How to Measure Brushless Motor and Propeller Efficiency?

Video tutorial

Why test 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 is a calculator that 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.

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.

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. To the right 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.

Stay tuned for more tutorials on how to analyze and interpret the results, allowing you to make smart design decisions for your designs.

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