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DC motors in parallel causing voltage drop

  1. Jan 12, 2017 #1
    Hey PF, first time here, I have a problem I can't seem to solve! Any help is appreciated.

    I am using some DC motors with peristaltic pump heads to pump water (picture). I was having issues so I've broken my circuit down to the bare basics:

    1) 12V regulated power supply
    2) 12V motor
    3) free-wheel diode

    That's all that's plugged in, the power pack is connected directly to the motor.
    The voltage measured across the power pack is 12V open circuit. For each motor I add (in parallel) the voltage measured at the power pack drops by 0.2-0.5V.

    I've measured (with a multimeter in current mode) 450mA being drawn by the motors. I've tried 2 power packs, a 12V 1.5A, and a 12V 5A, both regulated. Interestingly, while each pump dropped the voltage by 0.2V on the 1.5A supply, they dropped it by 0.5V on the 5A supply.

    The issue is that I need to control an accurate amount of water flow, which needs a consistent voltage regardless of the number of pumps used at any time.

    Other searching has returned that maybe the backEMF of the motors is to blame, but I'm at a loss how to solve this.
     
    Last edited by a moderator: Jan 13, 2017
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  3. Jan 12, 2017 #2

    Andrew Mason

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    The battery has an internal resistance. The voltage drop depends on the current it produces. This drop is ΔV = IR where I is the current and R is the battery resistance. The greater the current, the greater the drop in terminal voltage.

    AM
     
  4. Jan 13, 2017 #3

    NascentOxygen

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    Hi Dionysus2. :welcome:

    Sounds like the regulating circuit can't keep up with the gulps of current. How long are the power leads from the 12V supply to the motors?—they should be short and heavy gauge. That 450 mA is merely the average current, though the motors likely take it in big gulps, so I wouldn't expect the 1.5A power pack to work here. Try extra capacitance across your bundle of motors, say a 1000 uF, 25V electrolytic capacitor. (Also, add a 2uF polyester capacitor parallel to the electrolytic.)

    See whether the voltage remains stable with that.
     
  5. Jan 13, 2017 #4

    CWatters

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    0.5/12 = 0.04 or 4% If you need the flow rate to be accurate to better than 4% is it safe to rely on the motors and pumps being that consistent? For example the flow rate might change as the motors and pumps wear or if the pumps or water changes temperature? Might be necessary to measure the flow rate and set up a control loop to control the motor pump speed.
     
  6. Jan 13, 2017 #5

    jim hardy

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    Wall adapters and power supplies come in two basic flavors

    linear, with a big line frequency transformer inside and maybe a voltage regulator
    switching, with a small high frequency transformer inside and always a voltage regulator .

    You can tell them apart, switchers are small and light compared to linears.

    A regulated supply will hold voltage rock steady unless the motors take a "Gulp" of current larger than the capability of your supply.

    I'd try a five amp supply That'll let each motor take a 250% gulp. A switcher will be smaller and cheaper.
     
  7. Jan 13, 2017 #6
    Should still be an issue if I'm using a regulated power supply?
     
  8. Jan 13, 2017 #7

    NascentOxygen

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    Yes, because at moments when the motor tries to draw more current than the regulator can supply the power pack falls out of regulation.
     
  9. Jan 13, 2017 #8
    Hey NascentOxygen, thanks for your reply :)
    I've also used a 5A power pack with this setup, somehow it seems to get more of a voltage drop than the 1.5A one.
    I've measured the AC voltage over the motors and it is only non-zero on start and stop (a process which takes <0.1 seconds) and then the AC voltage is ~0. The DC voltage drop caused by the motors is steady and constant, not transient.
    The 2 power packs I'm trying are this 1.5A one and this 5A one.
    The distance from the power supply to the motors is pretty much the length of the supplied power cable (50cm) plus my wiring (10cm).
    I've moved everything to a soldered perfboard just in case it was the breadboard, though it hasn't fixed it.
    I live a bit remote and driving to the shops just to pick up a capacitor is a bit of a pain; I understand how a capacitor would fix a transient/fluctuating voltage issue, though would it fix a constant steady drop?
    Thanks for your help thus far :)
     
  10. Jan 13, 2017 #9
    Hey Jim, I've tried a 5A regulated power supply, and there is a voltage drop with just 1, and again with 2. Interestingly the voltage drop seems to be linear with the increase in motors, roughly 0.2V. If each motor is generating 0.2V in back EMF, could this cause this drop?
     
  11. Jan 13, 2017 #10

    NascentOxygen

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    If your measuring instrument has been a multimeter then its readings are meaningless when we are discussing transient conditions; it requires an oscilloscope.

    You may have a discarded piece of electronic equipment that contains a large electrolytic capacitor you could salvage?
     
