Power Limiting instead of Current Limiting

General topics and discussions about the VESC and its development.
ThierryGTLTS
Posts: 86
Joined: 09 Aug 2017, 11:10

Re: Power Limiting instead of Current Limiting

Postby ThierryGTLTS » 06 Sep 2017, 17:29

Ackmaniac has developped a special BLDC Tool for Watt limitation.

But you need to use the PPM input.

Thierry

devin
Posts: 253
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: Power Limiting instead of Current Limiting

Postby devin » 06 Sep 2017, 18:54

Ackmaniac has developped a special BLDC Tool for Watt limitation.

But you need to use the PPM input.

Thierry


..this raises an interesting question re: Ackmaniac's "special BLDC Tool"...

in the following thread on Endless-Sphere.com...

thread: "Anomalies Present in Enertion Video / Data Logging Software?"
https://endless-sphere.com/forums/viewtopic.php?f=35&t=88073&p=1289443&hilit=enertion#p1289443

Ackmaniac comments:

"The Raptor 2 video runs in FOC. And in FOC the duty cycle which is shown by the VESC firmware is not correct. In the firmware the duty cycle get's divided by 2/3 * ∛2 which is (0.86602540378).
In reality the duty cycle is then when you see 95% at 82.27% because you apply current to all 3 phases.
When you take that into account then you get much closer to the real value. And be ware of the float int issue.

BTW i am the developer of this app that is shown in the video.
@ Flat tire: sadly there is never a legit answer to any of devins questions. At least not for him.
"

any comments on this explanation from the experts? T/F?

BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Re: Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 20:25

Addy wrote:Why not limit the motor current instead of battery current? Then it doesn't really matter what the battery voltage is.


When I lower the motor current to about 30 A, it really disrupts the motor for some reason. I figured it might be due to current spikes that it is detecting, and the VESC is shutting it off for each spike over 30 A. But I only want to limit the continuous current.

I've attached a snip of the realtime data plot with a 30 A motor current limit and a 50 A motor current limit. You can see that in the 50 A snip (bottom) the motor stops much smoother.

For more info on my setup: I have the motor stopping once the pressure in the line reaches above a certain pressure limit. This is controlled using Arduino to read a pressure sensor and transmit a duty cycle via UART to the VESC.

I wonder how I can limit the power using the motor current, without disrupting the motor like this?
Attachments
motor_current_30A.PNG
motor_current_30A.PNG (43.64 KiB) Viewed 947 times
motor_current_50A.PNG
motor_current_50A.PNG (37.41 KiB) Viewed 947 times

BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Re: Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 20:27

ThierryGTLTS wrote:Ackmaniac has developped a special BLDC Tool for Watt limitation.

But you need to use the PPM input.

Thierry


Dang. I am using Arduino via UART.

devin
Posts: 253
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: Power Limiting instead of Current Limiting

Postby devin » 06 Sep 2017, 20:28

BlakeCarmichael wrote:and the VESC is shutting it off for each spike over 30 A


...that's why i mentioned setting the absolute max to 35A...

devin wrote:do an foc winding resistance detection... the true value will be double since it calculates to virtual ground point.

then (assuming, for example 0.1ohm detection, 24v pack)

0.1ohm*2=0.2 ohm lead to lead

177w=29.74^2 motor amps * 0.2ohm=7.37battery amps * 24V pack v

^simply assuming 177w max, 24v pack, and 0.2ohm lead to lead, the battery amp limit should be 7.37amps, the motor amp limit should be 29.74 amps, and the absolute max amp setting should be a hair higher... say 35a

BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Re: Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 20:32

devin wrote:...that's why i mentioned setting the absolute max to 35A...


Yes, I tried that. It completely brings the motor to a stop after a very short period of time, giving the fault code ABS_OVER_CURRENT, and repeats that.

I have to raise the absolute max to about 70-80 A before i get around the fault code.

BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Re: Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 20:41

Also, when I ran the FOC detection, I got R = 0.03448ohm, which can't be right because that would give me a max motor current of (edited) I = sqrt(177/(.03448*2)) = 51 A...
Last edited by BlakeCarmichael on 06 Sep 2017, 22:05, edited 1 time in total.

devin
Posts: 253
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: Power Limiting instead of Current Limiting

Postby devin » 06 Sep 2017, 20:44

i get (50.66A^2)(0.03448ohm*2)=177w

Addy
Posts: 7
Joined: 10 Aug 2017, 17:56
Location: BC, Canada

Re: Power Limiting instead of Current Limiting

Postby Addy » 06 Sep 2017, 21:31

Not sure why the motor current limiting is giving you trouble. It works well in my e-bike, during testing I found that motor currents over 75A caused my motor mount to flex too much, but by limiting the motor current to 75A everything is fine with full throttle applied.

How is your VESC set up? How long are the battery wires? What kind of capacitors are you using?

BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Re: Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 22:09

Addy wrote:Not sure why the motor current limiting is giving you trouble. It works well in my e-bike, during testing I found that motor currents over 75A caused my motor mount to flex too much, but by limiting the motor current to 75A everything is fine with full throttle applied.

How is your VESC set up? How long are the battery wires? What kind of capacitors are you using?


I bought the board from diyelectricskateboard.com (http://diyelectricskateboard.com/diy-el ... board-esc/)
I am running unsensored to the VESC, using only the 3 phase wires, and they are all about 14 inches long.
I have one of the hall sensors from the motor going to my Arduino Nano which I use to measure the RPMs, since I couldn't get the hall sensors working with the VESC when I tried initially.


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