yes with the assumed 60a motor amp limit and 30a battery amp limit settings, above the speed that the battery amp limit is reached, each vesc is drawing 30a battery amps, so total 60a battery amps with the dual setup vs 30a battery amps with the single setup.
I see, the battery amp limit is quoted per VESC instead of for the battery pack. Isn’t the apples to apples comparison then to do 60a battery amp limit in the single motor setup vs 30 for dual?
Here’s a comparison of the dual setup with 30a battery amp limit per motor vs single setup with lower resistance and 60a battery amp limit per motor…
The dual still has roughly double the thrust throughout most of the acceleration profile because, due to the 60a motor amp limit, you can’t draw anything close to 60a battery amps with the single setup throughout most of the acceleration profile as seen with the red line, bottom right chart…
Once you get to about 35mph, thrust is nearly equal with both setups, but still a bit higher with the dual (yellow and green lines, bottom left chart).
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Any chance I could get a copy of the spreadsheet? I dont understand where the battery amps calculation is coming from. I would have thought that the maximum current the motor could draw at a given board-speed would be: motorRPM / (Kv * motorInternalResistance)
which even at 5 mph, for a 200 Kv SK3 motor, 2.55x ish gearing ratio, and 90mm wheels is 380 A
the esc is going to limit the current likely. the vesc does maybe 100 battery amps max. but of course the motor amps max is what would be more relevant and it’s about the same max
the two motors will ultimately have more max torque potential (unless the one is as big as the two) because they have more iron and will therefore be able to produce a greater magnetic field and torque without magnetic saturation.
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Here are some of the equations used in the spreadsheet… the example values are from a separate thread. The motor and battery currents are limited below what the motor is physically capable of by software settings in the ESC, which vary depending on user preference and software limits imposed by the various ESC manufacturers.
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But idlers can prevent that…
Everyone here is talking about electrical losses and math when the real reason I like dual drive is because you have TWO WHEELs in traction with the ground and no torquesteer/skidding when you smash the brakes. The electrical efficiency is irrelevant to me.
Also when you have a tail, if the front wheels are off the ground, you basically can’t use the accelerator/brake until you put the front wheels back down – UNLESS you have dual drive
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Redundancy is important on a dual set up if you loose a belt then one slowing you down is better than none.
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I guess only the OGs can appreciate this burn hahahha
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Do you think a PhD that would help Jacob balance his audio levels? His tutorials were a nightmare when I started building (with a sleeping infant in the next room)
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Prof, sorry I dont follow. Specifically, where is this 56% duty cycle coming from? Do you have an article explaining the fundamentals that I could read over?
I thought that duty * battery_voltage * kv = motor rpm, but I must be wrong there
Sure, In this example, we already know the Pack Voltage ( A ), Back EMF Voltage ( D ), Winding Resistance ( E ) & Electrical Watts ( G ).
A = 50 = Pack Voltage B = X.XXX = %Duty Cycle C = X.XXX = Effective Voltage D = 24.20358 = Back EMF Voltage E = 0.07 = Winding Resistance Ohms F = X.XXX = Motor Current Amps G = 1500 = Electrical Watts (30a battery amps @ 50V)
We seek to determine the Duty Cycle % ( B ), Effective Voltage ( C ), and Motor Current Amps ( F ).
We need to combine & rearrange these equations to solve for the missing variables ( B ), ( C ) & ( F ):
A * B = C (C - D) / E = F C * F = G
Step 1: Combine the equations… we already know the values of all variables except Duty Cycle ( B )
(A * B) * (((A * B) - D) / E) = G
Step 2: Rearrange the equation to solve for B:
B = (sqrt(D^2+(4 * G * E)) + D) / (2 * A)
Step 3: Now that we know B, solve for C:
A * B = C
Step 4: Now that we know C, solve for F:
(C - D) / E = F
Final Values:
A = 50 = Pack Voltage B = 0.5591813 = %55.91813 = %Duty Cycle C = 27.95909 = Effective Voltage D = 24.20358 = Back EMF Voltage E = 0.07 = Winding Resistance Ohms F = 53.64978 = Motor Current Amps G = 1500 = Electrical Watts (30a battery amps @ 50V)
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Interesting. What is confusing me is, what happens to the 30a that the battery is supplying, that doesn’t go through the motor. I’m going to have to think about this for a while, but thank you for pointing me in the right direction.
the battery is actually discharging a series of avg 53.64978a pulses (55.91813% of the time) at a frequency on the order of 20,000-30,000 cycles per second which averages to 30a.
53.64978a * 0.5591813 = 30a
Gotcha, and for some reason “effective current” is not duty cycle * peak current, the way “effective voltage” is? That’s not intuitive to me, but I guess that is the only way the math works
Thankyou for some real world sense! calculate all you like but when you ride the bloody thing you will know!!!
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Yeah, it’s time for me to build and ride one to see what you guys mean, torque steer and traction could be deal killers
To be clear, I love me a 6374 single-drive too.
But I was saying that electrical efficiency or battery range is not why I like dual so much better. It’s because now you have two wheels biting the ground instead of one wheel. Also redundancy – if you have a failure with two independent drivetrains you probably still have braking ability and can probably still get home without pushing or crashing. (It’s of-note here that most dual builds are not two independent drivetrains)
single 5065 < single 6355 < single 6374 < dual 5065 < dual 6355 < dual 6374
So yea, I’d rather have dual 5065 than single 6374 even if it’s less power
Yeah the two points of traction thing is undeniably valid. Wondering if the ugly but light weight solution though would be to use a wider driven wheel (flywheels seem to be very narrow by modern standards) with a larger contact patch… Redundancy is a good point too, but I feel that if you know how to foot break (which every e skate rider probably ought to learn), and otherwise design for reliability, it might not be so crazy to trade the redundancy for weight.
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