X-File: Understanding VESC: the difference between "battery amps" and "motor amps"
#11
Now let's look even closer at this 1/800th second "on time" during which 20 A "Motor Amps", on average, is flowing.

When the switch turns on at the beginning of 1/800th second, does current jump to 20 A instantaneously?

actually no. since "the electrical load" (ie motor) features an iron core (stator) and magnets, it is considered a "inductive load".

 if the winding wire were straight, and there were no magnets or steel stator core, the "winding" would be considered a purely "resistive" load.

however, with the presence of magnets and a steel core, we must treat this as an inductive load.

an important aspect of an inductive load is current requires much more time to change. typically, (and in yet another dramatic oversimplification) the mere presence of this inductivity matters much more than the typical actual numerical magnitude of this inductivity.

since current (amps) requires much more time to change with an  inductive load, and we are using inductive load for 1/800 of a second, the fact is when the circuit turns on at the start of the 1/800 of a second,  The current does not reach 20 A immediately.  The 20 amp "motor amp" value is an average over the entire 1/800 of a second. As an average it has minimum, and maximum values, and total time (1/800 of a second). The minimum value is zero at the start of the 1/800 of a second. The maximum value is such that the average of minimum and maximum during the course of the 1/800 of a second equals the averaged 20 "motor amps" value for 1/800 second.

it turns out that when pulsed dc amps is graphed  on a chart, the minimum and maximum values of the "on time" combined with on-off duty cycle and fact of inductive load makes the graph resemble the appearance of a Saw-blade.

The technical term for this "waveform" is "a sawtooth wave".

here is a Wikipedia article detailing the characteristics of sawtooth waves:

https://en.m.wikipedia.org/wiki/Sawtooth_wave

Long story short, the DC amps & watts waveform coming from your battery is in fact nearly always a Sawtooth Wave.

The average amps during one "tooth of the saw" equals "MOTOR amps." (in this case with %50 off-time space in between each "tooth" averaging the 20 "Motor amps" value to 10 "Battery Amps")

",0,2,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/17,2017-01-14 03:45:07 UTC
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#12
Real World Test: 48/200/200 batt/motor/absolute

0.0415 ohm winding
35.2v pack voltage
1 hub motor
1660 watts
even acceleration
no hardware failure
little to no "cogging"

SUCCESS! :joy:

Implications: even-acceleration performance is only limited by the PC-Side VESC software arbitrary 200 "motor amp" max value = 1660w

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/19,2017-01-14 07:12:57 UTC

[Image: 48-200-200-1.png]

[Image: 48-200-200-2.png]



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#13
I consider the 48/200/200 test a success because the VESC didn't snap, @Hummie rode around the block, and the $100 I put down on the VESC is safely back in my pocket. (and low speed "cogging" is basically eliminated, acceleration is much quicker and steadier) Confusedweat_smile:

i asked @Hummie if he plans to go back to his old settings and his answer was "no."

",0,3,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/24,2017-01-14 11:50:50 UTC
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#14
Hummie, post:21, topic:15995 Wrote:I got Devin to ride my board once on the steep hill in front of my house and the cogging instantly bothered him and he hasnt been interested in riding since.

yes this happened many months ago. "cogging" is now solved with 48/200/200

no sensors involved.

the issue before wasn't the VESC having trouble identifying rotor position at very low rpms, the issue before was lack of sufficient electrical watts into motor at low speeds to initiate proper acceleration from standstill with the weight of a rider standing on it pointed uphill.

",0,1,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/25,2017-01-14 11:57:56 UTC
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#15
chuttney1, post:18, topic:15995 Wrote:I believe the waveform for DC from the battery to VESC is flatline and what you really meant is the output waveform from VESC to a brushless motor.

@chuttney1   %50 duty cycle literally means POWER ON for %50 of one second. (and by extension POWER OFF for %50 of one second) this applies to the wires between the battery and vesc, but the issue is even more complicated for the wires between VESC and motor (Pulsed AC).

in a nutshell, the DC current through battery wires is in fact pulsed dc with a sawtooth waveform and gaps between each tooth (the size of these gaps depends on the duty cycle which is not shown in the wikipedia image)

the AC current through the motor leads is in fact PULSED AC, also with a sawtooth waveform.

as a result of the nature of the pulsed AC waveforms, the size of each "tooth" on the pulsed DC sawtooth waveform would in fact be different (not evenly sized teeth with no gaps in between like in the wikipedia image, in other words some Pulsed DC "Saw teeth" would be "taller" than others, and the "gaps" would vary with "duty cycle %" and the frequency (hz) of the pulse width modulation)

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/27,2017-01-14 12:15:43 UTC
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#16
Ackmaniac, post:26, topic:15995 Wrote:@Devin And as a person who doesn't even own a VESC and rode a board only once you should not give such advice's

I owned the VESC during the test. @Hummie sold it to me for $100. Once the test was complete at 48/200/200 @ 35.2V @ 0.0415ohm @ 1660w max,(he rode uphill and around the block & no hardware failure) I sold it back to him for $100

",0,1,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/28,2017-01-14 12:18:25 UTC
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#17
Ackmaniac, post:29, topic:15995 Wrote:Problem i see is that you will spread this information in every thread now as you did already in the past. And i think it is very dangerous and might blow VESCs and hurt people.

uneven acceleration is very dangerous, people have been hurt, this will occur if the batt max / motor max are not tuned to the proper values in relation to desired max watts.

now i would like to return to the SOFTWARE THOUGHT EXPERIMENT (for thought experiment purposes, assuming all hardware is immune to damage.)

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/30,2017-01-14 12:38:07 UTC
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#18
fun software fact:

in the successful 48/200/200 batt/motor/absolute 0.0415ohm @ 35.2V @ 1 motor @ 1660w max ride-around-the-block test....

48
200<-- only thing limiting this number is probably just a couple lines of code in the PC-Side Software

giving:

- 1660 watt max limit for even acceleration
- 48 "traditional" amp limit @ 35.2

and by extension to achieve:

- 2000 watt max limit for even acceleration
- 56 "traditional" amp limit @ 35.2V

56 / 220 / 220 batt/motor/absolute amp limit settings are necessary

",0,1,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/31,2017-01-14 13:51:11 UTC
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#19
hexakopter, post:33, topic:15995 Wrote:You know that I already showed you how to solve it?

oh thank you! unfortunately I dont know how to make changes to the PC side programming language :disappointed:

hexakopter, post:33, topic:15995 Wrote:The only proof that it really works (and they hit the 200A) would be a recording of the VESC data.

the proof of the equations is a few posts back in the comparison of the formulas with the benjamin vedder data logging frame.

",0,1,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/34,2017-01-14 14:25:52 UTC
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#20
hexakopter, post:35, topic:15995 Wrote:I don't see 200A in this picture.

A few posts further back I described how to calculate the batt/motor/absolute settings from pack volt, winding ohm and desired battery draw wattage, with even acceleration requirement.

",0,1,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/36,2017-01-14 14:31:52 UTC
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