Help with Voltage on ST Head

[vdubnut62]

I'm not smart enough to make heads or tails of the leaflet that came with my 12kw.
Would it help to open the doghouse an snap a pic?
Ron

[BruceM]

Ron,
There was good documentation on the standard ST wiring, just nothing on the AVR for the ST-12. 
So no, doghouse photos would not help. 

If there were better documentation on the $50 AVR, it might be possible to reconfigure it to NOT use the harmonic for excitation.  Harmonic (Z winding) peak voltages are quite high- over 350 volts with a sudden surge load (measured on an ST-3).  Depending on their circuit design, a transformer on the ST output might replace the harmonic, and improve performance.  But I'd want to see more than just their hookup diagram.

BruceM

[BruceM]

Good news, I just got off the phone with Tom Osborne at Georgia Generator.  

Tom's $59 AVR has been tested to run off of 120V (no harmonic) excitation, for some of his customers with old ST's with failed harmonic windings.  This would be ideal for the ST-3 and ST-5 heads. (120V is perfect for excitation for these heads.)  Tom even offered to send me one to test, but I'm just not well enough for that right now.   If someone else with an oscilloscope and some interest would like to do an evaluation for the forum, I'd be glad to provide phone support.  

Tom is going to be adding the wiring diagram for the 120V excitation.  The harmonic is not connected, instead 120V is connected to the AVR excitation inputs.   If you are operating your ST head at 240V, then I recommend that you use a step down transformer for the excitation.  It should be at least 350VA.  You can run it off of one N-hot leg of the ST output, but I wouldn't (I don't like growling STs).  The voltage sense can be one leg (N-L1), though again using a very small step down transformer would regulate the total voltage, instead of just one leg.

Bruce M

[WGB]

Bruce do you think this AVR will do my 12KW?

Mine is 80 volt dc and think 6.4 amp, I'll check the amp draw tomorrow.
Thanks
Bill

[BruceM]

WGB, Yes, I think so.  Tom's number is on his ebay store (Georgia Generator), call and make sure. 478-457-5524 cell

Needless to say, I have no idea of the quality or durability of these units, and Tom has been unable to get schematics or other documentation for repairs.  At $59, you can't complain too much about that. 

Bruce

[vdubnut62]

Well the AVR on my 12kw sure is an itty bitty feller. With some luck "size doesn't matter"

Ron

[BruceM]

I just wish Tom had been able to get a bit more documentation on his AVRs- I normally wouldn't recommend a product so poorly documented but I don't see a lot of other choices and at least the price is a bargain. 

On the ST12, for example, 240V external (non harmonic) excitation would result in better regulation and a cleaner AC waveform.  I'd be surprised if the harmonic (Z) excitation for a surge load wasn't well over 500V.  (It's over 350DC peak after the bridge rectifier for a stock, harmonic excited ST-3.)  So in theory, an AVR which can handle the harmonic excitation on the ST-12 should be able to cope with 240V.  But I'm reluctant to recommend anything when I have zero information on the design... there isn't even a crappy user manual in pseudo-english.

Another oddity in these AVRs is that at least one seems to be switching excitation on the AC side via SCR's or Triacs.  (The unit is doing series regulation of the harmonic before the diode bridge.) This saves some cost but may cause some regulation performance problems.  Triacs and SCR's can turn on at any time, but can only turn off when the AC current goes to zero.  This could lead to issues with a change in excitation source waveform.

On the ST-12, I guess what I'd do is capture the peak DC voltage of the harmonic, after the bridge diode.  If it's 400 volts or better, then I'd try the AVR with 240V excitation, with no costly loads attached. 

I'd put a slow blow  inline fuse in the line to the field coil, on any of these units, just to protect the ST from a failure. 

[mike90045]

Are there other sources for reliable AVR's and bridge diodes ?   Or do I go to the parts bin at digikey.com and kit my own up?

[BruceM]

Bridge diodes are plentiful and everywhere, including Digikey, Mouser, Alliedelec, Jameco, etc.

AVRs suitable for ST heads-  I'd like to see some other sources, I'd be happy to at least scrutinize the documentation.  Anybody seen them??? 

[WGB]

I wish I knew what my sine wave looked like.
My bottom of the line 12kw ST came with what looked like a AVR.
The first time I spun her the unit smoked literally.
I just slammed a bridge on it, flashed the field, I was up and running.
At 59 to 61 hz I'm in mid 125 volt per leg, no growling or any problem that I'm aware of.
I think I want a AVR from what I've been reading.
Anyway I talk with Tom at GG today, great guy!
I think I'll try one just to see what how it does.
I'll put a fuse before the field.
I know I'm rambling just put in a 16 hour day.