X-File - The "Is My Math Right?!" Thread
#31
Ackmaniac, post:52, topic:17037, full:true Wrote:And what would be the formula when we have the H value? To make it easy lets do the math for hummies values.

@Ackmaniac we haven't measured his actual kv.

",0,0,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/53,2017-02-03 21:32:00 UTC
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#32
Ackmaniac, post:54, topic:17037, full:true Wrote:Doesn't matter. When we can calculate at which voltage he still could achieve like 1440 watts then we could do the math for any motor.

@Ackmaniac since i don't have ready access to his board, nor do i yet have logging abilities, since you have both, if we measure your actual kv, h, ohm and pack voltage you could do a log video to confirm the math.

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/55,2017-02-03 21:35:07 UTC
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#33
Ackmaniac, post:56, topic:17037, full:true Wrote:Let's do the math for hummies because i am not at home to check the values at the moment. Later the parameters can easily be changed in the formula to calculate it for any motor.

@Ackmaniac it's pointless if there's no way to confirm.

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/57,2017-02-03 21:41:30 UTC
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#34
Ackmaniac, post:58, topic:17037, full:true Wrote:Of course i can confirm later on.

@Ackmaniac i've already posted his H amd Ohm if you want to do the math, but as I mentioned we don't know his actual kv.

& besides, nobody has yet answered your original question.

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/59,2017-02-03 21:42:53 UTC
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#35
Ackmaniac, post:60, topic:17037, full:true Wrote:I have the values for my SK3 190 kv

R = 0.01627L = 13,12

@Ackmaniac how exactly was kv measured on your exact motor?

i think this could be calculated by looking at the no load rpm and pack v.

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/61,2017-02-03 21:47:48 UTC
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#36
Ackmaniac, post:62, topic:17037, full:true Wrote:The manufacturer was so kind to do that for me and he was so smart to print that on the motor for dummies like me.

@Ackmaniac ok please tell me the measured pack v & no load rpm. somehow I doubt it's a nice even 6840rpm @ exactly 36v despite any manufacturer's sincerest of labeling intentions.

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/63,2017-02-03 21:55:32 UTC
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#37
Ackmaniac, post:64, topic:17037, full:true Wrote:It is the voltage under load when i draw exactly 40 Amps from the battery.

@Ackmaniac to know the precise kv we need basically a log frame at no load rpm, recording the max rpm & pack v at that rpm.

it would also be nice to have a screenshot of H & Ohm, and Resting Pack V so everything checks out.

& the xx/xx/xx settings

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/65,2017-02-03 21:57:53 UTC
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#38
Ackmaniac, post:66, topic:17037, full:true Wrote:Ok i am out. A formula is a formula. To calculate the stability of a building you don't need to build it first. Seems i have to find out myself.

@Ackmaniac ok.... :confused:

so what is the formula for:

devin, post:30, topic:17037 Wrote:Which RPM does 1440W electrical = 95% duty cycle?

Ackmaniac, post:10, topic:17037, full:true Wrote:Correct. But lets imagine that the motor spins at no load with 94 % duty cycle. And now we want full power and feed the motor with 95% duty cycle. So my question is at which no load can i still achieve 1440 watts when i fed the motor with 95% duty cycle.

",0,0,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/67,2017-02-03 22:02:04 UTC
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#39
Ackmaniac, post:14, topic:17037 Wrote:Pack V = 36V, Ohm Resistance = 0.016, Batt = 1000 /Motor = 1000 /Absolute Max = 2000 Amp Limit Settings, and Motor KV = 190

Ackmaniac, post:60, topic:17037, full:true Wrote:L = 13,12

@Ackmaniac ok fine we can use this made up scenario which can't be directly verified.

devin, post:30, topic:17037 Wrote:Which RPM does 1440W electrical = 95% duty cycle?

Ackmaniac, post:10, topic:17037, full:true Wrote:Correct. But lets imagine that the motor spins at no load with 94 % duty cycle. And now we want full power and feed the motor with 95% duty cycle. So my question is at which no load can i still achieve 1440 watts when i fed the motor with 95% duty cycle.

devin, post:37, topic:17037 Wrote:@Ackmaniac I'll go so far as to say I'm still quite (95%) confident that:

1440w electrical @ 100% duty = 6718.93 rpm

I'm still not so sure yet about:

1440w electrical @ 95% duty = XXXX.XX rpm

Ackmaniac, post:60, topic:17037, full:true Wrote:L = 13,12

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/68,2017-02-04 00:33:33 UTC
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#40
devin, post:30, topic:17037 Wrote:Which RPM does 1440W electrical = 95% duty cycle?

@Ackmaniac

Assuming:

Pack V = 36V
Ohm Resistance = 0.016ohm
Batt Max = 1000A
Motor Max = 1000A
Absolute Max = 2000A
KV = 190kv
H = 0.00001312H

Assuming:

1440w electrical @ 100% duty = 6718.93 rpm

& Ohms Law:

1440W & 0.016ohm gives @ 0rpm:

Effective PWM V @ 0rpm: 4.8V
Motor Amps @ 0 rpm: 300A
Electrical Watts: 1440W
Winding Resistance: 0.016ohm

or simply:

4.8V
300A
1440W
0.016ohm

(4.8V / 36V) x 100 = 13.33% duty @ 1440W electrical @ 0rpm

In simple terms:

- ~0w electrical @ 100% duty = 6840.00 rpm <<-- ~0A "Battery amps" & ~0A "Motor amps"


- 1440w electrical @ 100% duty = 6718.93 rpm <<-- 40A "Battery amps" & 40A "Motor amps"

- 1440w electrical @ 13.33% duty = 0 rpm <<-- 40A "Battery amps" & 300A "Motor amps"

- 81000w electrical @ 100% duty = 0 rpm <<-- ~2250A "Battery amps" & ~2250A "Motor amps"

We could say:

"The 1440W electrical range is from 13.33% duty @ 0rpm to 100% duty @ 6718.93rpm."

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/69,2017-02-04 03:07:18 UTC
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