Brushless DC motors

[Jeff C]

In one of the issues of RC Microflight, there is mention of a DC brushless motor project that has been hand built. Does anyone know if the brushless motor from a computer floppy drive or a CDROM could be adapted to the controller shown in the article? An already assembled stator assembly would be a fantastic start to a very interesting project.

Jeff

[GHMBO]

Most computer cooling fan motors are coreless also. The controls are within the motor housing. I am sure that one of the electronic gru's out there could figure a way to vary the speed/oscillator. They are ball bearing and run very smooth.

[Jeff C]

I thought of that today as well. I took apart an old floppy drive and an old CDROM. The floppy drive didn't help much since the stator and magnets were spread out over a large area but the CDROM motor was very compact.

The CDROM motor's magnet assembly is rather confusing since it has 3 coil leads (easy) and 8 magnet leads (difficult) for 3 reed switches.

I'll have to look into the controller on the fans tommrow and see if I can mate them.

Jeff

[GHMBO]

Jeff
I am not an electronic expert,but in looking over the fan that I took apart for the ball bearing, I noticed several things. The one that I took apart had 4 coils and 3 drivers and one integrated circuit. I assume that the fan motor is driven by 5 volts and the IC is some sort of oscillator for constant speed. It would appear that the oscillator is a 3 phase one and provides the signal for the coil drivers. If we could figure a slick way to have a variable 3 phase oscillator to provide a signal to the coil drivers, we would have a suitable coreless motor.

[Jeff C]

I took apart a fan and a floppy drive as well. Found 4 good ball bearings so I'm probably going to just build a brushless from scratch. It seems to be easier to build from scratch with larger scale parts than it would be to reinvent the wheel and retrofit an existing motor to the controller circuit.

The motor at http://www.rcmicroflight.com/library/motor1.asp looks great but there really isn't any detail on what electronics are needed. It just says "build the circuit" and barely touches on the "timing board" which has the magnetic sensors in it.

I'll have to email the author for some more detail.

Jeff

[GHMBO]

Hi Jeff
You are probably right in building something in a larger size. I also thank you for pointing out the MicroFlight article that I had somehow overlooked. I believe that i saw an article mentioning advances made by the author, so you contact may be fruitful. I think that I will head for the local college library and see what I can find about brushless motors.

I have one more fan to dissamble and this time I will be more careful and try to draw the controller circuit. I would like to understand how these things work. Keep me informed on your progress. If the bulletinboard gets in the way, email me through my profile.

Jim

[Jeff C]

I've been taking apart some old hard drives for ball bearings and came across something interesting. The brushless motors in the platter motors have maybe 4 or 5 leads instead of 8 or 9.

I should email the author of that brushless motor article and get the schematics for his circuit boards.

Jeff

[GHMBO]

Jeff
I have been looking and reading (precious little on the web) and I think that now that I understand how they work. It would appear that the one in the article has a "magnetic feedback" switching network that drives the coils. He apparently uses an ESC for speed control. The drawback is that sometimes it must be started by hand. Most fans are "constant" speed and have an oscillator that runs at somewhat constant frequency. We have to design an oscillator who's frequency is controlled by the radio reciever output. I will keep looking and experimenting and let me know your progress.

Jim

[Jeff C]

I just took apart a spindle motor for a hard drive and oddly enough - no magnetic switches inside! The motor has 4 leads that are connected to the stator but there are NO magnetic sensors at all! These are very interesting to say the least but they may not be very useful since they weigh quite a bit and don't have a motor shaft but instead have 6 tapped holes (much like larger props) around a 1" diameter hub. This would be a decent motor since it was from a 5400 RPM hard drive and it was subject to spinning a large rotating mass.

Jeff

[GHMBO]

Jeff
The fans that I have been taking apart seem to be similiar to your disk drive motor. The magnets are in the hub that rotates and look like a strip of flexable "refrigrator" magnet. By running a steel pin around the inside of the hub I found 4 magnetic spots. This matches 4 coils that are electronicaly energized. I think that we have figured out how to drive these motors, but now we have to figure out what kind of signal comes out of the reciever and maybe we can come up with a speed controller. I have modified the existing drive circuit for one of the fans and can speed control it with a DC input to the controll circuit. The 5 volts or so are still required for the coil drive transistors.

Jim

[Taylorcraft]

My two bits worth, keep the change! :)

Usually the fans in a computer runs on 12 Volts and they're mostly regulated by voltage(!) The higher the voltage, the more RPMs, up to max voltage. The exeption here is the three-wired fans, that run a sensor wire back to the motherboard. As far as I know they can still run on full speed if you just connect them to 12 volts, the sensor is there just to provide a regulator on the motherboard with RPM info.

The question is if these motors are really suited for our use. At least we need something more than a few watts to churn up enough air to make our planes move, right?

Generally, a CPU fan motor draws 0.15 to 0.2 amps at 12 Volts. This adds up to approx. almost 2.5 Watts at the most. On top of this, these little blowers don't have any good power utilization, as far as I've been able to find out. They're more or less specialized to spin that little fan and that's it. Like, "Torque? What's that?" However, if one could put two or more motors "in series", IE on the same shaft, things could get more interresting? The only drawback will be weight. The larger case fan motors will probably be to bulky and heavy to use in small planes, even tho' they have more power.

However, this tickles my mind.
- What kind of prop should a motor be able to spin to be useful? - And at which RPMs? I should have a COX 6X3 collecting dust somewhere, can it be used for testing? How much trust will be enough to pull a, let's say, 30 inch plane thru the air?

BRG

Taylorcraft
______________________________________
Given enough power even a brick will fly.
But, the lighter the brick, the better it flies!

[GHMBO]

T-Craft
You are right about the cooling fans being too weak for our purposes. In looking for bearings, Jeff C and I noticed that they were moving magnet electronicaly switched coils (coreless)motors. Phil Smith ,as reported in MicroFlight, has a 150+watt guy running a prop at 9000 rpms. Phil uses a magnetic reluctance sensor in a "feed-back" loop to controll coil switching(and maybe speed). I am moving in the direction of switching with a voltage controlled oscillator. The advantage is that the motor current does not have to run through the ESC, as in convential motors, but only through a pair of switching power transistors. Hence the motor power comes directly from the battery and the speed voltage control(very little power) through the ESC. This way a small ESC can control a high watt motor. This is all in the early breadboard stage.

Your thoughts and comments are surely welcomed.

[GHMBO]

Jeff C
Check out some of the links in this thread. Looks like the German Helio boys have our answers.
http://www.*************/forums/show...&threadid=34755