / # of poles = frreq.  some say st heads produce poor power. so does every other joe blow on the block .  there are ways to "clean up" the juice from an st head but a lot is blown way out of proportion.  if marathon tells you watt you want to hear jump on and hang on. for the average back up power the basics geter done. in hard times the tv, video games, and computer is not not on the top of my list for gettin by. electronics are the most finiky about "clean power". if the pump gives water ,the fridge stays cold, and the sump pump does its job how much could you ask for in a power out?

[gpkull]

i forgot to add the bigger riods with a big st head power the range/oven, ac, hot water, clothes dryer, and a few others with no issue. my power may never work the fancys in a black out but i will GETTER DONE    thats all folks

[emerald]

And dont forget two pole alternators no matter who manufacturers them are or what technology they use, even PMG do not have the smooth rotation of a dynamically balanced four rotor pole alternator. There is uneven heating in the rotor which is the cause for this and no sales man can tell you otherwise....if they even know what it means.

I looked at the Lima range you mentioned and saw nothing there to make me buy one. Its a cheaper model by the looks of it. Flashing the field is mentioned in their service manual so it is like an ST. They use a rotating rectifier unlike the ST, and thats something I see fail more often in that type of design. Otherwise its basis is that of the ST.

[Doug]

Again I don't know what to add.

Efficiency drops as you add poles, its the laws of physics at play.

How many poles you have has very little to do with how smooth machine spins or the quality of the power output.

Carrier has for many years used a two pole 2000 watt alternator ( and I guess for those I am confusing by Alternator I mean AC generator ) with diodes and MOV's all spinning around at 3600 rpm. They are quite reliable and easy to service.....

I like the slip rings on the ST head its simple and cool and let you do things ( modifications )

But given a choice I would prefer a brushless unit with a 3 phase exciter seperate on the same shaft.
This too me is the simplest of all. As mentioned Leroy makes some small heads like this I am realy fond of.