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# RE: [femm] SRM Inductance plot at constant current

- To: <femm@yahoogroups.com>
- Subject: RE: [femm] SRM Inductance plot at constant current
- From: "Brad Frustaglio" <fuzz@wildrail.net>
- Date: Tue, 6 May 2003 09:50:17 -0500
- Importance: Normal
- In-reply-to: <3EB7C495.8090700@ieee.org>

Thanks for clarifying that Dave. Brad -----Original Message----- From: David Meeker [mailto:dmeeker@ieee.org] Sent: Tuesday, May 06, 2003 9:20 AM To: femm@yahoogroups.com Subject: Re: [femm] SRM Inductance plot at constant current `Brad Frustaglio wrote:` `> Hi All,` `>` `>` `>` `> I have been using FEMM for a few years now. Many thanks to Dave,` `> excellent program.` `>` `>` `>` `> I have question on evaluating the A-J integral and then calculating` `> the self inductance from the result.` `>` `>` `>` `> I am modeling a 4 phase SRM with 8 stator poles and 6 rotor poles. I` `> am attempting to extract the torque profile and inductance profile` `> with one phase excited at constant current as the rotor moves through` `> one stroke. The torque magnitude and shape looks reasonable. However I` `> am having trouble understanding how to calculate the inductance` `> correctly. The profile shape looks as expected I am just concerned on` `> the magnitude of inductance. My method is:` `>` `>` `>` `> 1. Highlight the blocks with the current flowing ( In my case there` `> are2 current blocks per pole (one on each side of the stator` `> pole) for a total of 4 current blocks per phase.` `> 2. Evaluate the A-J integral at each respective rotor position` `> 3. The self inductance as defined is int(A-J)dV /i^2) where i is` `> thecoil current` `>` `>` `>` `> I think I am running into trouble on the coil current term. What is` `> the correct coil current to use to evaluate the inductance correctly?` `>` `>` `>` `> The value of one current block or the sum of all current blocks. Also` `> this is actually amp-turns. Not the actual current flowing in the wire` `> itself. Right?` `>` `>` `>` `> For instance: the current in one block in the model is defined as` `> 11.15 MA/m^2. The coil area is 2.6903 x10^-5 m^2. So the magnitude of` `> current in the coil block is 300 A-T. Is this the current I use for` `> the evaluation of self inductance usingthe A-J integral?` `>` `>` `>` `> Regards,` `>` `>` `>` `> Brad Frustaglio` `>` `The /i/ in the equation actually /is /the actual current flowing in the` `wire itself. Like in your example, saythat your coil is made of 60` `turns of wire. Since you have 300 Amp*Turns, the current in the wire` `would be 5 Amps. The 5 Amps is then the value that you'd use as /i/ in` `the equation. There's also an example that might be relevant to you at` `http://femm.berlios.de/induct1/induct1.htm` `One last thought--switched reluctance motorsare typically run in a` `highly saturated condition. The quantity that you are computing with` `the A.J integral is (flux linkage/current), but it doesn't have the same` `relationship to stored energy as inductance in a linear problem.` `Furthermore, this "nonlinear inductance" appears in a subtly different` `way in any electric circuit equations that you might write. For example,` `the electric circuit equation that applies to your case is:` `D(flux linkage)/Dt + R i = v` `where the Dt is meant to represent the /total/ derivative with respect` `to time. If inductance, L, is not a function of current, this` `simplifies to the usual:` `L di/dt + R i = v` `However, if L is a function of i, we'd get:` `(L(i) + i*dL/di)*di/dt + R i = v` `Dave.` `--` `David Meeker` `email: dmeeker@ieee.org` `www: http://femm.berlios.de/dmeeker` `Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.`

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  - Re: [femm] SRM Inductance plot at constant current
    - From: David Meeker <dmeeker@ieee.org>

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