Real facts about the LRK (outrunner motor) - RC Groups
Nov 04, 2002, 11:53 AM
Registered User

# Real facts about the LRK (outrunner motor)

Here are my comments.
I am an electrical engineer, but my expertise in electrical motors is very limited. One of the few things I remember from the electric motors course will be mentioned later. I tried nevertheless to understand what makes the LRK so torquey and this was pretty easy and unexpected. The same old principles apply and the huge torque difference comes from two simple sources.
1. The induction in the LRK is assumed to be 1T as opposed to 0.8T in the classic motor. This 1/0.8=1.25 factor comes into play twice and this makes for a 1.56 magnifying factor.
2. The diameter of the LRK is 35 mm versus 18 mm for the classic motor. So we have here a 35/20=1.94 factor. This also is taken into account twice and therefore we get a 3.78 factor.
Now combine the two and here is the magic number:
1.56x3.78=5.89 higher torque for the LRK

This comparison is not fair. If the same magnets are used and if size is the same, then there is no advantage to the LRK-outrunner motor.

What is interesting about the LRK is the slippage between the rotating magnet and the rotating field. This makes it show up like a gearing effect that comes for free and is within the motor. Now I might not have a good grasp of these things, but I feel the slippage is not the most desirable thing. What I remember from school is that whenever high power and high efficiency is needed, the 3-phase synchronous motor is the one and only choice. One drawback is that these motors need other means (motors) to be started and brought to the synchronism speed and then they can run on their own. The motors used in RC do not need to deliver hundreds of kilowatts, but the same principles should apply. And electronics can easily make them run at synchronism at any speed and load.

Please find bellow a comparison table based on the fomulae shown on the LRK motor website: http://www.aerodesign.de/peter/2001/...ng.html#Anker1
There might be some inconsistency in the formulae, but they are the ones given by the motor builder, so I have no contribution here.

S55

(image cropped and changed to jpg by Andy W)

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Nov 04, 2002, 04:11 PM
Registered User
Interesting S55 ... I think the only reasons these motors are not THE new thing (yet), is lack of understandable Info (like this - even the LRK site has limited english info), No simple way of virtual testing eg Motorcalc & Ecalc and (Flame suit on) - there's no effective US distributor.

Here is a pic from the Weston Uk website - it fascinates me but even they dont have much more info than this.

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Nov 04, 2002, 04:17 PM
homo ludens modellisticus
Some extra information.

LRK is not a make or manufacturer, it's a design. The original do-it-yourself construction articles were published in the German quarterly Elektro Modell magazine, issues 2000/4 and 2001/1-2-3, http://www.neckar-verlag.de publishers. An outrunner stator is easier to diy wind than an inrunner stator. The aim of the design is to generate as much flux (= torque) as possible for a given statorgeometry/weight, hence the fact that only half the teeth is wound. And it should be relatively easy to build of course.

The construction articles are available on the internet, in English and German, including drawings, pictures and diagrams.

Magnets now used by lrk-type motor manufacturers range between 1.1 and 1.2 Tesla, maybe 1.3Tesla already.

There's no slippage, the rotor follows the magnetic field. It's a 'synchronous' DCBL motor, ynchronous in the sense of electronically commutated. The 12pole/14(or 10) magnet configuration makes that always the coils that generate the highest torque are activated i.e. the ones with the largest angle between tooth and magnet.

Quote:
 Originally posted by Mark Sanders ... No simple way of virtual testing eg Motorcalc & Ecalc ...
According to the German forums, the motormodels used by Motocalc and Ecalc are not suited (yet?) for lrk-type motors. Wilhelm Geck's program 'Elekroantrieb' features some commercial available lrk's.

Met vriendelijke groet Ron van Sommeren
lrk homebuilt outrunner e-motor discussion group.
E-fly-in & lrk meet, June 22nd, 2003, Nijmegen, the Netherlands.
Last edited by Ron van Sommeren; Nov 04, 2002 at 04:40 PM.
 Nov 04, 2002, 04:48 PM Registered User But the real advantage of of the LRK "aussenlaufer" (outrunner) is that you can do it by yourself and classic motor can't DIY.
 Nov 04, 2002, 04:53 PM Registered User But the real advantage of of the LRK "aussenlaufer" (outrunner) is that you can do it by yourself and classic motor can't DIY. I was make 6 of them (more or less successful)
Nov 04, 2002, 05:12 PM
Registered User
Quote:
 Originally posted by Mark Sanders Here is a pic from the Weston Uk website - it fascinates me but even they dont have much more info than this.
Yep. It's resting on the ground dammit! Guess they just couldn't get enough thrust
 Nov 04, 2002, 06:42 PM Registered User There is slippage. The field rotates in steps of 60 degrees each and the magnet rotor rotates in steps of roughly 9 degrees each. This is not a synchronous motor. S55
 Nov 04, 2002, 07:03 PM Registered User My \$.02 worth, A brushless motor (R/C applications) is a stepper motor not an induction motor. An induction motor induces current into the rotor to produce magnetic poles that the rotating stator field works against. ‘Slip’ (the difference between the Stator RPM and the rotor RPM) determines the torque. The more load, the more slip, the more current induced into the rotor and the more torque. A stepper motor rotor contains magnets and requires no induced current. (For a given current) The torque is a function of the angle between the stator poles and rotor poles. The outrunner motor (As I understand) produces high torque by creating a small step angle. It does this by the number of stator poles and the rotor poles. A stepper motor with 32 rotor poles has a step angle of 7.5 degrees. O.K., that’s as far as I can go off the top of my head. If I’m wrong I’m sure someone will correct me. Here, this explains it better http://www.aerodesign.de/peter/2001/...index_eng.html Jay Last edited by Happy Hobit; Nov 04, 2002 at 07:13 PM.
Nov 04, 2002, 07:26 PM
Quote:
 Originally posted by Mark Sanders Here is a pic from the Weston Uk website - it fascinates me but even they dont have much more info than this.
Mark,

Funny you should mention it!

