Hi Morgan,

I will have to think about this more when I get a chance, but essentially=20
this is what I am thinking:

You could drive one of these motors with three phase AC and it would work=20
well (as far as I know). So, therefore, our job is essentially to=20
approximate three sinusoidal signals, each 120 deg later in phase than the=
=20
last, with our three half H-bridges. A sine wave has no freewheeling=20
period, it always presents a rather low (zero, ideally) impedance to each=20
motor winding, but with a variable voltage. However, there comes a point in=
=20
the cycle (actually twice per cycle per phase) where the voltage of the=20
sine wave matches the back EMF plus inductive voltage of each phase and no=
=20
current flows in that phase. At this point in the cycle, it would not=20
matter if you disconnected that phase for a few degrees. Since our half=20
H-bridges can only produce high, low, or tristate output, we can=20
approximate a sine wave with just a square wave (high for 180 deg, low for=
=20
180 deg) but we can do it a lot better with a three state wave (high for=20
120 deg, tristate for 60 deg, low for 120 deg, tristate for another 60 deg).

Is this wrong somehow?

By the way, you are exactly right about using the back EMF to provide=20
feedback rather than the hall effect sensors. Many controllers do that.

Sean

At 02:14 PM 8/6/2002 +0100, you wrote:
>Hej Sean H. Breheny. Tack f=F6r ditt meddelande 01:29 2002-08-06 enligt=
 nedan:
> >Morgan is right, they apparently do expect you to tristate each phase=20
> for part of its cycle
> >(actually, this would be roughly the same as setting it to half voltage
> >since the wye neutral point sits around half voltage anyway, and this
> >happens when the back emf for that phase is near zero).
>
>I don=B4t think it is the same at all.  "Tristateing" Freewheeling it as i=
=20
>say, i mean shut of both transistors and let the inductance shoot current=
=20
>through the freewheeling diodes you should have across the transistors,=20
>regaining some current to the power supply / battery / other pahse drivers=
=20
>/ other electronic devices.
>
>I think holding it to zero will only turn that energy to heat in the=20
>motor, plus occasionally give a negative momentum.
>
>Also: Driving all phases strong, will probably induce lots of corrent in=20
>the windings.  The system is "overdefined" if driving more than two pases=
=20
>concurrently.  Ot works if you drive each with sinus 120=B0 apart.
>
>I once put a scopte to a large brushelss fan, and what i saw was:
>When the transistor shut off, the voltage wol go to opposite state, making=
=20
>current flow in the other transistors parrallel diode for a short while,=20
>then it will fall back to an EMF induced voltage (with some ringing)
>
>Hum... with an A/D and some clever algorithm this could be used to drive=20
>the motor without the hall sensors...?
>
>/Morgan
>
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