jiml

iTrader: (3)

Derek, I love ya babe, but you don't know the first freakin thing about electricity!

Electricity is governed by something called Ohm's law. Ohm's law states that the voltage in a given circuit is equal to the current times the resistance:

V=IR

where v=voltage, I=current, and R=resistance. In a direct current circuit, if you increase the voltage, and the resistance remains constant, then the current goes up! A brushed RC car motor is a DC motor, and the speed of a DC motor is driectly proportional to the voltage. The higher the voltage, the more power the motor makes, and the more current it draws.

The example you give is true for an alternating current motor circuit. The speed of an AC motor is directly proportional to the frequency of the voltage. In that case the power out of the motor is constant, so increasing the voltage will decrease the current. The formula for power is P=IV, or volts times amps.

DerekB

I Typed out some longer post, but the server made it get lost.


I'll list the current examples of similar problems and the solution


1:1 Scale Cars
Before the current 12-volt system 6-volt electric systems were used. As cars had more and more electronics they switched to 12-volts. Currently cars are so full of electronics, the automotive industry is trying to make the switch to a 42-volt system to provide enough power, and lower the amp draw for the system.

Solution was to increase voltage to help with electric draw problems.

Cordless Drills
If you'll notice, and read, that the reason we have Sub-C cells is because they are used in drills and other hand tools. If you've shopped for one lately most of them have gone from the old 6 or 7.2 volt sets to at least 14.4volts. Reason–more power, better battery life

RC Heli Copters
Most of the larger helicopters run off a 42-volt system. When run on lower voltage they get higher motor/esc temps and shorter runtimes.

Obviously you cannot just stuff more voltage into a system designed for lower voltage. I understand that our motor draw varying amounts of current, but there are many examples showing that lower voltage is NOT the solution. I also understand that using 4-cell will lower the available power in general, but like all these examples they don't solve the problem by reducing the available power, they change for more.

I have a pretty good grasp on how electricity works, and also a good grasp on the world around and the similar problems and solutions.

Bob-Stormer

iTrader: (8)

Fred, I was never against any change. Also, It's not that most of us aren't interested in or willing to look beyond the norm.

Where I had a problem was with what a few of us believed was a rule for the USA that was not asked for by the majority and could potentially be stuffed down our throats over scenarios that do not effect 99% of the racers in North America. And that the item being changed is not the item that is reportedly failing. And it's reportedly failing at a pitifully small level. As in so small, I think it's fair to say that 99.9% of us were unaware of it or had never heard of it. The issues for a select few in the .01% of racers rarely trickle down that far, nor are they necessarilly believed or even cared about.

Things just "appear" in the rulebooks. Look at brushless, and how it magically appeared in the rulebook.

If somebody says something is better for the sport, Perfect, I'm for that. But show me some numbers. Show me how it's going to help the sport.

vtl1180ny

I completely understand where Derek is going and he's correct... When you drop the 1 or 2 cells you are going to gu up a proportional amount of teeth to compensate. Now you're needing more current to get the car moving. It would be great to use the various formula's and say that 4 cell will me more efficient, but it won't....

AdrianM

iTrader: (4)

You are correct there is no efficiency advantage to 4 or 5 cell. No one said there was. The point to fewer cell is to make motors and speedos last longer. If you are fast in mod 6 cell right now you are going soft on the last two laps. You have to be pulling ~45A to do that to a battery.

If you run fewer cells sure you can gear up, time up and motor up to pull more amps but no matter what you still can't pull more 45A...or you will dump. 45A at 4.8 (or 6V) is fewer watts and less heat...so the motors last longer. Thats was always the goal...not efficiency.

Stock and 19T motors can only pull so many amps. Even on 6 cells there is a limit to the gear ratio they can pull. They can only generate so much power. On 4 cell the limit still exists so even with these motors you will never get them to pull more amps that they do on 6 cells.

SWTour

iTrader: (9)

Bob,

Art (ADVIL) mentioned the Johnson motors

We did this w/ the old Maubuchi's and Johsons a few years before we pushed for 4 cell in oval. It DID make racing really fun on the tight short ovals w/ 6 cell, but like happens way too often in this hobby...people who don't belong in a class decide to jump in and see if the can ruin a good thing.

There were a handful of us who ran just one single motor for nearly 2 years, the performance never changed..then a couple guys showed up trying to outspend this "FUN" class. They did the water break in trying to help seat the brushes...when that didn't get them anywhere they tried a magnet wrap around the can trying to make it have stronger magnets...they chucked the motors in a VICE and squeezed the cans to get the magnets closer to the armature (We had to have tech make a NO-GO gauge for the shaped of the can) . . and a few even figured out how to open one of those sealed motors and true the comm. FUN DIED..CLASS DIED

Just a short time before we switched to 4 cell, at the urging of Jay Kimbrough we were able to get the 5degree Stock motor made...This motor was an awesome "NOVICE" type motor w/ 6 cell....but it was way tooo slow for 4 cell. It would make a good compromise as a way to slow down tc's too

While I'm not for a SPEC based chassis class in really any form of R/C Racing, a SPEC battery and motor rule isn't a bad idea..something using a slower stock motor and maybe even some of the older "Cheap Packs" what are they like 1800 mah specs?

