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. This is true of both of the modeling programs we have used - they both overestimate force at low saturation and they both do so to roughly the same degree - that is they both give similar wrong answers.
We have gotten around this to some extent by simply overengineering the designs when the saturation is low, as well as intentionally designing for saturation or near to it. But it is an irritating phenomenon and I was wondering if anyone else has ever seen this problem. Our thought is that at saturation large errors in H cause only very small changes in B, therefore the force calculation is more numerically robust when problems are solved at saturation, versus at peak permiability where a tiny shift in H could cause a huge shift in B. But this would seem to suggest that we should see large errors of underprediction as often as overprediction - and we don't. It always overestimates.
Anyone have any thoughts on this? Thanks, Andy Reding
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