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# Re: [femm] variable inductance motor

- To: femm@yahoogroups.com
- Subject: Re: [femm] variable inductance motor
- From: David Meeker <dmeeker@ieee.org>
- Date: Sun, 29 Sep 2002 21:23:08 -0400
- References: <20020929204620.86707.qmail@web40613.mail.yahoo.com>
- User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.1) Gecko/20020826

bhaskar bollapragada wrote:

> `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
> `

First, you've set the length units to millimeters when, from the description of your geometry, things need to be in units of centimeters. Second, I think that you meant to draw your blocks as 2 cm by 0.5 cm blocks so that you'd get a total of 252 Amp*Turns with a current density defined as 2.52 A/mm^2. However, the coil cross-section is actually drawn as 2 cm * 0.4 cm, giving you less total amp*turns than you thought. Lastly, the mesh that you are using around the block is a little bit too coarse for really accurate force computations. You need to refine the mesh around the block a bit, e.g. by imposing a finer mesh density on the lines that make up the pole and block surfaces.
When I make these changes and run the analysis, I get about a 16 N/m force on the block. Multiplying by 1 cm in the into-the-page direction, yields a force of 0.16 N. This is in the same ballpark as the 0.2 N`result from the F=(m0*w*(N*I)^2)/(2*lg) formula that one would derive from magnetic circuit theory under no leakage / no fringing assumptions.
`
Anyhow, I've attached a modified version of your problem with the changes that I'd noted.
Dave.
--
David Meeker

Attachment: `zip00033.zip`
Description: Zip compressed data

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    - From: bhaskar bollapragada <bhaskar15656@yahoo.co.uk>

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