# Torque Calculation without a gauge

• A
Hi
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)

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CWatters
Homework Helper
Gold Member
Mount generator casing on bearings so it can rotate through a small angle.
Fix lever arm to generator casing.
Measure rpm an convert to angular velocity

Dead weight * length of lever arm = torque

Power = torque * angular velocity

billy_joule
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 ?

CWatters
Homework Helper
Gold Member
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).
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:
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
RPM = 2468

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

Is my Torque calculation correct or have i missed something.

CWatters
Homework Helper
Gold Member
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.