3573 replies to this topic

[havlicek]

Thanks Tom

-john

[Horsepower]

Somehow I thought size mattered, relating to the comm diameter. Oh,and that was the K&B Wildcat you were remembering. The Bobcat is the 36D version, or to be tech correct, a 600A.

[havlicek]

Hi Gary,

The way I look at it... the larger the com diameter, the longer the brushes are in contact with the segments that are "firing". If I think about how these motors work...well...it seems as though they shouldn't work at all and I get a headache If you take the stock com off a 16D and put the much larger Kirkwood on there (and bigger brushes to boot), they work just fine and I think it's because you still have three segments each representing 120° or one pole. Maybe one of the motor gurus can chime in with the physics Thanks for the information on the Wildcat too...I had so much fun with those things when I was a kid (and melted a lot of endbells back during the paleozoic era)

-john

[havlicek]

Back on the Russkits. All three of them made a nasty noise when I spun the arms either by powering them up or even by hand...kind of a groaning thing, very much like the noise the bathroom fan made in my first apartment. Replacing the can bearings completely eliminated that and they now spin really nice. Such a simple change has made a really noticeable difference and I hope the rest of the build goes as well.

-john

[havlicek]

I finished Russkit #1 today (at least winding the arm). The difference is pretty huge from the stock unit which sounded like it was on life support Even with the stock magnets however many decades old they are and the Mabuchi springs, the motor sounds great. Now I just gotta replace and zap the magnets, change out the springs and brushes, oil her up and send it out. Kickin' it oldschool




-john

[gascarnut]

Super cool!

What wind is on that armature?

[havlicek]

Super cool!

What wind is on that armature?

Hi Dennis,

It's a #29 awg and the owner had used this very wind back when it was "current", so he wanted it just the same. It's fairly mild, but sings really nicely, spins-up with authority and seems totally appropriate for the motor. Draws VERY little current and reads about 4+ ohms. I can see this thing doing really well in a period chassis, and I'm sold on these little Russkits! Wish I knew then what I know now

One more thing, the com said "tradeship" but is completely different from the other Tradeships I have here and was almost spot-on...REALLY nice!

-john

[gascarnut]

Thanks, John!

[havlicek]

Hey, no problem Dennis! What's cool about all this is that I had a baseline to judge the motor's performance against since I ran all three Russkits I have here as stock units. Even assuming that they would have run much better when they were new, the difference with the rewind was major, not at all like dewinding the things. Also, I'm stoked about these motors not in comparison to any other motor. They stand on their own from a historical perspective and while it's cool to wind-up a slamming #24 in a hot setup, this motor was "it" back then so I see it as part of a "class" of motors. Great fun to sorta go back in time and I really look forward to seeing the motor in it's new chassis. That will really be retro!

-john

[tonyp]

John is that mine?

[Dooner]

"If you take the stock com off a 16D and put the much larger Kirkwood on there (and bigger brushes to boot), they work just fine and I think it's because you still have three segments each representing 120° or one pole. Maybe one of the motor gurus can chime in with the physics"

John,
An advantage of doing this may be a cooler running motor, because the larger comm has more mass. A disadvantage may be slower RPMs. for the same reason.

Tom

[Jairus]

Greater surface area also results in greater friction....

[tonyp]

Tom, there is a ratio of brush to comm size that makes the most power. Ideally all 3 poles need to be touching the brushes at the same time, 2 on one brush and 1 on the other. As the comm rotates there should never be a point where the brushes touch all 3 poles at once. The closer you can get to this with out ever touching all 3 segments the more power your motor will make. This is why motors always run better with broken in brushes, the brushes wraps around the comm more...

Narrowing up the brushes reduces the timing and if too narrow you have an instance in the rotation when only 2 comm segments are touching and this makes for poor power. The reason the bigger dia. comms do not run as well with out a change to a wider or laydown style brush.

For less friction it is always better to keep the brush wider and reduce the height...

