X-File: Understanding VESC: the difference between "battery amps" and "motor amps"
#21
hexakopter, post:38, topic:15995 Wrote:Yes, but with the same P=U*I and U=R*I formula you can see that it should not be possible to measure 200A over the shunts. So @Hummie wasn't reaching the 200A in my opinion. Just a data log can show this.

using the formula

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

and the ohm's law solve for 0.0415 ohm & 1660 watts calculations i mentioned,

i predict full throttle accelerating under 200rpms with the 48/200/200 settings:

in the motor he would be reaching:

48 "battery amps" aka 200 "motor amps" aka 48 "traditional" amps aka 200 "amps" for 24% of sec avg

",0,1,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/39,2017-01-14 14:46:26 UTC
Reply
#22
i predict 24% duty cycle full throttle under 200rpms... in other word 200 amps 24% of the time otherwise known as 48 "traditional" amps. heating effects in motor should be equivalent to 48 "traditional" amps not 200 "traditional" amps... or maybe heating would be even less than 48 "traditional amps" since this 48 "traditional amps" is being divided between multiple phase wires.

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/41,2017-01-14 14:50:19 UTC
Reply
#23
Ackmaniac, post:43, topic:15995 Wrote:2000 watts at stand still doesn't lead to the same acceleration then 2000 watts at close to full speed.

for even acceleration @ 2000w electrical, you'd want:

- 2000 watts electrical into motor at standstill (duty cycle % very low here)

- 2000 watts electrical into motor at the lowest rpm at which duty cycle is %100 @ 2000w electrical <--this rpm is determined by back emf and motor kv

- at rpms above the lowest rpm at which duty cycle is %100 at 2000w electrical, 2000 watts electrical into motor is not possible. watts electrical into motor will diminish linearly from this rpm with increasing rpms, all the way down to just above 0 watts electrical into motor right around no load rpm. that's based on the motor physics & has nothing to do with the vesc software. specifically the spinning magnets generate a voltage (backemf) opposing the battery voltage, and this voltage increases as the magnets spin faster and faster. at no load rpm, the back emf votage is equal and opposite to the pack voltage, resulting in nearly 0 watts electrical entering the motor at no load rpm.

",0,1,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/44,2017-01-14 15:26:34 UTC
Reply
#24
Ackmaniac, post:45, topic:15995 Wrote:And that is wrong.

if i decide I want 500w max batt draw:

the goal of even acceleration is attained by steady 500w electrical into motor across the full range of rpms which 500w electrical is permitted into motor by physics. Higher than a certain rpm, 500w electrical is not possible. At no load rpm, nearly 0 watts electrical is the most which is possible.

At each separate rpm that 500w electrical is allowed by physics into the motor, a different duty cycle % is required to achieve 500w electrical into motor.

Accelerating at full throttle at very low (under 200) rpm, a very low % duty cycle will be used for 500w electrical. at a certain rpm, %100 duty cycle will be used for 500w electrical into motor. Above this rpm, 500w or more electrical into motor is forbidden by physics.

",0,1,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/46,2017-01-14 15:59:37 UTC
Reply
#25
As an on again-off again VESC owner, If my desired max wattage from the battery is 500w, I want full throttle to equate to 500w electrical into motor across the full range of rpms at which 500w is allowed by physics.

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/48,2017-01-14 16:54:56 UTC
Reply
#26
theory ?: vesc user should have the option for ""battery amp limit"" value to be dynamically changing with pack voltage, to achieve desired full throttle equivalent watt electrical value.

voltage sag? in this theoretical mode, when voltage sag happens, the programmed battery amp limit is adjusted on-the-fly to retain desired constant full throttle electrical wattage value.

step 1: user programs full throttle equivalent electical wattage value ( 500W )

step 2: user turns on dynamic battery amp limit mode

step 3: user enters motor winding resistance detection value ( 0.0415 ohm )

result:

motor amp limit setting is constant but battery amp limit setting changes on-the-fly with varying pack voltage.

this results in even acceleration, and 500w full throttle equivalence, even when pack voltage changes dynamically, such as during voltage sag, or when adding or removing battery cells in series.

