> 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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