St. George

iTrader: (7)

I'm using a 2S Lipos in a 2wd Slash with a 3300 KV motor. Unfortunately, I run it in grass a lot and the motor gets extremely hot. Gearing down helped a little but not much. Would I be better off with a 1900 KV motor and a 3S Lipo? I'd get about the same performance with that setup, right?

the rc guy

yes to some parts No to other points. Top speed would be down.but getup and go will be better.temps might be lower all depends how hard you run the 1900..Id look for a 2200 kv on a 3 cell...

St. George

iTrader: (7)

Thank you for that information. I'm also assuming 3s with a 1900 or 2200 KV motor would be a lot harder on my drive shafts? Also, why do you recommend 2200 KV over 1900 KV?

uDi_MP7.5

iTrader: (3)

3S with 1900kV motor should lower temps if gearing is kept the same.
However 1900kV is likely too low and you'll see reduced top speed, as the rc guy says 2200kV on 3S will maintain your current top speed. You can calculate this yourself, as motor kV multiplied by voltage will give maximum RPM attainable by the motor. In this case 2200kV with 3S (11.1V nominal) will give a close performance match to your current setup if gearing is kept the same. It shouldn't really be much harder on your driveshafts if the parts you choose aren't much heavier, maybe a little.

Just make sure physical fit is OK and your replacement motor fits your mount and you can get an appropriate pitch and ID pinion.
Motors that low in kV are often larger (for 1/8 scale) and use 5mm ID pinions. There may be limited choice in low kV motors matching your setup.

Juglenaut

iTrader: (19)

What is your current pinion spur setup??

Final drive ratio focus should be 10.5:1 I believe on a 2wd slash VXL brushless is 3500kv 3s capable and stock the slash should have come with gearing options as well not that the slash can’t handle 3s at 3500kv so that is where I would start and if you don’t like the speed adjust from there.

Juglenaut

iTrader: (19)

As for 3s temps will likely drop mainly due to that a 3s is more efficient than 2s.

Bry195

lets just generalize with some numbers that are close for allot of common motors. what is the max rpm the motor sees on your chassis on the terrain you drive? lets say its 10000 rpm or 10k for short. peak power will be a little over half the rpm and peak efficiency maybe 80% of the rpm.

if you drive the car for 5 minutes and it averages 5000 rpm it will come off hot. if it averages 8000 it will come off cooler.

so what is power and what is efficiency? its the torque at an rpm. if you have dont have enough power at peak efficiency you gear it to bring the average rpm closer to peak power. if you have enough power but its too hot you gear it closer to peak efficiency.

if neither situation and everything in between is unsatisfactory you get a motor that has peak power at 7000 rpm and peak efficiency at 10000 rpm.

the reason is peak efficiency is maybe 60-70% and peak power might be 40-50%. that might not seem like a big deal until you look at the efficiencies that you experience before you hit peak power or peak efficiency. if the car stops and then starts it runs thought zones that are 20, 30, 40 percent efficient. lets say it doesnt accelerate from 0 ever the way you drive it. would you like the car to spend allot of time in 30-40 percent efficiency or is it better for it to spend its time accelerating in the 40-50% region? if peak power is at 5000 rpm and peak efficiency is at 6000 rpm there is a strong possibility that that gearing the car or selecting the motor properly and having it spend more time at 40-50% efficiency it will have more power too.

there is a tendency to be more efficient at higher rpms. voltage alone does not make torque it allows you to extend the rpm out further which might result in higher efficiency just due to the fact that you are going to gear the car to use that higher rpm which will get you closer (stereotyping motors) to peak efficiency but if you understand the physics and setup a motor based on the physics it doesnt make it actually more powerful in the sense that you get more torque. current is torque and voltage is rpm. but it will feel like its more efficient just due to the fact that you will want to use that higher rpm by setting the car up around it. a higher voltage will extend max rpm out to 12 or 14k for example but torque and thus power and thus efficiency doesnt move.

Juglenaut

iTrader: (19)

I am not sure what to say.. my head hurts..

The Traxxas slash 2WD should be around 10.5:1 final drive and with a 3s on a VXL 3500 is 38,850 rpm, divide that by the final drive ratio and that is the wheel speed.

I am not about to toss around trigonometry anytime soon.

Ohms law.

Juglenaut

iTrader: (19)

power supply voltages effect temperatures because a lower voltage will cause the motor to draw higher current to deliver the same amount of horse power and the higher current means higher winding temperatures. A percentage of voltage drop yields about the same in increase of temp. Same with over voltage, can saturate the core rotor and overheat.

billdelong

iTrader: (33)

I don't have any data to share for 2WD, but will be happy to share my experience with a 4WD SCT (TEKNO SCT410) converting from 2S to 3S and how I used Watt's Law to make the conversion, for years I called it Ohm's Law but the math is pretty much the same, but technically it's called Watt's Law

Watts = Volts x Amps

I originally started with 2S powering a 3660-4300KV motor with a 15T pinion and was getting motor temps around 160°F after a 10 min run.

