Winding the ultimate Tower Pro BP-21 (BM2408-21) - RC Groups
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Sep 30, 2009, 09:43 PM
Engineer for Christ
IBCrazy's Avatar
Discussion

Winding the ultimate Tower Pro BP-21 (BM2408-21)


It seems many people in this forum use the cheap Tower Pro motors for many of their rewind projects. Since the Tower Pro motors are so easy to rewind and cheap, it makes for a great rewind motor.

I have found I can get more than double the power out of a BP-21 winding it as a 12 turn WYE with 22 AWG wire. I have several wound this way. It takes some practice, but you get a fairly good motor when you are done.

Original Specs:
Stock BP-21
kv = 1512 (18000 rpm @11.9V)
Io (no load) = @ 1.1A @ 11.9V
With APC 7X5 prop and 3S Lipo - 130 Watts @ 180 degrees F

Rewound wind (12 turn 22 AWG)
Kv = 1504
Io = 1.1A @ 11.9V
With APC 7X5 Prop and 3S Lipo - 210 Watts @ 140 degrees F

How to wind it:
Measure out about 4 feet of 22 AWG for each phase. Wind the motor as a 2 layer with all 3 phases. You should get 10 turns on each phase cleanly. Use a credit card or a propeller to compress the windings to get all 10 turns in there neatly. DO NOT CUT THE WIRE ENDS OFF YET!!! You'll need these for the last two turns.

Once wound, use the card/ prop again to compress all of the wires tight to the stator. Now begin winding your third layer starting with the phase you first wound twice around each of it's three coils. Do the same with the other two phases and then terminate WYE.

Pictures will follow. I have two motors I need to wind for a project.
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Oct 01, 2009, 03:29 PM
Will work for foam
wingnutt's Avatar
IBC - i too am a TP2408-XX fanantic. i have never tried your method for getting 12 turns of 22Ga on that stator before and it sounds interesting. using a pair of long/thin nosed pliers to carefully squeeze each layer and threading the last two turns, i can wind all 12 turns at once. are you using GB wire? I'm using Consolidated's 22Ga which must have a thinner layer of insulation. i also pack the hammer head and dont start working my way down until the 7th turn (if memory serves).

Your way sounds much easier though.

tight winds for all,
wingnutt
Oct 01, 2009, 03:46 PM
Registered User
Pics please!
Oct 01, 2009, 10:26 PM
Engineer for Christ
IBCrazy's Avatar
Quote:
Originally Posted by wingnutt
IBC - i too am a TP2408-XX fanantic. i have never tried your method for getting 12 turns of 22Ga on that stator before and it sounds interesting. using a pair of long/thin nosed pliers to carefully squeeze each layer and threading the last two turns, i can wind all 12 turns at once. are you using GB wire? I'm using Consolidated's 22Ga which must have a thinner layer of insulation. i also pack the hammer head and dont start working my way down until the 7th turn (if memory serves).

Your way sounds much easier though.

tight winds for all,
wingnutt
I have found that not all wire of the same gauge is alike. I start my downward run at turn 6.

I'm using polyamide/imide wire from a manufacturer I can't remember at the moment. It seems to be the same diameter as GB. I have a roll of 22 AWG I got from a rewind shop that is significantly thinner and I probably could get 15 turns in there. I'm not sure if that's manufacturing tolerance or if it is insulation.

Have you measured the power you can get from your 2408 after rewind?
Oct 02, 2009, 04:48 AM
Efficiency Freak!
Nice!

Subscribed!

,
Jay
Oct 02, 2009, 08:23 AM
Will work for foam
wingnutt's Avatar
IBC - manufacturing tolerances play a big part in winding these small motors. I have a roll of Belden wire that is labled as 20Ga but mic's out to a perfect 19Ga. For the while there i could not believe that after using every trick iv'e learned on these groups, i could only get 9 turns with it where i could always get 10 turns with another 20Ga.

As far as measuring power and heat on my previous winds, sadly no. I have just recently come into better testing gear which will allow such measurements. When I first started winding, I could only test current draw, RPM and thrust. Measuring power handling was the 'feel of the hand after running' technique. But you are right that it is AMAZING how much more power these motors can take with a simple rewind. Where the motor on stock wind would get uncomfortabley warm after a test, a rewound one would hardly heat up at all.

If you look at the numbers and compare the stock 3-strand 31Ga Delta wind to a textbook diameter 20Ga 'Y' wind, the 20Ga wind is equal to just over 4.6 strands of 31Ga (area of wire corrected for termination). 20Ga is slightly better than 8 strands of 31Ga. Divide 8 by 1.73 and you get 4.624 and the overall resistance is still lower for the 20Ga. So, it makes sense that it should handle more power. I just love hobbies within hobbies!
Oct 02, 2009, 08:44 AM
7000mw of raw power!
rich smith's Avatar
Have they redesigned TP motors recently? Last time I checked they had really thick lams, crummy bearings, and bells/shafts so soft you could almost bend with fingers. Not even close to BW or specially RBW with their super thin lams and shafts so hard they snap before bending.

