Rewinding brushless motors WYE or Delta
If you fly brushless you've probably cooked a motor or two. You also probably know there are many differnt types of motors. Similar motors when wound differently perform very differently. Whether you've burned the motor up, or just want to alter performance, rewinding is a cheap solution for a patient modeler.
For this tutorial, I will be using TheDon's KA450H (3200KV) motor, which had unwound on me whilst flying my Stryker. It is a Delta wound 8T (I think it's 8turn) quad wind.
The winding pattern described in this tutorial (called an ABC wind - ABCABCABC as you go around the stator) works for any brushless motor with 9 stator teeth and either 6 or 12 magnets and motors with 12 stator teeth and 8 or 16 magnets.
Motors such as the Eflite Park series and the Scorpion motors use a winding called DLRK. This winding pattern is AabBCcaABbcC where capital letters are wound one direction and lower case letters are wound backwards.
Definition of terms
Stator - The metal thing that holds the wire.
Rotor - The rotatin part of the motor. This is the can on an outrunner.
Armatuer - houses one winding. Each motor has a number of armatuers divisible by 3. 9 and 12 are common.
Turns - The number of tims a wire is wound around each armatuer
Delta - Most common commercial termination. has three leads and no neutral piont. Spins 1.7 times faster than a WYE termination.
WYE - Less common in commercial motors. More efficient in some cases. Has 1.7 times more torque than Delta
First, obviously you'll need to remove the old wires from the motor. Be sure to count the number of turns around the armatuers as this will give you an idea of how to rewind the motor. The direction is not particularly important at ths point.
You will also want to note whether it is Delta or Wye terminated. A Wye terminated motor will have three wires going to a central point called the nuetral, which is not connected directly to a motor lead. A delta has no such connection, just three motor wires. Often the neutral point ona WYE has a piece of heatshrink over it to keep it from shorting to the stator.
Starting the rebuild
Before you do anything, I highly recommend insulating the stator. Take it from the king of stator shorts, a stator short can easily destroy your speed control. I can not stress enough how much easier your rewind will be if you do this.
Most stators will be already insulated, but if you cooked your motor as well as I do that coating is toast, in wich case you'll need to reinsulate it. Start by using a small hobby file to smooth all the rough corners on your stator.
Insulating the stator:
The best thing to use is loctite 410 and a little CA kicker. It is available at www.gobrushless.com for $5. Be sure to coat the end turns well as that is where a short will occur.
Another method is the epoxy dip. Get a good high quality epoxy that hardens well (faster cure such as 15 or 20 minute will be easier) and mix in about 25% of a good dark paint. Wrap your stator in tape (don't forget to protect the bearing) and then dip in the epoxy and let it run off. Continue dipping to get a good coating.
A third method, is a thread wind. Wind a peice of thread several times around your stator teeth. You don't need to cover then entire thing completely. 10-20 turns is usually enough. Then soak the thread in thick CA and let it soak into the thread. After a minute or two, spray with kicker. This will leave a thin coating of CA over the stator with thread to keep it in place.
Ok, now to rewind. First you must choose the number of turns you want. My motor was 8 turns, and I liked it, so I'm going to rewind it with 8 as well. If you want to modify the performance of the motor, the physics and math are explained in my motor modification post .
Rule of Thumb - less turns is a hotter motor and will yield a higher kv and current draw. Go too low on this, however and the motor may not run as the speed control may not detect the motor's position.
You'll also have to choose whether you want a Delta or WYE termination.
Here's the quick and dirty of Delta and WYE:
Just remember a factor of 1.7. A delta with 1.7 more turns than a WYE will have nearly the same performance. My motor is an 8 turn delta, which is very fast (3200kv). If I terminated it as a Wye the kv would be around 2000 or so. In retrospect, theoretically, I could wind my motor 5 turns and WYE terminate it for the same performance as the 8 turns with Delta (3200 kv). It is mostly a matter of personal preference.
