I need help with brushless motor theory

[Goldenhour]

I desperately need to study how to design a proper power plant, gearing motor, power, head speed etc.
For instance, I have been running a copter from another mans design just fine. 10s, 740kv motor, 164/11,1800 head speed in governor mode.
I now need to lower my head speed to 1400rpm(620mm asymmetrical spinblades) and am at a loss on what motor to go to. I ran the blades at 1800rpm and almost amped out my nice set of batteries.

[Andy01]

If I am reading your post correctly, you are saying that your headspeed is;

740kV x (10 x 3.7V) x 0.9 (efficiency) x 11T pinion / 164T main gear ?

If so, the maximum sustainable headspeed (ie. towards the end of a flight, not with a fresh lipo) would be 1653rpm, so you would not be able to govern to 1800rpm I don't think - at least Castle ESCs are "limited" to governing up to 95%, which in your case would be around 1570rpm.

Are you sure the 11T pinion is correct ? It seems very small for a 600 heli, especially if running a 6mm shaft. Or, you are running a 115T main gear (as per your signature), not a 164T. If 115T, then the 100% headspeed would be 2357rpm, so if running 1800rpm, you are already governing to 76%, which is about as low as you would want to go. The only option I suspect would be either change back to 164T main gear, or get a slower motor.

IMO, your setup (if correct) would be 100% at 1650rpm, so governing to 1400rpm would be easy as it is around 85% governor, which should be fine for most ESCs.

Colin

[Goldenhour]

I am using a KDE 164 tooth main. I went from 13 to 11 pinion to increase motor speed at 1800 rpm.
What I want to know is this....from what I hear you need at least 30% on either end of your throttle for governing.
Is there a point at which a motor becomes hot or ineffecient due to low speed? I know with a car engine the power band is at about 50-80%, with my rice burner it is anyway. Car engine lugs at low rpm.
If you don't have a motor but you want to find the right one to put in do you go backwards from headspeed then gearing and then motor or the other way around?

[ahahn]

First you target your headspeed.

Nominally you want an outrunner motor to be spinning at ~80% of its no-load rpm (=Kv*Volts). That's just where the motor tends to run efficiently and at the same time actually produce some power.

To get the overhead you need when actually maneuvering the heli (not sure what you actually are doing while flying), you need to give yourself at least 10-20% of a voltage margin. Since you want that margin near the end of the flight too, that means you need to use less than the 4.2V/cell a lipo puts out. Castle Creations uses 3.7V per cell. What this means is that you should assume a voltage level that gives you enough overhead to be 85%*3.7V to use in your calculation, or 3.1V. Before anyone panics, this just means you are running a 85% throttle on the ESC, which is nominally consistent with the 3.1V that will give you the overhead you need at the END of the flight.

So putting things together.

1) Assume a voltage of 3.1V/cell to give you the overhead on throttle.
2) Assume the loaded motor rpm is 0.8 Kv*#Cells*3.1V
3) Head rpm is just this motor rpm/gearing ratios.

So starting at 3) and working backwards will give you Kv*# cells. You can decide either Kv or # cells, but once one is chosen, the other is more or less determined.

But that still isn't your motor. You now need to size it. What I would do is to look at the size of motor typically used in that sized heli. Once you do that, you just need to find a compatible Kv or # cells to actually be able to buy something from the motors in the marketplace.

Anyway, that's how I would go about it.

[Andy01]

My take on it is this;

An electric motor is entirely different to an IC engine, so forget any comparison with a car engine. It is my understanding that an electric motor develops its full torque at almost any rpm, which is very different to the torque curve on an IC engine.

Electric motors (especially the larger 600 and 700 sized ones) tend to have max efficiency of around 90% (some up to 94%) at about 90% of headspeed, but realisitically it would not matter much if you ran it at 70%. It is more likely that you would get heat issues with your ESC if running it at low throttle %s.

If you are running scale (an assumption based on the low headspeed and asymmetrical blades), then your motor is very lightly loaded anyway because scale flight uses very little power compared to 3D. Typically a 600 sized motor would be capable of close to 3000W, and most scale flying would use less than 1000W, so your lipos should never "amp out". The amps should be around 1000W / 37V = 27A or less with the odd short peak above this to perhaps 40A.

The range that Castle uses is roughly - don't govern below 70% of the "maximum" headspeed as per my calculation above, and don't govern above 95% of that "maximum" either. Thus your minimum headroom is 5%.

For scale flying or light circuits, you could probably get away with a little less than the 5% and run with the Castle "warning" up, whereas with hard 3D, you might find that you want a little more than 5%.

As said above, do not assume 4.2V per cell when working out a governing headspeed. The lipos will be at 4.2V for a few seconds only, and start dropping quite quickly - check out your Castle log to confirm.

With the 11T & 164T there is no way you will sustain 1800rpm.

