Confusing results from hacked-together VESC/motor dynamometer

General topics and discussions about the VESC and its development.
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sphereinabox
Posts: 2
Joined: 31 Dec 2015, 09:11
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Confusing results from hacked-together VESC/motor dynamometer

Postby sphereinabox » 04 Feb 2016, 03:44

So I had this great idea. Let's figure out how bad these cheap $19 motors are by using one to generate power, and one as a load, compare the wattage in and out to find efficiency, and be able to see what happens when motor stays under load unlike the inertial dynamometers that just spin up a big round thing.

Image

I've modified vldc-tool to have a "Benchmark" window that can connect to two VESCs, spin up one, then incrementally increase the brake current on the other motor, logging everything that would normally show on the "realtime data" tab.

But... The results I'm getting are strange. Here's an ODS file
https://drive.google.com/file/d/0B-Dzbm ... sp=sharing

The current_motor going into the drive motor initially starts higher than the brake current_motor, but at some point the brake current_motor exceeds the drive current_motor. Am I maybe assuming the brake motor is producing more brake power than the drive motor because I'm not seeing the brake voltage?

Do you know what else might explain what I'm seeing in these numbers?

How should I calculate the wattage in and out if not current_motor * v_in?

(At around 50A of braking the brake motor stalls and reports a much slower speed than the drive motor thinks it's going at. That's when I aborted the test)

Note: I only have it working well enough to start, abort, and save results (saves a CSV file). The settings area controls aren't used now, and closing the window won't disconnect because as I said, this is the first one working.

https://github.com/sphereinabox/bldc-to ... multi-vesc

benjamin
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Re: Confusing results from hacked-together VESC/motor dynamometer

Postby benjamin » 04 Feb 2016, 09:25

Attachments
log-50abrake-10hz-pub.ods.zip
(360.78 KiB) Downloaded 181 times
Motor_diagram.png
Motor_diagram.png (98.77 KiB) Viewed 6622 times

joe90
Posts: 49
Joined: 15 Dec 2015, 16:52
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Re: Confusing results from hacked-together VESC/motor dynamometer

Postby joe90 » 05 Feb 2016, 14:37

Wow,

I did a similar setup last winter but less automated and more basic installation.

It was just to see how much continious current the drives and motors could sustain.

I was not able to go over 50A phase current without problems.
My motor were SK3 6374. They should be big enough to go over 50A phase current.

would it be better to may the Y connection on the battery ?
(My wiring was like yours)
Michel Cote
Twin v4.7 + SK3 6374 149KV
Bicycle
2500W peak/unit on 12S

erwincoumans
Posts: 25
Joined: 16 Dec 2015, 02:16
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Re: Confusing results from hacked-together VESC/motor dynamometer

Postby erwincoumans » 11 Feb 2016, 17:00


Jacobbloy
Posts: 7
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Re: Confusing results from hacked-together VESC/motor dynamometer

Postby Jacobbloy » 12 Feb 2016, 08:28

Would love this feture in bldc tool

eXo
Posts: 4
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Location: Schweiz
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Re: Confusing results from hacked-together VESC/motor dynamometer

Postby eXo » 15 Feb 2016, 12:57


sphereinabox
Posts: 2
Joined: 31 Dec 2015, 09:11
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Re: Confusing results from hacked-together VESC/motor dynamometer

Postby sphereinabox » 09 Mar 2016, 16:57

Barely working code is in
https://github.com/sphereinabox/bldc-to ... multi-vesc

At the current time there's a "Benchmark" window available from the menu on the bldc-tool window.
Note that the settings for min/max current and step are not used (the values were hard coded to get a first run going), and if you close the window before disconnecting from the VESC, you won't be able to connect again.

The save results button saves a CSV file, with additional columns for time and whether the motor was a drive motor or brake motor. I associated rows happening at the same time in Excel.


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