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Electric motor power
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You are currently reading a thread in /diy/ - Do It yourself

Thread replies: 28
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Why do BLDC motors vary so much in power? You have small remote control plane engines like this putting out 15kw. But then you have larger and heavier ebike motors with only 2kw.

What variables cause one BLDC motor to have 10x higher power to weight ratio of another?
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Just guessing here, but the RC plane motors have excellent cooling and the run times (per run and the total run time) are pretty short.
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>>935624
its the difference torque stength
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>>935629
Why is the difference so extreme? For example this motor weighs roughly 4x as much and offers half the torque at the same voltage.
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>>935624
>>935655
Ho-man! Just wait until you discover the Halbach Array Motor - and through thay extension, the Halbach Array!
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>>935659
The axial flux motors are actually about the same power to weight ratio as the one in the first post.

Also has anyone discovered a good source for axial flux motors? Anywhere selling them for vehicles charges $5000+ but I have discovered some modern swimming pool pumps are using them so there can't be anything special difficulties when it comes to production.
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>>935624
are you getting confused because of RC faggots using KV to abbreviate "RPM(x1000) per Volt"?
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>>935689
Yeaaaaaahhhh. I was a bit flabbergastered. 15kW in an RC? Our dive club has compressor that has a 20kW electric motor. It would need one serious RC-plane to lift...
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>>935689
No people are sticking them on bikes and they go faster than the ebike hubmotors. I just don't quite get the enormous difference in power to weight ratios. Is it just a matter of cooling the ebike motors to get the same power to weight ratio or is there some basic difference in their construction.
https://www.youtube.com/watch?v=U-qhcfrPdk0
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>>935624
The variable is cooling. Bike motors have to be resistant against dirt and moisture, and are encapsulated in a case. This case, ob course, does not allow airflow. So while the motor would theretically be able to provide the same power, it would burn up as it cannot shed the waste heat.
Aircraft motors, in contrast, have all the cooling air flow they could ask for.
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>>935655
The major factor is how well the windings are made and what gauge of wire is used.

Magnetic field strength of a solenoid is B= permeability of core material x number of turns x current / length of wire

The thicker the wire, the more current it will draw for more power, but you also fit less turns with thicker wire. You can optimize the current draw with the number of turns to see what size wire gives you the greatest field strength for a given battery. But RC people usually like the hammer fisted approach of getting the thickest wire they can and simply pushing through as much current as possible to make up for the lack of turns.

If not for thick wire, they can also wind thin wires in parallel to decrease resistance.

>>935655
This motor might be made for efficiency not torque. If you are getting 4x more torque at the same voltage with the other motor, you are probably using more than 4x the power and your range is going to be significantly reduced.
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>>935749
Surely you could stick a fan on the rotor like electric drills do?

You'd easily have enough power to suck air through a filter.
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>>935764
>This motor might be made for efficiency not torque. If you are getting 4x more torque at the same voltage with the other motor, you are probably using more than 4x the power and your range is going to be significantly reduced.
Or is it just large outrunner type motors are not ideal? If you look at the hub motor it has much more iron and requires longer wires. Is keeping the motor as small as possible and carefully cooling it more effective? For the size and weight of the hub motor you could easily have a smaller motor with oil or water cooling.
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>>935769
drag. also depends on environmental conditions. a bicycle hub needs to be able to withstand a 2000N impact at least. say you drop the bike at speed or go over a bump. thats a fuck ton of force.
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>>936122
High torque and low speed is generally a bad combination for electric motors. Power is a product of the two, so to achieve high power at low speed you need lots of torque, which in turn requires a physically big motor.
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One trend I've noticed looking at various RC BLDC motors is that higher-KV motors with fewer windings (designed for high RPM and low torque) seem to have significantly higher power densities than their lower-KV counterparts (example: http://www.hobbyking.com/hobbyking/store/__15169__NTM_Prop_Drive_Series_35_36A_1800Kv_875w.html http://www.hobbyking.com/hobbyking/store/__14847__NTM_Prop_Drive_Series_35_36A_910Kv_350W.html same motor, only difference is # of windings and KV). Perhaps it has something to do with running current through more turns of smaller-gauge wire causing greater resistive losses and poorer heat transfer or something, I dunno. But in any case, a hub motor - being direct-drive, of a rather large wheel to boot - is going to have a very low operating RPM range, and thus require a very low KV in order to achieve any sort of efficiency.
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>>936313
Higher KV = higher RPM = lower torque for a given power = lower current required for a given magnetic field strength and diameter.
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I understand now, basically BLDC is constrained by temperature and switching delays at high speed if you have many turns. But high power density means giving up low speed torque.

