Can we talk large comm motors ?

[jeepinator]

Hello,
I have been a HUGE fan of the large comm motors (methinks the only ones ever made have been based on the Reedy Sonic design).
Here are the reasons I like them:
1) Easy on brushes. I do not have to change the brushes as often, or at least it sure seems like it.
2) Less actual wear. I have a lathe and take great care of my motors. I can get more run time on large comm motors before needing to turn them.
3) They are less sensetive to gearing errors (gear ratio). They run cooler when over geared as well.
4) I can gear taller with the same run time (this one I am the least solid on, but it really, really seems like it).
5) They seem to produce more low RPM power. This matches my driving style it seems, more than the regular motors. I am, simply put, faster (lower lap times) with the Reedy Sonic motors.

Anyone want to comment ? Any motor experts out there ?
I am looking for pro and con.

cheers !

EDIT: BTW, all of my comparisons are made assuming a similar motor (turns primarily). I was making generalizaions and thoroughly understand the differences in motor turns, winds, etc.

------------------------------------
http://www.theshreves.com

[ 04-30-2001: Message edited by: jeepinator ]

[Railman]

I have a Sonic 2, but I havn't run it as much as my sons. My read on them is that they are better for tight stop & go, offroad type tracks, & as a rule seem to have better torque. The torque thing may not actually be true though, as I think it just seems that way, because the large diameter com has more net friction to it, & reduces the free spinning of the motor....a more constant drag resulting in a more controlled acceleration. They have better brakes due to the drag, but lose some of that straightaway kick that stand ups have. I do believe that they are much more durable than stand up brush motors. Most of the guys that i've seen run them swear by them for their better cornering charactaristics, just easier to drive than stand up coms. They seem to be better suited to off road than on road.
Just my $.02 :)

[JP]

Bigger commed motors DO actually have more torque. But there are a couple bad sides to pure racers:

Less top end (RPM), may be slightly slower

More friction, slower speeds, even wear, but still a little more wear, even though it does seem liek it last longer.

Also have less brakes, which can really hurt yea.

[XXXER]

I have some friends that have a few bug block comms, they say that they can handle the "higher ampload" that is made in off road racing. He has to grind down the brush which is not as cool, but he is a fast guy.

BTW, jeep, no offense, but you are not supposed to post your web page in your posts, steve does not support it, you can have it in your pro-file, but that is about it.

[Railman]

I have to wonder why there would be less brakes, if it's an equivilent wind. There has to be more friction because it's a bigger diameter com. Sort of like having bigger diameter brake rotor or drum on a full size car. I however havn't run enough mods to compare them though. Are the springs that much lighter? I guess i'm just a little confused. :confused:

[jeepinator]

I don't understand the friction comments.
I am assuming you all are saying the higher resistance is due to greater contact area (between brush and comm).
The contact footprint of any two brushes will be very similar. In fact, stand up brushes have MORE contact area due to the higher curvature of the brush. Why would lay down brushes have more surface area contacting the comm ?
The resistance thing must be folklore. Think about it for a minute ...

tick


tock


tick


tock


OK, are you done yet ? :D

[Railman]

jeepinator, Don't be too close minded here or you'll get burnt. :rolleyes: On the friction thing, you can't see why a bigger com would create more friction? How about if we go to an extreme & look at a com thats 10 ft in diameter. Would that create more friction than one thats .30"? It's about the distance the brush is drug across the com. The greater the diameter, the more surface, the more distance, the greater the friction!

On another note about the advantages however, the increased surface spreads the amp or current load over a larger area, which should stay cooler.
Think about it. ;)
What's with the tick tock thing?

