W.E.T. CAD_CAM_EDM_DRO] : Stepper speed question

Hi Jeff and All:
Jeff would you explain the changes in the drive system that
would make the motor have to run faster to get the same travel speed
on a 10 pitch screw as compared to a 20 pitch screw. Are you
changing the pully drive ratio for torque? THANKS
GOD'S BLESSINGS
Bill


Hi,

I did chuck up a stepper once, but I used a battery powered drill
made for torque, not speed, and never got it close to 2000 RPM. I
don't remember the voltage I did produce, but I'll take your word on
the stepper's emf capabilites.

For the most part, people that use Xylotex systems are not spinning
their motors 2000 RPM. For a 20TPI machine (like a Taig or
Sherline), this would equate to 100 IPM. Even faster for a 10TPI
machine. These speeds typically are not achieved on most of the
machines the Xylotex system go into due to both the limited power of
the drive to get it going that fast, and to the limited travel of the
machine. I did get my Sherline X axis up to over 100IPM running at
24VDC. When it did finally stall (I think the max I got it to was
around 110 IPM) the drive didn't go 'pop'. The other axes weren't
running so there was only the on-board caps, and supply caps to take
the load. They did so without any problems - that I know of :-)

While what you suggest certainly is a possible problem if the drive
is run closer to its 35VDC maximum limit. I recommend a 'running'
maximum of 30VDC. The typical supply voltage used is 24VDC. This
does leave a good amount of "room".

One of the bigger problems I had in the past (before I started
selling the 24VDC power supplies along with the system) was people
looking at the mpja 24VAC/10A transformers for $18.00+/- and thinking
it would work great. Slap it together with a diode block and a big
cap and ready to go, right? Well, as you know, wrong.

The original 3 axis drives also did not have the on board DC-DC
converter for +5VDC like the new drives have. There were a
reasonable number of failures back then simply due to mis-wiring up
the 5V. Yep, it's true.

Finally though, some of the few "unknown pop"s cetainly might be
attributed to what you mention. Generally though, the users actually
know what they did, and it usually turns out to be a wiring error or
some other mistake they wish they could take back.

So hold on to your hat, you probably will not be eating it any time
in the near future. Mine's a cowboy hat, and I certainly wouldn't
want to eat it (though I have had my fair share of crow as food for
thought).

Jeff




--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
<mariss92705@...> wrote:

>
> Actually there is another, more significant effect. The most common
> drive failure report with low voltage drives is "The drive popped
> when I was decelerating my motor" or "The drive popped when my

motor

> stalled during a rapid".
>
> Step motors are unique in that can develop a back-EMF much higher
> than the supply voltage.
>
> For your own personal edification, chuck-up a 1A, 5V NEMA-23 motor

in

> a drill-press, Put a multimeter set to AC volts across a winding

and

> run it up to 2,000 RPM. If you don't read over 100VAC (141 volts
> peak), I'll eat my hat.
>
> Yet you can put the same motor on a 24VDC powered drive and easily
> spin it up to the same 2,000 RPM no problem.
>
> What happens? When you stall the motor, the kinetic energy stored

in

> the moment of ineria is released and returned. It gets returned to
> the drive as a current source with 100V+ of headroom. The power
> supply capacitor absorbs this energy at the expense of a voltage

rise

> across it. If the rise is enough, it exceeds the breakdown voltage

of

> the drive and pops it.
>
> Sound familiar?
>
> Mariss
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "jeffalanp" <xylotex@>
> wrote:
> >
> > Hi,
> > The largest cause of 3977SED drives "popping", when a SLA7062
> will
> > not, is when a person disconnects a motor when it is under

power.

> As
> > noted, the 3977 drive chip is specified for a max of 35VDC. As

has

> > also been stated in earlier post of this topic, when you

disconnect

> a
> > live motor wire you can get high (double?) voltage produced.
> > Running a 3977SED at about 30VDC would yield a voltage of around
> > 60VDC. Very bad for the chip. On the other hand, the 7062 MUST
> > handle the double voltage rating to cover the disconnected
> > transistor, because that is how unipolar drives work. So, the

guy

> > who innocently (or not so) blows up his 3977SED wouldn't even

know

> he
> > did something wrong with the 7062. There is nothing wrong with

the

> > A3977SED chip if it is run in its safe operating area. THE
> *absolute
> > largest reason* drives blow up is because people get excited at
> > trying the new hardware, don't follow instructions and do

something

> > wrong. Allegro makes thousands and thousands of these chips.

Most

> > probably go in to commercial devices like printers. These NEVER
> blow
> > up because they aren't subjected to undue stress (like

disconnects

> or
> > shorts or miswires or overheating, or...).
> >
> > The other main cause of a 3977 "popping" is when there is an
> > overcurrent. This can be from miswires or short circuits. I

don't

> > think the 7062 is immune to short circuits either. Since I

started

> > selling the 269 oz.in. motors along with the power supply and

drive

> > board in a "system kit", "blow ups" have dropped even more, since
> > people are not guessing at how to make motors connections or

power

> > supply connections; I provide instructions with each.
> Unfortunately,
> > people STILL don't read these sometimes, even though they are

taped

> > directly to the device :-(
> >
> > Anyway, both drive chips are good devices when properly
> operated.
> > The 7062 may however be more robust to user error than the 3977,

so

> > if you plan on making mistakes like disconnecting a live motor,

go

> > with the 7062. If you plan on following manufacturers guidelines
> > either will be fine.
> >
> > Jeff
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
> > <dave_mucha@> wrote:
> > >
> > > -
> > > > IMO the early xylotex were not as robust in actual shop use,

as

> > the
> > > > other two. There was also a noise problem in the step/dir
> inputs
> > > > which Jeff to my knowledge has fully corrected in present
> boards.
> > > > Saying they were not as robust may be an unfair assessment,
> since
> > > > their popularity meant a larger sample and response was
> presented
> > in
> > > > these groups where much of my NON-personal experience is
> gotten.
> > > >
> > >
> > > The A3977 chip is listed as 35 volts and 2.5 amps.
> > >
> > > as I understand, the drivers on the Gecko are rated at

something

> one
> > > the order of 100 volts, but the assembled unit has a manufacters
> > > rating of only 80 volts. a 20% de-rating of the components.
> > >
> > > In the early days of the Xylotex, many people tried to supply
> near
> > 35
> > > volts and with any back-emf, 36 volts was all to common.
> > >
> > > I find fault in the understanding of the parameters of the chip
> and
> > > Xylotex has since, as I understand, recomended 24 volts as the
> > desired
> > > power supply.
> > >
> > > Dave
> > >
> >
>

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