VESC vs "Native" controller

General topics and discussions about the VESC and its development.
rew
Posts: 904
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: VESC vs "Native" controller

Postby rew » 28 Aug 2017, 18:38

devin wrote:at 50% of no load you have half of max torque and half of max rpm giving max power output.
You are showing the curve of an oldfashioned motor. Those would tolerate being stalled and having the full U/R current running.

A modern motor has a "max current" rating that is MUCH lower than that.

Take for instance this motor: https://hobbyking.com/en_us/turnigy-aer ... tore=en_us

It has a winding resistance of 21 mOhm. They allow "12S" operation, so about 50V. Do the math and you get a current-at-stall of 2.4kA. or 120kW of power... All converted to losses inside the motor. This situation will not last for more than a few microseconds.

This motor has a KV of 150 RPM/V. Convert that to SI units, 15 rad/sec/V. Invert that and you get the torque constant 0.66Nm/A: So this motor generates 0.067 Nm per A of current. At 2.4kA that would mean 160Nm. That is ridiculous. In practice, the maximum is 70*.066 = 4.66 Nm.

The winding losses are 0.021Ohm * the current. Or 1.5V at the maximum current of 70A.

So when you configure your motor controller to allow the maximum current of 70A, the motor will be able to provide 4.66 Nm of torque up to 48.5V and only drop in torque the last 1.5V of the way up to 50V powersupply. If you want to plot against RPM, that is max torque of 4.6NM up to 7230 RPM, and then dropping of linearly to zero at 7450 RPM.

If you draw this graph, it is totally different from the one you showed.

It seems that most of the images you find on the internet are describing "oldfashioned" motors. One of the few showing the "correct" graph is figure 3.20 in: http://what-when-how.com/motors-and-dri ... nd-drives/

Ignore the "permissible 150%" torque. For the HK motor that I linked, the X-coordinate of the intersection between the dotted line and the torque line is at 97% of "percent rated speed".

Everybody draws the power curve as a parabola that has a maximum at 50% rated speed. For modern motors like the HK one you get a straight line through the origin, reaching a maximum at 97% rated speed and then dropping almost linearly (but in reality part of a big parabola) towards zero at 100% rated speed.

devin
Posts: 225
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: VESC vs "Native" controller

Postby devin » 28 Aug 2017, 18:50

rew wrote:You are showing the curve of an oldfashioned motor. Those would tolerate being stalled and having the full U/R current running.

A modern motor has a "max current" rating that is MUCH lower than that.

Take for instance this motor: https://hobbyking.com/en_us/turnigy-aer ... tore=en_us

It has a winding resistance of 21 mOhm. They allow "12S" operation, so about 50V. Do the math and you get a current-at-stall of 2.4kA. or 120kW of power... All converted to losses inside the motor. This situation will not last for more than a few microseconds.

This motor has a KV of 150 RPM/V. Convert that to SI units, 15 rad/sec/V. Invert that and you get the torque constant 0.66Nm/A: So this motor generates 0.067 Nm per A of current. At 2.4kA that would mean 160Nm. That is ridiculous. In practice, the maximum is 70*.066 = 4.66 Nm.

The winding losses are 0.021Ohm * the current. Or 1.5V at the maximum current of 70A.

So when you configure your motor controller to allow the maximum current of 70A, the motor will be able to provide 4.66 Nm of torque up to 48.5V and only drop in torque the last 1.5V of the way up to 50V powersupply. If you want to plot against RPM, that is max torque of 4.6NM up to 7230 RPM, and then dropping of linearly to zero at 7450 RPM.

If you draw this graph, it is totally different from the one you showed.

It seems that most of the images you find on the internet are describing "oldfashioned" motors. One of the few showing the "correct" graph is figure 3.20 in: http://what-when-how.com/motors-and-dri ... nd-drives/

Ignore the "permissible 150%" torque. For the HK motor that I linked, the X-coordinate of the intersection between the dotted line and the torque line is at 97% of "percent rated speed".

Everybody draws the power curve as a parabola that has a maximum at 50% rated speed. For modern motors like the HK one you get a straight line through the origin, reaching a maximum at 97% rated speed and then dropping almost linearly (but in reality part of a big parabola) towards zero at 100% rated speed.


rew wrote:This situation will not last for more than a few microseconds.


...an eternity, as they say, at the planck scale...

...however short a duration the situation would last, for calculation purposes, the mathematical relationships are still present...

Image Source: https://elementsunearthed.com/tag/f-orbital/
Caption: "Fractal image similar to p and s-orbitals"
Image

rew
Posts: 904
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: VESC vs "Native" controller

Postby rew » 29 Aug 2017, 03:42

I value a good discussion. But this is not it. To prevent others from thinking I might agree with what you are saying, I will from now on respond with a singele word: 'bullshit".

buran
Posts: 12
Joined: 28 Jan 2017, 18:02
Location: Russia

Re: VESC vs "Native" controller

Postby buran » 10 Sep 2017, 09:22

So, guys, I don't understand what are your hypothesis concerning the problem :)))

Rew (hello!) how can you explain the situation when observable ratio between max-speed and max-torque somehow depends on controller but not on motor constants?

thank you

rew
Posts: 904
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: VESC vs "Native" controller

Postby rew » 13 Sep 2017, 09:47

I dont understand your question.

The VESC has internal limitations that cause the maximum dutycycle to be 95%. This results in say 47.5V max motor voltage at 50V battery voltage. Another ESC may be able to deliver 100% dutycycle and achieve a higher unloaded RPM.

Also SUPPOSE that the VESC has a control problem that makes it commutate too slowly at high RPM. Then the motor would run slower than it theoretically could. And this depends on the controller.


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