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 |
