Little Torquemax

LRK350-12-24w for a thermal soarer

translation by Max Zuijdendorp



We dimensioned the first series of  LRK motors for 300-500 W power level. They have a 20 mm long stator and are described in LRK350-20 with rotating shaft, Design, Construction, Measurements .

In exactly the same way we have now designed and built a motor in the 200-300 W class, with a stator of 12 mm length.

The basic design has been calculated with the spreadsheet in What_winding and confirms the validity of the General Constant WL.

I wanted a new motor for my 8 cell 1,7 kg, 2,5 m soarer to replace my  Johnson HC683 with 2.8:1 gearbox. The old donkey weighs  283 g. This motor has served me well in the past. It could endure approx. 140 W, but had to be cooled after flight allright. The new motor  LRK350-12 weighs only 151 g and performs clearly better at a notably higher efficiency.

I wanted to turn a 15x8 propellor at approx. 150 W with 8 cells. The Design Spreadsheet suggested 24 windings at 0.85 mm wire diameter. Next year for the endurance competition I'll convert the motor to a star configuration to deliver only 50 W to a large propellor.

Firstly the 3 sides of the triangle were measured. They differed only 0.3 mOhm at 50 mOhm each. Not a bad figure for a slow running small motor. (The LRK350-20-11 has an ri=15 mOhm, but with only 11 windings).

No-load current

Then I measured the no-load current. I have used a DataLogger. The voltage of a power source was increased and decreased slowly. That is why some circles are above and some below the red average line.

Parameters of the motor derived from the no-load measurements:

  • ri=0.050 Ohm  
  • Io=1.2 A at 9V
  • ns=650/V

Measurements with 3 propellors

This motor was measured with 3 propellors. The graphs look like this:


For a shallow climb with 8 cells a 15x8 was used.

For a strong climb with 8 cells a 17x9 was used.

W - a parameter of the motor independent from number of windings and length of stator

In General Constant WL I calculated the constant for the LRK350-xx at 190.000-200.000 . The time has now come to prove this.

The constant  depends only on the typical construction of the motor, i.e. rotor diameter, stator lenghth, air gap etc., but not on the number of windings.

The product of no=ns(0) and number of windings N is a constant for a motor type, independent from the wind and from the wire diameter.


 no * N = W = constant


Stator Length = 12 mm

W * L= WL = constant

15360 * 12 = 184430

This figure deviates only 3% from 190000

How you can calculate the number of windings for a particular application is described in  How many windings does  my LRK-350-20 motor need? .


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