Re: Plasma table gear reduction

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "metlmunchr"
<metlmunchr@y...> wrote:

>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
> <dave_mucha@y...> wrote:
> >
> >
> > > The big problem with running servos slow is heat. For the same
> > output
> > > power if you halve the speed you double the torque required.
> > Current is
> > > proportional to torque so the current doubles. Now for the

killer -

> > the
> > > heat squares! (heat = I^2 * R). To make matters worse the

slower

> > the
> > > armature turns, the less effective the cooling is.
> > >
> > > Les
> >
> > I don't see that as a problem. An external fan would suppliment

any

> > low speed heating problem. But then knowing to solve a problem

is

> > half the battle !
>
> An external fan won't necessarily take care of that. Due to the
> I(squared) function, doubling the current results in 4X the heat
> generation. Heat transfer functions are complex, non-linear, and

far

> beyond the scope of discussion here, but suffice it to say that,
> essentially, the amount of heat transferred to the outer surface of
> the motor is a function of the temperature gradient from the

armature

> to the outer case surface.

> The only way to keep that higher
> temperature gradient is by allowing the armature to run at an

elevated

> temperature (which it will for sure). The problem lies in the fact
> that it won't run that way for long. Large DC spindle motors, etc
> which have to be able to run heavily loaded at a fraction of their
> design speed most always incorporate positive pressure centrifugal

fan

> systems for forced air cooling of the motor internals as part of the
> motor itself. You have to be able to cool at the source of heat
> generation, because there is no direct path for conductive transfer
> (the most effective means of heat transfer) from the armature to the
> case surface.

So, the armature (all the stuff spinning and the shaft also) have
only the conductive heat paths of the ends of the shaft. Pully and
encoder.

The rest of the motor has a only the bolts holding the coils in place
as the conductive heat path. The case gets hot from radiated heat.

It would be a simple matter of drilling a few holes and adding a
blower and filter to force circulate air.

But.... what I am hearing is that Servos are considderably harder to
use than Steppers although they do have some advatages.

I don't mind making a work-around for this or that, but it seems that
adapting servos to a gear rack gantry is mostly compromises.

Dave