Home-Built Brushless Models

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(DIY Brushless Motor specific)

For a more general electric flying FAQ, see:
http://www.ezonemag.com/pages/efaq.htm

 

Suppliers of Magnets, Wire, etc.

Tips and How -To's are at the bottom of page

 

Question

Answer

Why do I have to rewind a CD-ROM motor?  Can't I just use the standard windings that's on there already? The Kv (RPM per volt factor) of a standard CD-ROM motor is usually too low and the prop might not turn fast enough to fly a model satisfactorily. That means that there are too many turns (usually 30+) of too thin wire on the stator. Rewinding with less turns of thicker wire will result in a higher Kv and therefore the motor will turn faster while drawing more amps. The Kv of a motor is inversely proportional to the number of turns on the stator. A good starting point for a typical CD-ROM motor is say, 18 turns of 0.35mm wire with a Star (Wye) connection.
Why is it necessary to replace the standard magnet in a CD-ROM motor? In most cases (if not all), a CD-ROM motor will perform better if you install Neodymium magnets. Not only will the motor have more power, but it will also run cooler. I have noticed that starting problems also go away after installing magnets.
I don't know if I should use a Delta or Star connection for my new motor. What is the difference? With both Delta and Star (Wye), it's possible to achieve practically the same results with any 3-phase motor. With a STAR connection, 1.732 less turns need to be wound to get the same power (and Kv) as with DELTA. That is maybe the reason why most guys use Star when rewinding CD-ROM motors - there are less windings (of thicker wire) to put on. Conversely, when the wire you decide on is too thick to wind easily, change to Delta. The wire will be thinner, but you'll have to put 1.732 times more turns on.
How do I change the direction of rotation of a brushless motor? Just swap any two leads between the motor and the controller.
Why does my outrunner motor misfire / stop running and sometimes it doesn't even want to start ? The most common cause of a motor going out-of-sync with the controller is:
a) The magnets are too weak. The ring magnet that comes standard in a CD-ROM motor has to be replaced. Installing Neodymium magnets proved to be the single best method of getting reliable motor performance in my experience.
b) Other causes are winding-to-stator shorts, broken/intermittent wire leads or poor star connection joints.
c) The settings on the ESC are also very important. Changing the timing setting, sometimes make a huge difference when it comes to running CD-ROM motors.
d) Some ESC's just wont run CD-ROM motors properly.
e) The battery leads may be too long. The maximum length it should be is about 150mm between battery and ESC.
What is the best way of removing the magnet ring from a CD-ROM flux ring (bell)? Heating it gently with a butane flame (I use a pencil torch), will soften the adhesive and allow you to pry out the magnet ring. A sharp screwdriver tip may be needed to break out a piece of the magnet before removing the rest. Don't go overboard with the heat - when a little smoke is coming from the bell, it's normally a sign that the adhesive is burning and you can start removing the magnet. Some guys soak the bell and magnet in acetone (instead of using heat), but I haven't tried it.
I tested a motor with my small speed controller, but only used half throttle because the amp rating was too low for the motor. I did not exceed the ESC's rating. Why did my speed controller still burn out? Speed controllers work the hardest at part throttle. If the maximum continuous amp rating of the controller is lower than the max. current draw of your motor, DO NOT even attempt to use it. You may get away with it for a few seconds, but it's just not worth it. If you HAVE to test it, use a run-down battery and make sure the full throttle current is within the controller's limits.
When I make a new magnet bell for my motor, can I use aluminium, carbon fibre or some other non-magnetic material? It might run, but won't be very efficient. You should use iron (steel) for the flux ring (to complete the flux path) - the part of the bell that goes around the magnets. I have made a motor where the steel flux ring sits inside a bell made from aluminium, but on most commercial motors there is only a steel ring that is a press-fit onto the hub-part of the bell.
How should the magnets be arranged in the bell? Only use magnets that are magnetized through their thickness. In most cases, the poles of adjacent magnets should alternate, i.e. North, South, North, South, etc.  Keep in mind that there must ALWAYS be an even number of magnet poles in a motor.
How do I identify the North and South poles of magnets? When installing magnets in a bell, you don't necessarily have to know where the North and South poles are - you just have to be able to tell them apart. A good method is to take a stack of magnets and mark the top of the first magnet on the stack with a permanent marking pen. Then take the marked magnet off and put it aside. Now, mark the top of the next magnet as well and put it aside too. Mark all the magnets in the stack this way, taking care to make the marks on the same side of all the magnets. This way all the North (or South) poles are marked.
How much air gap is needed between the stator and the magnets? I normally aim for about 0.2mm on small (CD-ROM) motors. But unless one turns a custom magnet bell, you pretty much have to stick to standard magnet sizes. Most 20mm and 22mm (stator dia.) motors work well with 1mm thick magnets. Keep in mind that the smaller the air gap, the more power can be extracted fom a CD-ROM motor, but it should never touch. With large air gaps, say 0.8mm and bigger, you lose efficiency. 
I rewound a motor and it performs ok, but gets very hot. I've used the same number of turns as someone else who has the same size motor, but somehow my motor runs hotter. The primary cause of heat in a motor is copper losses. Make sure that you completely fill the slots with copper (wire). If there is still space left for another turn of wire after you've wound the required number of turns for a certain Kv, then you've used wire that's too thin. It's easy to calculate the correct thickness of wire needed, but it's just as easy to ignore this important step. Remember, the more copper you put in the slots, the lower the copper losses will be (less heat) and the result will be a motor with a higher efficiency rating.
I would also like to build my own Speed Control for brushless motors. Are there any designs available? The "Speedy-BL" design has been around for a while and is available here:
http://www.aerodesign.de/peter/2001/LRK350/SPEEDY-BL_eng.html
Also look at this site:
http://www.speedy-bl.com/speedybl-e.htm

