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How would you short 3 wires of a BLDC motor together? — Parallax Forums

How would you short 3 wires of a BLDC motor together?

T ChapT Chap Posts: 4,217
edited 2014-09-21 14:14 in General Discussion
One of the properties of a brushless DC motor is that if you tie all three leads together the motor does not want to spin, so you have a built in brake. I don't want to use relays, but am wanting to find out what other method I can use to tie the three leads together with a 3v3 control voltage off the prop. I am thinking about 2 n channel mosfets wired back to back off each wire to a bus. Any ideas on how you would do this? 24VDC, max is 3 amps pulled by the motor driver. I will add a method to detect that the shorts have been removed before allowing the motor driver to spin the motor.

Comments

  • GadgetmanGadgetman Posts: 2,436
    edited 2014-09-17 06:09
    A relay, maybe?
  • Mag748Mag748 Posts: 263
    edited 2014-09-17 08:12
    How are you driving the motor? Most commercial motor drivers have a braking function. I think an H-Bridge circuit would allow you to short the leads together as well.

    Thanks,
    Marcus
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-09-17 09:18
    Ummm... an H-bridge is a 2-wire motor reversing solution.

    Relays might be a good way to go. But I would look into snubbing circuitry on the switching side to provide longevity to the relay switching points. Everyone suggests a fly-back diode on the coils to protect the longevity of the processor, but circuitry on the switching side is often ignored.

    Most interesting is that you might simply drive one coil and has a triple pole, single throw switch set up on the other side. (you might have to settle for a triple pole double throw and not use half.)

    http://www.globalspec.com/ds/317/areaspec/spec_pole_triple
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-09-17 09:37
    I'd use three "PhotoFET" solid state relays, like this one, for example:

    -Phil
  • PublisonPublison Posts: 12,366
    edited 2014-09-17 11:10
    T Chap wrote: »
    I don't want to use relays, but am wanting to find out what other method I can use to tie the three leads together with a 3v3 control voltage off the prop.

    Why no relays? Phil's example seems to be a nice and small example.
  • Mag748Mag748 Posts: 263
    edited 2014-09-17 11:57
    How will the current be limited through the relays? If there is no series resistance with the coils in the BLDC motor, they will be damaged.

    Thanks,
    Marcus
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-09-17 13:07
    Why not just gorund all three leads via three n-channel MOSFETs (assuming the motor driver cannot do this on its own)? That effectively connects them all together and makes my prior suggestion seem a bit overkill.

    -Phil
  • tonyp12tonyp12 Posts: 1,950
    edited 2014-09-17 16:42
    Use a driver that has that as a feature:
    http://www.ti.com/lit/ds/symlink/drv10983.pdf

    9.4.3.4 Motor Brake
    The motor brake function can be used to stop the spinning motor before attempting to start the motor. The brake
    is applied by turning on all three of the low-side driver FETs.
    If the motor is spinning at a speed that is greater than the braking threshold (configured by BrkDoneThr[2:0]),
    then dynamic braking acts to stop the spinning
  • ercoerco Posts: 20,255
    edited 2014-09-17 21:51
    How about regenerative braking of said bldc motor?

    http://ww1.microchip.com/downloads/en/DeviceDoc/Regenerative%20Braking%20of%20BLDC%20Motors.pdf
  • RS_JimRS_Jim Posts: 1,762
    edited 2014-09-18 07:04
    erco wrote: »
    How about regenerative braking of said bldc motor?

    http://ww1.microchip.com/downloads/en/DeviceDoc/Regenerative%20Braking%20of%20BLDC%20Motors.pdf
    Very interesting !
    Jim
  • T ChapT Chap Posts: 4,217
    edited 2014-09-18 07:04
    Thanks for the great suggestions guys. The board does not have a brake input, however I see that the BLDC controller IC does have a brake input pin but it is tied to GND and was not used on this board. This is easy to lift and route to the Propeller as a hack. I appreciate the input as this will do the trick!
  • Mark_TMark_T Posts: 1,981
    edited 2014-09-18 18:11
    Phil Pilgrim (PhiPi) wrote: »
    Why not just gorund all three leads via three n-channel MOSFETs (assuming the motor driver cannot do this on its own)? That effectively connects them all together and makes my prior suggestion seem a bit overkill.

    -Phil
    And what prevents shoot-through if the motor driver is still active and pulling the phases high?

    This is a function for the motor-driver to perform really, it already has all the switches and it knows when its
    safe to do it.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-09-18 22:50
    Mark_T wrote:
    And what prevents shoot-through if the motor driver is still active and pulling the phases high?
    Nothing, except well-written firmware that disables the driver before braking! :) Do you drive with one foot on the gas and the other on the brake?

    -Phil
  • Dave HeinDave Hein Posts: 6,347
    edited 2014-09-19 11:31
    Phil Pilgrim (PhiPi) wrote: »
    Do you drive with one foot on the gas and the other on the brake?
    Only when stopped on a steep road, and I want to prevent rollback when moving my foot from the brake to the gas.
  • PublisonPublison Posts: 12,366
    edited 2014-09-21 12:48
    =Phil Pilgrim (PhiPi); Do you drive with one foot on the gas and the other on the brake?

    -Phil

    Well..yes. this is how I drive every day. :) I will say this is practice for racing where it is beneficial to have both feet in close proximity to the pedals for quick input.

    http://en.wikipedia.org/wiki/Left-foot_braking.

    After not having a clutch vehicle for a while, I had to do some workouts with my left leg to gain some strength.
    .

    Jim
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-09-21 13:54
    Wikipedia article wrote:
    When left foot braking is used to apply the brake and the throttle at the same time it is very hard on the car, causing extra wear on the transmission and brakes in particular.
    The mechanical equivalent of "shoot through," IOW. :)

    -Phil
  • PublisonPublison Posts: 12,366
    edited 2014-09-21 14:14
    Phil Pilgrim (PhiPi) wrote: »
    The mechanical equivalent of "shoot through," IOW. :)

    -Phil


    touche :)'
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