A high-accuracy torque transducer for small-scale wind and hydrokinetic turbine experiments

Abstract

The design, assembly, use, and analysis of a torque transducer for small-scale wind and hydrokinetic turbines is presented. The new transducer provides a calibration between the torque and electrical current produced by a model turbine that uses a DC motor as a generator. The transducer transfers the torque generated by the turbine’s shaft to a small beam instrumented with a full bridge configuration of strain gauges whose voltage response is calibrated to known torques. The design of the transducer, coupled with specific procedural considerations, yields torque measurements from 5 to 50 mNm and a calibration uncertainty of 1.2%. We believe this is the lowest uncertainty achieved to date for a torque transducer calibration of this type. The primary difference between the presented approach and previous approaches in the literature is that we use a control motor to impart the applied calibration torque as opposed to relying on the fluid flow, which improves both the ease of use and accuracy. The presented calibration allows for an accurate measurement of mechanical power by non-intrusive means. Measurement of mechanical power is preferred over other methods of quantifying turbine power as it makes the fewest assumptions and is most closely related to the fluid dynamics. The detailed calibration procedure is provided, including considerations particular to model turbines not previously discussed, such as the necessity of avoiding hysteresis during the calibration and the fluctuations in voltage characteristic of DC generators.