[jeepinator]

Closed minded ? Burnt ?
No, just a healthy discussion. The tick tock is representing "a minute" in my "think about it for a minute" comment. Get it ?
oh nevermind.
OK, here is the deal.
If the comm really is 10 feet in diameter the resistance would STILL be the same. The problem (or answer in this case) is not the comm size, but the BRUSH size. Assuming air itself is not a factor in your equations, resistance is only coming from the contact of two solid materials (in this case the copper of the comm and the brush faces).
If the comm is 10 feet in diameter and the brush is still the standard size, resistance will remain constant. You have not increased you contact area, therefore you have not increased friction !
You have, in fact, ever so slightly DECREASED friction because you have ever so slightly decreased the contact area.
See this illustration:
http://www.theshreves.com/pics/brushes.bmp
Brush "A" has greater contact area (greater friction) and represents a small com motor.
Brush "B" has less contact area (and less friction) and respresents a large comm motor.
The scale is probably not correct, but you should get the idea ... The distance along the curved surface is what I am talking about here.
Do not be concerned if the brushes stand up or lay down. This is irrelevant to this discussion (friction).
I hope this clears it up :p

EDIT: If I was being close minded would I have started this thread in the first place ? That close minded statement made me think for a minute, but now I am quite sure that I am not being that way at all. So, I guess I am saved from being burnt ! Woo hoo !

[ 06-17-2001: Message edited by: jeepinator ]

[Kevin_366]

i dont know much about this at all but, wouldnt B have more contact area unless u shaped the brushes??? j/w

[Traxxaskid]

yes Jeep you are right, but I haven't seen any large comm motor before, like how big are they over your typical comm?

[Railman]

jeepinator, It's not about the contact area of the brush. That is not a major factor. It's about the distance the surface of the com travels across the brush. A big wheel travels farther than a small wheel at the same RPM. The same thing happens with car brake rotors... a bigger rotor will give more stopping power than a small rotor because it's surface travels a farther distance across the brake pads, making more friction. It's the change in distance, not the change in contact patch. More distance = more friction. :( :mad: ;) :)

By the way your drawing would make sense if we were comparing to a laydown small com stock motor, but small com mods use standup brushes, not the same wrap! :D

[ 05-01-2001: Message edited by: Railman ]

[jeepinator]

Railman, thank you for keeping a level head. You make some good points. I had not considered the fact that the contact area would have more leverage on the rotor (because of the larger diameter).
I see your analogy with "normal car" brake rotors as well.
I can see, in theroy, how this would make a difference, but how well does this translate in real world performance ? I just can't see it making much difference, certainly not enough to FEEL !

BTW, my brush diagram was not meant to be accurate between laydown vs. stand up ... I was only illustrating a very specific contact area point. I very well know the difference. This is WHY large comm motors use lay down brushes; to keep the same wrap arch as standard comm's with stand up brushes.

OK, anyone else have any imput on the large comm vs. small comm debate ?

This is good stuff !

EDIT: spelling

[ 05-01-2001: Message edited by: jeepinator ]

[Railman]

jeepinator, I know I haven't run large com motors as much as you but I have talked to a lot of guys about them in the past, & observed a lot of them as well. I think the biggest problem with the big com isn't with how they work, it has more to do with the speed perception of staightaway kick. The small com motors are more free turning due to the reduced friction of the com to brushes. This gives a harder kick on the straightaways. I don't think that kick necesarily makes a faster off road motor, it only feels like it. But they still have to sell them right? The amount of difference in friction actually could be quite a bit though, maybe in the range of 30% more? It would be aproximately = to the differance in cicumferance. Right? ;) It's been fun!

[ 05-02-2001: Message edited by: Railman ]

[Car_Head12]

Railman, say whaaaa???... :eek: you lost me at the "net friction" part... :confused:

[nrgtrader]

There is more to the argument than just comm to brush friction.. if it was such a detriment, nobody would be selling large comm motors. The large comm contact areas are also larger thus applying voltage to the given field for a bit longer enabling the field to pull just a bit longer before being de-energized. You could almost liken the large comm to an internal "gear" for the motor. The large comm must spin faster (at the outside of the diameter of the comm) to generate equivalent RPMs, thus enabling the shaft (which spins slower than the outside of the comm, in terms of speed, not RPM) to react quicker to subtle changes in speed to the comm. I know it sounds convoluted, but I guess I can't really put it to words as well as it works in my head. I think that somewhat explains the torque difference though.

