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# Re: SRM Inductance plot at constant current

- To: femm@yahoogroups.com
- Subject: Re: SRM Inductance plot at constant current
- From: "fj_emtc" <fije29@sol.dk>
- Date: Sun, 11 May 2003 09:21:49 -0000
- In-reply-to: <3214.212.10.0.241.1052332848.squirrel@smith.iet.auc.dk>
- User-agent: eGroups-EW/0.82

    For a good explanation of calculation of torque in SR motors, have a
    look at "Computation of Torque and Current in Doubly-Salient
    Reluctance Motors From Nonlinear Magnetization Data" by Stephenson
    and Corda. It's in Proceedings IEE, Vol 126 from May 1979. It's a
    fairly old article, but SR motors are even older...(And if memory
    serves me right, Peter is using this principle also (or some kind of
    it))

    Peters programme is great, but at 75.000 RPM, I would be very careful
    with any programme. The iron losses will be huge, and these are very
    hard to model especially SR-motors. Be very careful

    Finn Jensen

    --- In femm@yahoogroups.com, <por@i...> wrote:
    > 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@i...) 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@i...]
    > > 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
    > >> 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@i...
    > > www: http://femm.berlios.de/dmeeker
    > >
    > >
    > >
    > >
    > >
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  - RE: [femm] SRM Inductance plot at constant current
    - From: <por@iet.auc.dk>

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