Re: rod-mill project - "mains" electric motor advice

"Jim Wilkins" <muratlanne@gmail.com> writes:

"Richard Smith"  wrote in message news:m1wmbd29er.fsf@void.com...
>
Hi everyone
I have been shown how to do this with power-electronics and
"off-the-shelf" gearbox, etc.  With all speed-control needed.
A friend had made a small welding positioner.  Hence adapted it:
* VFD - 1ph-to-3ph
  v
* 3ph induction motor
  v
* worm-drive gearbox
You turn a knob setting the output Hz.  Adequate speed range.
>
Having seen this - that it works and does everything wanted - will
simply do it this way.
Have friend's equipment on "unspecified loan' in my car to take home and
play with more.
>
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>
That is an excellent way to learn what works. The potential problems
are cost and custom machining. In that instance I'd look up and save
the components' data sheets and see how their max power ratings
compare to what was needed and what you intend to do. I've been burned
by guessing that something was good enough, my sawmill left a trail of
broken ball bearing innards until I switched to a more reputable and
expensive brand for the blade guides.
>
I hardly ever find the same device twice as second hand and must
decide on the spot if what I found is suitable, which may depend on
its power rating. People dispose of what they can't use, typically
because they are either inadequate or broken.
I tested that contactor for pull-in voltage and contact resistance.

I will go with "off the shelf".  Now seen what's needed.
Time si too valuable.

* have the thing working (hopefully!) and making inroads here in
  Cornwall

* free to get on with other things I have to do - would cost me more in
  what I have not done than I could save not "placing orders" for what
  is spec'd for the job.

One "blessing".
Gearbox must for sure have a torque rating.
Derive torque.
Found it's gloriously simple - relation of power, torque and revs.
P=tau.omega

P=power (Watts)
tau = torque (Newton.metres)
omega = rotation-rate (radians/s)
Latter makes total sense - well it does for me :-)  Radian is where a
radius is wrapped around the circumference.  Very often gives vast
simplifications (compared to working in angular Degrees or Revs Per
Minute, etc.).
Prompted that way, I can easily see from first principles how
"P=tau.omega" can be derived, giving total satisfaction in applying it.

So I can look through spec.sheets for "off-the-shelf" gearboxes.

I have a 30:1 gearbox on loan.
However, reckon 7:1 would make the drive "synchronous" (if ran motor at
mains frequency, would give the right drive speed).
Of course you need speed adjustment to make the mill(s) work exactly
right - but what you are asking is very achievable - say +-25% - which
an induction motor can do no problem.
[welding positioner - is so low power draw for small parts that eg. 10% of
the mains synchronous speed it is designed for is not a problem]
So going to have to get a 7:1 ish gearbox for mill.

Speed control should be wide enough to run rod-mills and ball-mills on
the same rollers-on-a-frame, no mechanical changes needed.

I calculated for the "metallurgical" part of the rod-mill - everything
which is and is within the "shell" ("drum"):
* 65RPM of shell rotation rate
* 127W of power draw