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Hello,

My name is BHASKAR. I am trying to realise the
following in femm but facing problem. I wanted to
realise a variable inductance motor which is actually
a pretty good model for a stepper motor. When the iron
piece is moved away from the gap, the cross sectional
area of the core will decrease, there by decreasing
the inductance.For a constant flux the internal energy
will increase and a force will be exerted on the iron
piece to attempt to restore it to its equilibrium
position. The force F is given by 

F = m0 w N2 I2 / 2 lg 

Where m0 is the permeability of free space , w is the
width of the air gap, N is the number of turns and I
is the current and lg is the length of the air gap.

For a force of 0.2 N, current of 1 amp, air gap of 2
mm and width of 10mm we get the number of turns as
252. I tried to realise this in femm but Iam getting a
force very less than this in the x-direction where did
I went wrong can any one give suggestions.I am sending
the fem file attachment.Please send me your
suggestions.

BBHASKAR


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Attachment: iron core new.FEM
Description: iron core new.FEM