Fly Electric!

Crocodile Motors

This page is about my 'Crocodile' motors. The stators in these motors are very special. They are made from the highest quality metal, each lamination is extremely thin (0.2mm) to avoid losses, their diameter is quite large (40mm) which allows thicker wire to improve efficiency and larger radius to improve torque, they have many teeth (18) which also improves torque, and the shape was designed with great care by motor design experts. The project was inspired by Ralph Okon and a talented team in Germany. Visit Ralph's excellent site for more info and the Crocodile Shop to place your orders.


Croc #3 36mm

Croc3 is the largest, most powerful and most efficient motor I have ever owned. It swings an 18x10 APCe at 6,300 from 1330W (33A and 12 lipos/36 nicads) with almost 90% efficiency. This allows a near-vertical climbout with my 9' 12.5lb RAE Hurricane. It is currently wound with 8 turns 0.9mm wire in a delta config. It has twenty 2.5x6x36mm magnets and weighs 498g (17.6oz). Notice the long square motor mount in the photos that follows; this makes it easy to mount and was needed for the narrow Hurricane cowl.

I have also had it wound with a not very dense 7T 0.7mm delta wind. This swung a 16x8 prop at 7,400 from 1350W (33A and 36 nicads). This compares very favourably against the excellent but heavier Actro 40-5 (556g/19.6oz) which swung a finer pitch 16x6 slower at 7,000 with a similar 1300W power (40A and 32 nicads). With a 10T 0.7mm delta wind croc3 swung a 20x12 at 4,700 with 1200W (30A). More details are in the table below.


Croc #2 20mm

Croc2 is even more fantastic than #1! Wound initially with a not very dense 6 turns of 0.9mm wire, it has the following characteristics:

* It turns a 12x8 at 7,700 from 19A and 20V (6 lipos).
* It turns an 11x7 3-blade at ~7,400, just 300 rpm slower than my Astro 40G but with ~400W instead of 700W (45% less power).
* It weighs 300g which is 30% lighter than the Astro.

* Wound star the same motor is capable of 1300W with 32 nicads!
* It turns a 16x8 at 7,000 which is the same rpm and total power with a similar prop that I used to run on my Actro 40-5 at 40A!


Croc #1 10mm

This motor turns an 11x7 at 7,000rpm with just 246W of power. This is 22% less input power for the same results from an Astro 15G. Along with it being 168g (6oz) which is 19% lighter, this croc is ~40% more efficient than the Astro. Pretty good! Six turns (delta) of three 0.63mm strands (trifilar) equates to a single strand of 1.09mm diameter which is a very good density for 6 turns. Efficiency is 84.4%.

I have tested a number of variations. Full details are in the table below which is summarised as follows:

Wire thickness - Thicker wire increased efficiency by about 0.6% for every 0.1mm of wire diameter. It also increases rpm, current and overall motor power. The 0.9mm wind was easy, twin 0.71mm a bit of a challenge and the tripple 0.63mm is hard and takes many hours to wind.

Magnet thickness - Thicker magnets increase flux which increases torque but reduces rpm. 3mm magnets yielded 90 less rpm than 2mm and reduced current by 1.4A.

Rotor wall thickness - Can have a similar effect to changing magnet thickness. So be generous with the thickness of the rotor ring if you want to maximise torque but don't make it too thick. In practice I recommend 50-60% of the magnet thickness. I seek zero losses as measured with a hall-effect sensor (more sensitive than the pin method).


Construction tips
Its easier to make the rotor ring from hollow steel tube than solid bar as you see here. However, I now use solid bar for my larger motors because this allows me to choose the steel I use (EN1A). A better description of the techniques involved in making the components with a lathe are described on my machining tips page.
The Croc stators have an 18mm hole and I machined my bearing/stator holder to 17.98mm (2/100th smaller). This provides a snug fit for accurate centering while leaving ample clearance for pushing and gluing the laminations on. I cut a few lengthwise channels with a dremel cutting disk to aid adhesion. I glue the laminations to the holder with epoxy resin as it is thin and has a good pot life. I apply it to the holder with a small brush after every 3-4 lams.
The Croc laminations are 40mm in diameter and my first 45 lam stack measured 40.15mm indicating slight mis-centering of some of the holes. I use this gauge to check all the lams. The central boss has a slight taper and the OD is exactly 40mm. Grind the central hole larger on the odd incorrect one; this will allow you to centre it between better ones.
The ends of the lams curl slightly; make sure you stack them all the same way (I don't believe there is any need to straighten them). You need a means of clamping the laminations (I made two ali disks with six 3mm bolts. Grab the teeth between hard 2.5mm ply and twist back and forth (like a key in a lock) to align the teeth.
The glass-fibre isolation plates work well to insulate the ends of the stack but need trimming to size. I have found it necessary to insulate the full length of the stator inside each tooth. This version is with silk and dope has worked well.
Here is Croc3 with self-adhesive Kapton tape. This is easier than silk and dope and worked very well. Cut lengths of Kapton 2mm longer on each end and fold the excess over the end insulation plates. Trim off excess tape on the sides of the stator either before (easier) or after winding.
Make a couple of tapered wedges to force the wire flat between the teeth. Use the best/hardest ply you can find. I found 1.5 and 2.5mm were useful sizes for this motor. Motor shown is 'Croc#2'. More detailed winding advice.
On longer motors there is a risk that windings may vibrate out and touch the magnets. Criss-cross thick cotton across each gap to prevent this.

Croc #1a 10mm

My first croc motor was a comparative test against a 'Ditto' drive motor. Not entirely comparible because the same winds produce different effects on the two motors. Bottom line is that the croc is 7% more efficient than the ditto motor and has huge potential for making great motors. Take a look at the chart for more details.


Other motor-related pages on my site are as follows:
* Examples of more motors (summary page)
* CD-Rom motors (easy)
* Machining tips (hard)
* Balancing motors (advanced advice)
* Winding Density (complex)

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