Hi Sean, first make sure you get any 3-phase wiring
details sorted, like if it soupposed to be star or delta
connected.

Second, brushless motors are current operated devices
(obviously) and you should operate them at the correct
spec current. Like steppers they usually have low-turns
(low ohms) coils, and are meant to be driven with some type
of constant current supply, like a chopper etc. In short,
you need some type of pwm etc on the motor coils to establish
correct running current.

If you don't need too much torque, the motors can usually
be run very efficiently with a tiny pulse on each coil a
correct time after the hall is triggered, ie 2 time delays.
This tiny pulse should give good speed regulation on most
flat type motors (decent flywheel) and use almost no current.
But, it will have lots of torque ripple and with any real
load attached the load will be moved in 3 "jerks" per
rotation.

Most VCR's flatmotors (capstans) have a cheap IC, available
in many styles, that interface direct to the coils, and
the 3 halls, and the chip accepts a train of speed pulses
from a digital input. The chips are sophistacted and drive
the motor well, with the "compensated" sinewave made by
correct PWMing of the coils. These are made to give very
smooth speed and torque with a cheap 3 coil motor. I can get
some chip numbers for you from some of our VCR manuals
if you want to go that route, and chip prices for the more
common chips. :o)

Since you already have the 3-bridge there are some options
for making it work with a PIC, but you haven't said what
this is going to run, which really determines the best way
to drive it. :o)
-Roman



Sean H. Breheny wrote:
>
> Hi all,
>
> 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.

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