3k st alternator just stopped mid work

[tyssniffen]

took me a while to even realize it wasn't the power tool that quit.   I'm just posting here because I need some ideas on what to look for after a first check for loose wires and finding none.

I was running a big old table saw (on 120ac)  when suddenly no power... well, after checking, I see that the alt is putting out about 8volts with the engine speed the same as ever, belt still tight..

nothing seems to be loose or disconnected (and how would it put out ANY volts if something wasn't connected?) so I'm already scratching a well-scratched head. 

would anyone know where I could find detailed photos of what all the wiring looks like on one of these chinese ST 3k alts?

Tys

[Tom]

Sounds like the bridge diodes for the exciter circuit failed. Something like this will work http://www.electronicsurplus.com/general-instruments-kbpc10-02-diodes-bridge-200v-10amp

[dieselgman]

Diagrams and support are available from Central Georgia Generator web-site. I am not sure what 3kW heads they have handled and there are a number of variances in the basic design - so don't assume anything without double checking.

dieselgman

[tyssniffen]

Sounds like the bridge diodes for the exciter circuit failed. Something like this will work http://www.electronicsurplus.com/general-instruments-kbpc10-02-diodes-bridge-200v-10amp

THAT is an awesome answer... but why do you come to that conclusion?  do these things just fail??  and why would it fail?

    and, that piece on the site you linked to does not look like something from my alternator... would the same piece be this, on mine?

http://g02.a.alicdn.com/kf/HTB1_SCsHVXXXXbqXXXXq6xXFXXXN/Free-Shipping-3-phase-font-b-generator-b-font-font-b-diode-b-font-bridge-rectifier.jpg

 miraculously, I have a replacement already.

thanks for the help!

[ronmar]

Well if it is a harmonically excited ST3 the brushes and the diodes are the only "active" components in the system.  The original chinese diode bridge, at least on mine was pretty cheesey.  The generic one he linked to is probably way better quality(i replaced mine with one).  A bridge is also pretty generic and although it may look different will perform the same.  There are two AC inputs and a pos and neg DC output, pretty straight forward.

[Tom]

Yes they just fail, as mine did on an ST5 the first time the air compressor was connected. And had the same low voltage you described. Replaced with a similar one to the linked one and it's been good for 10 years now. The rectifier you've posted a photo of is for a 3 phase generator/STC. It should work if connected correctly.

[M61hops]

Hi Tyssniffin, yes the origional rectifiers are notoriously prone to failure.  By the way the bridge in the photo looks to be fpr a center tapped transformer application.  The bridge rectifiers used for ST heads only have 4 connection wires.  The odds are good that a new rectifier solves your problem.                    Leland

[tyssniffen]

So, a couple things:

one, I replaced that old rectifier (http://g02.a.alicdn.com/kf/HTB1_SCsHVXXXXbqXXXXq6xXFXXXN/Free-Shipping-3-phase-font-b-generator-b-font-font-b-diode-b-font-bridge-rectifier.jpg )with an exact, new, duplicate, and the same thing is going on - that is, I'm not getting regular 110 out of it, but only about 8volts.

I will have to go and recheck all connections, but I probably didn't do it wrong if the same lame 8 volts is coming out. 

two, wait, rectifier?? I decided to look up what I'm working on and what I see seems to say that retcrifiers cut AC down to DC... ??  why would that be there? I guess I've always ASSUMED this thing was putting out AC... and it must be, right? I mean, I've run power tools off this alternator for years now.   what am I missing?

three, I see I also have an extra .. rheostat thing. (http://1.bp.blogspot.com/-D9hCk75zQ18/VbmpPExWPCI/AAAAAAAAGgI/OkA6c21g_EE/s1600/2015-07-28%2B18.30.42.jpg )  could that have failed, and created this problem?

still hoping for help
Tys

[dieselgman]

The rectifier converts only a portion of your ac output to dc for excitation of the field windings. The voltage level in this dc circuit in effect controls the AC output voltage of the main windings.

Your rheostat control does not look like anything I have seen on standard ST heads because it is enclosed like an add-on piece. Many of the ST heads did use such a variable rheostat control so that the operator could adjust output voltage and you may be right that there is an issue with that part on yours.

dieselgman

[Tom]

Ah a clue, that box says AVR which means Automatic Voltage Regulator. The standard ST3 comes with just a diode bridge is is self regulating of voltage. An AVR would typically take the 120vac output voltage and use that to power a circuit that controls the voltage. So in your case you can either get a new AVR box or switch the system back to self regulating. This assumes that someone didn't add the AVR because the Z windings are bad.

The way a generator controls output voltage is by changing the field voltage. On an ST5 the field voltage is around 48vdc. The Z windings generate some low power AC which is rectified to DC to power the field windings. See this link for more info http://www.utterpower.com/ST_manual.pdf.

[tyssniffen]

man. this is good advice. I will swap out that AVR thing and see if that helps.  So glad I asked you guys.  I need to really learn this whole thing once and for all.

I will report back when I find some time to rewire in a few days,

[Tom]

A picture can communicate a lot of things we might not know to when we're still learning how things work.

[tyssniffen]

Sooooooo... I swapped out that AVR thing too, and nothing changed.

BUT, just to be sure, I swapped out the carbon stick/brushes, and boom, kicking out plenty of juice.    seems really odd that the power would cut out mid work over wear on those things.  and shouldn't the springs keep those rods touching the wheel until they're too short? this looks like the wear - a 1/16th of an inch at most - was sort of uneven, on half of the stick. it's also odd that I can't move those things so they sit squarely on the wheel.

now I have a new re-wired set, and will begin to build a new box/platform for all those components, and I learned a bunch about alternators.

anyway, I'm back in business.

[Tom]

Amazing, changing the brushes did it. Glad your back in operation. Were the slip rings or brushes glazed? There's almost 2.5k hours on my ST5 with the original brushes. Could you post a pic of your slip ring/brush alignment issue? I'm also curious, if you take an OHM meter and measure continuity from the wear surface of your old brushes to the lead, what reading do you get. Perhaps this is a defective brush.

[tyssniffen]

so, yeah, weird.  AND worse, after kicking out 120 for a few minutes for one small job, I started it up a second time and only got 80volts now!?!?   

I'm not familiar with what you're talking about with glazing -though my rings (is that the spinning surface?) do look pretty shiny.  here's one side with the NEW brushes:

http://1.bp.blogspot.com/-TqBfaVJ4mYc/VcWVvb0hQhI/AAAAAAAAGgo/FSrOXBiw0hU/s1600/2015-08-07%2B17.52.51.jpg

and here's from the other side:

http://2.bp.blogspot.com/-e-6PT_tssiY/VcWVvVyWBdI/AAAAAAAAGgs/NgOIpzFU6mg/s1600/2015-08-07%2B17.53.17.jpg

I'm so confused, after years of not having any trouble from the alt, I've finally got the engine going well and now all this weirdness.