[Dooner]

Tony,
I think I understand better now.

"As the comm rotates there should never be a point where the brushes touch all 3 poles at once."

Would it be possible to cut a comm diameter so small that a wide brush would touch all three poles at once?

And do dual points on a car relate in any way to this?

Tom

[gascarnut]

I've read Tony's post a few times and for the life of me I still can't understand what he's on about. Perhaps Tony's still in his "comma" from Sunday....

Tony, are you trying to say that one comm segment should not be able to touch both brushes at the same time? This happens when the brushes are very wide in relation to the comm size and you get too much overlap. This short circuit is what kills performance and eats comms.

[Dooner]

My next slot car motor will have four brushes.

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[havlicek]

John is that mine?

Yes it is Tony.

"If you take the stock com off a 16D and put the much larger Kirkwood on there (and bigger brushes to boot), they work just fine and I think it's because you still have three segments each representing 120° or one pole. Maybe one of the motor gurus can chime in with the physics"

John,
An advantage of doing this may be a cooler running motor, because the larger comm has more mass. A disadvantage may be slower RPMs. for the same reason.

Tom

Hi Tom,

Yes, I've heard it said many times that the larger coms may run cooler...beats me. Since Tony knows a LOT more about this stuff, I'll leave it to him to 'splain On the lower rpms due to the mass of the comm, I doubt it very much. The difference in mass between a skinny com and a fat one is less the mass of a 7 tooth pinion gear. If your motor revs noticeably slower with a pinion installed than without, there's a huge amount of torque lost somewhere! Maybe the extra drag from a larger com & brushes will cause a drop in RPMs (as Jairus said) BUT also possibly an increase in torque (then again, brush springs play a big role here too), but I can't see how the tiny additional mass of a Kirkwood com would slow a motor down appreciably. Anyway, the com that Tony sent me to do the arm above is still small-ish, but seemed a real quality piece.

-john

[tonyp]

Dennis,

All three segments need to touch at the same time of course. The secret is to reduce the time it takes for the brush to cross over to the next segment. Ideally just as the positive brush (for example) leaves a segment the negative brush will come in contact with it. The more you reduce the "dead time" when the segment is not in contact with a brush the better the motor.

When we were designing a new R/C motor a few years ago a commutation engineer from Carbone came in to visit with our rep. He became very interested in the motors, the fact they were 3 pole and produced the power they did. Within 5 minutes he figured out the brushes were too narrow for the comm size. He got us some samples and that was how our bigger P-94 style brushes were born.

[tonyp]

Bigger comm and bigger brushes runs cooler. Less resistance in the brushes. If the Bigger comm/brush combo has a better ratiosize wise than the smaller comm and brushes the motor will be more efficient and run cooler while producing more power.

[gascarnut]

Cool, now we have it all nicely explained, thanks!

[Dooner]

Tony,
Would it also make sense that bigger comm diameter would make for better heat dissipation because of the larger area?

Tom

[HarV Wallbanger III]

I'll chime in here also and say that as a comm gets turned down the brush ratio gets larger. It is also true that switching speed increases as comm size drops as well. When I raced years ago I would have all new comms cut to .198 after I got them hot and bothered the first time. I think comms then were about .202" on a new arm? I'll have to measure one now to see what size they are new.
My fastest arms always were the best on the last cut. Sometimes when I would see plastic on a good arm I would try to find another setup were the brushes rode further down the comm just to get another trip on it before it got retired.

[havlicek]

The physics is lost on me...but it ALL sounds good Anyway, Tony's "23" is all done. I ran into a slight snag at the last minute when I realized the replacement magnets aren't grooved (rabbetted) for the Mabuchi can tabs. So I had to bend them flat, grind the inside and outside of the can flush, fill them and repaint the bottom. Motor sounds really nice and wants to run



-john

[gascarnut]

Sweeeeeet!

[Jairus]

Beautious!
(did you solder the brush hoods to the brush tubes?)