One hour CD ROM motor construction PART2 - Page 7 - RC Groups
Dec 15, 2003, 11:01 PM
Why not Delta?

# Re: Re: Re: Re: Re: Re: Re: Re: Brushless mystery uncovered - part two

Quote:
 Originally posted by rpage53 Interesting ideas, but since this topic is about converting CD-ROM motors in one hour could you please start another thread. You may want to move these posts over there.
As you wish. For you guys still intertested in that topic here is the thread I started:

RysiuM
Dec 16, 2003, 01:11 AM
Boffin

# FEMM Modelling

Quote:
 Originally posted by acvar Copper wire is definatelly cheaper than magnets
I get my CD-ROM's for free and 5X5X1 N45 magnets are only \$0.20 each. Can't get much cheaper than that.
http://www.engconcepts.net/Magnets/P...x_Magnets.html

I was going to see if the 2mm thick magnets were an improvement, but then the bell has to be bigger so its not really a fair comparison. I think I'll try some FEMM modelling. Anyone else try this and any comments on its usefulness (or should I start a new thread)?
http://femm.foster-miller.net/

Rick.
Dec 16, 2003, 07:05 PM
Why not Delta?

# Re: CD-ROM motor RC Prop. torque meter

Quote:
 Originally posted by Takao Shimizu I hope that my torque meter has good accuracy. The calculation(Torque x RPM) says that my CD-ROM motor's efficiency is 75%.
Please correct me if I'm wrong with units or calculation:

Power = Torque x rpm
Torque = Force x pivot arm
Force = indicated mass x gravity

So in SI units:

Power [W] = indicated mass [kg] * 9.81 [m/s2] * pivot arm [m] * 2pi * rpm [1/s]

for units more common:

Power [W] = indicated mass [g] / 1000 *9.81[m/s2] * pivot arm [mm] /1000 * 2pi * rpm / 60[/min]

so Power = Mass x Arm x RPM x Factor
where:
Power in [W]
Mass in [g]
Arm in [mm]
RPM in [rpm]
Factor = 1.03 * 10^-6

Is it right?

RysiuM
Dec 17, 2003, 09:56 AM
Registered User

# Re: Re: Re: Re: Re: Re: Re: Brushless mystery uncovered - part two

mmormota,

>This extremely simple sensing part is really enough for the firmware?

It is full runnig on may test bench without Rx interface power control software(I need a time to write the software).

Takao
Last edited by Takao Shimizu; Dec 17, 2003 at 09:59 AM.
Dec 17, 2003, 09:58 AM
Registered User

# Re: Re: CD-ROM motor RC Prop. torque meter

rysium,
Power = Torque x rpm

Power in [W]
torque in [g/cm]

Takao
 Dec 17, 2003, 10:47 AM Registered User RPAGE: Thanks for the info! Love this stuff, especially where to get stuff cheap for us poor flyer's!
Dec 17, 2003, 11:56 AM
Why not Delta?

# Re: Re: Re: CD-ROM motor RC Prop. torque meter

Quote:
 Originally posted by Takao Shimizu Power = Torque x rpm Power in [W] torque in [g/cm]
C'mon Takao, you can do much better than that

W (Watt) is unit of measure for power in SI, and = kg*m2/s3

Unit of measure for torque is Force times Distance. In SI it is N.m (Newton times metre) = kg*m2/s2. The measured weight (the force of the pivot on the scale) is in kg which is equal to the force of 9.81 Newtons.
Rpm is measured in 'revolutions per minute'. In SI the unit is [1/s] (one per second), so the conversion factor is 1/60

It's not as simple as you wrote. It's easy to get lost in that. What i'm trying to find out is, when you build a torque meter out of the postal scale (like you did) how to calculate the results.

I just run into website for torque calculation:
http://hem.passagen.se/suntool/utils/p2t_en.html

When I enter 0.001028 kW and 1 rpm I got the torque = 9.81 Nm that the force of 1kg weight (in earth gravity). So I guess the factor of 1.03 is OK, right?
And when the weight is in grams and distance is in cm then the factor must be divided by 10^5.

If the pivot arm = 1cm, and proppeler spinning at 1000 rpm gives 100g on the scale, then the power is 1.03 W
Correct me if my calculations are off. I even confused myself.

