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vedder.se forums • How should the VESC "feel" on a board? - Page 3
Page 3 of 8

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 09:12
by devin
@duggyboard

after pulling my hair out for months to attempt to fully understand the motor amps / battery amps / startup torque acceleration issue, i've finally gotten a really good understanding of the issue.

most of the surprising stuff I've learned i've summed up at

Here are some key tidbits:

The calculation for electrical wattage at full throttle standstill differs for battery amps and motor amps.

For example imagine you want 1000w electrical at full throttle standstill as well as all physically possible rpms from a single motor, and your typical battery voltage with sag is 46 volts, and your motor's winding resistance is detected by the vesc as 0.1ohm.

To achieve the 1000w electrical at full throttle standstill it is necessary to set the appropriate battery amp max / motor amp max / absolute amp max limits.

The battery amp limit is straightforward to calculate:

battery amp limit = desired wattage / battery voltage with sag

so

21.7a (22a) battery amp limit = 1000w desired / 46v pack volt with sag

The motor amp limit is less straightforward, but still fairly easy to calculate with some algebra.

motor amp limit x winding resistance = effective pwm voltage @ full throttle standstill

effective pwm voltage @ full throttle standstill x motor amp limit = desired wattage @ full throttle standstill


so

100a motor amp limit x 0.1ohm winding resistance = 10v effective pwm voltage @ full throttle standstill

10v effective pwm voltage @ full throttle standstill x 100a motor amp limit = 1000w desired wattage at full throttle standstill


I am told to avoid cutouts, the absolute max should be higher than both battery max and motor max, so to be safe we will say 120a for absolute max.

In simplest terms, from the above equations we see that for 1000w electrical desired at full throttle standstill with a 46v pack with sag, and a 0.1 ohm detected winding, the appropriate settings to use would be 22/100/120 batt amp limit / motor amp limit / absolute amp limit

These settings should achieve:

Full Throttle = 1000w electrical @ all physically possible RPMs

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 10:43
by benjamin

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 11:12
by benjamin

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 11:35
by devin

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 12:06
by benjamin

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 12:36
by devin

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 13:06
by benjamin
It seems like you are completely ignoring what I'm writing...

anyway, this is related to common misunderstanding that people have about motor control, so hopefully what I wrote will be useful for other people.

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 13:08
by devin
My point is I respectfully dont agree with you that motor heating would be equivalent to 71 amps in the example. I believe it would be equivalent to 22 amps.

71 amps for 30.73% of the time produces as much heat as 22 amps for 100% of the time.

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 13:10
by benjamin

Re: How should the VESC "feel" on a board?

Posted: 09 May 2017, 13:11
by devin
But I think 70.71 amps for 30.73% of the time produces as much heat as 21.72 amps for 100% of the time.

I dont think 70.71 amps for 30.73% of the time produces as much heat as 70.71 amps for 100% of the time.