My experience in rewinding Scorpion 40XX HK motor (12N8P) - RC Groups
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Dec 27, 2011, 05:17 AM
modisc's Avatar

My experience in rewinding Scorpion 40XX HK motor (12N8P)

Before i started rewinding my still working scorpions for fun, trying to achieve more copper coverage to boost up the maximum power the motor can handle, I tried searching in this forum for detailed rewinding procedures. The most detailed references I found are probably the tutorials by Powercroco presented in the scorpion official website. Unfortunately, although the tutorials are all amazing and inspiring, and including amazing results, it does not provide me with enough details to do a good rewinding job.

After learning from this forum, I finally have rewinded 3 probably nice scorpion motors (4025 and 4025, HK, 12N8P), with larger copper cross section area, up to 50% more than original scorpion winding (according to the scorpion numbers of turns, so the 50% may not be accurate, reason for this will be explained later).

I would love to share my personal experiences in rewinding these amazing motors build with precision, and the problems, solution, as well as questions that i had during this enjoyable process of rewinding.

1, disassembling the motor:
Many have found disassembling a scorpion motor is quite difficult without doing any damage to the stator and the two epoxy glass plates on the top and bottom of the stator which are essential to insulation during winding. There are several ways described to soften the epoxy between the stator and the stator holder, including using acetone and heat. I tried acetone first, it does not work for me. Later, I tried using heat, to be more specific, heat up the motor on flame (cooking oven) for approximately 3 mins, till the smell of epoxy comes out, and it was quite successful. After cooling the motor a little bit, for about 10 mins in room temperature, I punch the stator holder out of the stator with a metal cylindrical rod with exactly the same size of the stator hole (a wooden tool will be an ideal alternative), with gloves on. Remember to keep the small anti-rotating pin, which may still be sticking onto the stator holder, or drops off after the stator holder is punched out.
Then, remove the wires, and try to save the epoxy glass plates. With good control of the time of heating, the plates will not be damaged at all. with the epoxy plates, which has exactly the same shape of the stator plate, on the bottom and the top of the stator, you may simply just wind without modifying the stator. Otherwise, you will have to use sander paper, a knife, or whatever you are comfortable with, to make the all shape edges which will be contacted against with wires during winding, dull. Failure to do that may result in breaking the insulation paper at the sharp corners when you pull the wires with some force, even though 0.15mm Kapton insulation film is used.
Care is take while taking off the wires. Try not to damage the steel plate on both top and bottom surface. Sometimes one of the multiple strands of wires may pull the a corner of the surface steel plate up, resulting in deformation of the plate. It can make later winding more difficult.

2, Preparing new wires and insulation paper
There are many vedios and tutorials showing how to rewind with single thick wire. Single thick wire, which has a proper diameter that matches the size of the stator teeth gap, such as 1.6mm, 1.5mm, 1.4mm are all good options for 40XX 12 teeth stator, 2.5mm for 70XX 12 teeth stator, will produce the best copper cross section area. I also love that, but i find it very technically challenging. Flatten these thick, hard wires can be difficult, and insulation can be compromised during pushing a wire inside during 'zig-zag' last turn winding, or pushing already winded turns against the stator to flatten them and leave more room for turns on the adjacent teeth.
So what I am using for rewinding is multiple comparatively thick wires, 0.8 or 0.64mm, that are much softer than 1.4mm wires, and will produce similar copper cross section area.
These wires are usually winded on a small wire holder when you purchased them, and they can be not so flat after you uncoil them. Fattening them before and after making them a bundle of multiple wires is essential. As the pic shows, multiple wires are flattened, and soldered together at one end, to make your rewinding easier.
For insulation between wires and stator, I use Kapton 0.125mm film, which is much thinner than the 0.2mm white scorpion insulation paper. I tried 0.1mm Kapton once, during winding with 1.2mm wire, broken at a shape edge happened once. I also ordered some 0.15mm Kapton, which I feel, is a bit too rigid. My experience of using Kapton 0.125mm film for insulation during rewinding the three scorpion motors is excellent, never had to re-do the work because of a short. As shown in the pics, 12 pieces of properly resized Kapton film are made, and reflected to fit in the teeth gap.

