Author Topic: Lister 8/1 10kw ST head waveform smoothing. lots of pics.  (Read 7906 times)

gusbratz

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Lister 8/1 10kw ST head waveform smoothing. lots of pics.
« on: February 11, 2017, 05:39:17 PM »
for years we used my hobart champion 10kw welder/generator as a backup generator for when the power was out. we pretty much just run the whole house and show no mercy. usually i try to turn off the electric hot water heater and the pool pump but it has been known to run them both when i was at work.... this last summer the power was out for almost a week and the thing was killing me on fuel so i decided to build a lister cs generator over the winter. Got this built this winter and ran it a bit, I am very pleased with the power of the engine. i have been able to run the whole house and it only drops by about 1 hz. it seems to have plenty of power to spare. I may even change pulleys and lower it down to 650 rpm and see if it can haul the mail as good as it does now. any way it seemed like the lights on the computer power strip were flickering. then a day or 2 after a test run my well pump shit the bed. (it could be by chance as it was over 10 years old). sort of made us question the waveform of the ST head and wonder if it could be improved upon. So i bought a cheap o-scope off ebay from china and a 10x attenuation probe. took a round of readings, then added two 1000 uf DC capacitors to the DC going into the field. I thought maybe that would smooth out the waveform some.  I feel like you can see the capacitors effect on the Z winding and on the rectifier output but they ultimately seem to have no effect on the actual generator output. they don't seem to hurt and maybe they would help with starting the well pump. looking for insight as to weather or not this will help or hurt. here are my scope pics. it was fun anyway and the DSO138 oscilloscope was  well worth the 30$ i have into it, i would buy another one in a hearbeat.

grid power for reference,


hobart 10kw generator no load


hobart 10kw generator running the house


lister/ st head  output  no load, no capacitors


lister/ st head output no load,  2 capacitors on field


lister/ st head output running the house 2 capacitors on field.


Z winding output no capacitors


Z winding output with 2 capacitors on field


DC rectifier +DC output no capacitors on field


DC rectifier +DC output 2 capacitors on field




 

BruceM

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Re: Lister 8/1 10kw ST head waveform smoothing. lots of pics.
« Reply #1 on: February 11, 2017, 07:37:30 PM »
You seem to have gotten a very nasty ST head.  That much ratcheting of the output is as bad as I've seen, though I have see it in other waveforms sent to me after I made public my homebrew AVR design.  Since you have so much excess capacity in the head, you might try adding about 60 uF of motor run capacitors across the output in hopes of smoothing it a bit.  It is a crude solution that will cost you a bit in efficiency.  I would try to return this head if possible...it's output is truly awful.

Adding capacitors to the rectified harmonic output sometimes (sometimes not) removes "harmonic hump" distortion.  You don't have that as a primary issue, and so it makes no difference in the output waveform.  It will do nothing for surge loads or anything else, except that it will boost the unregulated RMS voltage.

The quality of ST heads varies greatly, but yours is in the worst 10%.  The stator laminated cores vary greatly, as well as the windings, wire used, and the quality of the case castings, etc.  I recently had 3 ST-3 stators on my workbench- all different and varying by 1.5 inches in laminated core length.  The latest replacements my neighbor bought from CGG (to replace a failed unit with aluminum windings on the rotor) had the harmonic winding so badly done that voltage was over 350 VAC.  It could only be used with an AVR.  The output waveform was however, quite good.

It's clear that the ST heads should be viewed with a great deal of suspicion.  Resellers/importers often buy from the low bid source or the same source may have little or no quality control.  The better built ones are still quite serviceable with replaced bearings and bridge diode.  Ignore CGG's claim of having done these, my neighbor's ST-3 bearings were groaning within 20 hrs, and the "improved" bridge diode was junk.  When you do get a unit with a very bad, ratcheting output, only replacement of the stator will correct it, and no one carries spares.  I would insist on all windings guaranteed to be copper, and waveform without gross ratcheting, and harmonic regulation alone withing spec., with free replacement if found faulty.  A dealer who actually inspected and tested his product (this is sadly NOT CGG) should be rewarded with a higher price.

I would suggest avoiding ST heads entirely except that there is no other 4 pole alternative withing a mile of price.  In larger heads Gary of DES has recommended Stamford clones.  For smaller heads (3-5KW it's tough and you're almost stuck with 2 pole heads like MeccAlt unless you are willing to play ST-roulette.  

Best Wishes,
BruceM




gusbratz

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Re: Lister 8/1 10kw ST head waveform smoothing. lots of pics.
« Reply #2 on: February 11, 2017, 09:52:36 PM »
hmm, bought it on ebay so return is really not an option. I did take it apart and clean and grease the bearing when i got it. so what did they do wrong in the stator windings that makes it do that? is rewinding the stator an option? would it hurt anything if i just run it like it is?

BruceM

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Re: Lister 8/1 10kw ST head waveform smoothing. lots of pics.
« Reply #3 on: February 11, 2017, 11:02:16 PM »
I don't know enough about the particular winding/stator problem that is causing this ratcheting to suggest how it might be rewound to fix it. Perhaps a rewind shop could tell you.  I'm told the more costly designs use skewed rotor or stator windings to avoid this type of distortion, but MOST of the ST heads don't have very much of it, and most generate a quite serviceable sine-ish triangle wave with a few relatively small spikey distortions.

Make sure you get a true RMS voltage reading on this head under load.  Most cheap AVR's don't do true RMS voltage regulation so with some non-linear loads, like a switching type battery charger, they can regulate way off (low).  If RMS voltage is OK with all your different intended loads, you should be OK to use it as long as your equipment isn't complaining. 




mike90045

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Re: Lister 8/1 10kw ST head waveform smoothing. lots of pics.
« Reply #4 on: February 11, 2017, 11:54:53 PM »
..... So i bought a cheap o-scope off ebay from china and a 10x attenuation probe...........

Slick scope.  Now I have scope envy, my CRT scope is dead and does not have all the nifty digital measurements.  Who makes/sells it ?  Then I can compare.

gusbratz

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Re: Lister 8/1 10kw ST head waveform smoothing. lots of pics.
« Reply #5 on: February 12, 2017, 01:48:08 AM »
the scope is a little electronics kit you build your self, they send you the board and all the bits in a bag. i guess it takes like 5 hours to solder it together. i paid an extra 5$ and some poor little bastard in china did it. then i bought and assembled the clear case, and also bought a cheap 10x attenuation probe so i could directly measure the line. dso 138 i think it was called.

http://www.ebay.com/itm/Fully-Assembled-DSO138-2-4-TFT-Digital-Oscilloscope-1Msps-Probe-Adapter-Case-/252531988039?var=&hash=item3acc145647:m:mkTeR0Hmzshj5rMw-h2NtBA

gusbratz

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Re: Lister 8/1 10kw ST head waveform smoothing. lots of pics.
« Reply #6 on: February 14, 2017, 05:00:24 PM »
so I showed my pics to the instrument tech at work and he said looks like the brushes my be pin-arcing. makes sense since it's all so new and they may need to break in. I think I will put some hours on it and check again next year.

BruceM

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Re: Lister 8/1 10kw ST head waveform smoothing. lots of pics.
« Reply #7 on: February 15, 2017, 01:25:39 AM »
You should shape the brushes with some 300 to 600 grit silicon carbide sandpaper.  Just cut a strip and wrap around the slip rings, under the brushes. The burnish the backside against the slip rings as you sand the brushes to shape.  You'll have to do one pair at a time. 

I forget that everyone hasn't already done this.   I'll be surprised but happy if it solves your problem.