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Old Dec 20, 2011, 07:53 PM
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modisc's Avatar
United States, MI, Ann Arbor
Joined Aug 2011
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A simple way of measuring Kv of unloaded motor with acceptable accuracy

To note that this is an original finding, I learnt this from a Chinese forum.

I have been using this method for measuring Kv of all of my motors in hand, with good precision. All you need is a laptop (with microphone) or any computer running windows system, with a microphone. Of course, the software is need: Acoustica (find it in attachment)

I have not seen any posting this in this forum, probably because most of the guys here may have specially designed tools or modules for measuring Kv as well as other data of the running motor. For people like me, who does not own these profession tools, this simple way of measuring Kv, or the actual rotating speed of a motor, both inrunners or outrunner, even brushed motors, is interesting with acceptable accuracy.


If you are using a laptop, the original microphone on the laptop is good enough. You do not need good microphones to do the test.

Unzip it and see if you can run the "acoustica.exe" execution file.

I assume this software should not need any registration file to run, but if so, you need to to try download a free copy of installation file of the software.

After opening the software, click "File" -> "New Recording", and set the "Sampling Rate" to "44100 Hz (CD Quality", "Resolution" to "16 bits per sample", and do not need to click "Stereo recording" under "Channels", as shown in Picture_1.

Click OK. Then Click the small red button located on the top left corner of the software window, under ‘Edit" and "View", for record. as shown in Picture_2.

Now it is not recording yet until you click the Record button in the popped up small window of "Record". You can get your motor ready to run now: connect motor to esc which is connected with a test servo, or your radio system, and battery. Just make sure you can run your motor at full throttle. It is good if you can measure the exact voltage during the motor running, but you can also use a lipo pack which is already in the platform voltage, such as 11.1V for 3S, then you will know the voltage would be very close to 11.1V when the motor is runner at no-load state.

Now click record button. And it starts recording. Keep the motor close to the microphone, and the room where you do the test should be as quite as possible, actually, just make sure you are not playing any music out loud while doing this test. Start the motor, turn it to max speed. hold it for a few seconds. then shut the motor down.

Then you can click the "Done" botton as shown in Picture_2. You should now see something like this as shown in Picture_3.

Then Click "Analysis" -> "Wavelet Transform ...", and make the settings match the numbers as shown in Picture_4. Click OK.

Allow a few seconds for the transform process. After done, you should see a popped up window with the transformed wave frequency/intensity pattern chart. as shown in Picture_5.

Now, if you did exactly as the previous steps, and recording is good, environment is quite enough, then you should have the image as shown in Picture_5, with a clear lower horizontal brown line, as shown in the black box in Picture_5. Move your mouse pointer to the line, it does not matter where you place your pointer horizontally, the reading of frequency, located at the bottom right corner of the window, will be same. But you need to place the pointer at the center of the brown line, in the vertical direction, as this will affect the number of frequency reading. As shown in Picture_5, the frequency reading is 174, when i place my mouse pointer which is not shown, on the brown line.

Formula: frequency reading X 60 / voltage = kv value.


As in the picture 5, the Kv will be 174 X 60 / 11.1 = 940.5



If you know something about sound physics, you will see how this works. But whatever, you will be amazed to find how close it will be to the number labelled on your scorpion, if the number is correct. I need to take a test now, post pics later. Hope you can get this before I post the pics.

Please note that picture 3 and picture 4 is just recording my cough!!! But picture 5 is the test result of my rewinded motor. So what you should get is something similar to picture 5, but not 3 or 4.
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Last edited by modisc; Dec 21, 2011 at 02:47 AM.
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Old Dec 20, 2011, 10:16 PM
Jack
USA, ME, Ellsworth
Joined May 2008
16,956 Posts
The acousitc Kv measuring is interesting.

The only thing I see that might be an issue is that you would ideally want to take the under load voltage reading at the same time that you are recording the acoustic data.

I have an eLogger with a brushless RPM sensor now so I can look at the simultaneous readings for the RPM and volts and I just divide them to get the raw Kv.

