| firstname.lastname@example.org wrote:
|> | http://www.st.com/stonline/books/pdf/docs/7209.pdf
|> I would not call that a DC motor. The motor is not getting DC; the
|> controller is. The motor gets modulated pulsed AC. It may well be
|> the standard practice to call this a DC motor, but if so, it is yet
|> another case of terminology misapplied. I'd call it a "pulse control
|> motor". A more clever controller could drive it from an incoming
|> three phase AC supply directly, by switching in the supply phase that
|> has the right voltage and polarity at that instant (in three phase AC
|> power, there's always at least one phase in each polarity at any time).
|> It sure would not be anything DC in that case.
| AC is alternating current because the current "allternates" the
| direction in which it flows. The current to the motor does not do so.
| The term "phase" does not only apply to one of the rotating vectors of a
| multiphase AC supply.
It sure looked like it was alternating from some of the diagrams. Sure,
not alternating in the way you get from the wall outlet. But the pulse
go one way sometimes, and the other way other times. See figure 3 on
page 7. See figure 4 on page 8.
The document showed an example where a simple north-sorth bar magnetic
was figuratively used as the rotor. I suppose a pulse sequence only in
one polarity could still pull it around in rotation and in sync. But it
would operate better if there was a corresponding reverse pulse to pull
the other end of the magnet. Additionally, reversing the field of each
winding just as the magnet pole passes, so it pushes away afterwards,
could help even more.
Also, even though the document describes "PWM" (pulse width modulation),
some of the diagrams seem to suggest it is instead using "PDM" (pulse
density modulation). PDM is where pulses at a very high rate are used,
each at a fixed time interval, but varying in how many are on or off in
a window of time, as opposed to a single pulse that varies in width.
PDM can be used to more readily vary the average current in the winding
over time, where desired. If the PDM rate is high enough, the winding
inductance smooths it out. You could make a decent sine-like waveform
that way. The advantage of PDM over PWM is that high switching rate
allows less reactive component to smooth it out.
I would envision a much more sophisticated system for certain uses like
independent automotive vehicle drives on all 4 wheels. The spacing of
permanent magnet poles could be different than the spacing of windings
so you don't have every pole aligning to every winding at the same time
(to reduce vibration). For example, 18 magnet poles and 15 winding
poles would allow all the magnets to alternate (even number) polarity,
and the interval variation would be replicated three times to maintain
a reasonable force balance around the rotor. In each 120 degree segment
there would be 6 magnets and 5 windings. The windings would be energized
according to which magnet poles are approaching/departing in rotation.
For vehicular use, I would definitely want a feedback to confirm where
the rotor is, since it is subject to variable load drag. There would
be at least 4 windings per segment, 12 for the whole stator, that could
be simultaneously energized.
| Everybody else calls them brushless DC motors, even the little ones in
| your computer case that drive cooling fans. These use Hall effect
| switches rather than back EMF for switching and do actually run on DC
| input. The current in their windings goes through "phases" as it is
| switched, but it's not AC. Three phase DC motors are used as the
| platter motors in hard drives, driven by the 12V DC supply. If this
| terminology is misapplied, it has been so for many years!
I guess the whole assembly can be called "DC" since it works on DC being
fed to the whole assembly. There's still AC going into the motor windings
according to the referenced PDF document.
FYI, this same design seems to be used in a VCR I once took apart to see
where the lightning killed it. It was the video head motor, too. It had
12 separate windings.
| What do you do for a living that gives you so much time to think about
| this sort of thing? :-)
It's a hobby thing. It's just like trying to figure out what chemicals
are needed to go boom. But in this case it's what electricity is needed
to go boom :-) Note that I don't actually build things unless I already
know what it will (most likely) do. And even then, I find more of the
fun in the thought process to design it. Electricity is not the only
area I do thought experiments in.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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