Treadmill motor / PWM vs .SCR

Thanks Mark,

I think if Les Newells' has been in use for a few years it's
worth a try. Like you say, the motors have many other uses.

Which invites a new question PWM vs SCR.

What's your view?

John

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Mark F <markotime@...> wrote:
>
> As an experiment, I used a Geckodrive 340 to run the spindle on my
> little Emco CNC5. Took some fiddling with acceleration (pulse train
> output under CNCPro control) but it ran just fine, thank you. THe G340
> has a pulse multiplier, which may not be necessary in an axis where you
> have mechanical drive ratio in place.
>
> Once I learned enough to abandon G-code spindle control <grin>, I
> used a KBIC DC controller on the same, ubiquitous tradmill motor;
> this works great *except* that under very low loads, carbon tends
> to build up on the commutator and the speed becomes erratic until I
> apply a percussively significant event to the motor housing (bang it
> with a screwdriver or something), which causes the brushes to make
> better contact again.
>
> Under higher loads, no problem, and it's a great combination.
>
> I agree that under high power operation, cogging would be a problem.
> In most cases, these motors will be operating under a fraction of their
> potential, so at minimum torque portions of their rotation, there should
> still be plenty to move to the desired encoder position.
>
> The second point I'd make is that reversing these motors *could*
> be a problem in an application like a spindle, where 99 percent of
> the motion is in one direction... This allows brushes to develop a
sharp
> trailing edge which, upon reversal, can snag in a commutator gap and
> cause a chunk of carbon to break off and *possibly* become lodged
> somewhere it shouldn't be.
>
> In an axis situation, motion will be bidirectional. Now, there is some
> finite slop between the brushes and their retainers, so there will
be some
> effective deadband at reversal time, but I dont' think this will be a
> problem
> either.
>
> My view - give it a try. The Geckos are totally reusable, the
motors are
> cheap and highly useful in other applications, and there's nothing
to lose
> if money' the object.
>
> /mark F
>