Re: 3 PHASE INDUCTION MOTOR DESIGNING FOR LOW FREQUENCY OPERATION

From: Yzordderex (yzordderrex_at_verizon.net)
Date: 08/20/04 

Date: 20 Aug 2004 13:18:50 -0700

Yes, surprising indeed. You really are going to have to get the
currents under control If you insist on a 3 phase induction motor I
will tell you that it can be done. You will run into some difficult,
but not insurmountable problems. A different motor would probably be
my solution as well. Not sure of the point Bill was trying to make
about low frequency currents, but the frequency of interest may be the
slip frequency. The difference between We and Wm -electrical freq and
mechanical frequency. That is what the rotor bars will see. If you
got 220v to play with then you should be able to design in plenty of
control headroom - Big current slewing no problem, but doesn't sound
like you will need much dynamic control.

Dimension constraints sound like they are pushing you into custom
motor rather than off shelf. (Why the airgap specification? Your
professor throw that in for good measure?)

Have you defined the speed regulation? VFD is basically useless where
you intend to operate anyway. Simple flux control is an option where
magnetizing flux vector is controlled to some degree. Abbondanti
wrote paper many years ago on an analog implementaton he had done.
Worked well. If speed control isn't a major issue then this may work
well for you. I think it maybe simple field orientation control.
Open loop vector (sensorless vector) control is a method where speed
is estimated and appropriate manipulation of pwm is used in order to
control direct and quadrature axis. DQ is just stator current broken
down into the discreet components with a transformation from 3 phase
coordinate system into a rectangular system. Components are the flux
producing current and the torque producing current. You try to
control them independantly and maintain an orthogonal system to
produce maximum torque per amp. Brush DC motors do this by way of
commutator. Closed loop implementations are a step ahead where flux,
speed, position information is feedback to controller in order to
precisley control the process. You can design some smokin control
loops and make the motor dance if that gets you off.

So there ya go!

Good luck with project.

regards,
Bob

PS. The fella who mentioned the weird sounding drive - Sounds like
synchronous carrier. The carrier is kept in synchronism with the
fundamental output frequency. You then have a discreet number of
modulation pulses per output cycle. As the motor accelerates the
carrier slides up as well. Shift points are used to keep the carrier
within a reasonable range. Yes sounds cool.

bill.sloman@ieee.org (Bill Sloman) wrote in message news:<7c584d27.0408200157.748c0f01@posting.google.com>...
> nasirtaxila@hotmail.com (NASIR) wrote in message news:<807d0973.0408152033.68b1c912@posting.google.com>...
> > I am working on the designing of a 3 phase induction motor for
> > operation at 20 rpmm. But i am not finding the exact
> > relationships/foumulas which hold true at low frequencies(i.e below 5
> > Hz). Could anyone guide me about a "booK" which deals with the abouve
> > mentioned subject or can give me some guidelines which should be
> > observed while designing at low frequencies.
>
> Considering the number of contributions posted to this thread, it is
> surprising that nobody had explicitly made the point that motor torque
> depends on the current through the coils, and the voltage applied
> across the coils determines the torque only as far as it determines
> the current.
>
> IIRR, induction motors are not suited for very low speed operation,
> because they rely on the current induced in the rotor by the driving
> currents in the static coils, and with very low drive frequencies, you
> can't put enough current through the static coils to generate enough
> dI/dt at the rotor to generate enough current in the rotor.
>
> Synchronous motors don't have this problem. which is why stepper
> motors - which are the devices used when you want really low rotation
> rates - are actually synchronous motors.
>
> Most stepper motors are two-phase synchronous motors, but there are
> some three-phase parts out there - usually described as 5-step
> steppers - which could offer smoother rotation, though a
> micro-stepping stepper drive should be adequate for most applications.
>
> -------
> Bill Sloman, Nijmegen

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