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Engine RPM

R/C Submarine modelers

Postby ThierryC » Tue May 06, 2003 2:04 pm

Hello all

I have just purchased a bunch of Mabushi RS-550 SH engines. I am looking for their max rpm on 12V and 6 V
I have been on various websites to try to find the right rom but each website was giving me a different values, ranging from 10,000rpm to 78,000 rpm (yeah right).
Would anybody know what is is the correct RPM on 12V and 6V so that I can get the right gear ratio on the prop shaft ??
Thanks

ps: I found a store that sells them for $3CDN a piece, I can pick up and send them if anybody is interested.




Edited By ThierryC on 1052244475
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Postby 112-1053369592 » Mon May 19, 2003 3:48 pm

It all depends... these motors are ALWAYS custom made in huge quantities for specific applications... depending on where they came from, they could be wound several different ways.
Quick trick: Generators are motors in reverse. Chuck the shaft in a drill press, connect a volt meter to the leads, run the drill at a known rpm, and note the voltage generated. Calculate rpm divided by volts, then multiply by 6v or 12v, and you'll be VERY close to the no-load speed of that motor at that voltage.
NOTE: You'll often hear that motors are most efficient at 50% of the no load speed... true, but you don't want to run there at full load! Not without water cooling at least. You should target full load at full voltage to run at about 75% of no-load speed, for the happiest motor.
If you find that your prop doesn't like to run that high a speed, then you need gear reduction to match things up.
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Postby ThierryC » Mon May 19, 2003 5:25 pm

Thanks a lot for the reply !
Using our method, I found that the engines are running at 17000rpm on 12V and at 8500rpm on 6V
The propellers I am going to use have a max rpm of 7000rpm.

Should I run the engines direct drive on 6V or should I run them on 12V with a 1:3 reduction gear ??
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Postby 112-1053369592 » Mon May 19, 2003 5:43 pm

It all depends!
Let's say you run 6v, direct drive. A teeny prop won't load the motor much, and it will spin at 8500 rpm, cavitating all the way. A huge prop will load it down, making it run slow AND pulling lots of amps. How slow? I have no idea. Experiment- make a rig with the motor, shaft and prop, and stick it in a bucket... if the motor gets too hot to hold, you are mismatched! Even better if you can rig a tachometer somehow (I build my own with a photo-interupter and read it on an oscilloscope).
If you run on 12v, you will need a gearbox... again, I can't predict how fast the motor will run then... but if runs at 75% (12,750 rpm), you'll want a 1.8 reduction.
Good luck!
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