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Jun 18, 2011, 08:20 PM
jackerbes's Avatar

Dualsky XM PR-40 LRK Rewind

I have Dualsky XM-PR 40 motor that came to me as a gift (thanks again Tony!) with one phase shorted to the stator but in otherwise good condition.

I had read a sort of horror story on rewinding Dualsky motors, about them being hard to disassemble and strip so I laid it aside when I got it.

A need for a 200 gram or so 1000 Kv or so motor came up recently so I looked at the motor again. I found it easy enough to get apart. Heating the inside of the bearing tube let me twist the stator off of the bearing tube. And to my joy, I found the windings were only very lightly epoxied, they came off nearly fully intact and very easily.

I found it wound 6 turns with 27 parallel strands of 0.21 mm (32 AWG +/-) wire. It was wound DLRK and terminated Delta and sold as a 850 Kv motor. The original specs rated it for up to 4S, 45-60A "efficiency", and 60A max so it seemed to be capable of being rewound to the 300-400 Watts or so I wanted.

I loaded the info into Turn Calculator 5 and started looking at the winds that might get me to a Kv of about 1,000.

I ran into some problems finding a wind with a decent size and amount of wire that would fit without running out of room in the bottom of the "V" between the stator arms. The stator arms are fairly short on this 35mm or stator.

I went through my available wires and the wind choices and in the end decided that using two strands of 21 AWG in parallel and a LRK wind with a Delta termination would be worth a try. The Kv was estimated at 957 for that wind.

The LRK wind only has windings on six of the twelve arms on a stator (has to be a 12 arm stator). If you've never done a LRK wind it is very easy to wind and terminate. I've done a couple now and they both turned out to have excellent power.

Before I did the wind I modified the way the stator and bearing tube are attached to each other. I filed a narrow groove on the inside of the stator and another on the outside of the bearing tube. That lets me use a short length of music wire as a pin to keep the stator from rotating on on the tube.

When the stator is put back on the bearing tube I put red Loctite into the grooves then tapped a snug fitting "L" shaped pin of wire into the aligned grooves to hold things in place. This method is an experiment at this point, I may decide to replace the pin with a small screw later if the pin does not work. The pin can be seen in place in one of the photos.

Winding the parallel strands went easy enough, as it happened 5 turns with two strands of 21 AWG was just right to fill the length of the stator arm. The strands were brought back on top in a second layer with 5 turns back. There was some minor crowding on the last turn in the bottom of the "V", I had to thread those through and put up with a little bit of "ugliness" there.

After I got the stator mounted and was working on terminating the winds, a short to the stator turned up. I found that was a combination of having lost some of the coating on the shoulder of one arm (nearly every turn crossing there nicked itself on the sharp corner).

A second issue was that I had put the transits too close to the center hole in the stator (that can be seen in the photo). So when the stator was seated on the bearing tube's shoulder, the transits were trapped between the stator and the shoulder and got crushed and nicked a little.

So there were my two hard learned lessons on this wind. I am again older and smarter...

It was wound again with a piece of Tyvek paper folder around the bared stator arm and the transits were spaced out away from the hole so to clear the contact point on the bearing tube.

I viewed all this as entertainment and an experiment. I really didn't know how it would turn out and what the motor would be like.

I did the Delta termination (another no brainer), put the ESC connectors on, did one more round of checks for shorts to the stator and decided it was time to play "you bet your ESC".

Got it all powered and and gave it a tiny nudge of throttle and away it went! Smooth and pretty quiet even.

The initial raw Kv showed it to have a 1018 Kv so that would just about what I was shooting for. The motor is drawing 4.2A on the no load which seems a little high to me but I'll keep an eye on that.

Ran the motor for a while and everything was staying cool. Then I used what I call my "test club" or "dummy load" prop. It is a little ugly to look at but it is well balanced and will put a load on a motor.

I run it up to about 12A and left it there for 15 seconds with averaged results of:

3,296 RPM, 11.86A, 11.01V 130.5W

The temp started at 68F and only rose to 77F rise after the end of the run.

Did a second run, this time I ran it up over 30A. A 15 second run gave me:

4,283 RPM, 32.8A, 9.05V, 310W peak

The temp started at 70F and peaked at 89F 10-15 seconds after the end of the run. As you can tell, my testing battery was running out of poop and needed a recharge. But this looks like an easy 300-400 Watt motor. It even looks like it might not have lost much or anything in the rewind.