Even using 4.2V per cell (ie. 42V for 10s);

740kV x 42V x 0.9 x 11T /164T = 1876rpm, so 95% of this (the max that Castlelink will "allow" without a warning) is 1782rpm

Have you set up the gearing correctly in Castlelink ? Is this a 2 blade or 4 blade ? If it is 2 blade, 1400rpm seems pretty slow for a 600 (even scale). If it is a 4 blade, you may find you could run a little slower than 1400rpm with 620mm asymmetrical blades (I run my 4 blade Seahawk at 1250rpm). One problem you may find with low headspeed is tail authority (or lack of).

Another warning is about the 11T pinion if it is a 6mm shaft size. I have never found a 6mm shaft Mod 0.7 pinion smaller than 12T, and had a 12T shatter on me. There are at least two reports in this forum of 12T 6mm pinions shattering. The problem with the really small pinions with a big hole up the middle is that there is very little meat between the root of the tooth and the hole.

If you want to run at 1400rpm with the motor lipos and gearing that you have then,

740kV x 37V x 0.9 x 11T / 164T = 1652rpm at 100%, so to get 1400rpm you would be governing at around 85%, which is fine. Your range for effective governing is between 1240rpm (75%) and 1570rpm (95%). Outside of this your ESC may struggle - too low and it may heat up a bit, and too high and it may not maintain the chosen speed.

Colin

[Goldenhour]

According to my eagletree data head and motor speed are constant. I use a 120HV Kontronik. I took the Spinblades off and put back on the Rotortech 620's until I figure out a motor replacement. For the Spinblades(asymmetrical wide chord) I use an asymmetrical tail blade that is wide chord as well for tail authority.
My motor is a Scorpion 4025-740 powered by 10s 5000mAh batteries. I use a KDE 164 tooth main gear. I'm pretty sure the pinion is 11 tooth.
This is a screenshot of my headspeed.

[misskimo]

Hello, What I would do if you are flying smooth smack 3d. Lower your voltage with same setup. With the setup you have. The way I figure it isn't 3.7 volts. This # with the 65 C is on the low side of the average voltage. Alot of people don't count this into play, and it don't fall under the same rule with different people's helis or gearing.
At 100% wide open throttle , your max rpms on the head will be around 2000 with 10s Now you drop the voltage to 8s 5000s Your max 100% rpms will be 1600. Drop the throttle gov down to match your wanted rpms to 1400.
Or. Get another motor with a lower Kv. A 450 kv on 10s for fair power running at 80%. That's where I like to run my kontronics.

Tony

[Goldenhour]

I use my 450 for fun, my 600 is for the beautiful pictures. With the set up I have now my throttle setting on my Futaba 10 chg is 40. I run the Kontronik in governor mode at 1800 headspeed. I was running the numbers on a 430kv motor and the head speed would be right(11-1400) using a few different pinion sizes.

[Andy01]

It may be worth double checking the number of teeth on your pinion.

Unless your system is running an efficiency that is more than 100%, I am struggling to see how your log could show a headspeed above 2180rpm (a short peak in your graph), even with a fresh lipo.

740kV x 42V x 1.0 (efficiency at 100%) x 11T /164T = 2084rpm at 100% throttle.

The only explanation is that it is not a 740kV motor, but perhaps an 890kV motor ?

I do not know how the Kontroniks governor works (or a Futaba Tx for that matter), so I have no idea what the 40% throttle curve means. I cannot imagine that the system is actually running at 40% throttle to achieve 1800rpm as it would imply 4500rpm at 100% (assuming it is linear), which would require a 30T pinion .

I assume from your "beautiful pictures" comment that this is an AP rig ? I would not be doing "smooth smack 3D" with asymmetrical blades I don't think - negative pitch moves could be interesting .

A 430kV motor on 10s would need a decent size pinion I think (or 12s lipos);

430 x 37 x 0.9 x 18T / 164T = 1571rpm, so governing to 1400rpm would be easy at 89% throttle (with a 18T pinion). A better choice might be a 550kV 4025, as it will be a more mid-range pinion (like a 14T). Another option might be something like an Xera 4030-470kV - I have two and they are quieter and argueably more efficient than a Scorpion 4025-550 I tried previously.

I still cannot understand why your current setup (assuming it is a 11T pinion and the 1800rpm is incorrect) can't be governed to 1400rpm, especially with a Kontroniks ESC, which I am told can "freewheel" at lower throttle settings.

If it is not a 11T pinion, but something larger (a 13T would reach 2180rpm with a fresh lipo), then you may be up for a smaller pinion or slower motor.

Anyway the calcs are there to be used - good luck with your setup.

Colin

[misskimo]

3.7 again. Yes that is nominal voltage or average, Yes. 100% throttle would be the headspeed i listed above with a fresh pack for the first min.
I been using this method since 2000 .
C rating change the first # though. I use 4 volts to figure out my 100% wide open throttle with 65c batteries. 35c would be a tad lower.
4 X no of cells X kv divided by ratio. X .94 for a kontronics esc
CC , schulze Dont X the .94.
Also. Did to calbrate your radio to the esc? That sounds like you didnt. Around 62 to 65 % throttle will be wide open

[Goldenhour]

Miskimo, I set the Futaba 10chg Tx with the Jive 120 to start motor at about 1.5% of full throttle. Is there another calibration procedure I'm not aware of?