I have found that I personally prefer WYE terminated motors as the speed control seems to sense the position better. I wound this motor before I really began experimenting. When I cook it again, you can bet it will be a 5T WYE. I also now prefer one strand of a thicker wire than multiple strands of thinner. It's a little simpler and allows for more wire fill in the stator for better efficiency.
Note in the pictures the different colors of wire. Each color represents one phase. The large wire cluster in the WYE picture would be the neutral point and would not be attached to the speed control.
Begin the rewind.
Now you need the winding pattern. This motor is an 9N6P (9 stator pole, 6 magnet). Therefore the winding pattern is ABCABCABC (each wire is wound every third tooth). This winding pattern will not work with the very common 12N14P motor. So before you start winding, count your magnets and stator poles and determine the winding pattern from the list below. Lower case letters indicate winding that tooth in the reverse direction.
9N6P - ABCABCABC
9N12P - ABCABCABC
9N8P (very rare) - AaABbBCcC
12N14P - AabBCcaABbcC <or> AaACBbBACcCB (I find this winding easier)
Now you can start winding. I used New-b wire from gobrushless.com for this. It has extra insulation to prevent shorts. I chose three strands of 28 gauge wire. So it will be an 8 turn triple wind. L lower guage will be a larger wire, so you won't need as many strands for the same current capability. I use higher gauges as the wire is more flexible and easier to work with.
Start winding with any pole you like. Go only in one direction (I went clockwise). Once you complete the number of turns you decided on earlier, skip two poles and continue winding the next. Repeat this process until one third of the poles are wired. It should look like the picture below when you are done.
If you are planning on terminating Wye, mark the ending terminal of the wire. You'll need to join the ending terminals of all three phases when it comes time to terminate the motor.
Now bfore you begin with your next set of armatuers, check for stator shorts with an ohm meter (multi tester). The resistance between the wire and the metal of the stator should be infinite (ie. not continuity).
If you don't get a short, good job. Move on to the next set of armatuers. If you have a short, unwind that entire phase get a new wire and start over.
Side note: When winding, do not tug on the wires too hard. 1-2 lbs is plenty. Winding too tight will result in a shorted winding to the stator. If you find that your wires are not snug against your stator you can use a non metallic object such as a broken prop, flat carbon rod, or my favorite, a credit card to slide between the stator poles.
Ready for another set? Start with a new wire on any other pole and repeat the above process. Make sure to test for shorts after each phase. I used different color wires for each phase. I'll call the green A phase, red B phase, and gold, C-phase. You'll notice the stator becomes crowded very quickly. You can clear some room with a dull object such as a speader from your kitchen drawer. At least that's what I used in this tutorial. I now prefer a credit card.
After that, you're almost done. Next step,
Just a side note: it took me 6 attempts to wind C-phase without shorting to the stator! A and B were perfect on the first try. The only reason for the shorts is I didn't do a very good job insulating my stator.
Wye or Delta?
From the pictures below, you can see the different terminations. If using the WYE, the bundle of three wire sets at the bottom would all be soldered together and covered with heat shrink tubing. Each of the colored wires sets would the get conneted to the speed control.
The other picture is delta. Note the multiple colors in the leads. A on one end is connected to B and connected to C on the other end. B is connected to A on one end and C on the other. In other terms: A-b, B-c, a-C (where capital letter indicates the start of the wire and lowercase indicates the end). The three connections are what hooks up to your speed control.
That stator looks like a Christmas tree:)
To test my new motor, I clamped the endbell in a vice and hooked the leads up to my speed control. I made an infra red tachometer from a fairchild semiconductor QRB1114 Emitter/Collector pair and a couple of resistors.
I placed two pieces of non reflective electrical tape on the can and set my multi tester for frequency. Upon revving the engine up to half throttle, thelectrical tape ripped free. So I then tried again, and it ripped off yet another time. So I then went to my favorite tape, Tyvek, which held just fine.
My multi tester showed 1.26 kHz. Which meant it was seeing tape 1260 times a second! That means my motor was spinning 630 revolutions per second, or 36,000 rpm! At 12 volts, this means the motor is 3000 kv. Not bad.
