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
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
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
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
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).
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.
of the motor derived from the no-load measurements:
- ri=0.050 Ohm
- Io=1.2 A at 9V
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
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.
Stator Length = 12 mm
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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