AdrianM

iTrader: (4)

Derek - In theory you are totally correct. However out of all of you examples the only one that supports your point is the one about full scale cars. Unfortunately the power requirements of an automobile are nothing like R/C cars so it a very apples and oranges comparison.

Every one of your other examples in reality (not in theory) go against the very point you want to prove.

Drills went up in voltage to get more power not to gain efficiency. The only efficiency gain from the higher voltage was higher productivity from the users of the higher power drills. In use my Dewalt 18v drill doesn’t last longer than my Black and Decker 9.6v but the Dewalt sure does sink screws into my deck faster.

You also missed the mark on the Helicopter example. I am putting together a Trex 600 right now. The standard power setup for that Heli is a 6s (22.2v) 4900Mah LiPo. That flies pretty well and runs for ~5 min. Soon after the 600 came out guys started putting 10s (37v) systems in it. Not to get more efficiency but to get more power. The guys running 10S are actually using smaller batteries (3300-3900mah) to compensate for the weight of the 4 extra cells. Guess how long a 600 flies on 10s....about 5 min! If you don't believe me ask Aaron. FYI, if you run the 10s big batteries the Heli is a pig and flies like crap.

That’s all great but once again comparing Helis to cars is apple and oranges. More power in a Heli makes it safer to fly. The power can get you out of trouble.

More power in a car makes it harder to control and is harder on the electronics. In mod we have more power than we need. More voltage would make more power and that would make things worse.

Mark Payne

Reposted from stuff we were disucssing over on RC Racechat (UK)

I Squared R Losses

--------------------------------------------------------------------------------

Hi all

I am out of the TC thing now. Enjoying 12th scale which is where I started as a kid.

Some perspectives. I have a degree in electronics and I can remember some of it so bear with me. I may also be wrong..... Not often ;-)

The issue is with the energy we are carrying in the cells. Over the years we have seen incresed energy storage. Dont mistake energy for capacity (we measure this in mAH), its not the same thing. Energy is mearured in Joules.

Joules = Volts * Amps * Time

You see, a set of 3600's with a much higher voltage could carry more energy than a set of 4200's with a lower voltage. The only constant in this equation for you TC boys is Time = 5 mins.

Now in general each cell development brings more voltage AND more capacity so the available cell energy is going way up and up.

BUT... you want to solve the heat problem by dropping out cells and reducing voltage... OK seems like a good idea but wait... wait 1 year or say two years and the available cell energy will be back where you started because of developments in cell capacity (mAH) which marches on.

So now what happens?... more cell capacity and available energy means that brushed motors will drop another turn or two and brushless motors will go from 4 star to 5 star .... to 6 star.... because we want all that energy extracted in 5 mins.

Now you have a problem. Look up in the air.......
http://en.wikipedia.org/wiki/Electri...r_transmission

Why do you think power transmission over the pylons happens at 110,000 Volts when all we want at home is 240V? The answer is that the cable losses (heat) are proportional to the SQUARE of current multiplied by resistance.

You minimise I-squared-R heat losses in the motor by doing electro-mechanical power conversion at a HIGHER VOLTAGE which minimises current for the same given energy delivery and thus massively reduces the heat generated in the process. This is good for the motor, the wires and the speedos.

So why are you blowing motors with the new cells? Simple: the rate of electrical energy (Watts) that the new cells can generate is greater than the traditional motors can take. You simply have too much available energy to extract in 5 mins. The motors cannot stand it, the cars apparently eat parts doing it and some drivers can't drive it.

If you drop a cell or two you will reduce the energy available BUT you will damage the efficiency of the motors and speedos, this will push you the right side of the motor_destruction threshold but it's a short term fix. When the cell guys come back at you with even more capacity (more energy) 1 year - 2 years down the road you will try and extract it and opps ... now you are in a worse situation. I-Squared-R is going to cook your balls off.

4 cell works in 12th scale because we push an 800g car around for 8 mins with almost no transmission drag.

4 cell may work for TC for a while but I predict more problems ahead because you are all about to do a really silly thing from an electrical engineering point of view.

Increasing the time to 6-8 mins might do it as you would start to dump and hence go more conservative on the winds (back to 10T and 12T maybe) but there is another problem here. Our 19T and open modifieds probably never reached a steady state temperature over the 5 mins, they were getting hotter all the time through the run. They are thermal time bombs. Today we don't go soft because of the cells, we run out of thermal capacity in the motors. They mostly dont melt in a TC in 5 mins but they may at 6-7 mins.

