Re: Old CNC controls

"D.F.S." wrote:

> From: D.F.S. <dfs@...>
>
>
> > From: Jon Elson <jmelson@...>
> >
> > >"D.F.S." wrote:
> >
> > > if it
> > > doesn't start correctly just one time, it will burn up.
> >
> > No, it will blow the breaker, pronto! It is VERY hard to burn up a
> 15
> > Hp motor!
>
> In all honesty, I'm a lot more concerned about toasting something
> in the controller.

Well, most controller manufacturers void their warranties if a rotary
phase converter
is used. The rectifiers are a non-linear load, and may seriously
unbalance your converter's
3rd phase. Of course, a 15 Hp motor will be a stiffer source of the 3rd
phase than a
smaller motor. You will likely have trouble starting the 15 Hp motor.
You may need to
use a pony motor, or a low-voltage starting system. I tried to hook up
a 17 KVA
motor-generator set at a friend's home, and we had all manner of
problems. We could
actually bring the motor up at the 480 V setting (there were relays
provided for
'reduced voltage' starting), but when we threw in the relays for the
direct 240/208
V connection, it popped the breakers every time. I don't know what the
idling
current for the 15 Hp rotary converter will be, but it will be
substantial. When you
add the exciting capacitors, the reactive current will greatly increase
the total
current (by making the power factor worse). When you add a KVA or more
of non-linear rectifier loads, it is likely to go through the roof! How
heavy a
240 V service do you have? If less than 200 A, you are in big trouble!

> Due to a spike or brownout or something I'll learn about after
> it's too late.
> One person mentioned a phase converter being jamed in reverse hard
> enough by the load to reverse.
> My controller appears very sensitive to the correct phase rotation.

How do you know that? Is it in the manuals?

> would a reversal toast it? any way to know for SURE until it happens?

If it uses SCR phase-control in the servo amps, then it is likely to
require the correct
phase sequence to operate correctly. It will probably not destroy
anything in the
amps, in fact it probably won't even blow fuses. It will likely cause
horrible buzzing
or chatter, and trip offline in a fraction of a second with an
overcurrent fault.
When idling, the servo would make tiny position or velocity adjustments
by
firing an SCR just before the zero-crossing, to turn it on for just a
few microseconds,
but because the phase rotation was wrong, it would end up getting turned
on
for about 2/3 of the half-cycle, causing a lot of current through the
motor.
But, these phase control SCR amps need to handle that, because noise
glitches,
etc. will occasionally cause this condition.

> It is old, probably unrepairable, and I have no documentation on it.
>
> Toast one 20 year old IC, and the whole thing is reduced to junk.

Well, given good documentation, most of this old stuff is repairable.
Without schematics, it can be beastly to fix anything, new or old.

> That fear, is a large reason to move to a EMC, as much or more
> that the added functionality.

Well, my experiences with my Allen-Bradley 7320 were that the older gear
(20 years old,
in my case) can be pretty unreliable. Not just power semiconductors,
but connectors,
switches, memory chips, video monitors, and always back to flakey
connectors, will
cause you headaches.

I debugged about 4 real hardware problems in the 7320, plus half a dozen
flakey
connector problems, in the 9 months or so I used it. Every time
something came up,
and I got in there and bumped the mass of cables, some buttons or lights
on the
front panel stopped working. I was glad to move to EMC at that point.

Jon