3, Winding.
A lot of techniques of winding are described by Powercroco et al, mostly for single thick wires. They are very useful and inspiring, and can be applied to multiple strands winding as well.
The following is what i have experienced and learnt during my winding.
-since i am rewinding for 12N8P, ABCABCABCABC, one direction winding is all I will have to do. Since i am right handed, i feel more comfortable with my left hand holding the stator, while using my right hand to wind and pull the wires, or use tools to flatten the wires. And I prefer counter-clockwise winding.
-Multiple wires need to be flattened as a bundle before rewinding. My way of flattening these wires is to use a piece of Kapton film, and make and angle of these wires and pull along, with the kapton film sliding along the bundle of wires. The angle made with your hand on the wires will diminish the unflattened fragments of each single wire, and more importantly, make the bundle of wire neat. Wires are side by side, no cross overs. I forgot to mention that this maneuver also need to be done on each single wire before putting them together as a bundle.
-I prefer to do the rewinding as exactly 3 turns, or 4 turns on each teeth, corresponding 6 turns or 8 turns in each slot respectively. In other words, there are no cross turns on any tooth. It seems that scorpion wind their motor in a different way, having 1/2 or 1/4 or 3/4 crossing turns on several teeth. In that way, the Kv may not be actually a "6 turns delta" Kv. I am not saying which way is superior, the key is that the Kv with my winding will not match the data given by scorpion, probably as a result of different ways of doing the ABCABCABCABC winding.
-I used to finish one wire 4 teeth, and then move on the the next wire. I find in that way, the wires are not arranged very nicely at the bottom of the stator, except that you are doing a delta connection, and you are OK with 3 wires coming out at 3 direction (120 between each other). For delta connection, it is also beautiful to wind as wire A on tooth 1, 4, 7, 10, followed by wire B on tooth 9, 12, 3, 6, and wire C on tooth 5, 8, 11, 2. But the major issue of finishing one wire and then move on to another wire, is that you may probably find it extremely difficult for winding the last wire, since the room are smaller and smaller, and you may have not flatten the winded turns nicely, which makes things worse. So I wind as from one teeth to another, and one wire to another, while the last one turn or 2 turns are not winded before the next teeth is winded with 1 or 2 turns. To be more specific, also as shown in the pic, for my 8 turns delta 4025, I wind as the following: (A, B, C stands for the 3 bundles of wires, 1 to 12 stands for the number of tooth).

a) A, 2 turns on 1
b) B, 2 turns on 2
c) C, 2 turns on 3
d) B, 2 more turns on 2
e) A, 2 more turns on 1
f) A, go to 4, 2 turns on 4
g) C, 2 more turns on 3
h) B, go to 5, 2 turns on 5
i) A, 2 more turns on 4
J) C, go to 6, 2 turns on 6
K) ......

You will see how i wind if you draw a pic following the above procedures. Basically, the winding is done in a "zig-zag" pattern on all the teeth, except for tooth 1. I find that in this way, multiple wires winding will be more easier. For example, for the 4025 12N8P motor, i can wind up to 48 strands of 0.64 wires in each slot without much effort, to make a 6 strands 8 turns (4 turns on each tooth), or 8 strands 6 turns, and 50 strands of 0.64 has approximately the same cross section area with 1.5mm wires 10 turns delta (5 turns each tooth), as Powercroco winded his 4035 kit.
The bottom will look nice winding like this, and you will be able to push the winded stator back on the stator holder to its original position.

4, remove excessive Kapton
I prefer to use a scissors to cut the excessive kapton, taking care not to damage the wires in between. A useful way of preventing this from happening is to push the wires in each slots a bit, as the multiple strands of wires can be very crowded in the slots.

5, Epoxy sealing
I prefer to use transparent epoxy to seal each slot after winding is done. This is essential to multiple strands winding. One or two wires may exceed the stator edge.

6, Check for shorts
I use a multimeter to test the resistance between the already soldered tip of each wire, and the stator, and between them. Infinite reading indicates that there are no shorts between all wires, or between wires and stator. Double check and make sure that there are no shorts. If any short is suspected during winding, or you are not sure, check for shorts after you finished several turns is a good idea. If any short is found, you may have a change to eliminate the cause without discarding all the wires. Even you cannot find the cause of short, or it is not possible to repair it, you will not be as frustrated as if you find there are shorts after all winding is done.

