So why are we not flying around the world on large motor drones? Small ones are so easy to make lift yet large ones with large motors and props struggle
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Because with land vehicles at constant speed, the weight doesnât affect the efficiency as it would on a flying vehicle that also needs to produce its own lift.
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So larger motors been less efficient donât matter?
Larger motors only matter if thatâs you limiting factor no point in going overkill it adds other factors in that are detrimental. You need to have a balanced system
Just simply calculate the I^2R losses of 2 motors (same kv) at a given speed and torque⌠a larger one with lower resistance and a smaller one with higher resistance. You will see the mechanical output (and motor current) is the same, but the I^2R losses of the smaller motor are higher, resulting in lower efficiency⌠you need more current from the battery to supply the same performance plus the additional losses of the smaller motor.
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That idle current is measured when the motor is close to top speed, which is why I said earlier electric car makers donât want their motors to go much over 10000 rpm because now youâre talking abut iron loss which has to be determined experimentally.
Voltage and KV determin speed not amps. Amps can bu used at any speed
iron losses close to (rpm/v) x max v with no load determine the idle currentâŚ
the same motor current with the same kv gives the same torque at any speed regardless of size.
the smaller motor with the higher resistance has greater I^2R losses at any operating point.
the mechanical output (nm * rad/sec) is the same in both cases with the same motor current but I^2R loss is higher with the smaller motor and that energy has to come from somewhere, and it comes from the battery.
you could argue the bigger motor has more iron loss when itâs close to top speed, but what percentage of the time are you constantly going close to top speed? that factors into the equation of overall efficiency.
if youâre constant speed at low speed or accelerating at low speed when iron losses in both motors are insignificant, then the larger motor wins the efficiency contest, because of its lower I^2R loss.
You only argument about 1 part not what happened in the real world. You canât say x loss counts but y dose not.
6355
65A 12000g pull =185g pull per amp
8085
90A 15500g pull = 172 pull per amp
Smaller motor is more efficient making motor power at the wheel unless itâs a limiting factor. Idle current is a direct representative of the losses in the motor. More losses need more ideal current to overcome.
iâm not familiar with âpull per ampâ - there is a kt torque per amp constant, which is directly calculated from the kv of the motor only.
Idle current only represents iron loss (which is only a factor close to top speed - thatâs the speed at which itâs measured).
If you also factor the I^2R loss you will
see this is the dominant loss until around 8000-10000 rpm depending on the details of the construction.
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Not the only loss going on Iv already said
Accelerating through ~15-25mph, I^2R is the dominant loss, iron loss comes into play above ~30mph.
The faster the motor turns above 10000rpm, the greater the significance iron loss will become, but below 5000-8000rpm roughly, the larger motor wins the efficiency pageant, because I^2R losses (of the smaller motor) vastly exceed iron losses (of either motor).
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Can someone ping me when this thread reaches its conclusion?
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Ideally, for best efficiency, youâd have half iron abs half copper losses. With the gearing at like 3:1 on a board you donât get close to that. Itâs almost all copper losses. But even before you get to a balance of iron and copper losses the motor will likely fly apart and would need some insanely large gearing.
The no-load current draw is a pittance you can see spinning it up with the low kv and low voltage.
For skateboards if u got a huge motor the iron losses will increase and maybe youâd finally get a balance but it would need to be huge. Impractical. Or spin ur little motor crazy fast(but generally there isnât the room for the gearing)
Look at the iron losses here at even 8k
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The bottom line isâŚ
âŚif you switch to a bigger, same kv motor and the resistance drops from 70mOhm to 20mOhm giving 10-12lbs additional thrust at 15mph w/ 30a bat / 80a mot limits per motor (2 motors, 12S, geared for 35-40mph)âŚ
âŚor if you hold a 12lb weight in your handâŚ
âŚyouâre going to feel it.
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On top of what? 10lbs extra on on 20lbs is quite a bit but 10 lbs extra on 500lbs is nothing.
12 lbs on top of 72 lbs. (72 lbs â 84 lbs @ 15mph)
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How about for these two motors?
the 6374 is 24 mOhm and the 6354 is 46 mOhm
Turnigy SK8 6354-200KV Sensored Brushless Motor (14P) | HobbyKing
Turnigy SK8 6374-192KV Sensored Brushless Motor (14P) | HobbyKing
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