Re: Plasma table gear reduction

--- 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.