​On your end, testing the endurance and efficiency of a modular small gearset at low output speeds (3–10 RPM) and torques up to 0.60 Nm requires precise braking mechanisms and accurate power measurement methods. Traditional brakes like magnetic particle and hysteresis types may not perform optimally at such low RPMs. One effective solution is the use of Positorq Dynamometer Load Brakes, specifically designed for high-torque, low-speed applications, capable of delivering smooth and controlled torque down to zero RPM. These brakes utilize oil shear technology to provide consistent performance without chatter, making them suitable for your testing requirements. ​

For electronic power measurement, implementing torque and speed sensors offers real-time data acquisition. Direct torque measurement using torque flanges or rotary torque sensors provides high accuracy by capturing torque signals directly from the drive train. This method ensures precise calculation of mechanical power output, which is crucial for endurance testing. ​

Additionally, considering a gear ratio adjustment which is important and in your test bench setup can be beneficial. By introducing intermediate gearing, you can increase the rotational speed at the brake interface, aligning it with the optimal operating range of standard braking systems. This approach allows for the use of more readily available brakes designed for higher RPMs while still applying the necessary load to the gearset under test.​

Would recommend these for review:

https://www.servicetruckmagazine.com/news/new-low-speed-high-torque-dynamometer-slashes-testing-time-a

https://www.forcecontrol.com/products/dynamometers/

the other option would be a water brake system. its relatively low tech but effective. and as Prinz suggests a overdrive gearing to the dyno would improve control ability.