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/51,2017-01-15 00:40:05 UTC
Reply
#27
theory?: turbo button

lets assume rider is using dynamic battery amp limit mode as described in the previous example.

in the example, the full throttle equivalent electrical wattage value was chosen at 500 watts.

step 1: user turns on turbo boost mode

step 2: user chooses turbo boost full throttle equivalent electrical wattage value (2000 watts)

step 3: user assigns turbo mode to a button on the throttle transmitter

result:

- when turbo button is off full throttle has 500w electrical equivalence.

- when turbo button is on, full throttle has 2000w electrical equivalence

- when insane button is on full throttle has 4000w electrical equivalence

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/53,2017-01-15 13:07:05 UTC
Reply
#28
theory ?: software independent hardware limit of VESC (unproven - test at your own risk)

assuming:

stock 160 A DC measurement shunt

50V Pack

PC-Side software change allowing unlimited motor amp values

stock mosfets rated at least 100 A DC continuous

this gives theoretical short-time hardware capability:

~100 A DC x 50 V = 5000 W

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/54,2017-01-16 01:46:40 UTC
Reply
#29
@Hummie assuming 0.0415 ohm, 50V pack & 5000 watt electrical full throttle equivalent with even acceleration limit this gives:

100/348/348 batt max / motor max / absolute max amp limit settings

duty cycle at 0rpm full throttle  = %28.80 duty cycle

in other words 100 "traditional amps" in battery leads and motors (heating effects equivalent)

in other words 348 "traditional amps" ON for %28.80 percent of the time full throttle under 200rpms for 5000 watts & 100 "Battery Amps", & limit 5000 watts electrical equivalent at all rpms.

at assumed 400hz DC pulse width modulation this equals:

(1 second / 400hz ) x (%28.80 duty cycle / 100) = 0.00072 seconds

In other words from the battery: 400 evenly spaced Pulsed DC pulses in one second, each lasting 0.00072 seconds is equivalent to 100 amps DC, when accelerating at full throttle while under ~200rpms with a 0.0415 ohm winding and a 50V pack, and 100/348/348 batt/motor/absolute amps max limit settings, equaling 5000 watts electrical into motor full throttle at all speeds 5000 watts electrical is allowed by physics into the motor, & electrical wattage dropping linearly with further increasing rpms.

So in actual reality the 100 "traditional" amps from 50V battery = avg 348 amps flowing %28.8 of the time for 5000w electrical into 0.0415 ohm motor full throttle under 200 rpms = 400 x 0.00072 sec evenly spaced DC pulses in one second.

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/55,2017-01-16 02:05:19 UTC
Reply
#30
If we lock the 0.0415 ohm hub motor in a clamp and put 5000w Pulsed DC electrical through any 2 of the phase wires, what exactly does one second of the DC "pulse train" in the battery wires look like, assuming 400hz Pulse Width Modulation?

%28.8 DC Pulse ON time implies %71.2 DC Pulse OFF time in one second.

(1 second / 400hz) x (%28.8 ON duty cycle / 100) = 0.00072 seconds ON time

(1 second / 400hz) x (%71.2 OFF duty cycle / 100) = 0.00178 seconds OFF time

so for 5000w @ 400hz DC PWM, over 1 second we end up with 400 x 0.00072 sec ON pulses, separated by 400 x 0.00178 sec second OFF gaps.

in other words, @ 5000w @ 100 "traditional amps" DC electrical @ 400hz DC Pulse Width Modulation @ 50V pack @ 0.0415 ohm inductive winding @ under 200 rpm, a typical ON-OFF-ON DC cycle consists of a 0.00072 second ON pulse, followed by a 0.00178 second OFF gap, followed by another 0.00072 second ON pulse

avg DC amps during ON% = 348 amps
avg DC amps during OFF% = 0 amps
avg 1 sec "traditional amps" = 100 amps
avg 1 whole second = 5000 watts electrical
avg pack voltage = 50 V
% time ON = %28.8
% time OFF = %71.2
rpm range = 0rpm thru under ~200 rpm
winding resistance = 0.0415 ohm
pack voltage = 50 V
Assumed PWM frequency = 400hz
Avg On length = 0.00072 seconds
Avg On length = 72/100,000ths sec
Avg Off length = 0.00178 seconds
Avg Off length 178/100,000ths sec
"MOTOR amps" = 348
"BATTERY amps" = 100
Duty Cycle % = 28.8

",0,0,http://www.electric-skateboard.builders/t/understanding-vesc-the-difference-between-battery-amps-and-motor-amps/15995/56,2017-01-16 03:50:42 UTC
Reply


Forum Jump: