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How to measure a motor's stall current safely.

Discussion in 'DIY Motion Simulator Building Q&A / FAQ' started by OZHEAT, Oct 16, 2016.

  1. OZHEAT

    OZHEAT Member

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    With all the different motors available on the market, a majority of the motors will not give a lot of info about the specs besides maybe voltage and a dubious rated power rating.

    In this post I will give you a safe way of finding out what the stall current is for your motor.

    Stall current is the maximum current the motor will draw when the rotor is at zero rpm.
    Stall current is the same as Locked Rotor Amps(LRA), Max Amps, & starting Amps.

    Measuring a high power motor for stall current is difficult as there are pitfalls in some methods of testing.

    On some high power motors, especially high geared motors it would be very hard to lock the rotor to even get
    If you try to run a motor at rated voltage and lock the rotor, your motor is pumping max current through the motor and will heat up VERY quickly and doesn't take too much time before bad things to happen.
    You can burn your windings or you can de-magnetize your magnets if you exceed 2-3 sec at stall.

    If you try measuring starting current by watching max amps on a current meter or multimeter you would have to be very fast to read the meter or have a max function on your meter. It wouldn't really be a reliable measurement as you can't be sure that the meter sampled the fast transition from stalled to non-stalled(moving). Given 10 measurements you will get maybe 4 different results, maybe accurate enough but which one is?

    How do you Safely test the motor for stall current with out damaging the motor?

    It is quite easy to do, what you want to do is instead of supplying the rated voltage(12v, 24v 36v etc) you want to supply a voltage well below rated voltage.

    The voltage doesn't really matter but the lower the better, a voltage ~2-5V would be fine. The motor may not even spin with the low voltage but that doesn't matter.

    People that use computer supplies can use the 5v output as long as it can supply enough current, even desktop comp PS will provide 5V@~30A.

    What this will achieve is that you can easily stall the motor and measure current without heating the motor excessively.

    Stall current is the same at different voltages.

    Now that you know your stall current you can determine which H-bridges are suitable for your motor and no you cannot reduce your stall current if it exceeds H-bridge specs.

    Cheers Andy
    • Informative Informative x 1
  2. Tim McGuire

    Tim McGuire "Forever a work in progress"

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    Hi, good post, but your method has a flaw in it. Stall current is not actually the same at any voltage, it will increase linearly with how much voltage you apply to the motor. What happens when a motor stalls out, is that the back-emf (Ea) being generated by the magnetic field of the rotor winding coupling into the stator winding of the motor disappears, which puts the full power supply voltage (Vt) across the coil resistance of the stator winding (Ra) (in parallel with the resistance of the rotor winding (Rf) through the brushes, but this resistance is large compared to the stator coil, so we can discount it in a rough estimate). You can think of back-emf as a voltage that counteracts the supply voltage of the motor, which gets larger as the motor spins faster, which is why current consumption goes down as the motor spins faster.

    Capture.PNG
    Instead what you can do is measure the stator (and rotor) resistance, and then calculate stall current from that measured value. If you have a very small motor, you might be able to get away with doing this using a multimeter at the terminals of the motor, but since our motors are pretty big, the coil resistance is probably going to be somewhere on the order of 0.1Ohms, which is too low for most multimeters to measure with any kind of accuracy.

    Instead, to get a rough guess, you can do this:

    1) Take a low voltage power supply (5V or below will work, as long as the motor does not turn when the voltage is applied, this is important) as suggested above
    2) Put it across the motor terminals with a multimeter in series with your supply, to measure the current. Write down the current value Again, make sure that the motor does not move, otherwise your results will be bad.
    3) Do the same thing as in step 2, except measure the voltage at the motor terminals with your meter (while the supply is attached and supplying current to the terminals). Record this voltage.
    4) Calculate the resistance using Ohm's Law: Resistance = Voltage / Current.
    5) Now to get stall current at the rated motor voltage, use Ohm's Law again: Stall Current = Rated Voltage / Resistance.

    This should give you a rough idea of what your stall current will be. If you want to get a more accurate reading, you can try slowly turning the motor to different positions between tests, and take multiple measurements, because the rotor winding resistance which is connected through the brushes will affect your value a little bit, but not by a whole lot.

    EDIT FOR POSTERITY: One more thing that I forgot to include in this post. Under certain conditions, it's possible for a motor to exceed stall current for a short amount of time, as explained in this stack exchange post:

    http://robotics.stackexchange.com/questions/613/what-is-stall-current-and-free-current-of-motors

    This can only happen when the motors' drive direction is switched back and forth rapidly (which does happen on occasion with a sim). If the motor is switched from full positive drive to full negative drive instantaneously, with a full torque load in both directions, it will attempt to draw up to twice the stall current, but only for a very very small amount of time, most likely not enough to trip the current limiters on an MM or a JRK.

    Sources:
    http://www.me.umn.edu/courses/me2011/arduino/technotes/dcmotors/motor-tutorial/
    https://forum.pololu.com/t/calculating-stall-current-ish/6249/3
    https://ece.uwaterloo.ca/~mte320/Files/DC Motors Final.pdf
    http://electronics.stackexchange.com/questions/256980/how-to-find-stall-current-of-servo-motor
    • Informative Informative x 6
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    Last edited: Dec 4, 2017
  3. Archie

    Archie Eternal tinkerer

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    Tried... to.... understand... :D
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  4. Sieben

    Sieben New Member

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    Hi. Can please explain to one dude, why there are 85A stall current with MY1016 motor, with 24v20A power supply. He claims that there is no need to put 2 bts drivers parallel with max 43A each. as there is no that power with power supply the motor is asking from driver.
    Is it this explanation: the motor will stall, the back EMF will be zero and the curent tends towards the supply voltage divided by the DC resistance of the winding.

    https://www.quora.com/Why-does-stalling-a-DC-motor-cause-more-current-to-flow-through-it
    Last edited: Dec 6, 2018