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Old 01-12-2021, 11:29 AM
Brushless motor guy
Join Date: Jan 2021
Location: Kawerau, Bay of Plenty, New Zealand
Posts: 5
Smile Instructions on brushless motor design.

This is a question, not a "How-to" - ? should be in the title.

I read the "Fly Electric" pages on DIY brushless motors, by David Theunissen, and I would like to find out more:
1) what are the "hot" cores for 550 and 2000 watt high speed motors currently?
2) how do you wind them to get the correct KV amps and watts?
I want to use 48 volts for now.
I would suppose a 550 w motor would need a 75mm drum, and a 2000 w motor would need about a 120mm drum.
Is DT still contactable?
Who knows the theory, or can point me to simple winding designs?

No-one seems to make motors for these 2 cases:

8x5 prop at 23,000 rpm, and 14x6 prop at 14,000 rpm.
This is using composite sports props.

If you have seen commercial motors that fit the specs below, please let me know.

I have a current application for the 8x5 props-using 2 of them on my current model, to get a vertical launch capability- and rather ballistic performance
requiring gyro nannies! Span is 44 inches and weight is 1.6 kgs planned. This is a belly launch VTOL, no undercarriage needed.
Good for a first RC plane! It will have all the learner nannies turned on (Ardupilot) and control sensitivity way down...

I had to shelve my current motors after I found out how to create dyno charts for them-they were just too small (BH 8208 1900KV)
-245 actual w -rated vs 550 w needed.

I had to derive a rating scheme based on W and A limits, plus their "ragged edge" benchmark test, plus motor similarity considerations.

The 14x5 is for a future proposed FW 190 lookalike stunt plane, also needing gyro stabilisation, otherwise you get lots of unplanned loops when you gun the motor!
Span is 50 inches, and it is based on 1:7.65 scale, with Lavochkin and Seversky addon mods, and a reduced wing aspect ratio.

This is to have undergear retracts, but no flaps. It should be practically impossible to stall on landing. It could also do a full vtol tail-dragger bellyflop landing (on grass!).

It should still do around 15 oz/sq ft loading. Weight may be over the 2 kg mark, due to the big motor and large batteries needed. It should generate over 5 kgs static thrust though.

I have drawn up suitable KV specs for each:

1) 8 inch x 5 inch propeller, 23,000 rpm.
This needs 16V, 1800KV, 550w, 34A rated spec.

2) 14 inch x 6 inch propeller, 14,000 rpm.
This needs 48V, 400KV, 2000w, 80A rated spec.

or 16v 1300KV 2000W , 240 A (based on 6x345w motors.
obviously, multiple batteries are needed there - maybe 3x 3000 mAh 4S 30C.
It shouldn't need to use full throttle much , as that gives 6000gf for a 2500g model.

Both motors are rated at 77% of the KV speed, and maximum load at 66% of the KV speed.

This loading scheme seems to be standard for model aeroplane brushless motors.

Cooling is from natural air flow from the prop end, from 20 m/s up to 44 m/s.

Thanks in advance for your replies.

<edit> - I have fund the sub-forum, thanks, and have a big list of resources to read. I will come back to you with more questions.

Looking at 12S batteries-can only find 1: gens ace 5000 mAh 45C, 134.99 euro!!

Anyone heard of any 1800 KV 550 w motors?
Here is one for consideration:
Spitz 3536-06 680w max, 50A max, 1300KV, props 8x4 and 9x6., 15V (4S)

1) that seems an odd prop selection? why not 8x5 or 9x5 ??

2) the max watts looks suspicious - usually max watts are 3 or more times the rated W.

3) the max amps also look suspicious - I estimated 6S 30 amps, max = 1.3 x normal = 39, x 24/16 = 58 amps.
now divide 50 /1.3 = 38 A
then expected w = 38 x 16 / 1.3 to allow for power factor-ac
=467w - too low - want 550w - what do you think of my calculations here?
An alternative is tying multiple quadcopter motors together with small toothed belts. this gives an assembly 68mm wide, which is a little clunky?

-maybe suitable for the "radial" engine model. The assembly is about 120mm in diameter, and you can gang up 6 motors, giving 2000 w static load.
I can use proper 1300KV 4S quadcopter motors. It needs a center spindle with ballraces, drive collar, thread, and spinner nut.
I will spec out the little belt drives, and check the center hole diameter in the 14 inch prop.

This total assembly weight is likely to be about 400g.-comparable to any 2000w worth of 150 KV 6s motors, which are too slow!

This may not be a sensible way of getting to 550w, 1300kv, 4sS, though.
<edit>correction-50 m/s belt speed is OK, so 20,000 rpm at 1 inch dia is good.
<edit> 13-1-21 11:17 pm local-corrected the motor specs.

Last edited by OwenB; 01-13-2021 at 11:17 AM. Reason: further info
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