Creating the optimal motor
Now that you know how to rewind the motor, it's time to get a little more advanced if you want. Below is how to calculate the optimal motor as well as much of the math and physics necessary to do so.
My example motor will be a Scorpion 3008 Kit Scorpion 3008 12N14P kit. I want an efficient 2200kv powerhouse from this motor. The formulas are not exact, but they will get you very close. Much closer than a blind guess.
A motor runs most efficiently when:
Rotational loss = Copper loss
Mind you, sometimes this occurs outside the motor's power handling capability, so it's up to you to decide whether it's possible to make it work in the real world. It can be estimated a motor dissipate about 1 Watt of heat for every 2 grams of motor.
Obtain original motor specs
Starting with the Scorpion 3008 - 32 specs we have:
Io = 1.14 amps
16 turns delta. (Scorpion uses wires per slot notation which is # turns * 2)
Determine field rotation
# magnets/2 = field rotation factor
Field rotation factor * kV = magnetic cycle/V
So with 14 magnets, field rotation factor = 7, thus field rotation = 7609 cycles/v
For 2200 kv:
14 magnet - 2200 * 7 = 154000 cycles/V
10 magnet - 2200 * 5 = 11000 cycles/V
8 magnet - 2200 * 4 = 8800 cycles/V
Determine rotational loss
Rotational loss can be estimated:
New rotational loss = (New rotation speed/old rotation speed) ^ 2
Again for 2200 kv:
14 magnet - (15400/7609)^2 * 1.14 = 4.67 Amps That's 56 Watts on a 3S!
10 magnet - (11000/7609)^2 *1.14 = 2.38 Amps or about 28 Watts - half that of 14.
8 magnet - (8800/7609)^2 * 1.14 = 1.35 Amps or 16 Watts.
Mind you less magnets might mean less rotational loss, but you also now need more turns which increases copper loss.
# of turn calculation
Original field rotation speed * # turns = new field rotation speed * new # turns
New Wind = (Original field rotation speed / Desired field rotation speed) * Original # of turns
Again targeting 2200 kV we calculate # turns for 14,10, an 8 magnets
14 magnets - (7609/15400) * 16 = 7.9 - We'll round it to 8 turns
10 magnets - (7609/11000) * 16 = 11.05 - We'll round to 11 turns
8 magnets - (7609/8800) * 16 = 13.8 - We can estimate about 14 turns.
Copper loss may be estimated by two methods:
Method #1: New Rm = ((New # turns/ Original # turns) * Original Rm) / (1+ % fill change)
Where % fill fill change is an estimate of how much more or less filling of copper you plan on fitting in the stator. This can be calculated or just estimated.
Method #2: Rm ~ (Length of wire per phase (in feet) / resistance per foot at 60 degrees C) * Termination Factor
Where Termination factor is 1.5 for WYE terminations and .66667 for Delta terminations.
Using method #1 assuming I can wind with 25% more copper fill:
Rm of original = .092 ohms
14 magnets - (.092 * 8/16) / 1.25 = .0368 ohms
10 magnets - (.092 *11/16) / 1.25 = .0506 ohms
8 magnets - (.092 * 14/16) / 1.25 = .0644 ohms
Calculate Copper loss
Copper loss = I^2 * Rm
At this point you need to make an estimate of what amount of power you want your new motor to produce. I'm going to target 30 Amps for simplicity's sake.
14 magnet - .0368 * 30^2 = 33.1 Watts
10 magnet - .0506 * 30^2 = 45.5 Watts
8 Magnet - .0644 * 30^2 = 58 Watts
Calculate Total Loss
Total loss = Rotational loss + Copper loss
As a motor is loaded up, it's rotational speed drops. Therefore to calculate this value, you need to estimate what percentage of no load speed your new motor will be running. 80-85% is a fair estimate for most motors. I'll use 85% speed. Remember that rotational loss is proportional the magnetic field rotation squared.
