dyno-like charts for brushless electric motors.

[OwenN]

Is there an existing thread or forum for this?
I have been working on charting the 2806.5 1300 KV,
applying some knowhow of Kv and Kt (si units)
It looks like the test benchmark is really misleading.
It shows 48 A, 777 W in, performing like a 9x4 prop at 16, 000 rpm.
However, it seems that real rated power is more like 356W and 22.5 A, and 0.15 N-m
The test benchmark is unsustainable, and was likely run at around 72 % rpm, where rating is at 77 % rpm- right at the possible limit!
efficiency is down to 60% and power is going vertical ballistic.
I also think the 2806 efficiency may be 85% at rated, and curve off more gently than this engine I have charts for, the 875w 1800 KV motor.
I am having real trouble nailing down the basic motor equations to model the motor behavior.
Kv, Kt, torque = Kt x A, rev = power/torque, don't seem to agree with each other. I fudged it by using 1300KV for rpm and 1000 KV for
torque calculation.
The 875w motor rates at 500 w 41 A real.
I am charting up a "gang of 4" motor for the 14 x 7 prop, so it will need a litle adjustment. At 38 amps in for each motor, it is a bit off the rated area.
It also runs at 11,000 and 14,000 rpm. which is 67% and 52% revs. the 875 will blow up below 67% !
Here is a mix-up chart. with "everything" plotted! I am trying to put these charts on a more scientific footing, but I need more motor theory, and less benchmarks!

 

[PsyBorg]

https://database.rcbenchmark.com/

and please edit the language mate this is family friendly here and there are young ones present

 

[slowjo]

https://database.rcbenchmark.com/

and please edit the language mate this is family friendly here and there are young ones present

 

[OwenN]

1) My calculation problems are caused by AC power-we have a power factor! so A can be high and W low.

2) I had a little look at that benchmark database-didn't see any 2806.5 motors.
3) I now have 2 separate problems:
3a) I swapped out the 2806.5 1800KV to the 2808 1900KV, and the 2806.5 was barely managing the 9x5 prop
static-10 sec full power only!!! (now 62% and 42% rpms)

The 2808 may run well over on amps. Can I restrict amps or volts by revs using the flight controller?
Will the motor even run if I do that, or just stall, or fail to spin up when flying?
At least a test restriction seems to be in order, possibly followed by a motor/ voltage swap??

I am planning on chopping the 9 inch down to 8 inch and running in a duct, so that may speed it up a bit.

3b) My plan of running a 14x6 prop at 1300 KV, 4S, seems to have hit a disallowed rpm range.
(now 67% and 52% rpms) (FW 190 look-alike future project)

Can I get a variable pitch prop. Are they even made for rc model aircraft?
Ditto the restricted volts or amps.
Otherwise, I have to go well down on KV and rpm, then it won't go very fast at all.

It would take a lot of power to spin it close to 14,000 static. I will check how much.

I found a whole heap of chinese "pancake" motors on ali-express, so I will look through them.
a 5 kW 500 KV 6s, 250 KV 12s etc might do the trick.
Or rewind a small scooter wheel motor with less turns, thicker wire?? ie 4 inch diameter +

 

[TheFlyingBrit]

Is there an existing thread or forum for this?
I have been working on charting the 2806.5 1300 KV,
applying some knowhow of Kv and Kt (si units)
It looks like the test benchmark is really misleading.
It shows 48 A, 777 W in, performing like a 9x4 prop at 16, 000 rpm.
However, it seems that real rated power is more like 356W and 22.5 A, and 0.15 N-m
The test benchmark is unsustainable, and was likely run at around 72 % rpm, where rating is at 77 % rpm- right at the possible limit!
efficiency is down to 60% and power is going vertical ballistic.
I also think the 2806 efficiency may be 85% at rated, and curve off more gently than this engine I have charts for, the 875w 1800 KV motor.
I am having real trouble nailing down the basic motor equations to model the motor behavior.
Kv, Kt, torque = Kt x A, rev = power/torque, don't seem to agree with each other. I fudged it by using 1300KV for rpm and 1000 KV for
torque calculation.
The 875w motor rates at 500 w 41 A real.
I am charting up a "gang of 4" motor for the 14 x 7 prop, so it will need a litle adjustment. At 38 amps in for each motor, it is a bit off the rated area.
It also runs at 11,000 and 14,000 rpm. which is 67% and 52% revs. the 875 will blow up below 67% !
Here is a mix-up chart. with "everything" plotted! I am trying to put these charts on a more scientific footing, but I need more motor theory, and less
"bullshit" benchmarks! View attachment 188796
Good luck

 

[OwenN]

The 2808 1900 KV will not do the job.- I now have two motors I have no current use for...
Continuous load is 6x5 prop, 22,000 rpm, 1045 gf thrust.
I want twice that, so back to the motor search.
this is rated at 0.1 N-m, 287w, so it is way less powerful than the 2806.5 motor-about 2/3!
The 2806.8 1800KV motor is too much on the ragged edge, so I want one with about 500 w rated load.
the 2806.8 is quite likely to go "poof"!
Maybe an advance to 6S might be in order, too.?
I will check through some of the x-motor pancake motors on ali express.

 

[OwenN]

A solid spec is 8x5 prop, 23000 rpm rated, 550w, 650KV, 15A, 48 V: 12S
<edit-rechecked>
No-one seems to make one like this. it is the same performance and rev-range. as 6s 1150KV 80 A
100-190 KV motors are not rated at enough volts to give the revs. 75 volts would be 352 KV,
200 V would be 132 KV. - getting into small car territory, although those are 5 KW +

The other spec is for 14x7 prop, 14000 rpm:
48V, 2000w, 400KV, 60A
I will talk to some people- You-Tubies?

 

[OwenN]

Looking at 12S batteries-can only find 1: gens ace 5000 mAh 45C, 134.99 euro!!
I will switch back to 6S for the smaller motor. Spec is now:
6S, 1300 KV, 550 W 30A normal rating.
This is more than the 2806.5 1300 KV at about 450 w.
I need to go up a bit to get my static load into the 66% rpm area.