Racerstar BR1102 rewind - RC Groups
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Oct 15, 2017, 09:20 PM
Brisbane, Australia
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Racerstar BR1102 rewind


I rewound these Racerstar BR1102 8000Kv motors and thought I would post some details and pics in case others are interested.

These motors weigh around 3.5gm. In original form they seem efficient and have good power for their weight. The original winding scheme is 15 turns of 0.2mm wire per tooth, single strand, wye terminated, and wound 1-7-4, 2-8-5, 3-9-6 (clockwise). It has the 9N12P configuration.

I bought 2 lots of 5 BR1102 8000Kv motors, the measured Kv's of the first lot were 9800 (not a typo, 9800), 8850, 8850, 8500, and 8400. The latter 4 were fitted to a small quad and I was very happy with the performance, happy enough to buy another set of 5. The 2nd lot of 5 had these Kv's - 9290, 9250, 8800, 8730 and 8680. These are the ones used for the rewind as documented here. Surprisingly, when I unwound the 2nd lot of 5, I found the numbers of turns to be very consistent on each tooth.

When I was measuring the Kv (I used a hand drill instead of a drill press, so the accuracy isn't that good but still good enough for ball park figures) I discovered that swapping the bells between motors produced different Kv. At first, I thought it might be different magnet strengths causing the differences in Kv. But now, looking closely, it also appears that the magnets are not lined up properly with the stators on some motors, so this could also be affecting the measured Kv. On some motors the magnets are higher than the stator, but there is not enough clearance to lower the bell by removing shims from above the upper bearing. So I had to raise the stator a little on the bearing tube on all of these motors when I reassembled them. Also during re-assembly, I noticed some magnets were not seated properly in position, so some sit a bit higher than others. All these minor niggles aside, they turned out fine after the rewind, but there sure is room for improvement

I wanted something around 7300Kv give or take 500Kv so set about rewinding the set of 5, I like to do 5 in case one doesn't turn out so well, and it's nice to have a spare if all 5 turn out fine. I rewound them with 0.21mm wire and 18 turns per tooth. I used the more common 1-4-7, 2-5-8, 3-6-9 winding scheme as I'm used to doing it this way.

18 turns per tooth gave the following Kv numbers - 7550, 7450, 7130, 7050 and 6990. Swapping a bell from a higher Kv motor onto a lower Kv motor gave the lower Kv motor a higher Kv and vice versa. Seems there is something going on with the magnets in the bells but I don't really know what it is. Still, the Kv spread isn't too bad and I fitted 2 motors with Kv around 7000 to the front and 2 motors with Kv around 7500 to the rear positions of the quad. Will update with flight reports in due course, so far, initial brief flight tests look good, and I'm waiting for some decent weather.

For more detailed info on winding small motors (1104) there is another thread here. Here's some pics of the 1102 rewind:
Last edited by Ian444; Oct 16, 2017 at 05:52 AM.
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Oct 15, 2017, 10:31 PM
Brisbane, Australia
Here's a pic of the quad the motors went into. It weighs 49.8gm with a 2S 300mAh battery. Props are 2.4" diameter, 3.5" pitch.
Last edited by Ian444; Oct 16, 2017 at 05:44 AM.
Oct 16, 2017, 06:03 AM
the people squad
schrodingers cat's Avatar
Great work as usual Ian. Very interesting how the different bells could change the kV rating significantly. Quite surprising really.

Did you do the second lot of kV readings with a hand drill too? How do you go about this?
Oct 16, 2017, 07:49 AM
Brisbane, Australia
Regarding the different bells, the only thing I can think of overall is the strength of the magnets, but you would think the average of the 12 magnets would even it out...so it's a mystery to me.

I use the hand drill all the time now. On my multimeter, it has a "Hz" 2nd function when AC volts (or AC millivolts) are selected. The frequency in Hz multiplied by 10 gives rpm on a 9N12P motor. Usually 11XX to 14XX motors are 9N12P. With the hand drill (it's battery powered), it can slowly drift down in speed so I check the rpm on just one phase of each motor. The meter I'm using is fairly expensive and can average the rpm over time (say 10 seconds) which helps, but even a cheap meter works well enough. I have a cheap meter too (cost around $35 AUD and it's close enough).

So basically you hook up the multimeter to 2 wires (one phase) of the motor, select AC volts, or AC millivolts if you have that option, and then press the Hz button (if it's there). X10 is the rpm. Kv = (rpm/1.344)/volts where volts is the output from one phase, usually it might be something like 80mV or 125mv so that would be 0.080V or 0.125V in the formula. I measure the 3 phases, then work out the average, then round it to the nearest 10Kv.

