Serial stepper, half or 71% or rating ? ( was Re: Stepper current -

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude
<davemucha@j...>" <davemucha@j...> wrote:

> I have seen the numbers in a couple places now. both
> the 71.5% and 50%.
>
> Is the half figure based on double coil area, without
> adding anything for losses ?
>
> any idea why the big difference ?
>
>
> Dave
>

My guess is that the 50% is based on magnetic flux. Full
coil has twice the turns of unipolar. Twice the turns
times half the current gives the same amp*turns and there-
fore the same magnetic flux. I think this is the correct
way to do it.

The 71% is based on DC losses. DC losses are I^2*R, and
when you change to full coil, you double R. If you keep
the current the same, doubling R doubles the heating. So
if you reduce the current to sqrt(1/2) (71%), the heating
is reduced to the original level. So if I^2*R heating is
the limiting factor, then use 71%. But this will result
in the magnetic flux being 141% of nominal, which may
saturate the iron and result in non-linear microsteps and
more hystersis heating when moving.

I'll use 50%.

John Kasunich