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.

Think AC Servos - Armature is a rotating magnet, the field coils sink
their heat directly into the case regardless of speed.

gary