Constructing Brushless Motor, hall effect sensors necessary?

Discussion in 'The Projects Forum' started by josephtc, Apr 16, 2016.

  1. josephtc

    Thread Starter New Member

    Apr 16, 2016
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    Hey everyone,

    I'm trying to build a 3 phase brushless motor, however I'm in a bit of a quandary. I'd like to make a simple 3 coil motor to save myself some time making coils, however I'm unsure of whether or not I need sensors. On all the diagrams I've seen of 3 coil brushless motors, 3 hall effects sensors are placed individually between the coils. Does a 3 coil brushless motor need hall effect sensors? Would a standard 3 phase motor controller for R/C applications work on said 3 coil brushless motor, provided it has the correct voltage and amperage?

    Thanks, hopefully you all can help clear things up.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Hall sensors can be used to commutate a BLDC motor, the other sensorless method is to sense the BEMF current in order to detect winding position.
    BLDC motor controllers switch current in two of the three windings at any one time, BLDC because they represent a DC brushed motor turned inside out.
    A motor that is intended to be used with a sensorless controller has no hall-sensors. A sensorless controller need to spin the motor up to ~10% of maximum rpm in synchronous mode before the back-emf is large enough to measure. Below this speed the torque isn’t very high.
    In general BLDC motors and controllers intended for RC toys are sensorless and use sensorless controllers.
    There is a subtle difference between BLDC and 3ph P.M. motor. This almost identical 3 winding motor is a fed with true 3 phase signal for synchronous control.
    Max.
     
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Are you say "only 3 coils" in the motor? Wouldn't a motor like that be both slow turning and very weak power wise? And you don't need to use hall sensors, opto sensors can also be used with a interrupter wheel to sense position.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    BLDC and AC servo's are mainly 3 winding stator with 4,6, or 8 pole P.M. rotor motors, they are capable of up to 8krpm often.
    They have lower inertia than a DC brushed and higher torque for the same NEMA size.
    Max.
     
  5. Sensacell

    Well-Known Member

    Jun 19, 2012
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    If you need your motor to produce torque from 0 RPM then you need a position feedback system- hall sensors or other.

    Sensorless motors need to be spinning already to commutate the coils, typically the controller chip "bumps" the coils to make the motor spin up enough to sense the back-EMF - to start the synchronous commutation going, this method only works if the rotor can freely spin without any appreciable load.

    A motor at 0 RPM is a total unknown to a sensorless controller, that's where the position feedback comes in.
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I know they use 3 windings. But the TS said 3 coils, not 3 windings. The 3 windings or phases are usually done on 6 or more stator "arms" or cores. Would it even work using 3 individual coils/windings? The distance between poles would be so great the motor would 'cog' so bad it wouldn't be very powerful or may not even start on its own.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    When BLDC servo's first appeared I had a salesman tell me that if I wished to use them for servo control, I had to ensure medium to high rpm as they would cog at low rpm.
    He was only partly right, if used in the open loop velocity mode then cogging is seen at the lower rpm's, but when using them in a high definition PID loop such as a CNC controller etc the perform exactly, if not better than DC brushed.
    I have been retrofitting PC based systems using motion cards such as Galil and previously the Acroloop PC slot cards, these operate the encoder feedback loop at up to 12Mhz so servo control is as perfect and as fast as one would wish.
    The distinct poles can be felt and counted on a BLDC 3 winding motor by shorting the 3 and spinning the armature by hand.
    You can also see the 3phases generated by 'scope when back fed.
    (see PDF)
    Max.
     
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