Hello fellow DIYer. Last February, my collegue and I posted a survey here asking if you'd be interested in a dynamometer for drone motors and propellers. We are now releasing the production version of the dynamometer!
For the release, here is a quick introduction to motor and propeller testing, as summarized in the video above.
Why testing your motors and propellers?
You must first ask yourself, what are your, or your end user's needs? This question is important, as it will help you know what parameters to optimize for.
Do you want to fly longer to film uninterrupted for longer periods?
Do you want to carry a larger payload?
Do you need more thrust and power to go faster, or to improve handling in strong winds?
Do you have overheating problems, and your application requires you to minimize failure rate?
The final choice of power system depends not only on the airframe and payload, but also on your application.
What parameters should I measure?
To fully characterize a motor, you need to measure the following parameters.
Throttle input (%)
Motor load or torque (Nm)
The RCbenchmark software automatically calculates the following parameters for you:
Mechanical power (Watts) = Torque (Nm) * Speed (rad/s)
Electrical power (Watts) = Voltage (V) * Current (A)
Motor Efficiency = Mechanical power / Electrical power
The output speed is function of the throttle, in %, and of the load (torque in Nm). If you want to completely characterize a motor, you will need to test it with multiple input voltages and different loads. The throttle is changed with the software, and the load is changed with the type and size of propeller.
For extracting useful propeller data, you need to measure the following parameters:
The RCbenchmark software calculates the following parameters for you:
Mechanical power (Watts) = Torque (Nm) * Speed (rad/s) ← same as the motor
Propeller efficiency (g/Watts) = Thrust (g) / Mechanical power (Watts)
Notice that the mechanical power is the same for the motor and propeller. That is because all the motor's mechanical power output goes into the propeller, since it is directly coupled to the motor's shaft.
The overall system
The overall performance of the system depends on a well balanced combination of motor and propeller. Your system will be very inefficient if these two parts don't match well together. Because these two parts have a common link (the shaft), the overall system efficiency is calculated as:
System efficiency (g/Watts) = Propeller efficiency (g/Watts) * Motor Efficiency
Where the system efficiency is in grams per watts of electrical power. Changing the motor, propeller, or even switching to another ESC will all contribute to changing this calculated system efficiency.
Moreover, the efficiency value will only be valid for a specific command input and mechanical load. In practice, this means that you will test you motor over a range of command inputs, and with multiple propellers to vary the mechanical load.
How to measure those parameters?
In summary, you need to simultaneously record voltage, current, torque, thrust, and motor speed, while at the same time control the motor's throttle. By combining these readings you can extract the electrical and mechanical power, which in turn will allow you to get the efficiency values.
The RCbenchmark motor test tool was built to reduce the time and cost associated with building a custom test rig. The tool is capable of measuring all the necessary parameters while controlling the ESC, and recording the data in a CSV file for analysis.
Test procedure for static tests
For now, we will only cover static tests (we won't talk about dynamic tests involving angular acceleration, estimating stall torque, etc...). Before starting your tests, we recommend:
Installing your propeller in pusher configuration, to reduce ground effects with the motor mounting plate
Have a reasonable distance between the propeller and other objects, again, to avoid ground effects
Having all safety measures in place to protect the people in the same room
Configuring your dynamometer to automatically cutoff the system should any parameter exceed its safe limit
A simple but effective test consists of ramping up the throttle in small steps, and recording a sample after every step. Before taking the sample after each step, we allow the system to stabilize for few seconds.
In the video above, we manually varied the throttle from 0 to 100% in 10 steps. This procedure could also have been performed using the RCbenchmark's automatic test or scripting feature, which we will cover in another tutorial.
You can summarize a lot of data points using any plotting software. Here is an example obtained using the CSV file linked above:
You can than compare this plot with other plots generated using the same method. Try comparing two plots, all with the same parameters identical expect one element changed, for example switching propeller.
We want to publish more tutorials, with more details about certain aspects, such as automatic tests, installation, automatic kV testing and pole counting, motor theory, dynamic tests, scripting, etc. Anything in particular you would like to learn about?
If you are interested by our dynamometer, have a look here. We offer 15% off until November 15 for DIYdrones readers using the code "DIY15".
It is an exciting time for my collegue and I, as this release is the results of almost a year of work! Please comment below, I will do my best to answer your questions!