RE: [femm] SRM Inductance plot at constant current

[<por@xxxxxxxxxx>]

Hi Brad and other interested users

For some time ago I made a software package ?SRDaS? dedicated to design a
simulation of SR-systems. Information about the software can be found at

http://www.iet.auc.dk/~por/SRDaS/SRDaS.html

If you send me a personal e-mail (por@xxxxxxxxxx) I will give you a link
were you can download the software free.

You can use external FEM results (from for instance FEMM) in the package
to time stepping simulations. But be very care-full to use 2D-FEM results
directly!. It is very common that SRM?s has large end effects in the
unaligned position, which for classical sized machines are in a range of
10-25 %.

The program is dealing static model parameters, which gives reasonable
results at typical operational speeds for electrical motors. I also have
good experience with the program and a practical single phase SRM
operating above 50.000 rpm.

I think you may deal with a very strange application (or some funny
limitations) since you are using a four phase machine were the iron-losses
may become a very serious problem.

Regards
Peter Omand Rasmussen



> I corrected my current value and I am getting the results I expected.
> The analysis I ran assumes constant current throughout the stroke angle.
> I am using this analysis as a first iteration design check to get an
> idea for the magnitude of current I need to produce a certain amount of
> torque. Here comes the hard part (I think).
>
> I am attempting to simulate the best I can, a high speed case. The rotor
> will be commutated at 75,000 rpm. I do realize the torque profile will
> take on a different shape at this speed. The current needs a finite time
> to build in the coil. I was wondering what is the best way to perform a
> pseudo time stepping integration.
>
> The inductance is a function of rotor position and coil current. This
> becomes an interesting differential equation to solve in my opinion. I
> was thinking of stepping the rotor in small finite angles in FEEM. Each
> time adjusting the current in the blocks to better simulate the "real
> world" case. Would it be sufficient to just re compute the current value
> at each rotor position with simple current rise time formulas, inserting
> the appropriate incremental inductance value each time? The conduction
> angle happens very fast when the rotor is spinning at 75,000. We are
> talking micro seconds in this case.
>
> I realize I may be trying to do something that may not be done easily in
> FEMM.
>
> Many thanks,
>
> Brad
> -----Original Message-----
> From: David Meeker [mailto:dmeeker@xxxxxxxx]
> Sent: Tuesday, May 06, 2003 9:20 AM
> To: femm@xxxxxxxxxxxxxxx
> 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
>> are 2 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
>> the coil 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 x 10^-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 using the 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, say that 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 motors are 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@xxxxxxxx
> www: http://femm.berlios.de/dmeeker
>
>
>
>
>
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