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Old Jun 12, 2005, 01:58 PM
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Will Someone Please Post Pictures of the Inner Workings of a Brushed Motor?

So I don't have to tear apart my first (and only, so far) Speed 400 brushed motor to find out what it looks like inside . In particular, I'd like to see what the "brushes" actually look like. Cutaway pictures would be helpful (you know, like the mannequins in human biology class). Pictures of brushless innards would be great, too!
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Old Jun 12, 2005, 02:32 PM
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Old Jun 12, 2005, 03:58 PM
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Your wish is my command. Here's a 7.2v 400 motor that sized up after many hours, (I think a screw must got caught in the magnets) and a black long can whose bearings had run and whose magnets were weak...look at the brushes. Still perfect.

Note the plasticky brush holders that can melt at high temps./current...also the brush direction which suits anticlockwise rotation.
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Old Jun 12, 2005, 04:51 PM
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Which components exactly are the brushes? The copper looking things on the white end caps?
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Old Jun 12, 2005, 04:58 PM
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Yes. The bronze flat bits are the springing, and the square lumps on the end are the bronze brushes. You can see how neatly concave they are to mate with the round commutator. Once you take the aramature assembly out the brushes spring together - in use they are forced apart with the communtator in between.

The armature runs in bronze bearings in the case front, and in the metal endbell. This is crimped into the cases. You can loosen the crimps and rotate the endbell a bit to time the motor.
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Old Jun 12, 2005, 05:41 PM
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Those look like some beefy brushes. When folks talk about the brushes wearing out do they wear all the way through those big blocks?

Thanks

Typ
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Old Jun 13, 2005, 05:14 AM
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Yes, I suppose so. But it has never been my experience. What I think happens is that if you operate at very high currents, the brushes get almost red hort, and several thiungs happen

- the springs go soft on you
- the brushes start to bounce
- the arcing adds to te heat and actually 'sparke erodes' the brushes and commutator leading to less contact area
- so the brushes get even hotter..

I have compared operating a 6v speed 400 at 12A on 8v (what many people say they do) with operating it at 11v on 9A (which is what I do)

In both cases the input power is just shy of 100W.

In te case of the low violtage high current case, the losses are estimated at 57W.!!! The motor is operating at a staggering 40% eficiency - even less when it heats up..

In the case of the 11v/9A case the losses are an estimated 36W. More like 64% efficiency, which is fair for a can.

In the first case the the output power is around 30W. In the second case its around 64W.

You can actually get more power out of a 6v can on 8v input by going down to 10A.

Note that going from 9A to 12A is nearly DOUBLE the heating effect on the brushes.

The moral is don't run yer 6v can 400 much over 9A if you can avoid it. better to add more cells by far than drag more current.
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Old Jun 13, 2005, 05:31 AM
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Vintage, can you rewind the brushed motors similar to brushless to get adjust current draw?
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Old Jun 13, 2005, 05:50 AM
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Its possible, but I would not bother.

You can get a sensible range of winds on a mabuchi 400 from the hottest (graupner 400 race 4.8v) through the MFA Rocket 400, the 6v 400 and the 7.2v 400 (everyones 400's) usually at less than $10 a hit, and its simply not worth the bother.

A race 4.8v 400 on 10 cells or 3s LIPO at 12A input is as good as it gets for a 400. I reckon on 11v input something like 12A draw and the motor screaming at 40K RPM and a bit...at 132W input and a total loss of about 35W, for nearly 100W OUTPUT...and over 70% estimated efficiency..you can get a heck of a power unit.

Needs gears of course. Lots of gears. Must get one and try it...
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Old Jun 13, 2005, 06:06 AM
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Thank you sir!
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Old Jun 13, 2005, 06:19 AM
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Well...I actually asked coz of these little toy motors I have lying around. Most of these are designed for 3v-6v. Now if I could rewind these for 7v-12v...hmm.

Ex.
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Old Jun 13, 2005, 07:35 AM
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Quote:
Originally Posted by vintage1
...The moral is don't run yer 6v can 400 much over 9A if you can avoid it. better to add more cells by far than drag more current.
Vintage, I seem to remember that you have a rule of thumb on how much to back off on the prop when going from 2s to 3s. When I tried 3s with the little LPS this I didn't have a smaller prop of the appropriate size so I intended to avoid full throttle. The performance improvement was impressive but my plan to avoid full throttle fell through in the heat of the battle with the wind and ... it lived a short but full life. Anyway, I plan to try this again, but now I will do all the homework up front; measure current draw and RPM's on the bench to make sure it's not overproped. There seems to be less margin for error on 3s.

Another question for you, vinatage, are the brushes an amalgam of graphite and bronze?

POD
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Old Jun 13, 2005, 01:14 PM
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Quote:
Originally Posted by ExtremeSkyes
Well...I actually asked coz of these little toy motors I have lying around. Most of these are designed for 3v-6v. Now if I could rewind these for 7v-12v...hmm.

Ex.

Spin them up and tell me how big they are, and waht RPM they do on a given voltage on no load, and I'll bet you I can tell you what load to put on them to make em fly without rewinding...
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Old Jun 13, 2005, 01:18 PM
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Quote:
Originally Posted by podavis
Vintage, I seem to remember that you have a rule of thumb on how much to back off on the prop when going from 2s to 3s.
Yep. Knock 20% off everything about.

So a 5x5 on 7 cells gets to be a 4x4 on 3s LIPO

Quote:
When I tried 3s with the little LPS this I didn't have a smaller prop of the appropriate size so I intended to avoid full throttle. The performance improvement was impressive but my plan to avoid full throttle fell through in the heat of the battle with the wind and ... it lived a short but full life. Anyway, I plan to try this again, but now I will do all the homework up front; measure current draw and RPM's on the bench to make sure it's not overproped. There seems to be less margin for error on 3s.
What motor and box was in that?

Was that a B2C motor?
Quote:
Another question for you, vinatage, are the brushes an amalgam of graphite and bronze?

POD
I have not a single clue. They LOOK bronze thats all I can say, and they are not brass or copper.

I am fairly sure there is very little graphite in them. But I could be totally wrong there.
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Old Jun 13, 2005, 10:21 PM
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Vintage, The motor was the RXC in the 'C' gearbox with an 8x6 GWS slo fly prop. Next time I stock-up on props I will try the 7060.

POD
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