Need to get my test battery recharged and try it with a real propeller...

To agonize over the stats a little, the original wind had 35 grams of wire (I weighed the stripped windings) and a total surface area of 0.86 mm2 of copper.

Rewound with the two strands of the 21 AWG the weight of the copper is now 30 grams and the surface area is 0.82mm.

The strand lengths were the same for both winds at 48".

So I lost a little bit of copper mass and surface area with the new wind. I think that the advantage, if any, of the LRK wind is that the LRK wind will have much better cooling and the resistance in the windings is probably slightly less than the original wind.

I'm not sure yet but I think this will get up close to the results that were reported for the original motor on 3S packs when it was new about five years ago:

3S 4000mAh Dualsky12C,
APC TE 13 x 6.5, 393W, 36.0A, 10.8V, 7710 rpm
3.2A no load at 12.2V

My rewind was pulling 4.2A on the no load tests, that seems a little high but I'll keep an eye on that.

I'll get this tested with a APC TE 13 x 8 or 14 x 10 and follow up on the results.

Last edited by jackerbes; Jun 19, 2011 at 08:24 AM.
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Jun 19, 2011, 08:17 AM
jackerbes's Avatar
Yep, this is definitely a motor that is capable of 400 Watts!

Ran it up with a APC TE 13 x 8 this morning using a charged and rested 7AH Sealed Lead-Acid battery and got the following results:

6,957 RPM, 40.6A, 9.41V, 381w

The temperature ranged from 79F to 86F during a 56 second run as I slowly increased throttle, the ESC's LVC kicked in at 9.0V with a two or so clicks of throttle left so I still have not gotten a sustained full throttle reading. But I'm pretty happy with what I see there.

I think I need a battery with a little more capacity or a 4S battery to see if I can actually get this guy warmed up a little...

I'm seeing some spikes in the wattage trace at lower RPM that make me a little nervous, I wonder if maybe I still have some issues with the stator coating and the windings to stator insulation. This was an experiment, I'll probably rewind it again and work on getting the stator insulated from the windings better this time.

Jul 01, 2011, 11:38 AM
jackerbes's Avatar

Rewound again - New stator insulating technique

I decided to rewind this motor again and when I got it apart I found a couple of places where the insulation coating had chipped or flaked off. Those and the sharp corners it left exposed lead me to wanting to renew the insulation on the stator.

My bottle of Loctite 410 (only a small amount had been used) had setup in the bottle, it was hardened to a rubbery mass and was unusable. What a disappointment! The bottle had been closed and stored well, it looks like maybe it has a short service life after opening.

I decided to try using Tyvek paper for insulation because I had it on hand (a used USPS mailing envelope made from Tyvek paper). I cut a strip of that so that it was a little more than 1" wide or 5-6mm wider than the 20.5mm stator height.

As seen in photo #1 I used a piece of bamboo skewer and a rubber band to hold the strip in place so that the extra material was even on both sides of the stator. Then I put a small amount of a Loctite Super Glue (a slower setting, thin to medium viscosity, CA glue) in the bottom of the "V" and spread it evenly across the bottom of the "V".

I pushed the Tyvek strip down onto the glue with a piece of thin plastic and and then used a rubber band to hold it in place for a few minutes. I worked my way around the stator and did all 12 "V"'s using two pieces of Tyvek because I had not cut the strip long enough.

Photo #2 shows the first two slots done, the skewer that was holding the Tyvek in place for the start, and the rubber band holding the paper down in the two slots.

Photo #3 and #4 show the top and bottom of the stator with the first phase of a LRK wind completed. I made sure the Tyvek was in place as the windings crossed down and in between the arms and the windings snugged the Tyvek down and into place as the turns went on. It actually conformed to the ends at the hammerheads in a very nice manner, I was very happy with the way it looked and worked.

The wind there is two wires in parallel, 5 turns from the inside out, 4 turns back, and then 2 turns on top in a third layer for a total of 11 turns. On the first wind I had gotten the same number of turns on without Tyvek so the Tyvek did not seem to take up much space or decrease the turn count on the length of the stator arm.

That wind is a not a benchmark for beautiful, perfectly spaced, windings. That is, in part, due to the last two turns on top. The first wind I did had too high a Kv at 1024 and I wanted to add two turns to the wind to get a lower Kv.

