ST Head Waveform Regulation

[WStayton]

  You folks have told me, and the opinion seems to be held by most of the rest of the electronics/generator world, as well that the ST generator head has a really awful  waveform with lots of harmonics and other "noise" caused by feeding the field coils of off the Z winding in the generator, so . . .

  What would happen if you threw away the Z winding, figuratively, and simply pulled the appropriate DC voltage, or really some voltage higher than what you really need, out of your battery string and ran that through  some filter appratus/circuiit to get the charger noise out of the system and then through  DC-DC converter and then through some dummy load, like a decade box, to chop it down to what is required to give you 120 Volt AC output from the generator?

   Would this work, or am I missing something here?

  Also, rcavictim has suggested that I use a three phase STC head to eliminate this problem, but 1)  I can't find anything that tells me how clean this AC is, though it IS, apparently, cleaner than an ST head, but 2) wouldn't the above method also improve the output from an STC head, since, I think it also has a field fed off of a Z winding, and wouldn't it, too, have an improved/cleaner waveform if the above was used?

  My thinking is that I'm going to have the DC Batteries there anyways, so if it improves the wave form of either head, why not use them?

  It would, of course, be an exercise in "chopping the legs off of a table" to find exactly the right voltage for the supply., but I don't think that is an insoluble problem, just a matter of a little "fiddleing". <smile>

  One thing that I am not sure about, however, is, is there a changing voltage requirement for the winding excitationn voltage to  maintain the same flux density in differnet conditions of current draw, and hence a constant output voltage?

  What did I overlook, this time?  <grin>

Wayne Stayton

[mobile_bob]

you can feed the field with a separate dc source but  you will have to vary the field current depending on load.
that is where an avr would be handy

the issue surrounding the Z winding is with the transformer effect that the Z winding has when it is coupled to the stator output winding
it distorts the flux in the air gap and backfeeds some distortion.

(basics, the actual effect is probably much more complex)

the upside of the Z winding is it is additive, in that as the current rises in the stator winding the Z winding also gains current (back to the transformer effect) which feeds the field more current which stabilizes the generators output voltage ( to an adequate extent)

you can also take  power off the stator , step it down and rectify it to dc, filter that well and feed it to the fields via an AVR, this will provide for good voltage stability over the full load range and also clean up most of the distortion because there is no Z winding interaction (provided you leave it disconnected).

bob g

[rcavictim]

Here is an o-scope capture showing the output waveform of my 24kW/30kVA 3-phase STC head under about 8 kW resistive load.

[WStayton]

rcavictim:

  Ok, I give up! <grin>

  How come the STC waveform looks SOOOO clean and the ST waveform is so dirty if they're both fed off of the Z winding (They are both fed off of the Z winding, aren't they?)

  What's the differance?  If I am just driving one set of coils to get a simple single phase or if I drive three coils to get three phase, aren't they both activated by the same thing?  And if they are both fed by the same dirty source, how come one is clean on one is dirty?

   BTW, shouldn't there be three wavesforms in the STC o'scope pic of the STC, displaced by 120 degrees from each other, or did you just show one of the three for clarity?

[mobile_bob]

RCA

refresh my tired memory, does your stc head have an avr, or is it Z winding excited?

bob g

[rcavictim]

My STC head IS Z winding excited and it does have an AVR circuit module.  I only showed one phase.  I checked the others. They all look the same.  Nice and clean. I don't have a 3 or 4 channel scope but I could have displayed two of the 3 phases in the photo. This waveform actually got cleaner under lower load.  The peaks got smoother.  Don't ask me how.  I dunno and I'm not worried about finding out if I have to divert any of my precious time from more important ongoing projects to find out.  Let someone else discover why and report back here.  

[WStayton]

mobile-bob;

 I think (always dangerous!) that it is Z winding excited, inasmuch as rcavictim has been telling me that the waveform is dirty/terrible and his, as per his o'scope, is pretty and clean and, per him, AVR controlled.  I also seem to remember from reading something from one of the head manufacturers in China, that the ST heads, at least from that manufacturer, were Z winding fed/controlled.  Also,  George Breckenridge has indicated that whatever brand of ST heads he was selling, at least in  the sizes that he was advertising on his website, were Z winding controlled.  I'm not sure if the manufacturer that I was communicating with was the same one that made the heads that George was/is(?) selling or not.

  So can I just order an AVR from some company and plug (ok, not really "plug", more like solder!) it in and solve the problem, instead of reinventing the wheel? (Which I seem to be good at undertaking! <grin>)?

  If so, who do you recommend as a source for AVR's?

[mobile_bob]

i am not sure i am a big fan of avr's, at least most of those i have seen so far

tom osborne sells an avr if i recall correctly that seems to hold up and do its job

if i were to use an avr i would want some means to disable the field if there was an underspeed condition
such as what happens as you run out of fuel.

some of the st fields can be damaged by an avr that is fighting to keep the voltage up while the engine is spooling down
over several seconds to a couple minutes.

bob g

[WGB]

Here is Toms info, great guy!

