BLDC servo design with unusual motor

Discussion in 'The Projects Forum' started by punisher454, Oct 23, 2010.

  1. punisher454

    Thread Starter Member

    Jun 29, 2009
    I am attempting to build a unique valve actuator servo which must use a particular brushless motor for which I have absolutely no documentation for.
    The original circuit board cannot be used and I need more user control than the original design anyhow. So I am committed to going all in with a whole new microcontroller managed unit.
    I have decided to try the Allegro A3931 for the motor control side of things, connected to an Atmega644 which also controls the touchscreen user interface and will handle positioning logic.
    The uC side of thing has progressed nicely, but the motor control side has frustrated me to no end.
    I've searched for days on the internet for any Allegro A3931 or A3930 circuit examples but have come up totally empty. One problem is that I am not sure how much current the motor will need. The motor is a proprietary design and I will not be able to get any manufacturer data. I have a few of them and cut one apart for a better understanding of what I'm dealing with.
    The motor is a brushless inrunner design with a 9 tooth stator. Only 3 teeth of the stator are wound, leaving 2 blank teeth between each winding.
    Each of the three coils have 45 turns of .0175" thick wire (25g?).
    Total unwound wire length for each coil is 99 inches long.
    The stator has 37 layers of lamination and a stacked height of .7" total
    The stator has an OD of 2.0" and an ID of 1.1"

    The magnets crumbled apart on the motor I "examined" but the rotor was almost 1.1 inches in diameter and .9" long (the gap to the stator was very small, maybe .005 or so). I'm not sure how many poles it had(maybe 3 magnets and 6 poles?). The remains of the magnets are black with a coarse crystal structure and very brittle, but the pieces dont have nearly the magnetic strength of some neodymium magnets I have laying around.

    I dont know for sure how many magnetic poles were on the magnet ring, but the hall sensors cycle through all six phases 5 times for one complete revolution.

    The actuator that these motors are in has a compound gear reduction and the motor will only see about 35 mechanical revolutions from lock to lock. Fast response in both directions will be important so I am going to stay with the hall sensors, even though sensorless is so very popular now days.

    Currently the 3 coils are connected in a wye pattern.

    On a 12-14 volt system what would be the proper capacity of the mosfets I will need given the above motor info? The original board had 6 faichild FDD16AN08A0 Mosfets's. I'm wanting something something with at least that capacity, that is easy to drive with the A3931 and is readily available.

    So far I let the smoke out of several fets and a few gate drivers. and then decided to try the Allegro instead. For me the programming is in the bag, but i'm struggling with high side charge pump cap values and such.
    If there are any example's of working A3930/A3931's it would help a lot!

  2. jsnook

    New Member

    Nov 10, 2011
    Have you tried contacting Allegro? As they have a demo board and schematics. Also, what ever MosFETs you select, you have to pick the correct capacitor for the phase legs to match the charge capacity. See the data sheet for details.
  3. GetDeviceInfo

    AAC Fanatic!

    Jun 7, 2009
    Curious as to why the FDD16AN08A0s are no longer acceptable, why Allegro's recommended charge pump capacitance is not acceptable, and why you wouldn't use Rsense to manipulate your currents to drive your anticipated loads while monitoring motor temps.