|Dec 19, 2003, 06:35 AM|
The propeller is just a load here, the propeller efficiency is not in any relation to the motor efficiency. Unfortunately the CDRom motors are not very efficient, the result is similar to my measurements: 40-50%, the 50 is at low power only, no more.
PJS motors: 55-70%, much better.
|Dec 19, 2003, 11:41 AM|
You are correct. When I test small brushed motors for micro planes to determine their efficiency I use sticks of different lengths as it is easy to adjust the load. I could use props, but in this case we want to just put a load on the motor and a stick is easier to adjust than props.
|Dec 19, 2003, 11:51 AM|
Re: Re: Re: CD-ROM motor RC Prop. torque meter
I run the same data through my prop calculator (a very nice Excel sheet based on HP calculator) and my results are very close to measured. That helps me regain the confidence in my calculator. At the same time I'm a little disappointed with the motor's efficiency. I may have a new idea why it's so low, but first I need to do some homework before I'll make a fool of myself
Thanks again Takao,
|Dec 19, 2003, 02:33 PM|
I have checked some theory and calculations and here is my conclusion:
We are saturating the stator's core.
The magnetic field in the core is:
B = u0 * ur * N * I / 2r
B - the magnetic field [T]
u0 - the permeability of a vacuum ( 4 • pi • 10 -7 in [Vs/Am]).
ur - relative permeability (for iron is is from 1000 to 10000)
I - current (in A)
N - the number of turns in the coil.
r - the radius of the coil (in [m]).
The saturation point for iron is 1.9 to 2.1 [T]
Here is my calculation (example)
u0 = 1.26 E-6
ur = 1000
I = 5A
N = 20
r = 2.5mm = 0.0025 m
Ther result is 50.24 T
It is over 25 times more than saturation point for iron. So the energy is just waisted.
Unless I made a mistake and place the decimal point in the wrong place or messed up the units, it may be something to think about. Maybe that's the reason CD-ROM uses very low current (like in miliamps) for the motor.
If I'm right, we need to think about coreless solution.
|Dec 19, 2003, 03:52 PM|
Re: CD-ROM efficiency
Bottom line: If your calculation is correct, every iron core motor fails, not only a CDRom motor. :-)
|Dec 19, 2003, 04:17 PM|
Re: Re: CD-ROM efficiency
Any other known motor has much more iron in there (I mean area) - much more, so the magnetic field density is much smaller. I example Razor 300 has a pole (around) 15mmx 7mm (about 5 times more than CD-ROM. And Razor is happy about 5-8 A.
Again, maybe I'm wrong, but 50% out of 40W is going somewhere. Not in the copper, because as I calculated it before it's about 1W. Where is 19W ??? It's not in ball bearings, not in the air drag in the rotor, so it must be in the iron. The main question is Why and What can we do about it
The challenge is to get 75% efficiency on 22g motor taking around 30W-40W (7V, 5A or 11V 3.5A) giving out 25 - 30 W.
|Dec 19, 2003, 06:01 PM|
The effective μ of a core with a gap:
μe = µr / (1 + (µr × lg / le))
lg: the lenhth of the gap
le: effective length
The 'effective length' of a core is a measure of the distance which flux lines travel in making a complete circuit of it.
In our motor the le is about 15mm, the lg is about 1 mm.
(Flux line: the length of the working tooth, the gap, the magnet, then a complicated path of the ring and the other gaps and teeth parallel but different weight factors, the closer the bigger).
Ok, let's count with 15mm and 1mm, ue=15, it is almost independent of ur if it is big enough.
Worth to mention that saturation alone is not not direct loss of power but hysteresis.
Unfortunately I am not familiar in motor calculations, I think it is very complicated if we want to calculate such things that efficiency.
On the practical side, I agree that probably a thicker (longer) packet is more efficient. The form of the magnets gives a higher then necessary gap on both sides (I mean toward the bell too), and I am pretty sure that the shape of the gap (circle to line, not equal distance and this is true on both sides of the magnets) is a source of loss too.
