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Motors in Series Vs Parallel

  1. May 1, 2009 #1
    I'm working on a project that needs to power two electric motors (PC case fans). These fans are rated 12V - 0.3 A. The power source is a AC-DC adapter that outputs 12V and rated for 0.3A.

    My question is simple: should I wire these motors in series or parallel maximum their speed?

    I have a (very) tenuous grasp on electrical engineering. I think that parallel would be better to maximize voltage across the motors but, from what I can gather, this would effectively split the current to each motor while a series circuit would have the same current throughout. Which is more important for an electric motor: current or voltage? Or am I looking at this completely wrong?

    Thanks for any help.
  2. jcsd
  3. May 1, 2009 #2


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    Just on the ratings, to run both motors at full speed would exceed the rating of the power supply.
    They take 0.3 A each and the supply is rated for 0.3 A.

    So, your choice is limited.

    You could try putting them in series and accept that they will run slower than normal.

    You could run just one of them.

    You could find a power supply that can handle 0.6 A.
  4. May 2, 2009 #3
    So hooking them up in parallel will allow both fans to run at full speed?

    In series, would they both run at half speed?

    I can always get a new AC-DC adapter than can handle the increased current if need be.

    [Edit] I know its an obvious question, but I'm unsure because they are motors that seem dependent on current, not a load like a simple light.
  5. May 2, 2009 #4


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    If you put two identical resistors in series each will dissipate one quarter of the power they would have if they were on their own across the same supply.

    But motor characteristics are not as predictable as that and they vary with load as well. You would have to try it.

    If you can get a better power supply, then try them in parallel to get full power out of them.
  6. May 2, 2009 #5
    Ok, that might explain it. I tried both configurations and it seemed that neither one would act as theory suggests (parallel: they would run slower than they did alone; series: one would run faster than the other). I'll look into a higher rated PSU, thanks for the help.
  7. May 2, 2009 #6


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    Yes, sounds like the power supply voltage was really suffering.

    Get a supply with about double what you actually need. Ratings on these are not very conservative and you could easily blow up a supply that is barely good enough.
    I'd be looking for 1.5 to 2 amps. Regulated is better even if you have to pay extra.
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