X-File: Understanding VESC: the difference between "battery amps" and "motor amps"
So What determines the optimal PWM frequency (hz)?

This is where my understanding begins to feel a bit fuzzy...

I read on vedder's site that the VESC uses variable PWM frequency...

Theory ?: Inductance value of the motor somehow determines ideal PWM frequency?

Since inductivity seems to affect the time required to change current ( A ) in an inductive load, the amount of inductivity in the motor maybe somehow affects the ideal PWM frequency?


Unit of inductance = H (Henry)

The more i think about it the more it makes sense that motor inductivity ( H ) value determines PWM frequency (Hz), but I don't really know how to calculate exactly which inductance value gives exactly which optimal PWM frequency... Confusedob:

I would assume adding inductance to the motor adds time (takes longer) to change current resulting in a lower optimal PWM frequency.

Theory ?: raising motor inductance (H) value results in lower optimal PWM frequency (Hz) value.

Theory ?: lowering motor inductance (H) value results in higher optimal PWM frequency (Hz) value.

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/58,2017-01-16 14:22:57 UTC
Theory ?: In other words, adding inductivity ( H ) to the motor by using different magnets or steel core should make the diagonal "ramp" lines in the Pulsed DC "Saw Teeth" graph into "shallower angles" or "less steep." To compensate for this, a different PWM frequency ( lower Hz ) would necessarily be required.

the detected ( H ) value for the 0.0415ohm @Hummie motor was "27.13uH"

I assume some mathematical relationship between the "27.13uH" and "0.0415ohm" motor detection values in some way directly determines the optimal VESC PWM frequency in (Hz).

It should be somehow possible to determine precisely how the VESC would chop up one second with ON-OFF DC Pulses through any 2 motor phases wires with the hub motor locked in a clamp for 5000w electrical if you know:

Pack Voltage: 50V
Winding Resistance: 0.0415 ohm
Motor Inductivity: "27.13uH"
DC PWM Duty Cycle: %28.8
Wattage: 5000w
PWM Frequency: XXXX hz
AVG Pulse On length: 0.XXXXX seconds
AVG Pulse Off length: 0.XXXXX seconds
Battey Amps: 100 A
"Traditional" Amps: 100 A
Motor Amps: 348 A
Motor Amps % ON Time: 28.8%
AVG DC Amps During ON: 348 A
AVG DC Amps During OFF: 0 A

What will the PWM Frequency (Hz) , AVG Pulse On length (sec) & Avg Pulse Off length sec values be?


The only thing we know is the AVG amps = 348 during the 0.XXXXXX seconds On pulse length.


0.XXXXXX seconds On pulse = 348 amps avg

the min value of the avg amps during a single pulse is 0, maximum value during the pulse is XXX Amps....

maybe the maximum value is double the average?

that gives 2 x 348 amps = 696 pulse Peak amps (possibly)

so how long does a single pulse take to reach 696 Peak amps?

696 amps x 50v gives a momentary peak electrical value of 34,800 watts :astonished:

",0,1,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/59,2017-01-16 16:06:18 UTC
theory ?: "efficient" motor design maximizes induction ( H ) value & minimizes resistance ( Ohm ) value

higher ( H ) value = more "efficient" motor design

lower ( Ohm ) value = more "efficient" motor design

@Hummie = "27.13 uH" @ "0.0415 ohm" "longie"

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/61,2017-01-16 19:00:01 UTC
theory ?: higher ("C" continuous not max) amp limit x CELL voltage AVG during MAX CONTINUOUS discharge (including V-sag) = AVG WATTs electrical...

AVG watts electrical x total dischage time = WATT HOURs

higher "watt hours" and/or "avg watts" electrical per cell equals more "efficient" battery design.

higher "watt hours" means more efficient for distance.

higher "watts" means more efficient for acceleration.

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/63,2017-01-16 19:12:35 UTC
@raphaelchang any idea how taking out the magnets would change the inductance value of the motor windings vs replacing the stator core with glass?

in other words, which contributes more inductance (H) value to the motor windings-- the magnets or the stator core typically? or would they contribute about equally? taking which away makes the value decrease further?

should the stator core and magnets contribution to H ideally be equal?

Theory ?: increased inductance value measured in the motor windings implies a stronger "magnetic coupling" between the rotor and stator?

Theory ?: ideally equal %50 of H value should come from the stator core vs from the rotor magnets?

Theory ?: there is never any more heating in the motor than would be caused by the battery amps value and not "motor amp" value.

battery amp = motor amp x (% duty cycle / 100)

wattage into motor = motor amps x (%duty cycle / 100) x pack voltage

wattage into motor = battery amps x pack voltage

^these equations prove motor heating is always equivalent to the so-called "battery amps", not the so-called "motor amps"

at times when the motor amps value is much higher than the battery amp value (very low rpm)... it means that the higher "motor amp" value is actually only occuring for fractions of a second, not a whole second -- averaging the heat for the whole second down to the value of battery amps.

",1,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/66,2017-01-17 03:03:33 UTC
@raphaelchang let's imagine the motor locked in a clamp with pulsed dc going through 2 of the phase wires.... (using all the values in my last detailed example)... @5000w electrical

turns out, the "motor amp" value is far higher than the "battery amp" value... but heating can't be more than 5000w = 50V x 100 A DC -- 100 battery amps

keep in mind this is 348 "motor amps" according to the thought experiment VESC software in this configuration

in other words 348 software variable "motor amps" causing the equivalent heating of 100 battery amps.

100 battery amps = 348 "motor amps" @ 28.8% duty cycle

in other words 100 "traditional amps" = 348 "motor amps" %28.8 of one second

in other words the 348 software variable "motor amps" are occuring for fractions of a second whereas the 100 "battery amps" aka "traditional amps" occur for the entire second... both in the batteries and the motor. (Pulsed DC both sides - motor in a clamp with only 2 active motor leads)

",1,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/68,2017-01-17 03:58:28 UTC

imagine continuous 50V @ Pulsed DC 100  "battery amps" = 5000w of heating in a clamped motor with 2 active leads.

please explain how a supposedly continuous "348 motor amps" would cause more than 5000w of heating.

348 motor amps = 5000w heating

100 battery amps = 5000w heating

",1,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/70,2017-01-17 04:17:29 UTC
so if the magnets start spinning the effect is BEMF voltage opposing the battery... if duty cycle constantly increases with inceasing BEMF at higher rpms to maintain 5000w, still a constant 100 battery amps will flow. at faster and faster speeds software parameter "motor amps" decreases until they are equal at 100% duty cycle at mid range rpms. at the 1st moment of 100% duty cycle... still 100 "battery amps" but also 100 motor amps. this results in less "motor amps" as the motor spins faster... not more.

at what point did the higher motor amp value (at low rpms) translate to more than 5000 w of heat... always equivalent to the constant 100 "battery amp" value.

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/72,2017-01-17 04:32:05 UTC
to me it seems simple logic that if in the first moment of acceleration 100 battery amps are flowing, and that is 5000w, and also 348 motor amps are flowing, that is not more than 5000w worth of heating.

in other words:

100 battery amps of heating = 348 motor amps of heating

in other words

motor heating is equivalent to battery amps

motor heating is not equivalent to software parameter "motor amps"

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/74,2017-01-17 04:44:09 UTC
one way to say it is motor heating is linear to

- battery amps

motor heating is not linear to:

- software parameter motor amps

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/76,2017-01-17 04:51:27 UTC

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