Questions about ST head operation and wireing

[Crumpite]

Folks,

I've got a 5kw ST head hooked up to my 6/1 metro.
I took the old flimsy doghouse off today to mount a new more sturdy unit.

To my surprise, the unit I took to be a solid state resistor substitute is clearly marked "AVR" on the side (where you can't see it until you disassemble the doghouse)
What is odd is that the AVR looks to be added on aftermarket, as the wiring is of a completely different style incorporating the use of black plastic tape to insulate the connections.
The AVR uses a couple of wires across the 120v lines (voltage sense and power ?) and it's other two leads are in series between one leg of the rectifier and a line going to the field on the rotor.
Does anyone know how these things work ?
I'd like to have a simple resistor to put in place in case of acts of God (otherwise known as lightning strikes around here...)
Anyone know where to get one and it's value ?

So I started to trace out the wiring so I could document it, and found something else I don't understand:

The little switch that turns the generator on and off is nothing but a switch in series with a small light bulb that lights when the unit is "on"
I looked for online wiring diagrams and found that the switch and light bulb are in series with a small winding in the stator.
Now how does a few mill-amps flowing through a winding turn the generator on and off ?

Thanks in advance,
Crumpite

[Ronmar]

Yea, the little light didn't make much sense to me.  It dosn't turn the gen on or off, the switch just turns the light off, perhaps to keep from burning it out.  The little winding powering the light is just to indicate the field is active. 

The description of the AVR circuit appears as if it is still in the harmonic winding circuit that powers the field, and it regulates the current flow from harmonic to field to maintain a steady output voltage.  My ST-5 dosn't have any resistors in the circuit.  It comes from harmonic winding to diode rectifyer to brushes and on to the field.  No other parts on mine. Most AVR circuits don't use a harmonic winding for power.  They take their power from the main output windings to feed the field. Since yours appears to use the harmonic winding to power the field,  It could be that you could disconnect the AVR module and connect the diode output wires directly to the field.  I would of course want to check the output voltage before trying to power anything with it wired this way...

[Lloyd]

Is it an "exciter" circuit...it sounds like a alt/gen light exciter?

Lloyd

[Crumpite]

Is it an "exciter" circuit...it sounds like a alt/gen light exciter?

Lloyd

The exciter on an automotive alternator feeds 12dc to the field to get it started.
This light just lights up from a winding on the stator, no voltage is fed anywhere.

I think that Ronmar hit it on the head, it's there to indicate that you're getting power to the field and that it can generate - no light = no field = no power.
It could be pretty handy in troubleshooting.
As to why the switch is there, it's either to extend lamp life, or just to give you a feeling of control. 
A bit strange, IMHO.

I've seen serious things written on the exact sequence of switches to throw to make sure you don't loose your residual magnetism.
Any such thing including that little switch on the panel must be just so much pure hokum. 

I didn't know that you could run without a resistor, everyone seems to mention them, but I did find wiring diagrams that didn't use one.
That's another good thing to know if your AVR craps out just when you need it.The resistor might be another one of those feeling of control things.

Thanks all, my confusion level is a bit lower now !
Crumpite

[mobile_bob]

i have seen the warning about never shutting down an st head with a load attached, because you will lose residual magnetism
and will have to flash the field to get it restarted.

my st7.5 has been shutdown many, many times with the load attached, it is a standard harmonic excitation and never fails to restart
although sometimes it take 15 to 20 seconds to do so.

of those heads that lose their residual magnetism, anyone just spin them up an wait to see if they will come back to life
or do you simply assume they need to be flashed and shut down too soon?

just curious

bob g

[potter]

I always shut mine down  with about a 1200 watt load (battery charger) on it the only time it failed to restart and I lost excitation  was when the slip-rings were dirty and I had to clean and flash. Also it has shut down under full load quite a few times overheat e.g. no problems.

    Potter
 

[JohnF]

It doesn't seem to be as critical a problem as some think it is.  I have had mine shut down a few times (I try not to do it) under load and they fired right up again with no problems.  I have also had heads lose the residual magnetism and have tried starting them and running over speed for a few minutes to see if they would come back - it has happened once or twice, but mostly I have had to flash.

[Ronmar]

The ST's that dont use a resistor are a balance act between harmonic winding and field load needed to get a particular output.  I would definitly try it without the AVR before you needed it.  It may be that the model with the AVR has a different harmonic winding than the non AVR ST, and won't make useable power without the AVR in circuit.

[Crumpite]

The ST's that dont use a resistor are a balance act between harmonic winding and field load needed to get a particular output.  I would definitly try it without the AVR before you needed it.  It may be that the model with the AVR has a different harmonic winding than the non AVR ST, and won't make useable power without the AVR in circuit.

I'd be a whole lot more worried about getting, say, 180 volts out, while my appliances fry !

Crumptie

[cschuerm]

Same concern here.  An overvoltage event could be REALLY expensive at my house.  I have a Timemark over/under-voltage sensor connected to a shunt trip breaker to protect things.

Chris

[Crumpite]

On the other hand, I consider it an unlikely failure mode on an AVR and a nearly impossible on a harmonic drive/resistor regulation system.

I *will* have to see how much one of those over-voltage relays cost though !

Crumpite

[bschwartz]

My ST-5 has an AVR that gets it's power from the 240 (my first got it from one leg 120).  It then connects between the field winding and diode bridge.  It gets it's power, and voltage input from the 240v and regulates the field.  It can be taken out and a high current rheostat installed in it's place.  I played with a 10 ohm 150 watt similar to this. 
http://cgi.ebay.com/7-5-Ohm-150-Watt-Rheostat-/250560907214?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item3a569807ce#ht_539wt_930

It worked, but I changed to a permanent magnet head.

Another link to look at is this:
http://www.utterpower.com/vreg.htm

[vdubnut62]

That darn little light and the on/off switch is a little weird and if I had not read up on them before I got an ST head , it could have gotten me killed,
or at least made me say "sugar" when I turned it off to connect or disconnect something.
Chalk it up to cultural differences, I guess.
Ron

P.S. Why do my italics not turn off? I assumed that what one types between the brackets would be italicized, not everything that is typed thereafter.

See, even the modifications that are posted are in italics

[sailawayrb]

I will have to see how much one of those over-voltage relays cost though!
Crumpite

Easily under $100 if you roll your own.  Here's how I did mine:

http://listerenginegallery.com/main.php?g2_itemId=351

see "Engine Speed and Voltage Protection System 1 & 2".

I was more concerned about "brown out" (low voltage and potentially high current) during 6/1 shutdowns.  I wanted to ensure generator disconnect from my house wiring should this occur during an automated emergency shutdown or if I just happened to forgot to manually disconnect the generator using the transfer switch during a routine shutdown.

The heart of the system is a Magnecraft voltage sensor with time delay settable contacts (use 831VS-120A for 120VAC or 831VS-240A for 240VAC):

http://www.bb-elec.com/product_family.asp?FamilyId=225

[Crumpite]

Oh, that's right, you're the one with the Mil-Spec setup ! 

That's a little out of my price range but I'm sure I can home-brew one out of parts on hand.
(I'm disabled, no money, lots of time...)
I'm used to industrial grade control systems design.

Crumpite