Dead st generator

[ronsmith]

I started my 12.5 kw st gen. head for the first time today and it seems dead. The red light does not burn and the panel meter is on zero. I removed the dogbox cover and checked the lugs and got 1.5 volts one way and 3.2 volts the other. I removed the back mesh cover and checked the brushes and they are all rubbing as normal. It is wired for 220v operation. I bougnt it from Muhammed at Motors, Generators and stuff and it is a Kawazi brand gen. Any ideas?

[dkwflight]

Try replacing the diode.
Dennis

[rmchambers]

The diodes can be tested fairly easily.  The rectifier is really just 4 diodes ganged together to direct the AC current one way on the plus side and the other way on the other side between the two halves of the power going each way you wind up with DC on the output.

Take your voltmeter/tester and check between one input and one of the output legs - it will either show current flowing or not, then swap the leads around, it should work one way and not the other.  Then whichever way it works on one output it should not work on the other output leg from the same input.

Do the same between the other input leg and the output legs.  If you find one that lets juice through both ways its bad (not the normal failure mode) or one that doesn't let current through either way (open, more common) then your bridge rectifier is shot and you need to get another one.  If you have some decent power diodes laying around you can cobble one over the failed part and "fix" the bridge.

My dad used to make them back when he worked for Texas Instruments.. big buggers, high amperage stuff and mil-spec too.  I wished I could have though ahead enough to have him bring a bag full home for "testing" when I had the chance.

who knew?

Robert
You could check them with a battery, lightbulb and a bit of wire too..

[adhall]

If you decide you need a recifier bridge, here's a suitable replacement:

http://www.allelectronics.com/cgi-bin/item/FWB-254/110/25_A_400_PIV_BRIDGE_RECTIFIER_.html

And while you're at it, a heat sink wouldn't hurt any:

http://www.allelectronics.com/cgi-bin/item/HS-141/270/HEATSINK,_1.57"#34;_X_1.26"#34;_X_0.75"#34;_.html

And don't forget the thermal compound:

http://www.mpja.com/directview.asp?product=16244+HK


Best regards,
Andy Hall

[ronmar]

You say this was the first start?  If the diodes check out OK, and you have a complete circuit when measuring from the diode output thru the brushes and field winding and back to the other side of the diode output, then there may not be enough residual magnetism in the rotor to generate the initial startup field current.  You could try "Flashing" the field to see if that gets the process started.  Briefly apply 12VDC to the output of the diode rectifier where it goes to the brushes.  This should do the same thing as the diode bank would if it was providing DC.  As soon as you build a field with the battery, the stator windings will begin to output, one of these feeding the diodes and field winding sustaining the process.

[adhall]

Please note: When flashing the field, it's a good idea to disconnect the field from the rectifier before applying battery current.

There are two reasons for this:
  -- The rectifier is spared having to bear the large voltage spike which will occur when you disconnect the battery from the field.
  -- You will not risk damaging the rectifier if you happen to get the battery polarity backwards.

You only need to disconnect one of the two field wires.

Correction 2/6/2007:
I realized today that I got it wrong in my suggestion above. I have since discovered that there is no risk to the rectifier when flashing the field provided you connect your power source to the field coil with the correct polarity. You must connect + to + and - to -. If you get this backwards, you will destroy the recitifier.

Also, you do not want to disconnect the rectifier from the field because if you do, you will experience a nasty spark when you disconnect your power source from the field coil.

By the way, I ran this through "peer review" so, even if you doubt my credibility, you should still believe this.

My apologies for for the error.

Best regards,
Andy Hall

"Crow's a dish best eaten warm..."

[ronsmith]

I applied 12v dc to the two brush wires, left and right. The wires sparked and got hot when I held them on the brush wires. I hooked the positive wire to the inside brush wire and the negative wire to the outside brush wire. Positive being the one closer to the front of the generator and negative being the one closer to the rear of the generator. I did so both while the gen. was running and before startup. No voltage present at the main leads. I pulled two of the brushes back away from the commutater and checked resistance there and got about 17.5ohms. So are my windings good? Someone told me to check the two heavy wires coming from the gen. to the regulator for ac voltage and if I got about 50vac or so It was a bad regulator.

[Doug]

I would run the generator and check for AC voltage into the bridge then look for DC voltage from the bridge.
You may have forward biased the bridge when trying to flash your feild and burned out a diode.

I doubt you did anymore damage than burn out a cheap Chinese bridge that would have let you down anyway...

[okiezeke]

Ron,
I seem to have the same problem on my st 15.  No output, no harmonic output to the diodes.  I've tried all the known methods of flashing the field to no avail.  How did your problem come out?
Thanks,
Zeke

[ronsmith]

I returned it to seller and he replaced it for me. The replacment st generated just fine.

[rcavictim]

Ron,
I seem to have the same problem on my st 15.  No output, no harmonic output to the diodes.  I've tried all the known methods of flashing the field to no avail.  How did your problem come out?
Thanks,
Zeke

Zeke,

I have an idea that you should try to get your ST head to go.  Try reversing the field leads on the rectifier output.  If the residual magnetism in the core is backwards to what gets generated when the bridge tries to make some juice and feed the field this magnetism will be neutralized and then all goes dead,  Same deal would occur if the core had a sneaky magnet buried in it to help start the field build up process.  Wrong field polarity connection would kill everything.

I don`t know if I have rationalized it correctly but i think it is easy enough to try reversing two field wires on the bridge rectifier + and - outputs.

[okiezeke]

I think you're on to something with maintaining a consistant polarity to the field.  It just makes sense.  I have the ST15 operating apparantly normally using a DC motor controller to feed the field.  I have normal voltage and frequency and ran fine with around 5KW load.  I'll mark the polarity on the feed wires and stay consistant, who knows, it just might regain some residual magnetism.
Zeke