[Railman]

It could be that the reason it seems to have more torque is that the com stays cooler. It has more surface area & mass to distribute the electrically generated heat, resulting in a lower temp com, giving a lower electrical resistance. Less resistance will allow more current to transfer, giving more torque. Just a thought.

[bryceanator]

i think the motor have more friction it just means you will need to lube more and use more lube and the large comm has a higher rpm and takes longer to slow down due to more moving weight but large comm has less friction on the brushes but in other places more

[XXXER]

Hoooooo dang, this is some interesting stuff guys! I think that Railman gets the win IMO.

[ 05-02-2001: Message edited by: XXXER ]

[jeepinator]

nrgtrader:
Can you explain why you think large comm motors have more contact area ? I am not arguing, only looking for supporting evidence to your claim.
I like your thinking on the "internal gear" idea. We must remember, though, that the torque is being generated by the coils of wire (turns), not the comm itself. The comm is only a mechanism to transfer current to the coils. As far as the brush wrap goes, they are identical (or very, very near it). I just took a reedy small comm and a Reedy large comm motor and mesured the brush to comm ration. Here are my measurements (using a Mitutoyo digital caliper last calibrated a year ago):
small comm diameter: .2900"
small comm brush width: .1445"
Ratio: 48.5% wrap
large comm diameter: .3885"
large com brush width: .1865"
Ratio: 49.8% wrap

Both these motors are fairly new, have little wear, and just got turned. The 1.3% variance (VERY small amount) could be a tad different with completely new motors.
So that settles the brush wrap debate at least. They are pretty darn even.

XXXER, what does Railman win ? I was not in a contest, I don't think. Do you have something to add ?

peace !

[ 05-02-2001: Message edited by: jeepinator ]

[nrgtrader]

Understandable question jeepinator.. I came up with that statement by using the tire theory.. at a fixed RPM at the shaft, the outside of the large comm is spinning faster than the outside of the small comm just due to the fact that the circumference is larger and to go the same speed it has to travel faster. Stay with me here, I am think aloud ;) .. If we measure the 3 seperate segments of copper that make up the individual sections by imaginarily laying them flat.. the sections of the large comm motor would physically be longer.

I guess the question is what the advantages are ,from a current delivery standpoint, does the large comm with a laydown brushes deliver more instantaneous current than a stand-up design. That is the only way we could be observing more torque. My guess is that.. MAYBE.. that is the case. Not that the conact patch has anything to do with it, maybe it's just the design.

Kudos on measuring the 2 different contact patches. You did finally put that to rest.

Can we liken this argument to the differences between slotted brushes and regular brushes. I know they are not exactly the same, but it may be in the same realm.

Remember these are my theories ONLY since I always had to come to terms on advantages of large comm over small comm by myself. Maybe a call to Reedy themselves could help us on this one. It certainly deserves to be answered.

Great discussion guys, really opened my eyes to some different angles to take.

[Railman]

I'll try again... It makes it more durable than a small comm. Lets look at what happens during one revolution on a small com vs. a large com. For arguements' sake lets say a small com has to tranfer 1000 units of energy, then these units would be spread out across that cicumferance. A large com would spread out 1000 energy units over a larger area. The result is less degradation to the surfce of the com at a given point. Think of it as arcing if you want. It just spreads the arcs farther apart in that revolution. It's just more heavy duty, just like oversize car brakes! Gotta go, later. :)

[nrgtrader]

My take on that is that the small comm brush delivers the 1000 units vertically whereas the large comm brush does so horizontally, but the contact patch of either deisgn is roughly the same.. just oriented differently. The large comm brush, if anything gets worn in better due to more revolutions at the outside of the comm at the same RPM. Maybe the fact that it seats better has something to so with less brush wear. The tolerance doesn't have to be as tight to consider the brush properly seated.