  12. Jan 13, 2017 #11

    jim hardy

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    Power pack is not very well regulated then, is it ?

    200 millivolts drop for 450 milliamp current increase is 0.44 ohms, about 22 feet of #20 two conductor wire.
    http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/wirega.html
    That's more wire than you describe.


    You've measured the 200 millivolts, how about the 450 milliamps ?
    A DC motor can draw a LOT more than rated current during load peaks.
    A peristaltic pump is hardly a steady load and motor current will follow its torque profile.
    A regulator will let voltage dip when current gets high.

    Do you know how to use your multi-meter for measuring current ?
    Can you measure motor current with pump pumping just air , no tubes connected,
    and again with pump pumping whatever it is you use it for?

    That'll eliminate one more unknown.
     
  13. Jan 13, 2017 #12
    I found some old caps, a 4.7uF electrolytic, and a 223k poly cap, they made 0 change. I'll try to pick some bigger ones up after work on monday. Any thought on it just being motor back EMF? Considering each additional motor adds such a consistent drop...
    I've tried 2 different regulated power packs, and I have even used a 15V pack with a 12V regulator, all producing the exact same output.
     
  14. Jan 13, 2017 #13

    jim hardy

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    Back EMF is your friend here not your foe .

    What is your understanding of back EMF in a motor?
     
  15. Jan 13, 2017 #14
    When I just have a basic resistor only circuit, the voltage stays steady at 12V, regardless of resistance.
    I removed the peristaltic head off the motor. This made the voltage drop less, but it is still there, additional with each motor.

    I've measured the current by putting the multimeter into mA mode, moving the probe to the mA hole, and adding it in series to a pump.
    Just measured again. With peristaltic head on, 450mA. Without peristaltic head, 90mA. Both produced a 0.2V drop.
     
  16. Jan 13, 2017 #15
    Honestly pretty minimal. My understanding is that the back EMF increases as the speed of the motor increases. Once the motor reaches a steady speed the back EMF should be constant. The back EMF acts like a voltage supply in opposition to the voltage supplying the motor. So my guess here is that these DC motors produce 0.2V of back EMF at top speed, and thus reduce the measured voltage across the supply by that amount. Each subsequent motor added goes through a transient state, then reaches a steady state of top speed, where it also produces a back EMF of 0.2V, further dropping the measured voltage of the supply.
    Would my understanding be correct or am I talking nonsense?
     
  17. Jan 13, 2017 #16

    NascentOxygen

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    Nothing to do with back emf.

    About the unexpected voltage drops you are noting—are you seeing these at the motor terminals or at the power pack terminals?
     
  18. Jan 13, 2017 #17
    At the power pack terminals, though everything is connected in parallel so it should be the same?
     
  19. Jan 13, 2017 #18

    NascentOxygen

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    It's just not a very good 5A regulated supply, then.
     
  20. Jan 13, 2017 #19

    jim hardy

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    Well there's a clue.

    Many regulated power supplies require some minimum load current , below which their voltage will rise somewhat.

    This note on your supply's datasheet at https://www.jaycar.com.au/12vdc-5a-dc-output-regulated-power-supply/p/GH1379
    suggests to me it is not a precision supply.

    You might try giving it a small load, like a small 12 volt bulb

    or try another supply
    this one is about fifteen bucks on Ebay, specs say 0.5% load regulation which would be +/- 60 millivolts .
    http://datasheet.octopart.com/RS-50-12-Mean-Well-datasheet-10901130.pdf
    http://www.ebay.com/itm/RS-50-12-Me...132323?hash=item1c759a52e3:g:q3cAAOSw5cNYL7BO

    if this is a for a medical application play it safe and use one of the medical rated supplies.
     
    Last edited: Jan 14, 2017
  21. Jan 13, 2017 #20

    jim hardy

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    okay so far.

    Back EMF opposes the supply. That's from magnetic interaction and it's how the motor makes electrical energy into mechanical work,
    (back EMF) X (armature amps) is the power being converted from electrical Watts to mechanical Newton-meters/sec

    The voltage left over to push armature current through brushes and armature wires''s resistance is (Supply - Back EMF).. It gets wasted as heat.
    I'd expect more like 10 volts of back-EMF and 2 vols across brushes and armature wire resistance.
    Draw yourself a picture - two voltage sources , brushes and armature resistance all in series.
    At stall there's no Back EMF so current goes sky high, and drops off as Back EMF comes up with speed.

    Do the exercise of making that drawing and talk yourself through . When it clicks, try rewriting that paragraph. Explaining things often crystallizes them in our mind.
    "Science is but language well arranged." a famous scientist of the 1700's.
     
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