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?

The motor

NTM propdrive 35-30 brushless motor and Quantum 13-4 carbon fibre prop.

To fully characterize a motor, you need to measure the following parameters.
  • Voltage (V)
  • Current (A)
  • Throttle input (%)
  • Motor load or torque (Nm)
  • Speed (RPM)
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.

The propeller

For extracting useful propeller data, you need to measure the following parameters:
  • Speed (RPM)
  • Torque
  • Thrust
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.

Dynamometer Thrust load cell

Test procedure for static tests

Dynamometer test of a brushless motor and a propeller

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.

The results obtained are shown in this CSV file.

How to use the efficiency results?

You can summarize a lot of data points using any plotting software. Here is an example obtained using the CSV file linked above:

plot trust and propeller efficiency

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.

What next?

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!
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  • Cyber Monday deal. 150$ OFF, only today!

  • Thanks for your feedback! 

    The software was updated to support burst current. The limits are 40 amps continuous and 50 burst for now, but we may increase those number. The device should in theory support 75 amps continuous, but we do not have the equipment to test it.

    We posted two videos of our (open source) GUI. I think some will find the second video especially interesting due to the scripting abilities. We hope in the future to integrate dynamic test with the scripts.


  • We are adjusting our launch price, and added a launch discount of $50, so our product is now $445. On top of that you can use the DIY15 coupon until November 15. Customers who ordered in the last 48h will be refunded the difference.

  • @Daniel Wibbing Yes. We decided to go with this system as it is simpler and easier to install. You do have to know the number of pole, but we documented how to count that properly. The software also has a small utility that will count the number pole for you. You just enter the manufacturer's KV, run the test, and you obtain the real (experimental) KV, and the number of poles!

  • @Charles Blouin
    Oh, that's great! So you are counting the commutation frequency with reference to the ESC's ground!?

  • @Daniel Wibbing . The rotational speed is measured electrically with a probe to one of the motor's wire. You see it briefly at 0:59 in the video, but is is explained more in details in the installation manual.

    You can mount the motor higher or lower. The load cell measurement is not affected by the torque due to offset.

  •  Charles Blouin, could you tell me how you measure the rotational speed (rpm)?
    What effect would it have, if the motor is not mounted in the center of the fixture plate of the dynamometer, but would have a vertical shift of about 50 mm?
    I am asking this, because I am wondering if I could also test my electric ducted fan with your dynamometer.

  • Developer

    Nice work.  By the way, for Copter 3.3.1 users, if you use this system (or any system) to determine the ESC/Motor/Prop combination's PWM to thrust curve you might get a touch more control by adjusting the MOT_THST_EXPO parameter to match.  How this parameter should be adjusted isn't documented on the wiki but we will add it eventually.

  • This is really excellent work Charles Blouin.  I share the frustration that others have expressed concerning manufacturers and their poorly published tech specs.

    My only wish is that you would make a future revision that has higher power handling capabilities.  I have many motors and almost all of them draw 42 amps or more at full throttle. (Several draw as much as 80 amps.)

    With that said, your device would still be very useful as it is for characterizing these larger motors at "cruise" speeds.  Again, excellent work!

  • Great Job,

    It would be great if every motor and propeler manufacturers make use of this , so we can get real numbers...

    Keep on the good job... Lâchez-pas !!

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