In this short tutorial, we’ll explain how to rewind brushless motor damaged by windings burnout due to the overload, short circuit, or sudden motor blocking at full speed. It’s not complicated when you get the full idea, and it can save you plenty of time (no waiting for new motors to be delivered) and money (yes, you can do it on your own).
Most common motor failure
There are a few ways to damage brushless motor windings:
- Overheating as a result of insufficient cooling
- Vibration and mechanical damage of internal winding juction and power supply wires
Motor overload is the most common cause of stator core burnout. If you load your craft more than the mass propellers can lift, the motor will heat up very quickly which will cause internal isolation (polish, enamel) melting.
As soon as the enamel is melted, there’s a short circuit inside the winding and that causes wire melting and motor destruction. Motor overheating can happen a lot if the rotor is poorly designed and the airflow which cools windings is insufficient for proper stator internal core cooling.
Mechanical damage and strong vibrations can lead to a break in the connection of a wire which powers the motor and stator internal core wires.
Selecting the correct brushless motors is key to obtaining the right performance for your drone. Motors can be optimized for long range efficiency, racing, freestyle tricks, tinywhoops, and much more. Using the wrong type of motor can lead to underperforming quads, and maybe even ruining sensitive components of the drone, costing you money.
parts of a brushless motors
When the motor is armed, the rotor is locked to the stator with a C-clip key which secures the shaft - it stops the rotor from the flying out of the motor bearing, along with the propeller and everything connected to the shaft.
The stator is made of tiny metal sheets and it’s protected by industrial varnish which is resistant to high temperatures, and usually olive-green. It’s made of sheets rather than one solid piece of metal because it creates a more efficient magnetic field. Just a few windings around every stator tooth creates a magnetic field controlled by ESC.
In three-phase motors, the number of stator poles must be divisible by 3.
As an example, we’ll take BARD Lil' Floaters with 9 motor teeth. However, the number of magnets on the motor can vary. 11xxx, 12xxx, etc motor sizes typically have 12 magnets and 9 stator poles. The marking we use (for this example) is 9N12P.
Three-phase motors have three motor phases and they’re being connected to speed controller (ESC) with three wires. The coils are made of three long wires, winded around stator teeth in a special way. There are two ways to do that:
Wye (Star) termination: 3 starting points of wires make a neutral spot of each of 3 coils. In motors we use in RC world, this point is protected by heat-shrinking tube and it’s inside the motor, usually under stator teeth. In Delta termination, there’s no physical neutral spot.
Delta terminated motor is rotated approximately 1.8 times faster than Wye terminated motor, while the Delta terminated motor has 1.8 times higher torque. It’s possible to convert Wye into Delta and vice versa, if needed (speed or power).
Brushless Motor Disassembly
For a start, you need to remove the C-clip key which holds the rotor shaft inside the stator bearing and is not allowing it to become separated while the motor is armed.
Some claim that keeping the rotor outside the stator for a longer time period is damaging for magnets because they start to lose their magnetization. However, no scientific theory proves or disproves this.
Next, you remove the rotor from the stator. It’s advisable to store the C-clip somewhere safe- it’s quite easy to lose it.
After that, you remove the winding part which represents the neutral spot and untangle it.
In this example (and every other motor of this type), it’s best to start unwinding from that point because it’s the easiest to count the windings.
It’s important to count them, if possible, because we’ll need that information for winding back. If not possible (burnt wires break easily and you’ll have a hard time separating and counting them), we can always email the manufacturer’s customer care and ask for the correct information about the wire thickness and number of windings.
The wire needs to be extracted very carefully; you need to avoid damaging industrial varnish on the stator teeth - it protects core wires from the short circuit. It’s advisable to use plastic tools for this. Varnish residue needs to be cleaned properly, along with the dirt and soot staining. If the industrial varnish is damaged, you need to use acrylic varnish or another temperature-resistant coating on the damaged area.
After the cleaning, you can start with first windings.
Brushless Motor Winding
When repairing the motor, it’s important to choose the correct type of wire. For motors, we use polished enamel wire with higher temperature isolation melting coefficient - enamel has been proven to be optimal for this.
On wires.co.uk you can find mostly all industrial wires needed for this job. Prices are low, delivery fast, wire is high-quality and can be ordered in more than one colour. 50g wire (and yes, they are metric, even in UK 🙂 ) spool is enough for dozens of motors so it’s best to order that one.
The price range is £2-£3, depending on the wire thickness. For this tutorial, we’ll use 0.18mm wire. It’s worth mentioning the wire thickness includes the coating thickness as well- that means the effective thickness of a wire is a bit lower. Thickness marking indicates isolation thickness as well (for mentioned website only). In any case, it’s always a good idea to check out the catalog provided by the website.
This motor has the most common winding scheme- ABCABCABC. You need to wind the windings very carefully, with first winding on the bottom moving upwards to the top of the stator tooth.
