



IMHO, given the same load, the higher Kv motor with the small pinion will ALWAYS be more efficient, if only because there is less wire for the electricity to pass through.
Higher Kv motors are capable of handling higher currents because of the shorter, and thicker, wire used in the windings. 





I'm still confused on this issue. I know from experience and all the reading I've done here that low KV motors with higher cell counts are always more efficient. Something to do with higher volts instead of more amps to achieve the desired watts per pound ratio being more electrically efficient.
Higher efficiency is not necessarily desireable however because of weight. The big appeal of high KV motors is that you get high rpms with just a few cells. This wasn't possible years ago because the battery technology wouldn't support the high discharge rates per cell necessary to run high KV motors. Since the batteries are the main source of weight, using fewer cells will definitely get you a better watts per pound ratio in a hurry with todays battery technology. Now gear your high KV motor up there by trading all those rpm's for torque so you can swing a large prop to gain efficiency in a different way. Now you saved weight over the low KV motor AND used the rpm's to get more prop efficiency. It's a win win situation which is why everyone in doing it. I've read several articles discussing fuel source (batteries) and electric motors. The fuel source is the most important choice to make based on the airframe and the demands you will put on it. After that is decided then choose the motor. There are lot's of different ways to skin that cat.... 





Jim, you are correct that a higher Kv motor with more cells, but the same pinion, is more efficient. However, in this case, we're talking about changing the motor and the pinion, not the number of cells.
In this case, input voltage and current will remain about the same, given that both motors are expected to spin the same set of blades at the same RPM, and they're using the same battery. The main difference in efficiency in this case will be due to what I believe are called "iron losses." Because the wires that make up the windings on a high Kv motor are shorter and thicker, they have less resistance than the thinner, longer wires in a low Kv motor. 





I've gone both routes on my planes, 40 amps through a endo on 8 cells geared deep with a big prop, and more recently 3 Li cells also geared fairly deep on a low kv ( ips) motor.
While the lower motor resistance does indeed come into play in overall efficiency, so does the efficiency gain you get by upping voltage on a higher resisance motor due to the much reduced amps you can have and still have adequte wattage. I think the thing thats going to tip the scales one way here is the new larger Li cells which make it easy to really step voltage up without a huge weight penalty. The low amp draw also has the added benefit of allowing cells voltage to remain higher through the discharge curve. Imagine a basic sp400 motor, normally run at 12 amps on 8 kan cells for 115 watts, but only 50% eff. due to the high amps for 60 watt output, or 7.2 volt sp400 run on 4s of the new etec 1200's at only 7 amps for 100.8 watts, but around 60% eff because the amps are way down, for the same 60 watt output. Now keep in mind, your swapping out a 6 oz batt for a under 4 oz one, and the motor run times will be about 2X what they would have been on kans. GSF makes 16 cell esc's now that have a 1.5 amps bec, so the missing building block is here now too. The future is bright! Dean in Milwaukee 





Low Kv is slightly more efficient...
Hi!
I'm wrapping up a comprehensive set of tests (will publish soon) using the GWS EDF75 fan unit. I tried three motors:  Speed 400 4.8V "race" (5 cells, 14A max)  Stock EDF75 (Speed 400 6.0V, I think) (8 cells, 9.4A max)  Speed 400 7.2V (10 cells, 7.5A max) With the same number of Watts in, the 7.2V motor produced the most RPM (about 5%) Now keep in mind that a 5% gain in RPM is more than a 5% gain in efficiency (exponential load as RPM increases) This result was the reverse of my expectations, since I had hoped that the 4.8V motor would offer more RPM... (it's around 13A 60s into the run, quite high) As alluded to by others, I had hoped to use five 4/5FAUP cells to run 2 parallel motors. But, there's really not much point doing this, I can have 10 KAN's at nearly the same weight and get more RPM! An interesting exercise though... 





Thanks for your input. I agree that lower kv, higher voltage => more efficiency. In this case however, as mkirsch1 stated, the voltage will be the same. So I think i can conclude that the higher kv motor will be the most efficient. I might buy the lower kv anyway, since it will be more useful with the 3s etechs in other applications, should i ever get tired of the hornet. I hope not, since it will cost me about 700$ for all the stuff






Quote:
Marc 






" Speed 400 4.8V "race" (5 cells, 14A max)
 Stock EDF75 (Speed 400 6.0V, I think) (8 cells, 9.4A max)  Speed 400 7.2V (10 cells, 7.5A max) With the same number of Watts in, the 7.2V motor produced the most RPM (about 5%) Now keep in mind that a 5% gain in RPM is more than a 5% gain in efficiency (exponential load as RPM increases)" Very interesting! The 5% increase works like this: 1.05% x 1.05% x 1.05% = 1.158 or about a 16% power increase, with no extra watts. Thats all efficiency baby! Also, I'm thinking a 3s2p pak of the new thunder power 1950's could run that twin edf combo very nicely at only 6 oz, and run 12 minutes wide open. What plane is this setup for? Dean in Milwaukee 





All other things being equal, it matters not a whit whether its a hot or a cool wind, provided you can adjust the volts to suit. In the end you end up with the same torque at the same RPM and the same iron losses and frictional losses. The copper losses are also the same because twice as long a piece of wire at half the cross sectional area gets you twice as many turns and doubles the torque for the same current, so you halve the current and double the volts and end up exactly where you started
Ultimate achieveable efficiency of the MOTOR is down to magnetic design, brush design and frictional losses. HOWEVER if you cannot infinitely vary the pack voltage, sometimes it makes sense to go for one wind over another. 5% variation is well withing production tolerences for ANY motor of the can persuasion, so I wouldn't place too much faith in that... It makes a little more sense to go for lower Kv with your average speed controller though, since these tend to have a fixed resistive loss. Although the MOTOR efficiency wasn't much different, when I looked at 2s/23s packs on speed 400's the 7.2v motor on 3s scored a tad higher  mainly because the lower currents didn't lose so much on the controller and cells. It was of course a shade HEAVIER due to the extra cell. However I couldn';t go down in cell rating cos it didn't fit what the manufacturers make. It also strains the BEC circuit to the limit, and loses you a watt or so extra there... Ultimately best efficiency is by revving the motor to the brush limit. Its probably kinder to do that with a few more volts and a bit less current. Then you up the load until the copper losses get to equal frictional losses (I think) and thats the best efficiency point. Lithiums make it a lot easier to get up to 10V plus. Having been on the point of ditching my 7.2v 400, I can now see a new lease of life Final point. Race/non race is irrelevant. You are optimising around what packs and controllers gearboxes and props you have. Motocalc is your friend. Just plot the watts per pound output for various combibnations and see for yourself. 





Quote:
Assuming I=1, R=1 gets P=1*1*1=1 (lets say watt) So doubling R and cutting I in half means; P = 2*(1/2)(1/2) = 1/2 (watts) So you cut heat losses P in half. Am i wrong? //Nicke 






As a side note, I chose the higher Kv motor, and had lots of flying fun before i turned down the gyro gain and lost control and crashed...
Im thinging about reviving it since I've gotten a full time job. That is needed if you want to keep your heli flying However, this time I'll power it with a cdmotor. Im planning for few turns of thick wire, for a high Kv 


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