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        Small LRK outrunner

#1 mcross Dec 26, 2003 07:44 AM

Small LRK outrunner
1 Attachment(s)
It runs but I need to tune my controller for more consistant startup and smoother running. But first I need to make a test rig to test these small motors.
16 Turns of 34 AWG, 2 ball bearings, 14 magnets, Weights 3.9 grams, turns 20,000 RPM no load. No thrust measurments yet, I may mount a prop over the weekend and do RPM with diferent props.

#2 mcross Dec 26, 2003 07:45 AM

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Back of motor

#3 mcross Dec 26, 2003 07:46 AM

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#4 mcross Dec 26, 2003 07:48 AM

1 Attachment(s)
Stator came from a very old and very small universal motor. I have half of the stack left.

#5 Gordon Johnson Dec 26, 2003 08:30 AM

Excellent work. Did you machine the bell, or is that the one that came with it? Weight is nice and low. What is the diameter and thickness of the stator?


#6 mcross Dec 26, 2003 08:45 AM

I had to machine all the parts except the stator stack. I have one of those mini lathes from Harbor Freight. The original motor had windings on the stator and rotor so I could only use the rotor. It weights 3.9 grams.

#7 clipclop Dec 26, 2003 09:39 AM

Looks good , nice n light to.

#8 mmormota Dec 26, 2003 10:14 AM

Higher turn numbers (30?) may help the controller keeping in synchron.
The more conventional "windings on all teeth" scheme should improve the parameters too.

#9 mcross Dec 27, 2003 09:08 AM

I don't have room for more turns unless I go down in wire size. I really was hoping to get a slow motor so I don't have to gear it. I think that a conventional winding may spin too fast. My controller is homebuilt with a Philips TDA5144 chip so I can tune the start and running timeing, if that is what it needs.
I would like to try it on a Pheonix 10 but mine came defective and doesn't even run a known good motor. The LED stays on and nothing happens. I guess I'll have to send it back.
This motor will be sitting for a little while so I can make a thrust testing rig that works with these small motors.

#10 mmormota Dec 27, 2003 01:19 PM

The more the turns the lower the Kv and rpm. 15 turns only on such a small stator means a very high Kv and rpm. There is a trade of course, the more the turns the thinner the wire.

The controllers are not too happy if the emf voltage is low, and lrk winding is a risk factor too.

I am not absolutely sure, but think so that this winding
is similar to the lrk in the (mechanical rpm)/(electrical rpm) value. There are several manufacturers of so called lrk, but most if not all of them producing motors with windings on all of the teeth. They do it for better efficiency and less controller misfires.

#11 mmormota Dec 27, 2003 01:33 PM

The thrust measurement is rather easy with an electronic scale. There is a small trap: if the propeller wind hits the scale, as high as 25% error is possible.

There is a picture of a simple torque measuring device (made by Takao Shimizu) in the Power systems/ One hour CDRom... topic, it should be interesting if you are about to measure efficiency too.

#12 billystiltner Dec 27, 2003 07:29 PM

There are propulsion test stand plans here.
I built mine smaller than dimensions on plans for small motors.
I also built this balance.
I use paper of known density (printer paper) to calibrate.
I guess I should get some standard weights for calibration.

Gordon Johnson uses a similar propulsion test stand which is simpler. He only uses the peice with the L on a pivot.
The one I made has a string on top of the L that goes to another part that has a pivot point which holds the motor. I think Gordon's version will have less room for error. The only critical dimension in the test stand design is that the L should have both the vertical part and horizointal part the same length. I have seen designs for test stands that use deflection of a rod to measure thrust and also ones that you connect a string to the tail of a plane to get thrust.

Nice job on the LRK.


#13 mcross Dec 27, 2003 07:59 PM

I don't have an electronic scale but I do have some very sensative pressure sensors, some displacement sensors and some strain gauges. My old test rig measured volts, amps, RPM and thrust on a PC, I think I'll resurrect that and replace the old thrust measurement part. I had a spring and displacement sensor on that but the piviot and spring could not be made to work on thrusts lower than about 25 grams. I'll start a new thead on it if anyone is interested.

#14 billystiltner Dec 27, 2003 09:07 PM

Maybe you could mechanically multiply the thrust x 10 to get 2.5g.
I'm interested in homemade electronic scale and thrust measurement using a PC. Sounds like an interesting project.


#15 Gordon Johnson Dec 27, 2003 11:43 PM

The arms in the "L" do not need to be the same length. I they are not you simply multiply the force measured by the ratio of the lengths of the arms.

I really don't think it matters which design you use, strain, connected with a string, "L-arm", etc. In most cases the conclusions we draw will not be tremendously affected by a small error in the measurment of grams of thrust. My first thrust stand was made from Tinker Toys and gave results almost identical to my current ball bearing pivoting stand.

In the case of motor development, the static thrust developed is an incidental by product. We are simply putting a load on the motor. Along the way we get some idea how much thrust we might reasonably expect. But, if we really want to know and optimize thrust, that's more a question about the prop and then a wind tunnel is the appropriate place to test.


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