I have found larger motors to have much more consistent Kv between phases, but the current Eachine 1104 6000Kv and 7500Kv motors that I tested have very consistent Kv too (an exception), although the Kv was not what they were supposed to be (8100 for the 7500Kv motor and 5600 for the 6000Kv Lizard motor). I rewound the Eachine 7500Kv motors to 6750Kv and they turned out really nice, the 6000Kv Eachine motors should turn out well too with a rewind, they (these 2 Eachine models) seem to have good bearings and good overall quality, and the hammerheads are thin allowing plenty of room for wire.
Last edited by Ian444; Oct 16, 2017 at 08:00 AM.
Oct 16, 2017, 03:03 PM
Registered User
Very fine explanation, Ian - thanks for posting! Since I read something about Halbach Arraysr, in,this special case concerning some new zmx motors, I have the feeling that the magnets have much more influence than expected. That would also explain the possible extreme difference in,performance of motors of the same size and KV. (e.g. 2204 from 2015 and 2017)
Oct 16, 2017, 05:29 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Neat

Did you check polarity of the magnets Ian?
N-S-N-S- ...

To keep track of number of winds you could measure out a length of winding wire before winding, or measure inductance after winding.

Vriendelijke groeten Ron
Oct 17, 2017, 02:20 AM
Brisbane, Australia
I didn't even think to check the polarity of the magnets Ron, I will do that this weekend, thanks

Keeping track of the number of winds - I cut the 3 pieces of wire exactly the same length and that helps, but still it is easy to make a mistake, I am not sure if I got them all at the correct number, but I did get them all to be the same number of turns I think (by comparing wire lengths). Also I wind the teeth sequentially, using all three wires at once, so I wind tooth 1, then tooth 2, tooth 3, 4, 5, and so on, so I can compare the 3 wire lengths after winding the 3rd, 6th and 9th tooth. But still I had doubts - measuring the inductance is a nice trick, I'll look into my options there too. So far the motors I've done only had 2 layers at most, usually one and a bit layers, this one had 3 layers (with many more turns) so it was a bit more difficult.

Thanks for your suggestions, always welcome.
Oct 17, 2017, 09:12 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Loosing Track of Turns Count ? - RCG

Vriendelijke groeten Ron
Oct 17, 2017, 11:30 AM
Jack
jackerbes's Avatar
Very interesting reports and good work too!

And when you are checking the magnets, can you measure the width of one of them? Or estimate the gaps between them? It would be interesting to see a photo of the inside of the housing...

The percentage of coverage that the magnets have on the side of the housing (or the size of the gaps between them) is a factor in motor performance. It has been said here for a Wye terminated motor that 75% or so coverage is better for some winding schemes and 95% or more better for other schemes.

Another consideration that I have wondered about is the number of magnets used in 9N motors. If you look at Dr. Okon's winding scheme table 9N12P is not as good as 9N8P or 9N10P. But try to find a motor other than 9N12P for sale anywhere...

Jack
Oct 20, 2017, 06:40 PM
Brisbane, Australia
Quote:
Originally Posted by jackerbes
Another consideration that I have wondered about is the number of magnets used in 9N motors. If you look at Dr. Okon's winding scheme table 9N12P is not as good as 9N8P or 9N10P. But try to find a motor other than 9N12P for sale anywhere...
I am always watching for a 9N10P but have not seen one yet. There could be possible gains with that configuration, I wonder if any manufacturers have tried it.

Something I forgot to mention in the first post, these motors have several shims between the upper bearing and the bell, sometimes when you remove the bell, one of the shims sticks to the upper bearing - easy to lose. You can see the shims in the 2nd pic (on the shaft inside the bell).

I removed a bell from one of the rear (higher Kv) motors to check the magnet polarity, which was all good, and good on the 2 spare motors as well.

Some measurements for Jack:
1102 bell outer diameter 14.02mm
1102 bell inner diameter ~13.14mm, difficult to measure because of the glue from the magnets.
Magnet face to opposing magnet face distance 11.2mm
Stator diameter measured 11.02mm
Magnet size (best I can determine) 2mm x 2mm x (0.8mm or 0.9mm)
Gap between magnets approx 0.9mm
Last edited by Ian444; Oct 20, 2017 at 06:47 PM.
Oct 20, 2017, 07:41 PM
Jack
jackerbes's Avatar
The shims are there to keep the magnet housing centered on the "sweet spot" where the housing locates itself over the end of the stator arms. If you take the shims out and push or pull on the housing you will probably find some movement in or out. And that would be movement away from the sweet spot and costs some loss in magnetic forces and power.

Thanks for the measurement info, when I use it I go to google and type in "circumference of a circle" and it bring up this calculator:

Then I would enter the inner diameter of 13.14mm and it gives me the circumference as 82.56 (that is mm as the output is always the same unit as you input).