[XXXER]

How can you measure the brush contact area after full break in, because each would bow in different, due to the surface that they are running.

jeep, it is just that i follow Rails posts a little better.......

[jeepinator]

nrgtrader and Railman,
Your last two posts were great.
Railman did a good job of describing why possibly the large comm motors wear better (less). Would this mean then that the brushes wear more ?

nrgtrader said:
The large comm brush, if anything gets worn in better due to more revolutions at the outside of the comm at the same RPM

I would say this is very close. Maybe you meant:
The large comm brush, if anything gets worn in better due to more distance traveled for the same amount of energy transfer at the outside of the comm at the same RPM as compared to a small comm motor.

I love this thread. I just wish a real motor expert would speak up ! Hey, where is Mike Reedy about now ? Mike are you out there ?
:rolleyes:

Anyone else have a new angle ?

[Railman]

The brushes may actually last longer, I just don't know. The reason they may last longer is that since a big com degrades at a slower rate, there is also less arcing of the brushes after x number of runs. This would have the same effct as undergearing a bit on any motor, which most would agree is easier on brushes. This would have to be weighed against the increased distance a big com's surface travel's across the brush. I would think it depends a lot on how agressively you geared each motor. The big com brushes would probably hold up better than small com brushes under agressive gearing. Maybe thats partially where the increased torque seems to come from, because they hold up to high amp draws better.
Nrgtrader, I get what your saying about the differance in the contact patch between the two. I was thinking of that slant earlier but got lost in other issues. I would think that a more square contact (as in larger radius brush face) would give a more even pressure across the face of the brush, resulting in more even wear & less arcing.

[ 05-03-2001: Message edited by: Railman ]

[Railman]

Everybody wins in a civil debate. It's the exchange of ideas that's important. It's not about winning, at least not for me. The give & take just brings ideas to the surface. Peace! :cool:

[nrgtrader]

Railman.. I think you hit it on the head. I would be willing to bet that the force of the spring is more evenly distibuted across the face of the laydown brush thus making life easier for both the comm and the brush. Kind of like the waterbed display where they would hold up the bed with nothing more than a bunch of paper coke cups to prove that although it was heavy, the weight was distributed evenly. BUT.. the vertical brush has the same basic contac patch, so what gives? I thought I would address that before someone asks.. I THINK, think being the key, that the placement of the where the spring pushes on the brush is more centered on the laydown than on the upright thus distributing the force more evenly.

Great thread guys, keep the creative juices flowing.

[jeepinator]

Thanks for the great discussion people !

Does anyone ELSE have anything to add ?

:cool: :rolleyes: :cool:

[jeepinator]

btt

[Splog]

um i read most of this post but not all...

a smaller Comm will mean the polarity of the motor can switch faster, hence the greater bang of a smaller com motor.

i have no evidence but it seems logical to me

bit like 2stroke vs 4stroke, the 4stroke (large com motor) waits longer between detonation (change in polarity)

Splog

[Splog]

maybe the slightly larger rotational mass of the large com motor gives it a bit more torque too

Splog

[jeepinator]

Thanks splog.
I am dredging this up just to see if anyone else wants a shot at it ...

[Railman]

It's about time to add one more thing to this discussion, & that's cogging. It's not my term, it's suppose it's Trinity's. It has to do with the percentage of time that all three poles are energized. If you look at the adds for the P94 mod motors it explains it very well. It also explains why the increased percentage of energizing the poles increases torque. Just thought I'd bring this back up for old times sake.:rolleyes:
By the way, was this your 1st post Jeep? I just now noticed you did it as a guest!
Joe

[brknwheel]

I don't wish to technicalize this discussion any further, but do wish to say that the sonic series motors have served me well in the past few years. They are killer motors on the off-road tracks I run on.

[TUCRACEMAN]

I like my bro's Silver Dot and my B&R Sportsman Mod. Not large comms, but blow everyone away.

~Dave

"Thanks for the Mickey Mouse phone SANTA!!!"