Most likely a high speed geared hub motor would be a much better option for pure performance.
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>>935769
Maybe that would work for middle motors, but those are limited to 250w anyways and work just fine without additional cooling. Much more power than that would shred the drivetrain. Hub motors do not spin that quickly, and a filter that is not soon clogged with dirt, mud and rain splashing against it has yet to be invented. There is simply no real market for overpowered electric bikes, at least compared to ~250W standard bikes. My city is called the "capitol of bicycles" in Germany, and for every >250W bike I see 100 ~250W ones. It's clear where the manufacturers spend money for research.
Some dudes over at the endless sphere forum have filled hub motors with atf oil and got a somewhat better cooling.
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Two things, mostly:

The most notable is cooling. Larger motors, ESPECIALLY the hub motors made for ebikes, tend to be cooled very poorly. They must be de-rated accordingly. Their sheer size hinders them further; remember that surface area goes up by a squaring factor while volume goes up according to the cube.

RC motors generally expect to have ridiculous amounts of air being blown over them by a propeller. They can simply open up the frame and achieve much greater cooling than other motors. And again, due to their smaller size, they have much higher surface area/volume ratios.


As was pointed out earlier, there also seems to be some advantage to using relatively fat wires and few numbers of turns. If I had to take a guess, I would say this has something to do with reducing the losses associated with the long, thin wires with many turns used to achieve high flux densities normally, and additional gains seen from lower inductance on the coils. The downside would normally be weaker magnetic coupling between the rotor and stator (and I guess the necessarily high RPM usually requiring gearing if it's attached to anything but a propeller or tiny wheel), but apparently the previous advantages more than make up for it.
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>>936348
>Higher KV = higher RPM = lower torque for a given power
Yes...
>= lower current required for a given magnetic field strength and diameter.
No, the exact opposite actually. KV and KT are inversely-related.
https://en.wikipedia.org/wiki/Motor_constants
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>>935690

15kw isn't that unreasonable. Some lipo 10s packs will put out more than 15kw and there are RC motors that handle it albeit not for long. RC motors spin hella fast and have relatively little torque. A 15kw RC motor will hurt you, a 15kw forklift motor will rip you in half, it's all about the torque.
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>>936867
I know the lifepo4 headway 15ah and a123 20ah prismatic cells are rated for 100A continuous output. so I suppose if you built a 150V minimum pack... you'd wind up with a 75s battery. at 600g each... thats 45kg
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>>936870

Headways are only 3C continuous, the good lipo's can do 65C continuous. Single cylindrical cells are not very space efficient and take too many cells in series to get the voltage to usable levels. 2 brick sized xSxP lipo's will get you 15kw easily.
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>>935624
Al these stupid answers, they might contain some truth but the real reason has not been stated yet.
The main reason is because you are comparing oranges with apples. A RC BLDC is able to do a lot of RPM, usually in the 10-20K, while a bike hub motor will probably drive at a about a max of 300RPM. Due to the nature of a bldc you don't want a low multiple of 3 steps per revolution to keep a smooth ride so you need many coils and that takes a lot of space. Also the current drawn is decreased that way, improving on efficiency.
The reason why an RC BLDC attached to a bike can be faster is that it's fucking connected trough some form of reduction, there actually are electric bikes with the motor connected to the pedals, and because there is already a reduction that way the motor can be waaaay smaller. At last: it does not make sense to give a bike 15KW of power as there are no batteries capable of letting you use that amount of power over a useful duration when they have to fit on a bike.
I fucking hate humanity for being so stupid that they cannot find out these fucking simple things themselves.
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>>936937
It's not for a bike, it's for a self balancing vehicle. It's an interesting problem, while they use less energy most of the time than a bike they need to be able to output huge amounts of power for a very short amount of time (500ms or so) when they travel over a bump.

With a small wheel and a low top speed the most they need in bursts is around 1kw. However newer models travel at 20-30km/h and have larger diameter wheels. To cope with a bump at those speeds they need 2kw peak and realistically 3-5kw would be safer. Because the only way they can get more power is to reduce the turns and increase current. But that leads to low speed crashes as the torque reduction causes cogging and the controller dumps more current into the cogging motor until it triggers an over current shutdown.

But reduction gearing also has it's issues, the direct drive hub motors are almost silent and results in less moving parts.
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Hub motors are less electric eficient than traditional electric motors. Because the high torque requires more current.
You may have more electric eficient motor if it spind at higher speeds, but you need a gearbox, so it will be less mechanic efficient.
You have to find the middle point.
Here, this may help
http://www.instructables.com/id/Make-Your-Own-Miniature-Electric-Hub-Motor/
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>>936870
ESCs above ~25V/6s are expensive; ESCs above 50V/12s are almost unheard of.
Parallel cells are always an option.
>>936930
>the good lipo's can do 65C continuous.
>Buying into bullshit marketing
LiFePO4 offers lower internal resistance and better power density. Even the best Li-polys will deliver fuck-all for voltage at 65C continuous.
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