When you count the windings (the reference should be the original motor), you skip two teeth and wind the next one. When that one is done, skip two teeth and wind the next one, and again, skip two and wind the next one as last of the A phase is done. Do the same for the phase B and C.
Typically, stator manufacturers mark some teeth with dots. You can use them as a starting point. First, you find a starting point and wind the correct number of windings.
As an example, we’ll use 11 windings. You need to distinguish the start and the end of the wire so we don’t get lost in them. In my case, the start of the wire is always longer than the end - it’s a great way to know which wires to join into a neutral point.
When the first three teeth on phase A are sorted, you move on to the next phase from the next first tooth clock-wise direction. It’s also important where do wires end because the internal wire must be connected to the external wire.
You need to be careful - as the mount pattern of the motor is already defined, if you miss the spot, the wire exit point won’t match the carbon frame arm and you’ll need to redo the whole motor.
After you finish phase two (B), you move on to phase three (C) the same way you did before with phase A and B.
Next, you connect the neutral spot (termination point) and separate wires with all three phases and move the wires away.
If you got to this point, congrats, the essential part of the process is done. You need to have a look at every tooth, check if any wire is crossing the tooth line (if it does, move it slightly to the centre).
This link: http://www.bavaria-direct.co.za/scheme/common/#prettyPhotois a great source of information about other patterns. The scheme for this example, 11xxx, 12xxx, and others is the same as the CD-ROM pattern in the link above.
Testing Brushless Motor
When you’re finished with winding the wire, it’s time for testing. First, you need to solder the termination point wires and make sure all three wire ends are successfully connected. To do that, you need to drench the tip of the soldering iron with the solder until you get a soldering blob. You pull the wires through the blob until the enamel has melted and wires are soldered together.
Next, you connect three ends to ESC (or, on a flight controller, AIO, as shown in this example). Wires are, at that point, not protected from the vibration and other mechanical damage so you need to be very careful when handling them. You need to check if there’s a short circuit and carefully put in the rotor. Termination point can be shortened additionally if needed.
In your favourite flight controller configuration software (Betafllight etc.), you can activate the motor to see if it works. If, when the battery is connected, you hear ESC’s initialization “melody”, you’re on the right track. You activate the motor and listen for any odd sounds, vibrations etc. In this photo everything’s alright. You can also try to test the motor with full power to see if there’s overheating.
After you made sure everything is okay with the motor, you cover the windings with a very thin layer of transparent nail polish to protect them from vibration and movement. Then, you shorten the wires and prepare them for connecting with the main wires. Be careful when cutting the wires- if you cut them too short, you’ll have to go through the whole process again- which includes removing fresh nail polish you just applied.
You solder all wires together so the smaller ones don’t stick out; solder joint must be parallel and approximately 2mm length. This way of soldering prevents vibrations from breaking the wire connection.
The neutral termination point should be shortened as much as possible (you need to be careful not to unwind it in the process). Heat-shrinking tube will protect the termination point and three main wires. The diameter needs to be the smallest as possible- too wide one can get caught in the rotor and that certainly needs to be avoided.
After warming up the heat-shrinking tube with a hot-air gun, when the tube is the tightest around wires, bend the termination point and tuck it between the stator and motor base so it doesn’t hit the rotor magnets.
Also, you need to check if unprotected wires are touching the stator teeth and adjust the wire tension if that’s the case. When the fixation is finished, cover them with a thin layer of nail polish and let dry.
When the nail polish is finally dry, and the wires are fixated on the bottom of the stator and motor base, you’ll protect them with a heat-shrinking tube. Be careful not to pick too wide tube- when shrunk, it shouldn’t touch the rotor. After you make sure all wires are in order, you can assemble the motor.
Don’t forget to put the C-clip to the bottom of the shaft before you test the motor.
When the motor is assembled, along with C-clip on the bottom of the shaft, you can solder the wires to the FC and try to turn on the motor. You also need to check the sounds the motor makes- make sure to give a few speeds a try and wires isolation is not touching the rotor.
In essence, motor winding isn’t as complicated as it seems. Until the winding technique is perfected, you’ll spend hundreds of meters of wire but, once you get it, it will go seamlessly. Practice makes perfect. If you’re having issues with separating the stator from the motor (that can often be problematic), you’ll also have trouble with winding teeth the closest to the bottom of the base. The chances of aesthetically pleasing windings are quite low.
The most important parameters mentioned are: the number of windings (you don’t want to have a different number of windings on teeth and you must make sure they’re all the same) and the thickness of the wire (too thin means quicker overheating and too thick one means not enough space for desired number od windings- you need to balance the two).
When your motor burns out and bearings and the rotor seem fine, hang on to it and fix it. You might fail the first time, you might count it wrong, but when you wind your first motor and your craft successfully flies, you’ll never throw away burnt-out motor ever again.
Author: Davor Kustec
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