X-File - The "Is My Math Right?!" Thread
#1
The information in this post was recovered from a back-up archive of posts by user @devin after some of the information was hidden, deleted or removed from the Electric-Skateboard.builders forum.

Link to Original Thread @ Electric-Skateboard.builders

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The "Is My Math Right?!" Thread

It was suggested by @treenutter forum moderator, that, since I often observe mathematical errors and misstatements (and respond to them), maybe I should start a thread devoted to Math Mistakes and Misstatements.

So here it is.

In an effort to avoid "ruining other people's discussions with math," whenever I see misstatements being made about products I care about, I will post my observations here, preserving the "sanctity" of other people's threads.

@devin
2017-02-01 17:17:11 UTC
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#2
Ackmaniac, post:4, topic:17037, full:true Wrote:Then i have a math question here. When i have a resistance of 0.016 and a maximum duty cycle of 95%. What is the highest duyty cycle for the motor where it still can produce 1440 watts. And let's say that the battery is at that moment at 36V and 40A = 1440 A.

@Ackmaniac

Question: What is the highest duty cycle for the motor where it still can produce 1440 watts (electrical)?

Simple Answer: 95% Duty Cycle

Extended Answer: The one and only RPM at which 95% Duty Cycle = 1440 W electrical is determined by VESC-detected ohm resistance, pack voltage, user chosen batt/motor/absolute amp limit settings, and motor KV.

@devin
2017-02-02 10:56:12 UTC
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#3
Ackmaniac, post:6, topic:17037, full:true Wrote:Your right, sorry. My question was wrong. I meant what is the highest no load motor voltage (no load duty cycle) at which i still can achieve 1440 watts at 95% duty cycle.For example at a standstill the motor has a no load voltage of 0V. And we feed i with some volts and amps to achieve 1440 W at a stand still.But now i want to know at which no load motor voltage i still can achieve 1440 watts when my maximum duty cycle with that i can feed the motor is 95% (34.2V).My calculations tell me that this is somewhere around 92,33% duty cycle (33,24 V) but that seems to be wrong.

@Ackmaniac

Question: What is the highest no load motor voltage (no load duty cycle) at which i still can achieve 1440 watts at 95% duty cycle?

Answer: The pack voltage is 36V The highest duty cycle which you can achieve 1440 watts electrical is 95% at one particular RPM determined by VESC-detected ohm resistance, pack voltage, user chosen batt/motor/absolute amp limit settings, and motor KV.

@devin
2017-02-02 11:45:54 UTC
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#4
Ackmaniac, post:8, topic:17037, full:true Wrote:When the motors no load is already at 95% duty cycle and you power the motor with 95% duty cycle then the power output is 0 W. Think if a ideal world without any resistance. The means when you spin the motor with 1000 rpm then it will spin forever with that speed.A 100 kv motor will spin at 95% (32.4 V) no load with 3240 rpm. When you power that motor now with 32.4 V it won't spin any faster. You remember the donkey/carrot picture. So in this scenario the donkey reached already the carrot.

My question is now at which no load duty cycle (actual motor speed) you can still get a power output of 1440 W when your maximum duty cycle is 95% (32.4 V). Because when the motor spins already at 94.9 % duty cycle you can't achieve 1440 watts anymore (at least not when you have a realistic resistance).

@Ackmaniac

I am confused by your use of the term "no load duty cycle".

 My understanding is the term "no load" generally refers to a maximum RPM the motor reaches with no mechanical load.

This RPM is determined by:

No Load RPM = Pack V x Motor KV

@devin
2017-02-02 11:59:58 UTC
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#5
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.

@Ackmaniac

Question: At which no load can I still achieve 1440 watts when i fed the motor with 95% duty cycle.

Answer: Very close to No Load RPM, you can only achieve maximum ~1 W, even with 95% duty cycle.

Extended Answer: Very close to No load RPM, the voltage produced by the spinning magnets, opposing battery voltage (called Back EMF V), is almost equal to Pack V. As a result of this, even with 95% duty cycle, only maximum ~1 W will flow to the motor.

",0,2,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/11,2017-02-02 12:20:23 UTC
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#6
Ackmaniac, post:12, topic:17037, full:true Wrote:That's not a answer to my question. You repeat what i explained already.

Ackmaniac, post:10, topic:17037 Wrote:So my question is at which no load can i still achieve 1440 watts when i fed the motor with 95% duty cycle.

@Ackmaniac

Since you said "no load" I understand this to mean "no load rpm".

At no load rpm, you can't achieve 1440 W @ 95% duty.

If you are asking:

Which RPM does 1440W electrical = 95% duty cycle?

The answer is determined by Pack V, Ohm Resistance, Batt/Motor/Absolute Max Amp Limit Settings, and Motor KV.

",0,5,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/13,2017-02-02 12:37:09 UTC
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#7
devin, post:13, topic:17037 Wrote:If you are asking:

Which RPM does 1440W electrical = 95% duty cycle?

The answer is determined by Pack V, Ohm Resistance, Batt/Motor/Absolute Max Amp Limit Settings, and Motor KV.

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 this is a good question and I'll admit I don't know how to calculate the answer precisely.

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/15,2017-02-02 12:40:56 UTC
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#8
Ackmaniac, post:16, topic:17037 Wrote:Me too
10 chars

@Ackmaniac

What I do know how to explain is W Electrical @ Full Throttle Standstill

with these settings:

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

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/17,2017-02-02 12:46:09 UTC
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#9
devin, post:13, topic:17037 Wrote:If you are asking:

Which RPM does 1440W electrical = 95% duty cycle?

@Ackmaniac

I'll bet it's roughly somewhere in between 0rpm and No Load RPM. :yum:

",0,0,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/19,2017-02-02 12:51:38 UTC
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#10
Ackmaniac, post:20, topic:17037, full:true Wrote:I just experienced that the power output at higher speeds gets less the more resisance the motor has. But on the other side the motor has more torque at very low speeds of you limit the motor max to for example 50 A.So at low speeds the motor max plays a role. But at close to 95 % duty cycle the motor max can't even be reached.

@Ackmaniac

In my view, this is really the all-important equation:

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

in simple terms battery amps is a whole second avg, motor amps is an avg for "ON" duration only (seconds).

battery dc is pulsed.

or you could say:

battery amps: ON + OFF time AVG

motor amps: ON time AVG


[Image: avg-on-amps.jpg]

",0,1,http://www.electric-skateboard.builders/t/the-is-my-math-right-thread/17037/21,2017-02-02 13:02:44 UTC
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