How to Measure Brushless Motor and Propeller Efficiency

Evaluating the performance of your motors and propellers through testing is paramount for identifying areas of improvement in your design.

This process enables enhancements in efficiency, directly impacting factors such as endurance, flight duration, payload capacity, and beyond.

This article will delve into both the methodology and significance of motor and propeller testing.

The significance of testing your motors and propellers lies in understanding your own or your end user's specific needs. This awareness is crucial as it guides the optimization of relevant parameters.

Why Test Your Motors and Propellers

Considerations such as:

- Do you aim to extend flight durations for uninterrupted filming?

- Are you seeking to increase payload capacity?

- Do you require enhanced thrust and power for speed or better maneuverability in adverse weather conditions?

- Are overheating issues a concern, necessitating a reduction in failure rates for your application?

Ultimately, selecting the appropriate power system isn't solely dictated by the airframe and payload; it's also influenced by the demands of your particular application.

What Parameters Should I Measure?

Brushless Motor Efficiency

To comprehensively assess a motor and ascertain its efficiency, it's imperative to measure various parameters:

1. Voltage (V)

2. Current (A)

3. Throttle input (%)

4. Motor load or torque (Nm)

5. Speed (RPM)

These parameters are pivotal as they facilitate the calculation of key performance metrics:

- 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 contingent upon both the throttle input, expressed as a percentage, and the applied load or torque, measured in Nm. To comprehensively characterize a motor, testing should encompass varying input voltages and loads. Throttle adjustments are achieved through the controller, while load variations are accomplished by altering the type and size of the propeller.

Propeller Efficiency

Determining propeller efficiency requires measuring the following parameters:

1. Speed (RPM)

2. Torque (Nm)

3. Thrust (g)

These measurements enable the calculation of the following:

- Mechanical power (Watts) = Torque (Nm) * Speed (rad/s) (same as for the motor)

- Propeller efficiency (g/watt) = Thrust (g) / Mechanical power (Watts)

It's important to note that the mechanical power remains consistent for both the motor and propeller. This consistency arises because all of the motor's mechanical power output is transferred to the propeller, given their direct coupling via the motor's shaft.

Overall System Efficiency

The overall efficiency of your system hinges on the harmonious interaction between your motor and propeller. Even if each component individually boasts high efficiency, the system can suffer from inefficiency if they are not well-matched.

Given their shared connection via the shaft, the system efficiency is expressed as:

System efficiency (g/watt) = Propeller efficiency (g/watt) * Motor Efficiency

Here, system efficiency is measured in grams per watt of electrical power. Alterations to the motor, propeller, or even switching to a different ESC (Electronic Speed Controller) can all influence system efficiency.

Furthermore, it's essential to note that efficiency values are specific to particular command inputs and mechanical loads. Consequently, comprehensive efficiency characterization necessitates testing the motor across a spectrum of command inputs and with various propellers to vary the mechanical load.

How to Test Motors and Propellers with a Thrust Stand

Testing motors and propellers for efficiency requires simultaneous measurement of voltage, current, torque, thrust, and motor speed. These readings allow for the extraction of both electrical and mechanical power, essential for calculating efficiency values.

For streamlined and accurate testing, employing a thrust stand is highly recommended. This specialized equipment is purpose-built for characterizing motors and propellers, providing an efficient and effective means of conducting tests.

Test Procedure for Static Tests

This article focuses solely on static tests, omitting discussions on dynamic tests involving angular acceleration or estimating stall torque.

Before commencing your tests, it is advisable to:

1. Install your propeller in a pusher configuration to minimize ground effects with the motor mounting plate.

2. Ensure a reasonable distance between the propeller and surrounding objects to mitigate ground effects.

3. Implement all necessary safety measures to safeguard individuals in the testing area.

4. Configure your thrust stand to automatically cut off the system if any parameter exceeds its safe limit.

A straightforward yet reliable test method involves incrementally increasing the throttle in small steps and recording a sample after each step. Before recording each sample, allow the system to stabilize for a few seconds.