Just yesterday one of our clubmembers, Andy, showed up at the field with this very same combination, bought from Weston UK as package, and gave it its maiden flight.

Stunning!

A take off run of about 10 feet, then straight up into a vertical climb, rolling as it went. (Yes I know it was a maiden flight, but Andy is nothing if not an exhibitionist!). After several minutes of throttle-managed 3D aerobatics, the Cougar would still prophang convincingly.

Although we didn't time the flight, it was several minutes in length.

The few specs I have are as follows; AUW 3lb 15oz, 8x3300 NiMH, Static draw 57Amps (checked with a Wattmeter - it must unload a hell of a lot in the air!).

Dear Santa...etc...........

tim
Nov 04, 2002, 07:48 PM
Heli Bouncer
Quote:
 Originally posted by S55 There is slippage. The field rotates in steps of 60 degrees each and the magnet rotor rotates in steps of roughly 9 degrees each. This is not a synchronous motor. S55
Actually it is is a synchronous motor, as my pancake motor will actually stall and the ESC shutdown when the rotor stops matching the fields guidance. There is an excellent animation of the way an LRK works at the site that Happy Hobit posted

http://www.aerodesign.de/peter/2001/...l#Anker1164462

Looee
Nov 04, 2002, 08:14 PM
homo ludens modellisticus
Quote:
 Originally posted by S55 ... This is not a synchronous motor ...
That's why I put 'synchronous' between quotes. All (?) of the brushless motors in e-flight electronically self-commutate.

For defination's sake: there's slipping, the magnetic field rotates faster than the shaft and there's lagging, the shaft/rotor runs behind the magnetic field at a constant angle but at the same speed?

Quote:
 Originally posted by S55 ... There is slippage. The field rotates in steps of 60 degrees each and the magnet rotor rotates in steps of roughly 9 degrees each.
You're right, that was a very inaccurate remark I made: "The stator follows the magnetic field". There's always the 7:1 (or 5:1) ratio.
As the field rotates, you don't keep looking at the same tooth/magnet combination, now another tooth/magnet combination does the work, the torque-generating spot 'jumps'. Hard to describe. How about this: there's mechanical slippage (a lot!) but no electrical/magnetic slippage? Have a look at these 7 pictures of a complete cycle at 0, 8.57, 17.14, 25.71, and 34.28 and 42.85 degrees will s:
http://www.aerodesign.de/peter/2001/...tic_field.html

The first one, at zero degrees:
Last edited by Ron van Sommeren; Nov 04, 2002 at 08:43 PM.
Nov 04, 2002, 08:42 PM
homo ludens modellisticus

# Some background information

The sps (separated phase-sectors ) principle used in lrk-type motors was originally developed for magnetic levitation trains, see:
http://www.aerodesign.de/peter/2001/...0_Wroclaw.html (English)

The LRK acronym stands for Lucas, Retzbach and Kühfuss who adapted the concept for our modelling purposes. The original Elektro Modell magazine do-it-yourself lrk design was done by Christian Lucas, he designs motors for busses and armoured vehicles (www.magnetmotor.de, also in English).
 Nov 04, 2002, 09:20 PM Registered User I’m sorry Ron but I feel uncomfortable calling the Gearing designed into a stepper motor Slipping. Slipping, in an induction motor, is a function of load. More load = more slip = more induced current = more torque. The Gearing in a stepper motor is a function of design (number of stator and rotor poles). The torque output of a stepper motor, at any moment, is a function of Lagging is not Slipping. If a stepper motor starts slipping, as Looee noted, it shuts down. Gearing may not be the best term to use. It’s not mechanical gearing as we know. It does reduce the RPM of the rotor relative to the RPM of the stator while increasing the maximum torque available for a given voltage / current (by reducing the step angle). Am I beating a dead horse? If I am I’ll shut up. Jay
 Nov 04, 2002, 10:15 PM Registered User … slippage… this might better fit a trading discussion… OK, I chose the wrong word. I only meant the rotating field spins faster than the magnets. But why do stick on this aspect when the whole point is somewhere else, at least this was my intention. The LRK has higher torque because it has 1T magnets versus 0.8T and 35mm rotor versus 20 mm for the classic motor it is compared to. Use 1T magnets on a 35 mm conventional brushless and I expect to see the same torque. S55
 Nov 05, 2002, 04:24 AM Registered User Thanks Tim, Whilst these guys discuss the theorectical stuff in english and german - my visa card is twitching. This is just the sort of confirmation I wanted. Coming from Weston, it just sounded too good to be true. Now is this THE next thing ?