RysiuM
Dec 17, 2003, 12:04 PM
Why not Delta?

# Torque meter

I'm sorry to get the whole discussion about this torque to power calculations started. But it's not out of the topic at all.

I need to know that because my new CD-ROM motor (that one with new magnets) gives me very poor results on the bench. When I calculated the Power-In (U x I) and Power-Out (using HP calculator) I got 50% efficiency. I need to build such torque mettering stand to know exactly, what is going on.

RysiuM
Dec 17, 2003, 01:31 PM
Registered User

# Re: Re: Re: Re: Re: Re: Re: Re: Brushless mystery uncovered - part two

Quote:
 It is full runnig on may test bench without Rx interface power control software(I need a time to write the software). Takao [/B]
Great results!
According to the schematics, using small smd parts, it could be as light as 2g without wires!
Dec 17, 2003, 01:38 PM
Registered User

# Re: Torque meter

Quote:
 I need to know that because my new CD-ROM motor (that one with new magnets) gives me very poor results on the bench. When I calculated the Power-In (U x I) and Power-Out (using HP calculator) I got 50% efficiency. I need to build such torque mettering stand to know exactly, what is going on. RysiuM [/B]
I think your calculation (constant) is correct.

The usual CDRom motor efficiency is rather low, I compared my cdrom motors to a PJS300SF and found that the PJS is much better (the PJS efficiency itself is not very high).
Dec 19, 2003, 03:03 AM
Why not Delta?

# Re: CD-ROM motor RC Prop. torque meter

Quote:
 Originally posted by Takao Shimizu I hope that my torque meter has good accuracy. The calculation(Torque x RPM) says that my CD-ROM motor's efficiency is 75%.
Tako, what are the results of your experiment with torque meter? I mean after you use my calculation.
I'm going to build my own torque meter just to see how are these brushless motors doing in example compared to razor 300.

It is important to me, because I want to use BL motor on my 75% Depron Edge 540 Profile, and saving 20g on motor and another 20 on battery makes a big difference (compared to Razor 300) on 24 in wingspan 3D plane. But if there is no way, I can get enough power out of CD-ROM, then I'm in big trouble. I didn't see any other 20g BL motor that will pull 10oz static thrust with 2 or 3 340 LipPo.

RysiuM
Dec 19, 2003, 05:17 AM
Registered User
mmormoto said
Quote:
 The usual CDRom motor efficiency is rather low,
Why is that? I too have found my motor on the low side. (50% at best it would seem). I wound the motor with thick wire ... 0,63mm and managed 13 turns per stator before all space was filled. Using the thicker 2mm neo 45's as well.

Is it prehaps because that stator length is too short? Would love to know reason for this.
Dec 19, 2003, 05:58 AM
Registered User

# Re: Re: CD-ROM motor RC Prop. torque meter

I have checked the balance of mass and wiring torque mistake.

Number of winds 39
Wire size 0.32mm, Delta
Magnets used 5x5x0.9 N45 (from Miki Hobby)
Loaded voltage 6.95V. 2s etec 700
Amps 5.7A, 9720rpm at full charge
Input watts 39.6W
prop size 5.5x4.5 APC, DD
RPM:9720rpm
Torque:189g*cm:1.9N*cm
Output Power:18.0W
Effciency:45.4%
ESC JES 04-3P

Static thrust:135g(APC 5.5x4.5),173g(GWS 6050)

### Images

Last edited by Takao Shimizu; Dec 19, 2003 at 05:29 PM.
 Dec 19, 2003, 06:21 AM Registered User Takao, your torque results asume you have no prop losses !!!! From what i have read , the best prop is only about 72% efficient , smaller props are less so . I beleive the motor is getting much better than you are measureing with your setup . Stewart
Dec 19, 2003, 06:29 AM
Registered User
Quote:
 Why is that? I too have found my motor on the low side. (50% at best it would seem). Is it prehaps because that stator length is too short? [/B]
Unfortunately I don't know the reason.

The PJS motors are very similar to a CDRom motor. While the PJS efficiency is not the best compared to other outrunners, much better then the CDRom motor.

Differences (possible reasons):
- stator length longer, as you wrote (maybe it is important, PJS moved from the short&big diameter 500 to the longer 550)
- extremely small airgap, smaller then possible with 12 stock magnets (machined magnets)