who will NOT build it?
by Peter Rother, Aug.2001
An fantastic idea has been born! This ideal motor has much
higher torque, than everything seen before. But much more important:
you can build this engine at home, so to speak, in the corner
of your living room. Ludwig Retzbach, German professional author
on electric gear for the rc-modeling, has presented this motor
in the German periodic electroModell, 4/2000 and 1/2001 (please read it before you continue
here). After these two articles many people have built number
of engines, sometimes with modification, sometimes without. All
this realizations have proven, that this motor isn't that difficult
to build and that the achievable results are fantastic. This
was only question of time:
When will I build my own strong
300-500 watts motor for my electric glider?
We have discussion on this motor all the time in German rconline
. You should know this Links
, if you want to deal with LRK-bossiness.
One word about naming: LRK comes from Lucas,
the authors. LRK350-20-15 mean diameter 35.0 mm, stator
length 20 mm and 15 windings per slot. Torquemax
means an engine with extreme strong torque.
For sure, you can buy everything
for money today. Plettenberg and Actro are selling outside running
brush less motors. But, ....it was my dream to build my own motor,
that will run a good size prop 14" to 17" without need
of any gear box - Wow! By the way, I don't like this gear-box-sound
and the energy waste in it either.
I've talked to my friend and
rc-pilot Jochen about this motor 3 weeks ago. I gave him all
literature on it and send him all links. A day later the decision
has been made. We are in business. We'll design our own Torquemax.
It should be very robust for years of service to come and run
400 W (short period 600 W) with easy. The most important thing
for Jochen was: it must go into fuselage. The appearance of his
nice Semi-Scale Models can't be disturb by an outside-fuselage
sitting motor (as original LRK).
While the rotator
with magnets resides behind in the fuselage, the rotating shaft
must go through the stator
to the front. The shaft will
be very hard steel with diameter of 5 mm (0.2"). Both
ball bearings will be placed as far from each other, as possible,
much further, than in original design (much higher mechanical
stability, less vibration). The rear ball bearing is bigger (16x5x5 mm)
and front one smaller (11x5x4 mm).
One more goal was to be able
to take apart the whole engine. This was the reason to design
the stator with thread joint. All parts can be replaced.
These nice parts were manufactured
in a masterly way (thank you Jochen). Every day we were discussing
and changing some details. There was continues fight between
be robust and light-weight. Despite very strong and heavy ball
bearings we have achieved an weight of 185 g,
before winding. Here is the weight table and dimension.
Here some photos before winding:
Here after winding with Schematic:
The calculation for the first motor was done to achieve about
10-20% slower running motor. I wanted to run target props of
about 14x8 but also wanted to try bigger props with better efficiency.
The LRK motor is very strong from slow
rpm up to high rpm. The maximum is very broad. I
am using 2-pole inner-runner AVEOX1010/2w with 4.4:1 gear box
on 14x9" prop and a very fast inner-runner from MAXON with
14:1 gear box on 18x11 (endurance flight). Therefore I was interested
in comparison between these commercial good engines and this
new design. The huge advantage of LRK is, that you don't need
gear box. You save therefore 8-15%.
controller, self made
The was only one controller for
this beautiful engine: SPEEDY-BL from Jo Aichinger , distributed by the shipping
company Megra es a kit. Why? I wanted to test this
circuitry, but also wanted to be able to change the angle of
ignition. Three other
commercial BL-controller were used to drive my motor.
Here ready-to-go combination
LRK350-20-15 and SPEEDY-BL
nice thing about such project is, that you can measure your motor
with relative simple tools: ampermeter, voltmeter and revolution
After completing the motor I was measuring with these simple
tools all the day.
Later on I'll show how to measure such engine with 14-bit
self-made test equipment. The Aeronaut company supplies all props
with n100w numbers, so we can calculate, how much power at given
revolution this props takes.
I've measured voltage with 0.1% accuracy and current over
2.5mOhm shunt resistor (4-point-measurement). The revolution
has been measured as frequency of MOSFET-Gate drive. The advantage
of this method is much higher accuracy than the classic optical
prop method. The normal optical revolution counter has an accuracy
of 100rpm, in best case of 10rpm. It easy to measure MOSFET-Gate
frequency within 1 Hz. I've measured this frequency of about
400 Hz within 0.4 Hz.
The whole measurements are
to big to be posted here. For each propeller up to 8 working
points were measured and the specific revolution/volt calculated.
@ 25 to 30A, about 300W
The motor was operating very
cool, even after minutes of service at 300 watts. This engine
was much cooler than Ultra 1310-10 at the same torture. The whole
work was done with single goal: to determine windings for two
engines, one for 8 cells and one for 10 cells NiCd at 300 watts
with 14x8 prop.
Example, Excel spread sheet
Here you can download all spread sheets with
tables and 9 diagrams. The angle of ignition has been vary and
efficiency observed. Here
you can download program for prop selection.
Thrust has been measured in 3.6kg ASW24 (span 3.2m) containing
LRK350-20-15 or an Ultra 1300-10. Prop was Aeronaut CAMCarbon
15x8. Battery 10 cells 2400mAh. Diagramme und Tabelle hier.
What comes next?
We are planning to build few
different types of this engines. Construction is not finished
yet. At the end we hope to have a good design, that can be duplicated
Who are we?
Jochen was studying precision mechanics some years ago. He is
working on IC testing equipment at Agilent Technologies today.
18 years ago he was designing combustion engines for rc-modeling.
Today he is flying only electric gliders and normal gliders.
Both semi scale. He is also working with passion in 3-D CAD systems.
All work on milling-machine has been done by him.
Peter is working in the same
company, but in a different division. He designs measurement
equipment for the last 20 years, specially for medical, chemical
and internet area. He has worked for NASA on Jupiter Orbiter
"Galileo" years ago and for the Hewlett-Packard Labs
in California. He likes very much to design, construct and measure.
But he likes also to bike or to chat in rco .