A more radical view would be to redesign the motors to be more efficient and require a HIGHER voltage. Now maybe go to a smaller capacity higher voltage LIPO or stay with 6 cells. Why is it assumed that we must keep carrying bloody Sub C sized cells in our cars?

Cheers

Mark

DerekB

Actually what I said is true and you even typed it. Your drill has more power, and the same run time. So if you actually don't use all that power, you get more run time, you'll also notice your drill doesn't get hotter with that extra voltage. When run on the 6-volt system, the batteries died quicker because they were being overtaxed, and almost dead shorted when the couldn't drill in.

The heli example, I'm basing it off our Heli editors experience and comments. He's been flying for many years and says what I say. Yes more power helps it pull large helicopters, but it also gets more run time (again more power and the same runtime is actually more efficient) and electronics don't get as hot, and again you can say it's because the lower power struggles to pull the load, causing more heat.

All examples of why more volts is more efficient.

Just like in nitro you can actually run cooler with more nitro. Hard to understand, and probably wrong in theory. But in real world examples it does.

SWTour

iTrader: (9)

MAKITA Cordless IMPACT driver

9.6v Variable speed (0-2,200 RPM) for controlled driving starts
(Torque 608 in. lbs) Battery Ah 1.3 NiCad

18.0v True variable speed (0 - 2,600 RPM) for controlled fastening
(Torque 1,260 in. lbs.) Battery Ah 2.6 NiMh

From the Makita web site, these are two of their Cordless Impact Drivrs.
The first one uses the older 9.6v 1.3 (1300 mah) Ni-Cads, the second is one of their NEW 18v units. The 18v unit uses 2.6 Ah NiMh batteries. (I have a couple w/ the 3.0 Ah packs)

They DOUBLED the VOLTAGE, DOUBLED the Mah of the Batteries, and DOUBLED the Power. You can bet in order to achieve this they also reduced the internal gear ratio by close to 50% since the rated RPM is almost the same.

A third unit utilizes the NEW Li-On 18v pack, this unit is also rated similar to that of the 18v Nimh powered unit
(Torque in.-lbs. 1,280) Battery Ah 3.0 Li-On , but this unit has way more run time and utilizes a 45 minute FAST Charger, and weighs a lot less.

FUNNY I don't see ANY 3800's or 4200's used in POWER TOOLS, maybe we will in 3 or 4 years.

Bob-Stormer

iTrader: (8)

I think you're both wrong on the whole drill thing. I think the changes were made for marketing purposes. Both within the one company and within the competitive world between brands. "were, 9.6v...", "OH YEAH, We're 12 volts", "oh yea? We're 14.4 volts"... etc... etc...

Why would a person upgrade to a new drill if they already have a perfectly good drill, and all the stuff to go with it? I have no problems with the power, torque or usefullness of the stuff I have, and it performs perfectly, every time.

I have a completely acceptable Makita setup, actually I think I have 3 makita drills and maybe 6 packs, some flashlights, a saw, etc. BUT NOW, I see all the new stuff, and well.... I WANT IT!! It's fast, flashy, new, and is not going to help me do a better job of screwing in drywall... It's just fun to have the new stuff.

My other drills are just fine. This whole scnario sounds vaguely familiar somehow?

Bob-Stormer

iTrader: (8)

Sounds like it was fun.

Problem is you can't stop everybody. The group we club race with the most is VERY good about keeping racers where they belong. Gotta point people where they belong. I really enjoy stock. Our club won't let a lot of us race it anymore. I suppose that could be for the best. But 19 and mod are to close together, and people can run the same car in both. The classes need more seperation to be effective. I liked to run stock and mod. Two different experiences. Mod and 19 are kind of the same experience, more or less. This could be debated, and it's not necessary, but for the average club guys, it's the same.

asw7576

That's why electric trains runs at 10.000 volts, so the cables can be made from thin wire and thousands kilometers long. Ditto with electricity for cities.

I always suggest longer runtime for Racing. Racers should race more than 5 minutes, so they have no choice except to use milder motor ( slower ) in order to be reliable during the race. It's brain & strategy ( like real racing ).

For eg.

For qualifying racers could use his fastest motor ( 5 min qual time )
For final racers should use slower motor ( 10 minutes ? does it make you happy? ).

just imo.....

SPC

Great post! I've been watching and waiting for someone to correctly address the energy issues involved. Thank you.

jiml

iTrader: (3)

Biggest reason you can't compare heli's to cars: today's heli's use alternating current motors, better known to us as brushless motors. And heli's are going to higher voltage (much higher voltage) for more power neede for 3D manuevers.

As far as gearing up, racers are doing that now. The higher voltage of today's cells means more power, and that means you can gear higher for a given track. Right now you have racers burning up 19 turn motors after only a few runs.

Reducing the number of cells will slow the cars down alot without killing lap times by much. Less voltage means less current, and that means motors last longer.

However I don't know if I agree with this. Slowing down touring cars indoors on carpet is a good idea, but outdoors I think the cars will be too slow.