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Why DC motor draws more current with friction?

Discussion in 'Sensors and Actuators' started by NuLED, Jun 25, 2016.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    the "wall plug", unless it is regulated will exhibit the same effect.
     
  2. skenn_ie

    skenn_ie

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    A motor generates back EMF dependent on it's speed. If speed is zero, no back EMF, so it is only the DC resistance that limits the current. Free running speed generates slightly less back EMF than the voltage the motor is rated for, so you get the input voltage, less the back EMF at that speed, divided by the DC resistance, gives you the free running current. If you slow the motor, the back EMF drops, giving you slightly more of a difference between EMF and back EMF, so the current goes up.
    Battery voltage drops because they have their own internal resistance. As the current goes up, you lose more voltage across that resistance. No device is purely a battery, resistor, capacitor or inductor. A battery has capacitance, series resistance, inductance, just as anything else has, and the parasitic bits often aren't even linear. One example is capacitors that are used for smoothing rectified power. They have low ESR, which is Effective-Series-Resistance.
     
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  3. NuLED

    NuLED

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    So, I used a bench top power supply. BK Precision, capable of 5 amps.

    Voltage STILL dropped.

    So my question still is, why would voltage drop (at the power supply end)? The amps did go up, but why would voltage drop, given LESS back EMF.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    This is the fault of the power supply not the motor.

    I can't say why it happened (maybe the motor demanded more than 5A?) but that's the reason.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Can you measure the DC resistance of the motor and remind me again what voltage you're running it from.

    Then use ohms law to determine the stall current.

    Is this < 5A? Was the power supply set to have current limiting?
     
  6. NuLED

    NuLED

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    With due respect, surely that can't be the case?!

    At 5 amps (if it ever reached that) my thin little DC wires would have heated up and burned the insulation.

    The peak amperage was showed to be less than an amp (I measured it during the experiment).
     
  7. NuLED

    NuLED

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    Note that the motor is not stallED but stallING.

    I will make the measurements next round.
     
  8. Minder

    Minder

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    What gauge are you using? 20 gauge is good for 5 amps.
    It would take well over 10 amps to burn the insulation,
    M..
     
  9. NuLED

    NuLED

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    The wires don't feel hot at all. Anyway I will make proper measurements again just to be sure.

    I can't believe I am the only one seeing this?

    Don't you guys have 3V or 4V DC motors to replicate this experiment?
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    no, but we understand what's happening.

    it would also be useful to know where you're measuring the voltage.

    and how you're measuring the peak current.

    there are no spooky actions which will allow a motor to tell a power supply to reduce the voltage.
     
  11. BobK

    BobK

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    Indeed I do. I tried one of my little motors, ran it off a variable 2A supply and I see no drop in voltage when it is is running, either at 5V or 12V (it ran pretty slowly at 5V).

    Does your supply have a current limit? If so, and it is set too low, the voltage would drop when you run the motor to keep the current at the limit.

    Bob
     
  12. BobK

    BobK

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    Update: Also tried a 18650, 4.0V unloaded 3.8V when running the motor. As expected, it drops a little due to the internal resistance of the battery.

    Bob
     
  13. NuLED

    NuLED

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    Bob, what happens when you use your fingers and pinch the motor shaft, causing it to run slower due to friction?
     
  14. Minder

    Minder

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    See the tail end of my post #11
    See the empirical test I did on a much larger DC motor than the one you are using, rated at 2Hp.
    Off load it drew 2 amps all the way through the rpm range from 0 to 2000rpm.
    The only time I saw a large momentary current was if full voltage was applied, it was almost too fast to see it on a ammeter.
    As soon as the armature starts to turn it generates BEMF.
    On a miniature motor it would be Much faster.
    M.
     
  15. BobK

    BobK

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    I just did it, and I get exactly the result we have been telling you you should get.

    With the power supply set to 5V.

    Running freely, I measure 5V 50mA
    Pinching the shaft to slow it down partially I get 5V and varying current around 120mA
    Pinching it to stop the shaft I get 5V and 250mA.

    The power supply has a 2 digit meter and it always read 5.0 no matter how hard I pinched.

    bob
     
  16. BobK

    BobK

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    If you are seeing even a few % change in your supply when drawing 500mA from a 10A capable supply, it is a pretty crappy supply.

    Bob
     
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  17. shlomo1234

    shlomo1234

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    Speaking about motors,
    The high curent whene we start AC motor does also come because no back EMF?
    Like the DC one?
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The current in an AC motor is also limited by its inductance.

    Load on an AC motor can also change the phase relationship between the voltage and the current. This may not affect current very much, but it will affect the power.

    All three factors come into play with an AC motor.
     
  19. Minder

    Minder

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    Upon switch on an AC induction motor represents a transformer with shorted secondary turns.
    As the rotor catches up with the rotating stator field, the difference of the induced slip frequency becomes progressively less as the rotor catches up, it can never catch up completely as there would be zero induced current in the rotor, so this slip frequency is no less than around 5 cycles.
    In a synchronous AC motor, the rotor rotates at the frequency of the applied field.
    M.
     
    Last edited: Jul 10, 2016
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  20. NuLED

    NuLED

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    You guys are probably right. I need to check with yet another large amperage supply, just to rule things out. Either my supply has a bug, or I set the current limiting inadvertently (I need to double check that with the manufacturer).

    This was confusing because it overlapped the dry cell (AA batteries) and small wall socket supply, which obviously did not have sufficient supply capacity.

    In the meantime, is it safe for me to wire up 3 or 5 of the smaller wall socket supplies, in parallel, to boost their amperage? (Will find ones with the same voltages). Since I do not at present have yet another bench top supply to check?

    Thanks.
     
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