holding a DC motor at stall current

Discussion in 'The Projects Forum' started by kutalinelucas, Dec 12, 2011.

  1. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    hey guys...

    I'm just wondering how damaging it is to hold a motor at its stall current for a period of 10 to 20 seconds...

    Any ideas would be brilliant

    cheers
     
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    at rated voltage without current limiting, very.
     
  3. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    ah...ok I do have a (firmware) way to get around this, but just out of intrest, my motors have a stall current of 500mA; and they're being driven by L293D H-bridge drivers with a current limiter of 600mA per channel, would this still do some damage?
     
  4. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    Maybe not to the circuit, but the windings in the motor will overheat very quickly and die.
     
  5. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    lovely...thanks for the info
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    You could add a PTC fuse rated for 450mA.

    I see that the stall current IS 500mA

    What is the running current?

    When a brushed motor stalls, the current typically shoots up to what is allowed by the DC resistance of the motor windings. Add a fuse to blow when that current is met or exceeded before the windings melt.
     
  7. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    the running, no load current is ~300mA (due to a 76:1 spur head on the output).

    Putting in a fuse wouldn't really solve the problem as my system knows when the motor has stalled through feedback from a H.E quadrature encoder so I can protect the coils...but ideally I'd like to maintain some holding torque when the load on the rotor stalls the motor...could you think of a way to do this?

    For instance, could I increase the mark-space ratio in the applied pwm signal so a lower average voltage is applied across its terminals?
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    If you have a 76:1 reduction, how much torque is needed on the output to backspin the motor? Maybe you could somehow short the motor leads and let the generator effect provide holding power.

    (ED) If you have encoder feedback, can't you use it to generate forward/reverse commands to hold a position?
     
    Last edited: Dec 12, 2011
  9. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    it does to a certain extent...the (4) DC motors are being used to actuate the upper/middle knuckles of a robot hand, where translation of the joints is achieved by using the output of the DC motors to twist two parallel strings which creates a tortive effect, reducing the overall length of the tendon. The beauty of this approach is high output torque, but also inherit compliancy, so the hand can grasp most objects without a prior model of its attributes. I have sensors placed over the hand which informs the control system of any contact, but heavier objects seem to require some amount of grasping torque...so really I'd like to maintain some power accross the motor without burning the coils, ideally anyway. Its not crucial to be honest but it would be nice to manipulate heavier objects
     
  10. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    Fantastic, can't believe i didn't see that...thanks so much for the help
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    That's the difference between open and closed loop servo systems.
     
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