That damn thing has a low eff. at low power too. Replacing the magnet ring increases the effective power range but not really the averall eff. That means that the CDRom motor has some inherent problems. Maybe we have to take a look to the better outrunners, the axis or orbits, or even the DIY lrk-s to find out, what is better there. When we build a really good CDRom motor, to be honest we keep only the stator iron: new bell, new bearings and new tube for the bearings, new shaft... :-)
Maybe better to find a small 12 teeth core and build a small lrk...
As I think now, if one need a light, silent, direct drive outrunner for cheap, just use the CDRom motor in delta or rewind it. Enjoy the flight with it, but keep in mind that there are more efficient motors.
But if one need a really good small motor, and ready to use lathe and other pro machines whatever is necessary, probably better to find another motor type, lrk or who knows...
|Dec 19, 2003, 07:10 PM|
So getting to the original thread let's find some way to get a good power from that CD-ROMs
|Dec 19, 2003, 07:20 PM|
Joined Nov 2002
The copper resistance loss
The resistance of 0.32mm dia. UEW wire at 2.6meters is 0.8 ohm at Y wiring.
Then, the copper loss=5.7A^2 x 0.8ohm=26W???
I need more accurate ohm meter.
|Dec 19, 2003, 09:00 PM|
Re: The copper resistance loss
The current allways flows through only 2 series windings (as seen on your diagram on the schematics of your controller), so the series resistance is only 0,8Ohm * 2/3=0,53Ohm.
My PJS300SF has about 0,35 Ohm measured between any 2 pins.
This could be a good reason for the higher efficiency.
|Dec 19, 2003, 09:26 PM|
Industrial motors have to work for years or tens of years, and they rate and use them in a fairly conservative way.
I think very rare if our motors are running more then 100 hours in their life, and most of the time not on full power, so we can take bigger risk.
Maybe that "low power" industrial motors could give us multiple times the rated power...
A guy is making interesting experiments with a pour little maxxon in the indoor and micro group.
|Dec 20, 2003, 03:00 AM|
I'm being confused more and more every time I play with this CD-ROM thing.
What I did is:
swap the rotor from one motor to another stator, so the combination is:
1. Stator 18T 2x28G
2. Replaced strog magnets (12 5x5x1)
3. Everything on GWS B drive (4.43:1)
First I noticed, that I have bigger gap between stator and rotor (because the other pair doesn't fit together).
Cogging is much much lighter. It's kind of stronger than original magnetic ring but not as strong as on the other stator.
I got improvment - the same prop/power gets more RPM (and more current).
- Prop GWS SF 9x4.7
- 2 cell 1200 Lipo
The results are:
rpm = 4620
I = 5.4A
P IN = 38W
P OUT = 26W
Loss = 12W
Copper loss = 3.4W
efficiency = 70%
On my previous results I got screw up by my Amp-meter. What I noticed the same condition with Amp-meter in the circuit I got only 4020 rpm, so my calculations were off because the voltage on ESC was much lower than on the battery.
Anyway, the same motor and gearbox with GWS SF 9x4.7 put 4.5A, 5150 rpm, 7.29oz thrust and efficiency went up to 75% (loss about 8W, the copper loss is 2.3W).
That fits me for now. I will put this motor on my little Edge.
|Dec 20, 2003, 06:09 AM|
Joined Dec 2003
Hello everyone , I'm new here
I wonder if there is an ability to use the original CD-ROM controller ( with the sensors ) to drive the improuved CD-Rom motor.
Is the original CD-ROM controller able to vary speed , if not how much rpm is x50 ?
As i undestand all we need is to:
1. Rewind , remagnet the motor ( and gently return it back to its place ).
2. Replace power elements in the controller.
3. Make some ( simple ?? ) interface between Rx to CD-ROM controller input.
Thanks , Dany
second time posting this , am i totaly talking rubbish ?
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