Well, thats the point I dont like, you don’t lose power by not using the full power band! You lose a little bit of efficiency (which also is variable with load). But since driving at lower speeds than topspeed (not maximum dutycycle) is basically the same (your ESC will step down voltage to deliver motor a lower voltage than battery voltage), I don’t see that as an argument.
Not using the full power band has the only disadvantage of carrying more weight and a bigger motor (than theoretically necessary).
To continue, Spoiler Alert: having a gear adds weight and makes your propulsion unit bigger.
Let me guess, mathematic or physic background?
Coming from a engineering background this makes absolutely no sense: efficiency decreases with:
rising (input) rpm (regardless of reduction, just by additional bearing losses/heating and viscose friction of the lubricant (no, you cant use magnetic bearing, magnetic reducer and vacuum for a gearbox in the real world))
rising reduction ratio (by having e.g. more stages or different tolerances due to sizes).
So your 1 million rpm motor might melt bearings, but wont have any practical use.
Also inappropriate example in our case, since the human body only can do very limited cadence (RPM) and variable gearing is not an issue in water since acceleration of the propeller is not the issue. High torque demand comes at with high rpm in water. For acceleration of a bicycle you need high torque all the way to topspeed.
Still the powerlifter will win in a (endless) long race… on the flat, without wind
I am excited to see you cooling concept (of motor and gearing of course).
Finally after a lot of reading i was thinking that it is even not a matter of power but more size and resistance of the winding , bearings and vibrations
So more about how much amp the motor’ s winding is capable of taking before heating to much , and the field of the magnets (size)
So kv gives wiring size and resistance so max amp , voltage is just given for the limit of the rpm ( structural purpose)
I wanted to build a esurf at first but it needs high amp (thrust : torque…) , since i wanted speed for fun i choose efoil and under use the motor
But since now i have the battery pack and the controller i am thinking about it again with 2 direct drive 56123 / mhz 52-58 jet
Well, roughly seen motor power is dependent on weight of copper inside the motor (sure there are constraints, but weight and copper density is what determines a well designed motor).
Power is ~ RPM x Torque (mechanical) and also ~ Voltage x Current (electrical). So since power is given by weight, you can either design the motor along RPM preferences or Torque. Therefore you have geometry, poles and winding:
Geometry: high rpm inrunner, or high torque outrunner.
Pol: low pole count high rpm (ERPM ~ RPM) or high pole count low rpm (ERPM >> RPM)
and winding: many turns and WYE => high voltage low amps, or less turns (thick wire) delta winding for low voltage high amps.
Still winding doesnt change RPM and torque. Geometry and poles does change maximum RPM and Torque.
Winding on the electric side trades current for voltage. Since power (torque and RPM) is fixed by design, you just change the KV(RPM/V) and KM(Nm/A), where KM ~ 1/KV.
So e.g. a SSS 56104 with 360KV has max power of 11kW, so lets say it has 130V max voltage you have max torque at 85A and max RPM at 130V. So if you dont need 50kRPM but only 12,5kRPM and max torque you can easily use 32V and 85A but then you wont get 11kW but only 2,8kW.
On the other hand you have the SSS 56104 with 1000KV, same max power but only 50V max voltage, so max torque at 220A. So if you need max torque you have to use 220A. If you still need only 12,5kRPM you will use only 12,5V and also use 2,8kW.
In the end it comes down to if you can use the “won” higher RPM. Personally I cant. I dont have a gearbox capable and direct drive you will have cavitation. So I rather stay with 12,5kRPM and max torque at 85A (since losses/efficiency is dependent on amps), instead of beeing able to use the full 11kW but have to use 220A to have max torque.
Thanks, great information. One additional question, it looks on the specifications that larger diameter of the motor seams to increase the torgue, is that a part of the “geometry” factor to consider ?
exactly, the diameter of the airgap and length of the stator determine torque mostly.
Is a good place to get started.
Somewhere in university I learned T is proportional to Diameter x Diameter x L.
A 100mm diameter motor will, all else being equal, produce twice as much torque as a 50mm diameter motor.
Right now I can not see it in the formula directly : " T = 4 * N * B * L * R * i " I guess the second “x Diameter” hides inside N (number of turns) and/or I (current). Like with bigger diameter (more space) you can either do also more turns at same current or more current at same turns and thus get the dependency of R².
I think using your own example will help prove my original point. If you only use 12k rpm out of 50K possible, direct drive, and draw 85A on that 360KV sss 56104, it would appear that is better than using a 1000KV winding that requires 220A for full torque. However, I can gear that 1000kv motor 4:1 to get to 12k RPM, using full 50k rpm. Now my torque is 4x at 220A, or at 1x only 55A, which beats direct drive.
If you’re an engineer, its a little puzzling why you would argue that leverage is somehow inferior to non leverage. That’s what gearing literally is, rotational leverage.
I would also add that the 1000KV motor is capable of generating 10KW of power at 48V@220A, instead of the 360KV at max 4KW at 48V@85A.
We choose to limit our efoil voltage at 48V for safety, so we MUST push current to get the wattage. Maybe this point is what was missing in our argument. Because of the fact we limit ourselves to 48V, using a motor that is capable of 130V means we’re leaving power (ie wattage) on the table, ie losing it.
