How to Measure Brushless Motor and Propeller Efficiency

How to Measure Brushless Motor and Propeller Efficiency

By Charles Blouin, last updated: 27-10-2020

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 which parameters to optimize. 

  • 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

alt Motor and Propeller Tutorial

NTM propdrive 35-30 brushless motor and Quantum 13-4 carbon fiber 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 (Nm)
  • Thrust (g)

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 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 your motor over a range of command inputs and with multiple propellers to vary the mechanical load.

How to Measure all Parameters

In summary, you need to simultaneously record voltage, current, torque, thrust, and motor speed, while at the same time controlling the motor’s throttle. By combining these readings you can extract the electrical and mechanical power, which in turn will give you the efficiency values.

alt Dynamometer Thrust Load Cell Calculator

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 at each step, 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.

alt Dynamometer Test Brushless Motor in Lab

How to Use the Efficiency Results

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

You can then compare this plot with other plots generated using the same method. Try comparing two plots with identical parameters but one element changed, for example, switching propellers.

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

Liked this tutorial or have comments? Let us know below.

alt Plot Thrust and Propeller Efficiency

Comments

Lauren Nagel - RCbenchmark staff - April 27, 2021

Hi Shane,
Thanks for your comment! Your problem sounds like just the type of frustration our products were designed to solve. The prices for our smaller products can be found in our online store (https://www.rcbenchmark.com/collections/all), but if you need a larger test stand (>5kgf), send an email to our sales specialist Joffrey (sales@rcbenchmark.com) and he can set you up with a quote.

Shane Anderson - April 27, 2021

Hello!
Well done…i am so frustrated by lack of motor data. Your product may just suit me. Is it affordable?

Shane

Charles Blouin -RCbenchmark staff - February 22, 2021

@Ashmita Unfortunately, the correlation between throttle (in%) and voltage is not well defined. It can change depending on the firmware and the ESC manufacturer. You can try a linear approximation, but that is usually off.

Charles Blouin (RCbenchmark) - February 22, 2021

Hello! @John: You are right that the efficiency of the propeller changes with the wind speed. Static tests will provide the propeller efficiency at hover. For dynamic testing, you can use a wind tunnel such as the one we offer, or test on a moving vehicle (we have an article about that here:https://www.rcbenchmark.com/blogs/customer-stories/rcbenchmark-tools-help-fpv-quadcopter-designers-in-two-projects)

john - February 22, 2021

Generally, static tests do not provide efficiency for the propeller. It’s confusing to denote “propeller efficiency”, it should not be defined for such tests.

Muthukumar - February 22, 2021

Hoe to calculate thrust and torque of the BLDC Motor?

Ashmita - February 22, 2021

Let’s say we only know the PWM signal input to the ESC, how do we calculate the throttle percentage from this information? I would like to finally note the input voltage to the motor after the ESC.

Joffrey Ferry - RCbenchmark staff - September 30, 2020

Hi Santanu!

The mechanical power (output power) is measured by multiplying the torque with your motor’s rotation speed (mech. power = torque x RPM).

The motor efficiency is the mechanical power divided my the electrical power (motor eff. = mech. power / elec. power).

The propeller efficiency (kgf/W) can be calculated by dividing the thrust by the mechanical power (propeller eff. = Thrust / mech. power).

Both thrust and torque are measured by the stand’s load cells.

santanu mondal - September 29, 2020

How to measure output power and efficiency of the BLDC motor?

Leave a Comment

Comments must be approved before appearing.

* Required fields