But I wonder if your measurement on the magnets is right? You say they are 2mm wide so 12 of them would cover 24mm of the 82.6mm inner circumference. And that would make the magnet coverage only 29% or so (24 / 82.6 = 0.29 or 29%)

I used irfanview and zoomed in on the image of the housing and used the Paint Tools dialogue to take some measurements in pixels. And I came up with the inside diameter being 611.5 pixels in diameter or 305.75 pixels in radius, the circumference calculator says that a 305.75 radius has a circumference of 1921.08 pixels.

The magnets in the image measured 93.06 pixels wide so 12 of them would cover about 1117 pixels of the circumference.

1117 is 58% of of 1921 (1117 / 1921 = 0.580) and that implies that the magnet coverage on the motor is only 58%. I don't know very much about it but I recently read here that 75% magnet coverage is good for LRK wound motors and 95% or full coverage (no gaps) is good for dLRK motors. So it may just be that you have a motor that is not well designed.

If you really wanted to try to figure it out you would have to buy new magnets of a width that gave you a better coverage percentage and/or change the magnet count from 12 to 10. But that motor's dome has pockets for the magnets that would prevent fitting wider magnets or a different magnet count.

If you really want to fool with it you would have to find wider magnets and change the size of the pockets to take the bigger magnets.

Jack
Oct 20, 2017, 08:10 PM
Brisbane, Australia
Jack, I think you may have entered the diameter as the radius in the calculator so the circumference calculated was double. The magnets are indeed 2mm square though, or very close to that. Another way to do it might be to use the magnet face to magnet face distance (11.2mm) as the diameter for the circle as this is the circle that the faces of the magnets sit on. That would give a circumference of 35.2mm, leaving 35.2/12 = 2.93mm for each magnet plus magnet gap. Magnet coverage would be 2/2.93 = 68% which is still low. However, in practice, these motors give good power for weight and good efficiency compared to other small motors I have. It would be great if spare blank bells were available so one could try different magnet arrangements. Which reminds me, some time ago a saw a link in rcgroups pointing to an excellent source of neodymium magnets for motors (very big range of magnets and sizes), but I cannot for the life of me find it again. Do you happen to know of this link Jack?

Regarding the shims on the upper bearing, and the raising of the stators to better line up the magnets with the stator - I didn't explain it in much detail previously, but the problem with removing any of those upper shims, was that if I did, the bell would rub on the motor base. So I had a choice to (1) raise the stator on the bearing tube or (2) remove some material from the upper skirt of the motor base. I chose (1) to keep the original appearance of the motor and it was probably less difficult
Last edited by Ian444; Oct 20, 2017 at 08:20 PM.
Oct 20, 2017, 08:46 PM
Jack
jackerbes's Avatar
Quote:
Originally Posted by Ian444
Jack, I think you may have entered the diameter as the radius in the calculator so the circumference calculated was double.
I'm a bonehead at math, but I think I got it right. I was working in pixels and the radius I used was half of that or 305.75.

Quote:
The magnets are indeed 2mm square though, or very close to that. Another way to do it might be to use the magnet face to magnet face distance (11.2mm) as the diameter for the circle as this is the circle that the faces of the magnets sit on. That would give a circumference of 35.2mm, leaving 35.2/12 = 2.93mm for each magnet plus magnet gap. Magnet coverage would be 2/2.93 = 68% which is still low. However, in practice, these motors give good power for weight and good efficiency. It would be great if spare blank bells were available so one could try different magnet arrangements. Which reminds me, some time ago a saw a link in rcgroups pointing to an excellent source of neodymium magnets for motors (very big range of magnets and sizes), but I cannot for the life of me find it again. Do you happen to know of this link Jack?
Looks like you have the numbers right at 68%. my bonehead math messed my numbers up.

Was the magnet place this place? https://www.kjmagnetics.com/

I like that place for the info and all, and eBay will have a lot of them too. I have never found a need to move or replace magnets as I worked on motors so I'm plowing new ground here.

If I wanted to do that on your motor I would probably grind the arms that define the pockets off on a disk/belt sander and then locate and space the magnets manually and dribble drops of medium think CA adhesive down into the opening behind each magnet. And maybe some CA in the gaps too. But that is just a guess.

The document is the instructions for a Komodo kit motor from some years ago, it implies that 80% coverage is a good number for a 9N12P motor.

Jack
Oct 20, 2017, 11:14 PM
Brisbane, Australia
Yes, that link might have been the one, thanks. That Komodo motor manual has some very good winding pics and instructions. I just found this store too.
Oct 21, 2017, 07:36 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Ian, did you determine one Kv, measuring one voltage, or three Kv's, measuring all three voltages? In the latter case, were all three voltages the same, as they should?

edit: nevermind, you did, I missed it somehow.

Prettig weekend Ron
Last edited by Ron van Sommeren; Oct 21, 2017 at 07:55 AM.


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