Not arguing that your method is not good, just that not getting and voltage measure more accurately and at the right time seems like it would maked the reading less accurate.

Jack
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Old Dec 21, 2011, 01:27 AM
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manuel v's Avatar
Mexico, BC, Mexicali
Joined Aug 2004
5,217 Posts
Precisely, that I wanted to ask another for.
As you get the 11.1 volts?
If lipo battery, says Jack, you have to measure it.
The voltage of a lipo battery is continuously declining, although the burden is minimal.
rarely see 11.1 volts can be measured.

Check out these measurements I made a few days. the voltage is quite different in each measurement.

Post no. 1225.
http://www.rcgroups.com/forums/showt...777998&page=82

Unfortunately the program is not working.
Mark that the trial period has expired .
You could put the original link to download?
Thank you.

Manuel V.
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Old Dec 21, 2011, 02:38 AM
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modisc's Avatar
United States, MI, Ann Arbor
Joined Aug 2011
887 Posts
Of course measuring the voltage while motor running is good. I am just trying to keep things simple. The platform voltage of single Lipo is 3.7 or 3.8v, I am suggesting to discharge the lipo a bit, till it will output platform voltage even under small load, such as use a lipo with 11.5V voltage.
This is just a relatively precise way of measuring Kv. The method itself, is flawless theoretically. the biggest error of this method, does not only come from inaccurate measurement of the battery, but also from the fact that the "brown base line" has some thickness...and you will need to point your mouse to the center in vertical direction.

Solder 2 wires from the battery out, and you can take readings via a multimeter/volt meter while motor running. I have no doubt that this will make the results more accurate. But my point is, keep it simple ^^
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Old Dec 21, 2011, 02:44 AM
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modisc's Avatar
United States, MI, Ann Arbor
Joined Aug 2011
887 Posts
Quote:
Originally Posted by manuel v View Post
Precisely, that I wanted to ask another for.
As you get the 11.1 volts?
If lipo battery, says Jack, you have to measure it.
The voltage of a lipo battery is continuously declining, although the burden is minimal.
rarely see 11.1 volts can be measured.

Check out these measurements I made a few days. the voltage is quite different in each measurement.

Post no. 1225.
http://www.rcgroups.com/forums/showt...777998&page=82

Unfortunately the program is not working.
Mark that the trial period has expired .
You could put the original link to download?
Thank you.

Manuel V.
I directly zipped this file from my "program files", so i guess you need the registration file, since you encountered "trail period expiration".

The new version of this software is a paid software, not a free one, just check the original website. I have the old version here, which is free, I hope sharing this will not violet anyone's right.

The original link of this very old software is in Chinese...you will not be able to read the page. But you can check the acoustica official website.
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Last edited by modisc; Dec 21, 2011 at 02:49 AM.
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Old Dec 21, 2011, 05:29 AM
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Brazil
Joined Feb 2006
34 Posts
Many years ago, Sergio Massara (Brazil), based on data published in http://www.badcock.net/ThrustXL/ ,a software created in C# that runs on a Microsoft Framework platform called "PCTaco".

A microphone should be used, positioning it a 1.1/4" away from the front and towards the end of the propeller (suction).

Still available for download in http://www.e-voo.com/forum/viewtopic...er=asc&start=0 (Portuguese).

You will get rpm, thrust and pitchspeed, measured every 1 sec.

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Old Dec 21, 2011, 05:47 PM
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Fourdan's Avatar
Antony (France)
Joined Sep 2003
2,965 Posts
Hi
I prefer to use a SAMPO tachometer DT2234 for example (25$)
+ patches on the motor's rotor, to get precise rpm.
For many tests (with PC recordings) my PowerAnalyzerPro + rpm phase sensor is also correct (but not so accurate as the DT2234)
If I declare 2 poles (instead of real 8P or 10P or 14P) + a division .. accuracy is better.
Louis
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Old Jan 19, 2012, 02:20 PM
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United States, NY, New York
Joined Dec 2009
487 Posts
very interesting KV measuring method. thanks
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