 

 

Suppliers of Magnets, Wire, etc.

Magnets  -  South Africa Techco Power Magnets  -  Randburg
Tel: (011) 886-4122
www.magnets4u.co.za
Magnets  -  Abroad Super Magnet Man  -  USA (Alabama)
Mr. George (Mizzel)
E-mail: magnets@engconcepts.net
www.engconcepts.net/List_Of_Motor_Magnets.asp
Note: Mr. George is my favourite magnet supplier. He gives great service and will take orders on any size magnets.

Air Craft  -  Japan
www.aircraft-world.com
Note: They only carry a small selection of sizes, but they do have magnets for CD-ROM motors. Excellent service and very fast shipping (typically 5 - 8 days to South Africa).

Wire  -  South Africa Communica  -  Midrand & Pretoria
Tel: (012) 657-3500
www.communica.co.za
Heat Shrink  -  Abroad Air Craft  -  Japan
www.aircraft-world.com

 

 

Tips and How -To's

How to strip the enamel coating off copper wire Burn off the coating with a cigarette lighter or better still, a pencil torch. A heatsink will help avoid burning the wire back too far. It's like a thin alluminium clothes peg. Electronic supply store may still have theml.

Burn the wires about 1cm back. Hold the bunch of wires flat over the edge of a table and rub with an ink rubber to remove most of the remaining carbonated coating.

Use Bakers Fluid (from a hardware store, for soldering galv gutters etc,) on a cotton ear bud. This rips off any oxide when heated with a dry soldering iron tip.

Tin with solder.

Longer shafts for CD-ROM motors Most CD drives have at least one 3mm shaft that the optical read/write head slides on. They're quite hard and great for replacing the short shafts that are in the motors. It's easiest to cut these shafts with a Dremel cutting disc. 

 

I post as Skylar on RCGroups.com
and also on
LRK-Torquemax


E-mail: Christo van der Merwe   ---   Skype name: Christo1636