As I worked, my fingers pressed and flattened the Tyvek but it stayed glued down and in location. It was easily pushed back into place with a toothpick as I got to the places where windings would cross it. I made sure that the Tyvek was in place when windings or transits dropped into or came out of a slot as those are where I have most of my issues with nicked wires and shorts.

Photo #4 is the bottom of the stator (as it will be mounted) and you'll notice something different there. Those are the ends of the first phase but there is no transit run there! But if you look at photo #3 you'll see a transit run on the wrong end of the stator and there are no start and finish ends the for wire there.

I had decided to try putting the transits on the top of the stator. I checked the clearance to make sure the bell would not hit the transits and there was plenty of room there on this Dualsky motor. So I wound it upside down with the starting end on the bottom but looking at the top, then I added add a half turn of wire at the finish to bring all the ends out on the bottom of the stator.

After all three phases were wound I used an Exacto knife and small scissor to trim the Tyvek away from the hammerheads and other places where it was sticking up and could be snipped away. I worked carefully to avoid nicking the wire but it went OK. As you can see in photos #5 and #6 it is not the neatest looking job in the world but I'll get better at it!

So in the end I had the transits at one end, the terminations at the other and it made for more room and easier work in making the terminations. Unfortunately I forgot to get a photo of the termination end before I remounted the stator...

The motor was Delta terminated by bundling the four strands from two phase ends together and soldering them, then a piece of 14 AWG stranded wire was soldered to each of those. Two layers of heat shrink and it was a done deal but there was plenty of room for it all with the transits on the other end.

The wire I used for this wind was from Tech Fixx, I got it from his "store" on eBay (seller "tech-fixx") and it was reasonable enough, shipping was cheap, and the service was good too.

This is U.S. made Essex-Superior wire, the 21 AWG green wire I used (rated for 155C) measures just slightly smaller than Micro Dan's wire (0.74mm compared to 0.75mm) and I suspect the difference is in coatings and that maybe the MD wire has a better coating.

The motor was fired up after making all the important "you're betting your ESC that you got it right" checks. A no load test showed that I now have a nice 882 Kv motor.

I've run it up several times now with several props and it is a monster of sorts for me, bigger than anything I've been messing with. I am getting half throttle or so numbers like this out of it but I'm reluctant to firewall the throttle in static testing in the shop because it scares me!

Averages - 28 seconds at half throttle
my "dummy load" prop - 4,080 RPM, 9.55V, 28.25A, 269.85W
temperature stable at 98F

Averages - 5 seconds full throttle
Master Airscrew 13x10 folder - 6,899 RPM, 42.3A, 10.1V, 427W
temperature 81F and climbing slowly

I want to use the motor on a glider I'm building (96" WS scratchbuilt foamie) and had found that the 200 gram weight was perfect for getting the planes CG in the zone. The power is a little more than I need but I'm cool with that. I want to use it with a larger folding prop and at lower RPM, I tested it to half throttle this morning with the largest folder I have on hand:

Averages - 22 seconds at half throttle
Master Airscrew 15 x 12 folder - 3,509 RPM, 20.2A, 10.41V, 210W
temperature rise from 81.3-85.3F during run

My glider is going to fly at around three pounds/1500 grams or so I think so this will have plenty of power at a half throttle 200 Watts reading. I want to fly it with a smaller battery, like a 2200 or so 3S and that motor and prop should do fine on the shorter and less than full throttle runs. I think this motor will push 400 Watts or so with this prop at full throttle and a battery that will stay up around 10V or so

I'm sort of looking forward to seeing what it will do at higher throttle settings and even with a bigger prop on it.

LRK winds are amazing, so much more room for wire! And the torque is pretty amazing too. If you like bigger props and running motors at mid throttle the LRK wind is definitely worth a try.

Last edited by jackerbes; Jul 02, 2011 at 08:40 AM.
Jul 15, 2011, 02:42 PM
Registered User
i did rewinding once before about 20 years ago in high school project with a small brush motor. lots of hard work and patient. End result is a powerless motor and can spin a small prop with 0.1 miles of wind. seems like u have good result. Nice information u showing here.
Jul 15, 2011, 02:54 PM
jackerbes's Avatar
It's pretty amazing how much more you can get out of these motors with rewinding. That, and the ability to fine tune the Kv to a specific need is really nice.