Tom Osborne
Milledgeville Aviation, INC
Generator Division
216 Airport Rd NE
Milledgeville, GA 31061
478-453-9358     
478-453-0642 fax
478-457-5524 cell
www.stores.ebay.com/georgia-generator
www.centralgagenerator.com
Email:  info@centralgagenerator.com
"We appreciate you considering us as your Supplier!"

[WStayton]

I sent Tom off an email last night - but in my typical style, I probably "buried" him in all my questions!!   I am known for not being able to say "Hello" in less that 500 words!!! <grin>

My email is down/not-responding at the mement, so I can't tell if he has answered yet or not - sometimes cheap is its own reward!!!  I suppose the answer is "What do expect for $5.50 a month?" <smile>

Oh, BTW I beg your indugence for my terrible typing - a stroke and couple of mini-strokes have left my finger co-ordination a little lacking - if it something where typing/spelling is critical (think email to your congressperson) I use a wordprocessor which nicely underlines all of my typing fubar's, but for everyday emails, I am lax enough to just type them direct and  hope that the readers are understanding /discerning enough to read around them.  If it REALLY bothers folks, let me know and I will revert to word processor and transfer.

On another side note, if you think my typing is bad, you should see my handwriting!!!  The stroke/mini's have left that SO de/mis-formed that I can't even decipher it - The bank has told me it would be better if I paid everything possible electronicly, since each of my checks, when they come thru, get kicked out of the automation process for human intervention/examination to verify the amount and confirm that it was indeed properly drawn and not the random idle scibblings of some 1st grade child!

I'll let everybody know what Tom Osborn had to say when I hear (and get his permission to tell all!)

Regardz,

Wayne Stayton

[Horsepoor]

I'd like to have an O-scope to read the wave patterns on my various generator heads, and those of my nearby friends, but I dont know very much about the type of scope I would need for such a basic operation. I went on line, did a little reading, I think an 8 bit analog handheld will do. I dont think I need more than 2 channels, etc. Does anyone have a suggestion? Like to spend no more $100 to $200 to accomplish this goal. Is this possible?

[rcavictim]

I'd like to have an O-scope to read the wave patterns on my various generator heads, and those of my nearby friends, but I dont know very much about the type of scope I would need for such a basic operation. I went on line, did a little reading, I think an 8 bit analog handheld will do. I dont think I need more than 2 channels, etc. Does anyone have a suggestion? Like to spend no more $100 to $200 to accomplish this goal. Is this possible?

I've seen some of those inexpensive pocket o-scopes on ebay and the problem with all of them is the lack of adequate input range scaling to safely deal with the kinds of voltages you will be looking at in measuring power circuits. These things often use a miniature headphone jack as the probe input.  That is just a junky toy. IME all these miniature jacks seem to be plated with some sort of metal this is non conductive in order for them to stay shiny. That causes nothing but grief when trying to measure voltages. They are sort of OK I guess for looking at logic levels and audio signals, etc. while you are in an airplane washroom at 30,000 feet or somewhere else where it would be just as inconvenient to have a real oscilloscope in front of you, but beyond the novelty factor what's the point?  Not all pocket scopes are junk however. If you can find a used Fluke ScopeMeter that would be ideal for you.

[BigGreen]

Search your local craigslist or allofcraigs.com or your local pawn shops. There are some good deals out there, you just need to do the legwork. Epay if all else fails. You don't need anything more than your basic 2ch 20Mhz scope like B&K, LG or an older Tek. Stay away from the vintage stuff unless U just have to have it. Yea, they look cool but are very touchy.

I searched allof craigs this morning and found a nice EZ here in Houston for $200. Too bad I'm not in the market. THere was also a nice 460 somewhere out there. Digitals have there advantages and disadvantages but 4 what you want to do anything will work.

If you do get one be careful not to blow it. I was using a 6.5V xformer but it was pointed out here that I was altering my signal with that and sure enough I was. I built a resistor voltage divider to measure true signal. The Xformer cleaned it up a bit whereas I am seeing the real signal across the resistor now.

[WGB]

I'd like to have an O-scope to read the wave patterns on my various generator heads, and those of my nearby friends, but I dont know very much about the type of scope I would need for such a basic operation. I went on line, did a little reading, I think an 8 bit analog handheld will do. I dont think I need more than 2 channels, etc. Does anyone have a suggestion? Like to spend no more $100 to $200 to accomplish this goal. Is this possible?

If you get one it will drive you crazy! LOL
It did me!

[Thob]

IF you're handy with electronics you can use the sound card in your computer and some software for a cheap oscilloscope.  Search for "sound card oscilloscope" for some ideas.  You should be able to easily get 2 channels that will cover the frequencies involved for line power plus harmonics.

If you don't already know the dangers of line voltages, then don't attempt this.  You could blow up your computer, start a fire, or worse, electrocute someone.  You'll need the resistor voltage divider that BigGreen discussed above - DON'T ATTEMPT TO PUT LINE VOLTAGE STRAIGHT INTO THE COMPUTER!