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How to Measure Brushless Motor and Propeller Efficiency

By Charles Blouin

Last updated: 27-10-2021

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.

Note: for our tests we used the Series 1580 Test Stand:

series 1580 thrust stand in a safety cage

 

What Parameters Should I Measure?

BLDC Motor

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

Figure 1: 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)

BLDC Motor Calculations

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.

Further reading: How BLDC Motors Work and How to Test Them

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.

Further reading: Why You Should Test Your Drone's Motors and Propellers

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.

Series 1585 test stand
Figure 2: The Series 1580 test stand with brushless motor and propeller

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.

Further reading: Automated Propulsion Test Scripts for Your Drone

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.

brushless motor test stand

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.

Further reading: Brushless Motor Power and Efficiency Analysis

Motor speed and propeller thrust graph

11 Responses

Lauren Nagel

Lauren Nagel

May 10, 2022

Hi Anya,
Thanks for your question. The cut-off values are meant to prevent you from drawing excessive power and burning out your motor or ESC. You can get a rough idea of a safe operating range from the specifications provided by the manufacturer of your battery and motor. This video may help you set your cut-off values (start at 3:30), and if you need any more help feel free to send us an email: https://www.youtube.com/watch?v=eA0wLIQhi70&ab_channel=TytoRobotics

Anya

Anya

May 10, 2022

Hi,

I am new to this and I am doing a project on propellers using thrust rig 1580 model.
I am confused about how to set the cutoff values.
I am using “overlander batteries of 2200mAh capacity (11.1v 3s)” and “Propdrive 28-30s 800KV” 12 poles motor.

Thanks

Lauren Nagel - RCbenchmark staff

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

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

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)

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

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

Muthukumar

February 22, 2021

Hoe to calculate thrust and torque of the BLDC Motor?

Ashmita

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

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

santanu mondal

September 29, 2020

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

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