On Sep 30, 6:55*am, "RogerN" wrote:
"DoN. Nichols" wrote in message

...





On 2010-09-29, Karl Townsend wrote:

In pic 2, it looks like label 1 _could_ be a 3-phase common mode choke,
to keep the switching transients of the SCRs/triacs out of the power
lines.

Tim suggested checking for continuity. Theres 0 ohms accross leads, so
its not an isolation transformer.

0 Ohms from side to side along one coil, or 0 Ohms between
terminals on the same side of the transformer?

If the former, then yes, it is a choke.

If the latter -- and high resistance between sides, it could
still be a transformer.

* * * * * * * * * * * * * * * * * Any use for a chocke, or just toss
it?

Keep it. *You'll reduce the hash which you put into the power
line (especially if you brew your own circuitry to turn on the SCRs),
and it will help keep the circuit from blowing away in a hurricane. :-)

But, as others have said, without the control electronics, you have
a pile of probably not very valuable spare parts.

[ ... ]

Looks like i spent the morning making a large hunk 'o junk into a pile
'o junk parts. But at least I learned a little bit. Thanks everybody.

If you have the manuals -- maybe you can restore the control
board to the SCRs and wind up with a nicely regulated power supply to
drive the servo amps.

If not -- you could perhaps hook up some wall warts to power the
gates of the SCRs to turn them on full time and at least use them as
rectifiers. *The wall warts will probably be cheaper than that many high
current rectifiers would be. *The main trick is determining how many of
those you can wire to a single wall wart (through current limiting
resistors to each gate). *I suspect that you can get away with three
wall warts -- one per bank of SCRs.

Are you going to be running this from true three phase? *If not,
you can probably get away with fewer wall warts.

Hmm ... I wonder about those banks of SCRs. *If all are the same
part number, then you have three rows of common cathode ones and three
wall warts for three phase. *However, this does not sound right for a
proper three phase bridge -- so you either have half of them with a
different part number (anode to case instead of cathode to case) in
which case the number of needed wall warts increases, or some strange
setup.

A lot better if you can get the original control board working,
and each set of terminals connected to the right SCR's terminals.
(Proably coupled pulses through small transformers on the logic board,
so you get proper isolation between SCRs.

I've got a very old VFD I'll tear down next. I know it has
electrolytic caps and I'll look for diodes.

Does the VFD still work? *What kind of horsepower is is supposed
to drive? *If it still works, I would be tempted to keep it intact. *And
if it doesn't, it is likely that the rectifiers were what failed -- or
the failure of the output MOSFETs took out the rectifiers too.

The diodes in that are likely to be combined in fewer packages,
one package being three rectifiers in a single package with common
cathode, and the other three rectifiers in a single package with common
anode instead.

The real question is whether they will handle enough current for
your needs. *Based on the looks of that original supply, it was to drive
some hefty motors. *(Hmm ... could it be that big supply was purely to
drive the spindle servo motor and the axis motors were some other
supply?

Good Luck,
DoN.

--

DoN,
I don't remember SCR's all that great but I remember once the gate voltage
goes high the SCR turns on and latches on until current goes below some
threshold. *A person gave me some small hockey puck SCR's to use to make a
rectifier for my welder. *I didn't get the details but I thought he said you
could connect the gate to the (?anode or cathode) and the SCR would turn ON
and operate as a rectifier. *I'll have to experiment before I recommend to
Karl but as soon as the SCR turns on it prevents the gate voltage from going
any higher than the (I think Anode ?) voltage. *I could try hooking up one
using a resistor to connect the gate to the anode (?) and seeing if there
are any voltage or current spikes showing up at the resistor.

What I do remember of SCR's is that you can turn them on in the rising part
of the sin wave with a simple voltage divider, and you can use a resistor
and capacitor to get a phase shift to be able to turn the SCR on almost
anywhere during the sin wave.

RogerN

SCR tutorial:
http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

The gate may have a maximum current rating and should be protected
with a series resistor.

jsw