Fly Electric!

DIY Motors

At the end of 2000 Christian Lucas, Ludwig Retzbach and Emil Kuerfuss published a design of a home-made brushless motor in the German magazine elektroModell. Although commercial brushless 'outrunners' had been available for some years already in Europe (eg: Actro), these guys demonstrated how anyone with a lathe could make these motors themselves. This style of motor has now become known as 'LRK' after their names.

Things have moved on since then and the 'CD-Rom' revolution has arrived. Suddenly it became possible to rob your old PC for parts to make incredibly powerful motors, often without even a lathe. This caught my imagination. Aided by the efforts of GoBrushless.com, new components became readily available and production started...

I have a number of pages on this site to showcase this technology. On this page I list some of my more successful motors, and you can find links to more specific pages below. Although CDRom motors are fairly easy to make, expanding to larger sizes can be quite hard due to machining complexities. Having mastered these I can now build any size, fine-tune them to suit the model and fix them easily when they break. I derive great satisfaction from flying with my own motors and would not turn back now. The LRK discussion group is a good resource for more advance information.

* CD-Rom motors (easy)
* Crocodile motors (high efficiency)
* Machining tips (accurate machining)
* Winding Density (advanced advice)
* Balancing motors (advanced advice)
* Milling machine (DIY mill from a drill)


'Crocodile' motors

The 'crocodile project' has created very high efficiency motors using a special 40mm stator. My first is 10mm thick and weighs 168g (6oz). It turns an 11x7 at 7000 off 3 lipos at 30A and is 40% more efficient than an Astro 15G. My second has a 20mm stator and is 75% more efficient than an Astro 40G and equal to an Actro 40-5 with a 16x8 prop turning 7,000 rpm at 1300W. Read more here.


400W 'Floppy Drive' motor

This is my most powerful motor to date (although not as efficient as the ditto and croc motors). It turns a 9x5 prop at about 10,000 rpm. It draws 35A from 3 LiPoly's. It weighs just 138g (4.9oz). This is lighter than a feeble Speed 500 (153g/5.4oz) and more powerful than a geared Astro 15 which weighs almost twice as much (248g/8.7oz). Impressed?

Completed motor

Most of the bits for my first Floppy Drive motor

250W 'Ditto' motor

It is quite rare to saturate the iron in a stator but can happen with floppy stators (due to their large diameter and relatively long and thin teeth). A better choice for larger motors are stators from Iomega Ditto drives and other 'tape streamers'. These often have 18 fairly short teeth which makes for a very torquey motor. Some have 9 or 12 teeth. You can buy them for just 99p on ebay.

Ralph Okon is "Mr Ditto" and his site is an outstanding demonstration of what can be achieved. My best with these stators so far is a motor which turns an 11x7 at 6700rpm and draws 26A from 3 Lipos. It has similar thrust to the above motor with 2/3rds of the current. Nice!

I have used two 18 tooth ditto stators (10x36.5mm) and twenty 3x5x10mm magnets. I found that 3mm thick magnets drew 13.5% less current than 2mm magnets with almost no loss of RPM.

Motor on Speedy Bee

Experimetal winding

Double CD/'S400' can motor

This one is similar to the Axi 2208 series. It turns an 8x6 APCe at about 9,000 rpm drawing under 9A off 3 LiPoly's. It is a perfect match for my 36" Spitfire so I have not bothered trying to improve it yet.

Two GB 22.7mm stators form the core with 14 turns of 2 strands of 0.375mm wire, star. The clever part (not my idea) is the can which is made from a Speed 400 casing. This is vastly easier to create than making a bell from scratch. It is also very light which requires less need for precision. This in turn runs smoother and results in fewer balancing / resonance problems which can be a problem with DIY rotors.

I started with 24 1x5x5mm magnets in a 12 pole configuration but now use 12 1.2x5x10mm. This reduced the air gap from a gaping 0.56mm to 0.36mm and are easier to fit. I've read of people using 1.5mm thick magnets but I thought the 0.15mm air gap would be less forgiving.

Despite the fact that the Speed 400 based can is so easy to make, I was concerned that the thin wall thickness would have a detrimental effect on efficiency. It was clear from measurements with a DIY 'hall effect' sensor than considerable flux escapes through the sides. So I made my own bell with much thicker walls. I can't make a direct comparison because I used 2mm thick magnets. Higher flux results in lower rpm so the 8,000 measured at 7A came as no surprise. I would expect that if I optimised the windings for the 'proper' bell with 2mm magnets it would perform better than the simpler Speed 400 can version. However, the ease of construction of the latter has won so far!

Speed 400 can motor

Home-made bell with 2mm magnets vs Speed 400 can

'Standard' CD-Rom motor

If there is such a thing as a standard CDRom motor it is the single 22mm-ish diameter stator, standard bell, 12 1x5x5mm magnets and 20-ish turns. I use the GB 22.7mm stator and 23 turns of 0.45mm wire, star. This produces about the same power as a geared Speed 400 at 1/3rd of the weight and almost half the current. Not bad! Take a look at my CD-Rom motors page for more details.


'Empty' core plus completed motor

How the winding starts

Speed 400 replacement for fast models

The standard CDRom motor can be assembled with 6 instead of 12 poles. This yields a motor which will turn the small plastic Gunther prop (5x4.3) at a higher RPM than a good Speed 400. Static current draw may be a tad higher (13-14A) but it results in more speed and longer flights in a fast model. I use the standard 8xAr800 packs I have always used on 400's. In addition to the performance and duration benefits, the motor is a third of the weight. See my CD-Rom motors page for more details.


Main components with a couple of test cans

Some of the magnet options tried


More information (same links as above):
* CD-Rom motors (easy)
* Crocodile motors (high efficiency)
* Machining tips (accurate machining)
* Winding Density (advanced advice)
* Balancing motors (advanced advice)
* Milling machine (DIY mill from a drill)

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