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A Torque Calculation without a gauge

  1. Oct 21, 2016 #1
    How can I measure, shaft power (watts or HP) of a vertical turbine using dead weight blocks

    Turbine diameter - 18 inches
    RPM - 450 ( Zero Load)
  2. jcsd
  3. Oct 23, 2016 #2


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    Mount generator casing on bearings so it can rotate through a small angle.
    Fix lever arm to generator casing.
    Add rope from lever arm over pulley to dead weight.
    Measure rpm an convert to angular velocity

    Dead weight * length of lever arm = torque

    Power = torque * angular velocity
  4. Oct 26, 2016 #3
    Thanks a ton. A little more help.

    My Turbine is a reaction turbine of 6 inch diameter. It rotates at 2500 RPM generating 15N.m of Torque. The mentioned RPM is measured for shaft rotating (without coupling it to any other machine). Once a Generator is coupled, what will be the final RPM. Will the RPM reduce ? If yes, is there a formula to calculate the final RPM ?
  5. Oct 26, 2016 #4


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    That statement is inconsistent. If there is no load on the turbine it cannot generate any torque (or power for that matter).

    What usually/sometimes happens is that as you increase the load the torque increases and the rpm reduces. If you use the right units.. Power = Torque * rpm. So you really need a graph of Torque vs rpm to workout the best operating point (peak power) for the turbine.

    Perhaps 2500rpm @ 15N.m is the max power operating point?
    Last edited: Oct 26, 2016
  6. Oct 26, 2016 #5
    The RPM has been read with a non contact tachometer, while the Torque has been calculated theatrically based on the input forces (Water Pressure and Flowrate)

    Motive fluid = Water
    Pump Gauge Pressure = 29.86 PSI
    Flow rate = Water leaving the turbine = 10LPS @ 19.70 m/s (linear Velocity)

    Force = 10*19.697 = 196.95 Newtons
    Turbine radius (arm length) = 3 inches ( 0.0762 meters)

    Torque = 196.95*0.0762 = 15 N.m
    Angular Velocity = 258.49 rad/sec
    RPM = 2468

    Power = Toque*rad/sec = 15*258.49 = 3877 Watts

    Is my Torque calculation correct or have i missed something.
  7. Oct 26, 2016 #6


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    Perhaps I miss understand your calculation but force isn't mass * velocity.

    It might be easier to use...

    Power = pressure * flow rate

    ..to calculate the power available.

    Which I make 2058 Watts.

    If the turbine utput isn't connected to anything then this isn't the output power. It's the power needed to spin the turbine with no load. Eg its the power loss.
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