Once you have a few sizes of wire on hand, it really saves you money and avoids having to order and wait for a new motor to show up in the mail.

Jul 24, 2011, 01:53 AM
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two XM-PR 40's ?

You must have received two of those motors because i clearly remember sending you one along with four other smaller motors. Good job. jim
Jul 24, 2011, 05:06 AM
jackerbes's Avatar
No, I just have the one Dualsky. This would be the one you sent me wouldn't it?

It had one shorted winding, but is doing fine now. And thanks again!

Do you have any other dead motors? I'll rewind them and send them back to you if I can.

Jul 24, 2011, 05:41 AM
jackerbes's Avatar

I should tell you also, most of the little motors got resuscitated too. They all got a little stirred around in the process so I don't know which one cam from who any more but I have about eight 24xx motors that I've rewound and they are all the better for it.

With the wire, new bearings, replacement shafts, conversions from the old style shaft collars to the new and one thing or another I probably have the most expensive collection of Tower Pro 24xx motor in the western world! But I love them all!

I'm off on a sailboat delivery this morning, be gone for a couple of days if I don't reply here...

Jul 24, 2011, 09:16 AM
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Jack- after bugging you to get the specs for rewinding a 2730 to 1500 kv (and failing miserably) i bought a couple from west michigan park flyers but sure could use 10A or two esc's. Have fma M5 rx's to trade or the two bare 2730's-one missing clip.
I was probably pushing 500 watts on the XM-PR 40 when i ran 5 A123's and two much prop.
Jul 26, 2011, 08:39 AM
jackerbes's Avatar
Originally Posted by jim e View Post
Jack- after bugging you to get the specs for rewinding a 2730 to 1500 kv (and failing miserably) i bought a couple from west michigan park flyers but sure could use 10A or two esc's. Have fma M5 rx's to trade or the two bare 2730's-one missing clip.
I was probably pushing 500 watts on the XM-PR 40 when i ran 5 A123's and two much prop.
I haven't wound (nor am I flying) any 2730's yet...

On this Dualsky, I'm running a Master Airscrew 15 x 12 folder on it right now on a 3S LiPO. The battery is limiting me to 20A because the glider does not have much battery space in it. And I only need short runs anyway.

You can see your old motor flying a little bit here:

Big Blu 96 Glider - maiden flight:

Big Blu 96 - 96" KFm9 Winged Glider Maiden Flight (5 min 18 sec)

But I tested the PR 40 with larger battery and at 10V under load it is pulling around 20A for 200-220W on that big prop. I have not tested it at a higher voltage yet. It is not even warming up at 220W though. I think it will do fine a 400W, not sure about 500W on 16V or so. That would be 31A at 16V and I think it would handle it.


I just shot you a PM with a question, no rush on replying.

Jul 31, 2011, 08:04 AM
Closed Account

pic of XM-PR 40 on Stick 40+

And thanks ++++ for all the VX 1100 packs Jack!! cheers jim
Jul 31, 2011, 08:18 AM
jackerbes's Avatar
Originally Posted by jim e View Post
And thanks ++++ for all the VX 1100 packs Jack!! cheers jim
You're more than welcome!

Did that 3S A123 fly it OK? If it did and you liked it, you'll probably love it on a 4S A123!

Have fun!

I flew my PR-40 rewind again this morning on the Big Blu 96 glider. Propped it down from a MA 15 x 12 to a MA 13 x 10 folder and that kicked the RPM up nicely and gave me a better power band. Looks like that is a better prop choice for that plane.

I'm using a 2200 LiPO there because of the cramped confines in the fuselage. I was thinking about trying to fly it on a A123 2300 mAh 4S pack but it is just too bulky and heavy...

Nov 21, 2011, 09:22 AM
Flazo's Avatar
thank you sharing.
Nov 21, 2011, 10:24 AM
jackerbes's Avatar
You're welcome, Fred.

Do you have one of these? I like it and it has proven to be a good motor for me.

I had it on a glider that went into a beaver marsh (radio failure I think) and it took me two days to find it and get to it. The motor was submerged in water for two days but I took it apart, cleaned it up, and it is running fine again now.

The photo is the plane as I found it...

Jan 17, 2015, 06:53 PM
Registered User
what is the height of those stators? I just tried wraping two layers of 22 awg on my radian motor I can only get 3 wraps actual 6 but 3 parallel. I will be lucky to get 11
Nice video by the way big blue looking good

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