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Newbie needs some help kw/rpm relation

  1. Aug 18, 2008 #1
    theory check: i have a hydro site that has generates 2000Nm of torque at 100rpm. this has been calculated at 20.95 kw. i am looking to attach a low-rpm PMG making 20kw at 250 rpm. if i use a gear box to achieve the 250 rpm (leading to a gecrease in torque 2,5 times=800Nm) will i be making the 20kw that is rated at rpm according to manufacturer specifications? the generator has 93% efficiency, so in the end i will be making 93% of the 20kw , or 18,6 kw.

    please let me know if my calculations are correct

    big 10q
  2. jcsd
  3. Aug 18, 2008 #2


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    Staff: Mentor

    Minus losses in the gearbox (which could be substantial), your logic about the power sounds correct. Note, though, the rpm gets divided, but the torque multiplied by the gear ratio. That's how the power stays the same (p=rpm*torque).
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