Re: some proposals for future developments

["David Meeker" <dmeeker@xxxxxxxx>]

--- In femm@xxxxxxxxxxxxxxx, "atreding" <atreding@xxxx> wrote:
> Hi Dave - I'm new to the group but I've been watching the posts for 
> a while now and I downloaded the "beta" version of Femm 4.0 - my 
> question is since the lua scripts from 3.3 are no longer going to 
> work with the 4.0 version anyway, have you considered moving to Lua 
> 5.0 for the new program? It would be a good time to do it since 
> scripts need to be rewritten anyway.
> 
> P.S. - This program is great - fully the equal of commercial magnetic 
> software I have used. Thanks!

I'd considered moving to Lua 5.0, but it's not clear to me that 5.0
offers much of an advantage over 4.0 for 

> Also, for everyone else - just a question that has been eating at me 
> for a while... We have been using Femm as well as Ansoft Maxwell for 
> modeling solenoid actuators. The results agree well with each other 
> in general, and when we have actually built the devices for test the 
> models do a pretty fair job of predicting forces. However, in 
> solenoids where the iron is not very strongly magnetized (B field far 
> from saturation, ie. 1T or less) the results overestimate by a large 
> amount, as much as 30%. While for saturated designs, the accuracy can 
> be 5% or even better. [...]

Well, my two best guesses have already been covered. The BH curve can
be a very strong function of heat treat. The apparent permeability
for minor loops can also be a lot less than the initial permeability
in some materials. 

If that's not it, I'd also guess either extra parasitic air gaps from
where parts butt together, or extra air gaps due to tolerancing in the
mechanical construction. These effect things in a big way when the
iron isn't saturated, because there, the airgaps are typically the
lion's share of the reluctance. When things are highly saturated, the
reluctance becomes dominated by the iron instead, and as you have
noted, the saturation level isn't too dependent on the heat treat--the
effects of these parasitic air gaps wouldn't be so detectable then.

One last thing to consider is that force in solenoids is notoriously
hard to measure. This is mostly due to side force and friction. 
However, at low flux levels, the forces are also low--any offsets in
the measurement setup are more significant relative to the total force.

Dave.