Have these changed that much?
Oct 02, 2009, 04:42 PM
Engineer for Christ
IBCrazy's Avatar
Quote:
Originally Posted by rich smith
Have they redesigned TP motors recently? Last time I checked they had really thick lams, crummy bearings, and bells/shafts so soft you could almost bend with fingers. Not even close to BW or specially RBW with their super thin lams and shafts so hard they snap before bending.

Have these changed that much?
Actually, they have changed. They now use .35mm laminations (as opposed to the old .5mm). Not great, but definitely an improvement. It cuts magnetization loss in half.

They've also increased the size of the front bearing (I think it's a 10 mm now). It used to be 8mm.

Unfortunately, the magnets aren't quite as strong and the shaft are still soft as can be. I guess you take the good with the bad. Definitely not a high quality motor, but you can make it into a decent one with a good wind.
Oct 02, 2009, 04:47 PM
Engineer for Christ
IBCrazy's Avatar
Quote:
Originally Posted by wingnutt
IBC - manufacturing tolerances play a big part in winding these small motors. I have a roll of Belden wire that is labled as 20Ga but mic's out to a perfect 19Ga. For the while there i could not believe that after using every trick iv'e learned on these groups, i could only get 9 turns with it where i could always get 10 turns with another 20Ga.
That's likely not a manufacturing tolerance issue but probably a multi layer insulation issue. I use Belden and Dearborn CDT wire as well. I can't even get 9 turns in there with 20 AWG. The insulation is too thick. I use the Dearborn 22 AWG for this particular motor. GB 22 AWG Newbie is just as thick. Single insulated wire you can get more turns in.

With single build insulation you might be able to use 21 AWG and get 12 turns. I'll have to try that
Oct 02, 2009, 05:49 PM
Registered User
Gotta try it!
Oct 03, 2009, 05:11 AM
7000mw of raw power!
rich smith's Avatar
Quote:
Originally Posted by IBCrazy
Actually, they have changed. They now use .35mm laminations (as opposed to the old .5mm). Not great, but definitely an improvement. It cuts magnetization loss in half.

They've also increased the size of the front bearing (I think it's a 10 mm now). It used to be 8mm.

Unfortunately, the magnets aren't quite as strong and the shaft are still soft as can be. I guess you take the good with the bad. Definitely not a high quality motor, but you can make it into a decent one with a good wind.
Thanks for the info. It might be worth giving a couple of those prop/esc/motor combos another try.
Oct 04, 2009, 05:16 PM
Engineer for Christ
IBCrazy's Avatar

Rewind with pictures


I finally got some pictures of the rewind. I used double insulated polyamide/imide 22AWG wire from Dearborn CDT. You may also use Go Brushless Newbie Wire at 22 AWG as well for this wind. 12 turns is the maximum you can get in there with 22 AWG double insulated wire. With single insulation, you might get 15 turns with this technique.

I ran out 5 feet of wire per phase and wound 2 layers, 5 turns on each layer and left a good 2 foot tail hanging off the end. I tucked this underneath the wrapped wire to hold it in place.

I then repeated with phase B and then phase C. Each time tucking the finished end underneath the crossover wind in the back of the motor to hold it down to keep it from coming loose. This really only needs to be done with phase C since phase A and B can be trapped down by phase C's wire if wound properly. I also found myself compressing the wires on turns 8 and 9 to get a good second layer. I used a worn out propeller for this.

Once all three phases had 10 turns, I went back to phase A and starting with the last pole I wound, I added two more turns, then went and wrapped the other two poles. When finished I tucked the wire underneath the cross strands in the back to keep the wires tight. I repeated this with phase B and C. I recommend tucking all of the wires underneath with this third layer as it tends to be a bit sloppy.

The result is a fairly efficient motor packed to the hilt with copper wire that can produce more than double the power of the original.
Oct 04, 2009, 09:23 PM
Registered User
Perfect timing, as I've got a dead BP-21 on the bench right now. Thanks for taking the time to document this. It looks like you've really crammed the copper in there. If you wind another, some clearer closeup pics with the camera in macro mode would really be helpful - for neophyte rewinders anyways!
Oct 05, 2009, 04:10 AM
Registered User
And how is this better from just winding it with 12 turns from the begining?
Oct 05, 2009, 10:58 AM
Registered User
Quote:
Originally Posted by ALEX_BZ
And how is this better from just winding it with 12 turns from the begining?
These are prewound motors, they don't come with 12t of 22g.

Keeping the Kv about the same while lowering resistance makes the motor more efficient and able to handle more power due to smaller % losses.


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