7, connect the wires
Remove the insulation sheath on each of the wire with a knife or other tools that you are comfortable. I do not recommend using fire to burn the insulation as it can be pretty dirty, and even after burning off the insulation, you may still need to use a knife to remove the black carbon powder. I have tried burning off the insulation sheath on the 130 degree 0.8mm wire (QA-1-130 wire, 1 stands for thickness of insulation, 130 is the temperature tolerance), it works well. But for the 0.64mm wire, QZY-2-180, the burning does not actually do a good job, and it takes quite some time to burn, while a lot of black carbon remained on the wires. Actually I used a 260degree iron solder to scratch the QZY-2-180 wire for about 10 seconds, the wire insulation seems not be damaged. Although it may be a bit time consuming, I still prefer to remove the insulation sheath with a knife.
Connect the wires as delta or star.

8. Raw test
At this point, i will push the stator back on the stator holder. You will probably notice that one of the 4 holes on the stator holder is bigger than the other 3, and here is where wires are supposed to be lead out. However, because of the anti-rotation pin, the stator can only be assembled onto the stator holder at a particular direction. So, it will be a good idea to take this into account before start winding, to make sure the wire ends are closest to the bigger opening on the stator holder.
Without using any adhesives, the stator is back on the stator holder. Hopefully it is tight! So another hint is that when you disassemble the motor, there might be some epoxy left on the stator holder. Do not sand the stator holder too much, otherwise, the stator will be too loose on the stator holder.
Try pushing the stator back to its original position (depth). If you are not able to do this, take out the stator, and adjust those crossing wires on the bottom of the stator.
Put back the bearings. Try not to apply force on the inner ring of the bearings.
Put back the rotor, and try running a a low voltage to see if everything is good: motor starts without any problem, no abnormal noise which may indicate there are some Kapton exceeding the stator, and etc.

9, Finalizing the motor
If everything is ok during raw test, you may now use some epoxy to permanently stick the stator back to the stator holder. The assembled motor is tested as desired.

Original Scorpion with same number of turns:
4035, 12N8P, 27 strands of 0.29mm wire, 6 turns delta, Kv 800
4025, 12N8P, 23 strands of 0.29mm wire, 8 turns delta, Kv 890
4025, 12N8P, 24 strands of 0.25mm wire, 10 turns delta, Kv 740

My test results of my 3 rewinded motor:
4035, 12N8P, 8 strands of 0.64mm wire, 6 turns delta, Kv 1028 (50% more copper)
4025, 12N8P, 6 strands of 0.64mm wire, 8 turns delta, Kv 1170 (32% more copper)
4025, 12N8P, 3 turns of 0.8mm wire, 10 turns delta, Kv 930 (27% more copper)

Note that for the two 4025, 8 X 1170 = 9360; 10 X 930 = 9300.
Last edited by modisc; Dec 27, 2011 at 02:58 PM.
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Dec 27, 2011, 02:36 PM
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manuel v's Avatar
The method appears to be the 3 wires at the same time.
post 185-193.

Clearly, Scorpion is adding more turns per slot and not the account.
How many and how is not clear.
because in the 4025-8 has a kv of 1170 and 10 has a 930 kV.
then its 8 turns approximately 10.5 would be more twists and turns his 10 turns would be about a 12.5.
I do not think that these motors have so many extra turns, so somehow the 1 / 2 extra turn out of place es must adversely affect kv.
I suppose the performance.

Manuel V.
Dec 27, 2011, 05:07 PM
modisc's Avatar
Originally Posted by manuel v
The method appears to be the 3 wires at the same time.
post 185-193.

Clearly, Scorpion is adding more turns per slot and not the account.
How many and how is not clear.
because in the 4025-8 has a kv of 1170 and 10 has a 930 kV.
then its 8 turns approximately 10.5 would be more twists and turns his 10 turns would be about a 12.5.
I do not think that these motors have so many extra turns, so somehow the 1 / 2 extra turn out of place es must adversely affect kv.
I suppose the performance.

It appears we did the same winding. I just figured it would be preferable to wind like this, easier to squeeze in more turns. Sadly I do not have any model to fly these motors on, but I love the sound these motor makes^^

Manuel V.
I also calculated the numbers of the equivalent turns in original scorpion motors. Yes, it looks like 2.5 more turns for the 4025 motor than standard number of turns. However, for the 4035, if you compare with the original scorpion Kv 800 4035HK 6 turns delta, you find the number of scorpions turns appears to be around 7.7, if i remember right. It does not match the results from comapring 4025 motors. So...really, not sure what is going on.

Still, if someone else is doing the same rewinding as I do, it will be a good reference! I am not very sure of the results since it is just me doing this. More data from various people is needed to get some conclusion.

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