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Handling high motor stall currents

Discussion in 'General Electronics Discussion' started by rasen58, May 4, 2021.

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  1. rasen58

    rasen58

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    0
    Apr 30, 2021
    Is there any general advice on how to handle motor stall currents? I'm making a robot with two large motors powering the wheels and the motors take 4A normally but have a 15A stall current each.

    Are there certain things I should be doing in either hardware/software to make sure that the stall currents aren't there for prolonged periods of time or if there are, that I protect the circuitry somehow?
    For example I think that if I powered the motors at full speed in one direction and then suddenly reversed in the opposite direction, that could create a large emf which could damage my circuitry or motors?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    Use a PTC in series. Low resistance at low temperature, high resistance at high temperature. If the motor stalls, current rises leading to an increase in temperature of the PTRC leading to increase n resistance, leading to reduction in current.
    Or use a resettable polyfuse, based on the same principle of operation.
    Or measure the current and turn off the driver when current is too high for too long.
     
  3. Tolaneva12

    Tolaneva12

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    May 4, 2021
    PTRC have better usability
     
  4. rasen58

    rasen58

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    Apr 30, 2021
    Thanks. What is PTRC?
     
  5. rasen58

    rasen58

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    Apr 30, 2021
    Also it looks like the PTC would be great for protecting the motor from over current. But will a PTC also protect my motor driver from back emf current if I suddenly switch off the power going into my motor driver?

    This is the simple motor driver circuit I have (two motors and two motor drivers). So should I add a PTC on the OUTB or OUTA pin connecting the driver to the motor?

    upload_2021-5-4_14-42-29.png
     
  6. bertus

    bertus Moderator

    1,864
    707
    Nov 8, 2019
    Hello,

    From the drivers page:
    Output current: 13 A continuous
    Active current limiting (chopping) with default threshold of 30 A (can be adjusted lower)
    https://www.pololu.com/product/2992

    When needed there ia a stronger driver available:
    https://www.pololu.com/product/2995
    Output current: 21 A continuous
    Active current limiting (chopping) with default threshold of 50 A (can be adjusted lower)

    Bertus
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    No, that protection is usually part of the driver.
     
  8. rasen58

    rasen58

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    0
    Apr 30, 2021
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
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    Nov 17, 2011
    It doesn't have to state that explicitly. This is an inherent function of the MOSFETs' body diode.
     
    rasen58 likes this.
  10. rasen58

    rasen58

    9
    0
    Apr 30, 2021
    I'm also using this motor driver which doesn't have a current limiting functionality https://www.pololu.com/product/760
    Is there anything I can do to be safe when using this one?
     
  11. ratstar

    ratstar

    486
    20
    Aug 20, 2018
    If you could measure the velocity of the motor, you need to cut out the battery when the velocity is low.

    Would a constant current circuit work for back emf from dc motors? Its actually not resistance load, its actually back generating out from the motor, but would it work anyway?
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    You will then face the issue of starting the motor. At 0 rpm the battery will be cut out and you need an override circuit - which in turn will need to be protected from a potential high stall current ...
    No. Motor current varies with load.
     
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