There have been LOTS of discussions on this subject on the Electric Vehicle
list.  I haven't been there for years now but the topic always intrigued me.

-Derek

> -----Original Message-----
> From: Olin Lathrop [mailto:olin_piclist@EMBEDINC.COM]
> Sent: Monday, August 05, 2002 8:02 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Brushless motors
>
>
> > One of the things I'm working on involves controlling
> brushless DC motors.
> > As a first step to understanding them better, I built a three half-H
> bridge
> > driver and I am trying to just get one of them to run,
> driven by my driver
> > circuit. The motor I'm using is a Maxon EC45 with Hall
> effect sensors. I
> > was under the impression that each hall effect sensor
> output simply told
> > you what polarity to feed to it's particular winding
> (ideally, you would
> be
> > feeding it with a sine wave but I think that a square wave
> should be close
> > enough for a rough test if not for most applications).
> I.e., all I need to
> > do with my three half H-bridge circuit is to connect each
> winding to +
> > voltage when its hall output is high and to gnd when its
> hall output is
> > low. When I do this, the motor runs, but at about 1/4 of
> the correct RPM
> > and drawing about 40 times the correct no-load current. I have tried
> > swapping the sensors around in every combination possible (i.e., use
> sensor
> > 1 to control winding 2, etc.) and even tried inverting all the hall
> outputs
> > and then trying all combinations again. The best I can get
> is to have it
> > run at about 1.5 times the correct rpm for my input voltage
> (!) drawing
> > about 7 times the correct current.
>
> The whole subject is too complicated to go into here.  But
> briefly, here are
> some points to keep in mind.
>
> 1  -  The motor also works as a generator all the time.
> Shorting undriven
> windings shorts the generator output, which will act like
> viscous drag.
>
> 2  -  The windings are inductors and can store significant
> energy.  At the
> least you have to make sure this energy doesn't fry something when the
> winding is turned off.  A much more efficient scheme is to
> recycle this
> energy.  The best scheme is to transfer the energy in one
> winding to the
> next one in the sequence.  An alternate strategy is to dump
> the energy back
> into the power supply, although this can be trickier than it may seem,
> especially while avoiding the viscous drag from #1.
>
> In short, you need to study up on this stuff.  None of it is
> magic, but
> there are lots of issues the novice might overlook, and lots
> of wrong ways
> to do it.
>
>
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>
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