Loaded magnetic loss ~ % no load speed ^2 * no load loss
Loaded magnetic loss:
14 Magnet: 56 * .85^2 = 40.5 Watts
10 Magnet: 28 * .85^2 = 20.2 Watts
8 Magnet: 16 * .85^2 = 11.56 Watts
14 Magnet: 40.5 + 33.1 = 73.6 Watts
10 Magnet: 20.2 + 45.5 = 65.7 Watts
8 Magnet: 11.56 + 58 = 69.6 Watts
% Efficiency = (Power in - Total Loss) / Power in
My 3S Lipo should be around 11 Volts when discharging at 30 Amps. So my Power in is 11*30 or 330 Watts.
14 Magnet: (330-73.6) / 330 = 78% Efficient
10 Magnet: (330-65.7) / 330 = 80% Efficient
8 Magnet: (330-69.6) / 330 = 79% Efficient
Thus we can conclude the 10 magnet configuration is the most efficient.
Calculating peak Efficiency
As stated in the beginng of this post, peak efficiency happens where copper loss = rotational loss. We can estimate that rotational loss will remain about the same.
Calculating where peak efficiency occurs:
Peak eff Amperage = (Rotational loss/Rm) ^ 1/2
14 Magnet: (40.5/.0368) ^ 1/2 = 33 Amps
10 Magnet: (20.2/.0506) ^ 1/2 = 20 Amps
8 Magnet: (11.56/.0644) ^ 1/2 = 13.5 Amps
From here you put you obtained values into the % efficiency calculation and you get:
14 Magnets: 78% Peak Eff @ 33 Amps
10 Magnet: 81.5% Peak Eff @ 20 Amps
8 Magnet: 84% Efficient @ 13.5 Amps
This should get you fairly close to the actual values you will see in your motor if your estimations are close. There will be more dramatic of a spread in motors with a lesser quality stator.
Any questions or comments? Send me a PM or post if you need more details. I might have left something out. As a final note I'll give some examples of motors and how they are wound.
Tower Pro - bm2409-18 - 18 turn double wind Delta
Tower Pro BM2409-21 (BP21) - 21 turn double wind Delta
Just Go Fly 400DF - 8 turn WYE
Gobrushless triple stator:
- 4 turn Wye = 2100kv
- 6 turn WYE = 1550kv
Gobrushless double stator - 6TWye = 2150kv
I'll post more as I destroy more motors. It's only a matter of time :D
Also from experiment I have found that a speed control senses the position of a WYE wound motor more easily than a Delta. At higher rpms my delta winds get choppy than comparable KV WYEs. For this reason and the fact that WYEs are more efficient, I almost always terminate my motors as WYEs.
Excellent job, Alex! You should post this thread over in the Power Sources forum as well. I might link to it via a few threads on another website I have going. I've only skimmed it for now but will read it in depth when I'm not so tired. Good job again...
A few final notes
When I first wrote this thread my experience with winding motors was limited (hence the editing) but I find my technique improving more than anything. I made the edits as my knowledge and skills got better, but the general idea hasn't changed. These are just some notes that I've found from my experiences:
I find I get the best winds with a single strand of the largest wire I can fit in the stator. However, 26 AWG will work in nearly any motor well.
I find I like WYE terminated motors more, even for higher kvs. The speed control seems to sense the WYE terminated motors position better than a delta.
Sometimes motor winds (especially with high gauge wire) will come loose and the insulation will wear out or the wind will physically free itself and hit the spinning rotor. To avoid this I put white glue or clear paint on my windings after I wind the motor to keep them in place. Don't use CA glue as it eats the wire coating!
Again, I'll add more as I learn more. I might be no expert, but I'm learning fast and making some incredible motors in the process. The above posted 4T WYE triple stator from GoBrushless absolutely knocked my socks off. I used 3 strands of 24 AWG wire and built an incredible powerhouse. I have also built up some Scorpion kits for my gliders and got over 5 lbs thrust on a 3 cell Lipo!
I'd like to add my tip...I just ask Alex over to the house under the impression that we are going to cut out some foam wings and then talk him into re-winding a motor or two for me. :) Seriously, I'll be re-winding my BP21 soon but will wait for you to come over and walk me through it. I need to learn by doing it myself. Sinks in better that way.
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