Help with Voltage on ST Head

[bantor]

Hi guys

During a recent test run of my setup - Cat 0.5 driving a ST5 head, I noticed a problem.  I was getting 88V on each leg.  My ST has a AVR, so I removed the "voltage sense leads" and voltage came back up to 125V.  The AVR is still in the loop, but I suspect is just passing the Z winding voltage through.  I origninally had not planned on ordering one with an AVR at all.  It was just a surprise that it was in there.

My questions are:

1) Is 125V (@60.5HZ) too high?  - No Load
2) If I turf the AVR, How do I lower the voltage?  Or do I even need to?

If it safe, I will run it in it's current config.

Thanks for your help!

[BruceM]

Your AVR isn't hooked up right, or is defective, or you'd never see 88V.   If you post a link to the AVR manual, or scan and post the manual's AVR hookup schematic page, perhaps we can help get you sorted out.

[bschwartz]

It sounds like the AVR isn't needed if you can maintain that voltage (minus normal droop) under load.
From my understanding of these ST AVRs, they can only reduce the z winding voltage (or current, I don't remember which) to bring the output voltage down, not bring it up.  125V at 60.5 Hz seems about perfect to me.

[BruceM]

Edit-  1/4/2010 Bschwartz is right, and I was wrong-  there are some new AVRs which regulate the Harmonic output.  It's a very poor way to do an AVR on an ST head, but they are being sold cheap.  What they will do is reduce the no-load overvoltage.  Read the rest of the thread for more. 


An AVR shouldn't be hooked to the harmonic winding of an ST head.  If Bantor did that, that's why his output voltage was so pathetic (88v).  The AVR gets hooked to the 120 or 240VAC output, and uses that for excitation.  Since the harmonic winding distorts the AC waveform on the ST heads, this is a good thing;  AVR controlled heads will have a much better looking waveform.

If Harmonic regulation is good enough for your needs, great.  You can adjust the overall voltage higher by adding some capacitance after the bridge diode.  You can adjust it lower by adding a dropping resistor in series with the harmonic before the bridge.  You can't fix the voltage droop with load, that's just the nature of the (primative) beast.

[WGB]

An AVR shouldn't be hooked to the harmonic winding of an ST head.  If Bantor did that, that's why his output voltage was so pathetic (88v).  The AVR gets hooked to the 120 or 240VAC output, and uses that for excitation.  Since the harmonic winding distorts the AC waveform on the ST heads, this is a good thing;  AVR controlled heads will have a much better looking waveform.

If Harmonic regulation is good enough for your needs, great.  You can adjust the overall voltage higher by adding some capacitance after the bridge diode.  You can adjust it lower by adding a dropping resistor in series with the harmonic before the bridge.  You can't fix the voltage droop with load, that's just the nature of the (primative) beast.

Bruce do you have a suggestion on a good AVR, or a schematic of a home grown?
I just read about that in Bill Rogers book, I would like to add one to my 12kw and drop the Z winding.
Thanks
Bill

Never mind Bruce I found it in another thread!
Thanks

[BruceM]

Bill,  I don't have a design for the ST-12.  That's a big beast, and the simple AVR I did for the ST-3/5 uses a very simple push button start (no 12V supply needed) that isn't OK for the ST-12.

There are some commercial AVR products out there...

[bschwartz]

Bruce, you said "An AVR shouldn't be hooked to the harmonic winding of an ST head." 
I believe on my ST, the AVR is wired to the harmonic winding.  It also connects to the 240 (or 120 depending on the version) output of the generator.  The AVR measures the output and reduces the throughput of the harmonic winding to lower the voltage output if it gets above the set point.  I don't know if it has the ability to raise the current/voltage to raise the output if it starts to drop too low.  These aren't the highest tech AVRs on the market.

[WGB]

Bill,  I don't have a design for the ST-12.  That's a big beast, and the simple AVR I did for the ST-3/5 uses a very simple push button start (no 12V supply needed) that isn't OK for the ST-12.

There are some commercial AVR products out there...

OK thanks Bruce.

[BruceM]

bschwartz,  The typical ST stock harmonic winding doesn't have any (or not much) extra excitation current to be down-regulated, so on those ST heads, trying to regulate the harmonic output would be disappointing, at best. 

There's a bunch of ST manufacturers, so it's possible that one put some extra turns on the harmonic, and then offered a basic AVR to down regulate it.  But it's never been reported anywhere before.  It would be great if you had some documentation on yours and could post it...it may be something new in the world of ST generator heads.

[bschwartz]

I don't have any documentation, but this is what I have.
http://cgi.ebay.com/ST-STC-Automatic-Voltage-Regulator-AVR-1-or-3-Phase_W0QQitemZ150394075298QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item23042f78a2

[Crumpite]

I don't have any documentation, but this is what I have.
http://cgi.ebay.com/ST-STC-Automatic-Voltage-Regulator-AVR-1-or-3-Phase_W0QQitemZ150394075298QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item23042f78a2

bschwartz,

That AVR is driven off of the 220 volt outputs of the alternator, it's not driven by the Z winding...

Daryl

[bschwartz]

Why is it in series with the windings as opposed to bypassing them?

[Crumpite]

Why is it in series with the windings as opposed to bypassing them?

That will teach me not to expand the picture and take a close look at the diagram... 

You're right, it appears to sample the AC output and then regulate the field via the Z winding...

It's better than just a Z winding for regulating, but not near as good as one that regulates off of the rectified AC output...

I wonder if the Z winding is modified in your ST5 vs. what we have in a standard ST5 with just a resistor ?

Sorry for the confusion !
Daryl

[vdubnut62]

Guys, I just bought a new 12kw st head from Tom @ Central Georgia Generator and it has an AVR built in. What the heck it was the same price with or without. Would it help to post what I got in the way of paperwork? On second thought, I think Tom has documentation posted online.
I just checked, he does, hope this helps.

http://www.centralgagenerator.com/documents/documents.php

Ron.

[BruceM]

Thank you bschartz, for posting the info on the "harmonic regulator" type AVR.  I never would have believed that an AVR designer would be so ignorant as to do such a thing- but now it's clear that at least one has.

The  $50 AVR wiring diagram and ebay ad text does indicate a Z (harmonic) winding connection.

This defeats most of the benefits of an AVR on an ST head.  Normally an AVR does not use the harmonic, which results in significantly improved waveform.  (Current flowing on the harmonic winding in an ST head causes a characteristic distortion of the AC waveform, known as the "harmonic hump".)  For the smaller heads (ST-3 and ST-5), with a decent AVR (not using the harmonic) you can also expect some flicker reduction, as the excitation is powerful enough to compensate for the compression stroke voltage droop.  

Using the harmonic as excitation on ST heads with too low a voltage under load (Z winding not providing sufficient excitation),  you'll have to add some capacitance after the bridge to try and boost the loaded voltage.  This  $50 AVR device can only reduce the harmonic excitation.

The only benefit of this device is that no-load overvoltage will be eliminated.  If you were expecting the other benefits of an AVR, you'll be disappointed.  For example, my washer and other appliances "growl" when run off of the ST-3 with harmonic excitation, but are quiet and cool on my AVR which does not use the harmonic windings at all.

I could not find any documentation on Central Georgia Generator's AVR.  I hope it is not another "harmonic regulator".