[Tutorial] Motor and propeller efficiency testing with a dynamometer

[Jebarus]

Hi guys,

I am a recent graduate of M.Sc. Mechanical Engineering from Ottawa, Canada. My colleague and I designed a dynamometer and motor tester specifically from UAVs and quad designers. We actually started this project after spending weeks to build our custom solution.

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For the release official product release, here is a quick introduction to motor and propeller testing, as summarized in the video above. The tutorial is done with our RCbenchmark dynamometer, but is should be applicable to DIY solutions too.

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

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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.


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. Here 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.

What next?

For the launch week, we are offering 15% off to multirotorforums readers. Use the code "MRF15" until November 15th.

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?

 

[Old Man]

So essentially a prop/ motor combination mounted on a lever arm that incorporates a transducer, right? The most important factor is always missing on a reaction dyno when measuring propeller efficiency. Blade stall that always occurs when propeller is not moving forward through the air mass.

 

[Jebarus]

Hi,

You are right, the propeller test is a static test, similar to hovering for a quad, unless you are using a wind tunnel. In practice though, prop data are usually well known in moving air for standard profile. The biggest unknown is often the motor. In that case, the propeller is only used as a load on the motor.

 

[fengshuidrone]

What Old Man says being true pretty much makes the results of the tests as far as prop efficiency go are maybe not necessarily accurate? Or was that blade stall taken into account during testing?
Have you done any marketing research? The segment seems extremely limited at those prices (as I mentioned in your new member intro/sales pitch.) What do others here think? Can they sell enough of those to make it profitable? What you have is easy to make at less cost by those with a little arduino skills, fabrication skills, and hardware. I can't help but wonder. Any others have any feedback?

 

[Old Man]

The set up these guys have is sound as far as it goes. However, measuring propeller efficiency only goes so far when dealing with static testing and theoretical calculations obtained from general reference material. We do prop designs all the time, and put them into use, and have in every instance found that when the moving air mass at varied velocities are not taken into consideration we end up designing a propeller two or three times more. Without the moving air mass factor the test results can be off as much as 35%. Something to consider, even in a stationary hover a Multirotor or helicopter has an uninstructed air mass above it and unless maintained in ground effect has the air mass below it generally unimpeded. However. Relatively accurate test results can be obtained if the propeller is far enough above the ground to negate ground effect in hover type testing. Controlled velocity air flow can also be introduced under those conditions to establish propeller performance at relative airspeeds that better simulate loading.

I like their test concepts in the general sense but the practical needs just a little more work.

 

[Jebarus]

Prop efficiency will not be accurate for airplane propellers, as they are designed for an incoming airflow. If you need that information, it is possible to use reference tables, or wind tunnel tests (from which the reference tables were generated). For example, this paper has experimental and theoretical models. For static tests however, the results will be quite similar to what you would obtain with a quad hovering.

Once you choose a propeller, you know the torque and speed required. That is where the dynamometer is useful for motor selection, as it is possible to completely decouple the propeller test and motor test. If you know that, for an airplane, at cruise speed, you need 5000RPM and 0.4Nm of torque, you can to a static test on the ground that will load the motor with 5000RPM and 0.4Nm, and obtain your efficiency.

I understand your concerns, it is something we though a lot about, and I'd like to have other people's feedback. Most of our beta clients are small businesses, manufacturers and universities. With all those extra features, high precision, and low volume, we do have a price point more oriented towards this audience. It is a tool that saves a lot of time and money, as the other solutions are basically all custom hardware. Our first prototype was a bunch or arduinos, hundreds of lines of MATLAB code, and sensors from many, many suppliers.

That being said, we understand how difficult it is right now for DIY people (we have been there too) to test motors, and part of our roadmap includes solving that problem. Right now, the only tools available are basically glorified scales. So if you can't afford our tool right now, stay tuned, subscribe to our Youtube channel, and sooner than later there will be a version better suited for the hobby market.

 

[Old Man]

I wasn't saying what you had was bad. Far from it because very few take the time to build a test stand at even basic levels. For general testing and making selections for general purposes your set up and tutorial is head and shoulders above anything else widely available. We test out VTOL use propellers too in what could be viewed as a high volume environment. Not publicly commercial but we move a lot of them on different small aircraft and engine configurations

 

[fengshuidrone]

That being said, we understand how difficult it is right now for DIY people (we have been there too) to test motors, and part of our roadmap includes solving that problem. Right now, the only tools available are basically glorified scales. So if you can't afford our tool right now, stay tuned, subscribe to our Youtube channel, and sooner than later there will be a version better suited for the hobby market.

I'm not so sure there will be a hobby market. Most of us little hobbyists get by using an ecalc. It works for my builds.
I applaud your efforts to accommodate the high end users who need exacting specs though. Most of us little hobbyists just pretty much know what motor and prop combos work together. I always overbuild thus ensuring success.

 

[Jebarus]

We test out VTOL use propellers too in what could be viewed as a high volume environment. Not publicly commercial but we move a lot of them on different small aircraft and engine configurations

That is pretty interesting!

 

[Jebarus]

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 coupon MRF15 coupon until November 15. Customers who ordered in the last 48h will be refunded the difference.

 

[Old Man]

Looking at your site I found the test bench to actually be pretty reasonable in price since it incorporates a load cell. Eliminating the need for a 90* arm or a straight bar on a gimbaled fulcrum and a gram scale makes for a cleaner and much reduced bench area test fixture. i might try talking some of the management into obtaining one to see how it stacks up and handles modifications.

 

[maxwelltub]

whats the max thrust this thing can handle?

 

[Old Man]

add to that largest prop diameter and maximum RPM.

 

[Jebarus]

Sorry, I really though I was watching this thread... The max thrust is 3kGf. The max RPM is 200 000 rpm on 2 poles, 33 000 on 12 poles. The largest prop we tested is 16 in, but again, the limit is the thrust.

PS: The max torque is 2Nm. We never reached that with props.

 

[Jebarus]

I have been pretty busy working on a motor model last week. Here are some examples. The powers are in watts.

The motor is an NTM 28-30 800KV.

Also, here are the constants I obtained:

KV: 800 (experimental) The most known motor constant. It came out to exactly 800 surprisingly. I tested a lot of motors, and the manufacturers are usually not that close, it is probably chance. There is often more than 10% of variation. It is simply the max RPM divided my the voltage.

Kt: 0.0119 N.m/Amp This is a ratio of torque to current. In theory, the current is proportional to the torque. In SI units, KV and kt are equal. Some researchers are recommending to calculate Kt with a fit on the current vs torque plot. It is suppose to give a more accurate model. I tried it. The result is a bit different, but the problem is then that the KV you obtain is different from the manufacturer KV, and it will confuse everybody on a database.

b: 7.3795e-008 N.m.s Friction constant on the motor at no load (from bearings, etc...). It has a very small impact on the model, only at very low torque. It is important for modeling efficiency though.

R 0.3116Ohm. Our dynamometer measures the motor resistance as 0.1648 ohm per phase. The resistance constant here includes the ESC. Was calculated with a fit on the data.

I0: 0.33A Current at no load, or the current required for the motor to spin at no load.

Imax 20A Max current due to the motor wires. Could be higher for short periods of time.


I used matlab to analyse all that data. Wouldn't it be great if there was a database with all this info? We want to work on that, but allowing people to generate that data is still a complicated problem. First, interpretation of the data is not simple. We have to develop tutorials.