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mipevi

C-mod RPM Drop

I replaced the metal brushes of stock Elite semi-auto blaster motors and Rapidstrike motors with carbon brushes harvested from these. I was a bit surprised to see a significant drop in RPM at a given voltage, can't remember reading about that anywhere. Pretty much the RPM I got with stock metal brushes at 2S now requires a 3S power source. I guess the motors (brushes) are still supposed to last longer, even if I need to use a higher voltage to achieve the desired RPM..?

The carbon brushes I got are apparently copper carbon, I suppose it makes sense a mixture like that would have higher resistance than just copper. I might get some silver carbon ones just to see how they work.  Smile
SSGT

It's not something I've noticed before but, then again, it's been a while since I've needed to drop a set of carbon brushes into something. Friction-wise I'd expect the opposite unless, for some reason, the carbon brushes are being pressed against the commutator with significantly more force than the stamped copper brushes were - carbon tends to be self-lubricating and carbon-on-copper contact should have a lower coefficient of friction than copper-on-copper contact. Higher electrical resistivity is a possibility but that shouldn't make that much of a difference especially once the motors are up to speed and drawing minimal current - I certainly wouldn't expect 3.7V worth of voltage drop across the brushes.

How did you measure the RPM and have you let the carbon brushes bed-in? It might also be worth lubricating the bushings at each end of the motor can end ensuring that they are seated correctly and not misaligned.
mipevi

The RPMs were measured by sound with an app called Giri (seems pretty ok for what it is). I've broken the brushes in by running them extensively at a low voltage in methylated spirits, but maybe I need to do that again without a medium to get them lubricated? I just realized I have not cleaned the commutators after running them on metal brushes, so I guess that would be my next step... Rolling Eyes

After the methylated spirits bath, the motors were pretty squeely on 2S, so I have applied a bit of sewing machine oil at each end, it took care of that.

EDIT:

Well, I lightly cleaned one motor's commutator and let the carbon brushes run for a bit on 1,5 volts. No difference, I even switched the metal brushes back into one of the motors that soaked in the methylated spirits for a control, so essentially the only difference between them are the brushes. According to Giri I got approx. these results on motors from a Rapid Red:

Metal brush, 2S: 24k RPM
Metal brush, 3S: 33k RPM
Carbon brush, 2S: 22k RPM
Carbon brush, 3S: 24k RPM

EDIT

Don't double post, edit your old one. -Boff

SSGT

I'm not sure how much I'd trust a microphone-based app for measuring motor speed. Did you make multiple runs and take an average of the measurements? The figures for 2S are almost close enough to be within the sort of margin of error I'd expect for that sort of measuring device but the 3S results, on the other hand, don't make a whole lot of sense. If the 2S results were accurate the 3S results for copper and carbon brushes should be 36kRPM and 33kRPM respectively unless something else changed between tests. Unless it's down to a difference in the sound generated with the different brushes, or simply that the app isn't accurate or reliable, I'm not entirely sure what might have caused that discrepancy. I'll try to look into it when I have the time, I have a tachometer and I've been meaning to do some motor testing for a while anyway. For the time being Stryfe and RR motors should give best performance on 3S regardless.
mipevi

Yeah, certainly possible the app failed to get accurate RPMs. I'll do some FPS comparisons after I get a rewire done. Smile

EDIT: Chrono results pretty much confirm the motor RPM drop. I get a bit under 100 FPS using 2x IMR and a stock Rapid Red, now with the c-mod I need 3x IMR to achieve the same.

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