Electric motor power comparison

[mickvk]

I have viewed a bunch of information about electric motors. I always found it hard to understand which ones were better, worse, faster, slower, stronger or weaker based on kV ratings. Nobody ever lists power! After a while I started comparing max watts - so I converted watts to HP and graphed it for common boat motors.. This may not be a perfect science but at least it gives some relative view of how a Neu 1515 compares to a Feigao XL. I also threw in a reference to nitros since I had it handy. Interesting, huh?

http://www.krocko.com/ftp_bucket/forum_image_upload/HP-chart2.jpg

[Ub Hauled]

nice graphs, thanks for your time in developing this.
One question though, are you posting results for the NEU motors based on 80% efficiency as well?
NEU motors have an efficiency higher then 80%, some of them go over 90% (as per FlyDMA), the lowest efficiency I've found was 85% (as per FlyDMA)

[NitroVal]

I think a stereo guy explained it best to my as to why a cap is almost always needed in a stereo system and how you can almost never judge a speaker by its advertised ratings. He said electricity from any power source is delivered "dirty", meaning that there is never a constant or steady flow. It is always fluctuating with temp, conductivity of wire, surges, etc.. That could be my only guess as to why there really is not a whole lot of HP ratings on electric motors.

[mickvk]

nice graphs, thanks for your time in developing this.
One question though, are you posting results for the NEU motors based on 80% efficiency as well?
NEU motors have an efficiency higher then 80%, some of them go over 90% (as per FlyDMA), the lowest efficiency I've found was 85% (as per FlyDMA)

Yeah, I did all the continuous and peak watts times 80%. Sorry.

[sailr]

I have a real problem with using WATTS as a good comparison. WATTS is a moving target. Watts is Volts X amps. If you change the amps or the volts, you change the watts. If the motor is not very efficient or if you have a draggy driveshaft, too big prop, etc. etc. etc. then the WATTS will go up and be wasted as heat. A lot of companies who advertise watts just multiply the max volts times the max amps the motor can take. That is totally unrealistic.

I learned this a long time ago in the brushless aircraft world. Far more important than watts is THRUST. The actual work the motor will do.

Just my opinion guys.

[Steven Vaccaro]

Nice graph. How did you get the numbers on the feigao motors?

[mickvk]

I have a real problem with using WATTS as a good comparison. WATTS is a moving target. Watts is Volts X amps. If you change the amps or the volts, you change the watts. If the motor is not very efficient or if you have a draggy driveshaft, too big prop, etc. etc. etc. then the WATTS will go up and be wasted as heat. A lot of companies who advertise watts just multiply the max volts times the max amps the motor can take. That is totally unrealistic.

I learned this a long time ago in the brushless aircraft world. Far more important than watts is THRUST. The actual work the motor will do.

Just my opinion guys.

What up Sail... Yeah, it's not perfect, but to me the max efficiency amp rating is useful as a rough way to do a relative comparison. For example, in my first boat I got a 9L in a 32" Delta Force that over heated like crazy. I was way over the 68A max eff line. Now I have a better understanding why. And yes, you are absolutely right, we can't always work at the voltage where "max" is measured at, so yeah, it's not a dyno. But it shows me if a motor is just a little different than another or very different...

[domwilson]

This is interesting. But I am curious, how did you arrive at these numbers? Was this thru actual testing?

[mickvk]

Nice graph. How did you get the numbers on the feigao motors?


That's a good question... I forgot I had to derive that. Since they rate Max Efficiency Amps, kV and RPMs, you can take max RPMS - 50,000 and divide by the kV. For a Feiago 540 9L for example, that's 50,000RPM / 2713kV = 18V. Multiply that times Max Eff Amps of 68 and you get 1224 watts. That's close to another place I had that series of motors listed at 1300 max. Then 80% of that for heat, then divided by 745 watts per HP, you finally arrive at 1.3 HP.

I don't really mean for this to be anything official, just my swag at understanding what's relative across motor choices.

I'd appreciate any input.

[mickvk]

I have a real problem with using WATTS as a good comparison. WATTS is a moving target. Watts is Volts X amps. If you change the amps or the volts, you change the watts. If the motor is not very efficient or if you have a draggy driveshaft, too big prop, etc. etc. etc. then the WATTS will go up and be wasted as heat. A lot of companies who advertise watts just multiply the max volts times the max amps the motor can take. That is totally unrealistic.

I learned this a long time ago in the brushless aircraft world. Far more important than watts is THRUST. The actual work the motor will do.

Just my opinion guys.

But motors dont make thrust, propellers do, so I guess that would quantify an entire setup.

I have no idea idea how thrust would be derived or measured though, do you?

[ozzie-crawl]

i have access to a proper rc dyno, and hopfully over christmas will get some testing done
the dyno is for 2-4 wheel drive,will do 1/5 -1/10 scale
but it also has the capacity to bolt the motor straight up with out it being in a vehicle
will be testing a few diffrent electric motors in vehicles and straight bolt up :buttrock:

[questtek]

A test system for actual boat thrust I have used is a digital fish scale attached to my dock and a line at the CG of the boat thru a Y harness. Since I also have an Eagle Tree set up I can work the fish cale registered digital force against RPM, amps, volts, watts, etc. Granted this is a static test but it simulated to the best of my knowledge, true lbs of forward force. The digital fish scales go to 50 lbs and are only about $15 at WalMart. When I was expermenting with larged ducted fans on cats this test set up proved very valuable. Joe

[ozzie-crawl]

thats the problem with boats static testing is to inaccurate for real world applications as there is to many things come into play
if you test 2 diffrent motors and one is producing more watts than they other can meen bugger all when actully running the boat
if you data log a run and you see your producing x amount of max watts this really meens little in performance as max watts is produced when the motor is under max load,usally under take off or accelerating out of a corner etc. when the boat is actaully running full speed the amps drawn hence watts produced is a hell of a lot less
so a boat producing 1500 watts or around 2 hp under load may only produce 800 watts or just over 1 hp when running at full speed
just my thoughts

[sailr]

On a boat, no, but it's real easy on an airplane. A digital fish scale secured to the tail with a stake in the ground and let her rip. You try different props until you hit on the right amp draw for the motor/battery combination. You just read the pounds and oz. of thrust on the digital fish scale.


But motors dont make thrust, propellers do, so I guess that would quantify an entire setup.

I have no idea idea how thrust would be derived or measured though, do you?