Staying practical you might need more close to 4-8kRPM, so gearbox still be in the range of ~ 1:10 so still you need a gearbox which does not exist.
Leverage is a nice tool, but only makes sense if you cant have things directly. And leverage in classical sense almost neglects efficiency (nobody cares about the efficiency of a vice). But for power applications you have to stay practical. You know what the high ratio neugart gears typically are used for? Servomotors.
Ever had/heard a chain at 50kRPM? I can tell you, you will have a hard and noisy time. I only went up till 20kRPM max but was willing to switch to belts soon.
Well, for efoil, do you really need more than 4kW? With propeller propulsion I never saw that demand.
If you do, sure then you have to push the amps, still I don’t think high rpm and big reduction is the way to go.
Feel free to prove me wrong! I’d be happy if you change my mind with a working prototype.
This very informative reading fro me thanks. I am still little puzzled about the Torgue change in relation to KV. Here seams to be 3 otherwise identical motors but the kv( and obviously max RPM) and here torque clearly changes to the better with lower kv:
Vs
So probably the 100kv has higher pole count and that’s why torque is rated higher?
Well, if the factor is exactly 1,7 it is most likely due to star / delta winding.
So exact same motor, just diffrent wiring of the phases. So 100KV is 1,7 x Nm/A and 1,7 x higher Voltage than 170KV, but also 1,7 x less maximum amps
So what would be the best battery / ESC compromise : bigger battery / weaker ESC or weaker battery / stronger ESC ?
1.7 times less KV = 1.7 times more current === ie less expensive battery and more expensive ESC
OR
1.7 times more KV = 1.7 times less current === ie more expensive battery and less expensive ESC
Well, generally I would go higher voltage, less current.
ESC gets more efficient with less current. And overall strain on components is less by less current (since isolation is for ~200V anyway, there is not really a advantage in design for lets say 30V vs 50V).
But I just had a closer look at the specs: it is not just delta /star, also torque is different and they have same current and voltage rating. So for the sake of higher efficiency at bigger slower prop I would choos the 100KV one since it has more torque.
In general, if you are limited at voltage (like: I dont want to go above 50V in water, or above 15S ESCs get damn expensive), choose the highest voltage you can do and then choose based on RPM…unless you like to use some kind of reduction/gear.
Current then settles with power demand. E.g. if you need 3kW to get out of the water, your battery is 50V, your motor should do at least 60A.
But also keep in mind who “made (up)” the specs you use for decision…
I’m going jet drive, with impellers, which typically operate at far higher RPM than props. So they’re happy at 10-20k RPM. The smaller the impeller, the higher the optimal RPM. So the 4:1 gearing is perfect for my needs.
Of course leverage has losses. The transmission on a car usually suffers 10-20% loss of power between engine and the wheels. Yet its worth having it because direct drive would result in worse efficiency. Even Tesla cars have a single 10:1 gear reduction. And Tesla designs and has their motors built to exactly their needs. And yet they still have a single stage gear in there.
You have me here. No I haven’t run a chain drive at 50k. Maybe inside the board some of the noise will be muffled. But I haven’t built my chain drive to experiment, so maybe this all goes to shit.
That’s the thing, I’m not building an e-foil. I’m building an e-surf and I want it to be able to hit 35+ mph. The plan is twin 64mm jet drives driven by 4:1 geared 1000kv 56104 using the full 50k rpm to hit target 12k rpm of the 64mm drives.
If you haven’t bought the jets yet, I would go for a pair of jets from youngsters jets (YouTube). He has designed a 53mm impeller and it works really well. He can even run the jets off 6S batteries. Smaller jets allow you to use cheaper components all round.
I’ve seen youngsters 53mm(?) jets, but its really unclear what his status is. No website and so no clear way to order them. Plus, I get the feeling they might be expensive.
He’s also running smaller motors and direct drive on his 53mm jets, so I get the feeling my larger geared motors might be overpowered for the 53mm jets. So I’ll go 64 and then adjust the pitch as needed.
I’m not going to get the Mhz drives. Too expensive. There are some free designs out there that I want to try.
I understand the foilers want to keep the engine diameter small, but for a jetdrive surfer engine will be inside the hull(usually). Is there benefits of running only 56 diamaters, torque would be probably the double on a 100 mm + engine? Do You know how much Torgue the engine need to produce for running 64mm impeller at 12000 rpm at 35 knots?
There is no specific benefit. The only reason I’m running 56104 is that they’re cheap and even with 2x they’re cheaper than a single larger diameter motor of equivalent power. The 2 1000kv 56104 14kw peak motors I bought totaled under $300 USD. Whereas a 24kw peak motor is anywhere between $600-$1k USD.
But…a commercial jet drive, like MHz is $600+ per drive, in which case it would have been cheaper to go single drive, larger motor. But because I found some free designs, the jet drives were almost free, thus making the 2 smaller drive/motor approach more economical.
He is shipping now. You need to email him directly.
I have 2 onean 64mm impellers that I am going to use for something. Not quite sure what yet though. Probably design a unit that can be used in both a surf and foil.