While this was acceptable, after 4 months of racing I started having excessive resistance on my battery terminals where the wires would become unsoldered at the 5mm bullets going into the battery which consequently damaged the battery.

I have since invested in a bottle of conductive lubricant to clean the connectors and ensure that resistance is no longer a factor:

I also chose to improve efficiency by converting to 3S with the following notes:

Watts = Power, idea is that I wanted to keep the amount of power drawn a constant, so by increasing voltage, I will reduce current (Amps) or the flow of electricity reduced which will improve efficiency and reduce temps.

Going from 2S --> 3S is an increase of +33% voltage

I then swapped to a 2500KV motor which is 58% of 4300KV (this is reduction of 42% less current draw)

The net speed difference would be (3S/2S) x (2500KV/4300KV) = 87%
So to get the same speed(power draw) I need in increase my 15T pinion +13% = 17T pinion with the new setup to get roughly the same speed, while reducing the current draw by 42%

The 3S battery I have chosen is a 5.2Ah pack which will produces 65.52Wh
The 2S battery I had before was a 7.7Ah pack which produces 64.7Wh

Both batteries produce the same amount of work/energy but the 3S pack will be more efficient and should produce longer run times which I have not calculated yet... more to come on my testing

After the upgrade I saw my motor temps drop to 135°F from 160°F = reduction of 17%

*** You should be able to plug in the same numbers related to your 2WD setup to get similar results, good luck!

Bry195

no trig required but that Frank Trig had a hell of an uppercut. LOL

To break this down would require allot of background that would make me want to knock myself out. In the end it would still boil down to taking the time to follow the explanation, trust in (me) random internet dude, or conduct some good ol fashioned thread war. Your probably better off just trying whatever it is you want to try and report back. When you come back and say yo random internet dude you were wrong I’ll remind you that I said higher voltage means higher rpm and I’ll ask you if you used it. then i will remind you that during the ramp up of power you come to a place on the ramp that is known as peak power and 20-30% rpm later you come to peak efficiency. since you extend your rpm with higher voltage you are probably going to use gearing to use the higher rpm. you will be running in a cooler more efficient area of the curve.

you guys are connecting allot of things together that makes the physics difficult to explain. you can confirm 1 data point and it will clarify my point or you will still not understand but get my point. look up the definition of a torque constant. if the answers are still too exhausting (im not trying to be an a**) search for torque constant of PMDC motors. what you will find is that all PMDC motors have a torque constant. its a formula or a simple relationship. for every amp you dump into a motor you get a torque. when comparing identical motors but 1 is at 2s and another at 3s what happens to current at 5000 rpm on the 3s? voltage goes up and current goes down by the same amount. the power remains the same. if the max rpm is 10000 rpm the power is the same with an exception. as the voltage you supply approaches the voltage (back emf) you cant go faster. this creates the heat at max rpm which by the way you never drive at for more than a very short time. this is a low efficiency zone on the curve. so you supply it with more voltage and it takes more rpm before the back emf and forward voltage meet. the losses from forward voltage meting back voltage are much less. this is where it helps to understand Kv. this is really the reason for having a Kv. its not for measuring power. its for estimating losses or knowing the max rpm of a motor.


when your car (real life) is running a generator is filling the battery up. when the battery is full the generator is making heat. same principal. the generator always generates the same power that it is possible to generate when the battery is full or empty. it does not turn into a 1000w generator when the battery voltage is low unless its 1000w when the battery is full.

power is power because as voltage goes up current goes down. you can use this fact to change wire diameters, resistance losses, and extend rpm but it does not make power. to make more power you need to go from a 100w motor to a 200w motor.

most people get lost when contemplating the curves of power and efficiency but a 100w motor never turns into a 200w motor and its watts that spin the wheels.

uDi_MP7.5

iTrader: (3)

There are two separate factors being discussed here, which are important to clarify - though I may not be the best person to do it - I'll try.

Bry195 is talking about motor efficiency in relation to rpm, and has published a very interesting thread in regard to this if you search under his username. It is worth skimming, even to understand that keeping a motor within its peak efficiency rpm range will reduce temperatures for equal performance. If your gearing, voltage, and driving habits keep the motor in a lower-efficiency part of its curve, it will generate more heat than it needs to. Scrolling through his data gives a general idea of where peak efficiency and peak power are for any motor, and made me wish manufacturers would publish a rough/average curve for motors they sell. I am not an expert on this topic but it is very cool to see it tested and discussed here.

Juglenaut and billdelong are discussing the results of an interesting effect called Joule Heating, which states that heat generation through a conductor (i.e. any copper wiring) is a function of current (I) and resistance (R), not of overall power (P). This means that contrary to many views expressed (in the past, by people who don't understand this concept), running higher cell counts can indeed net higher efficiency and lower temperatures (with P held constant), purely as a result of reduced self-heating in copper wiring. This includes connecting wiring, internal traces of the ESC, and motor windings. For this reason alone, switching from 4s to 5s/6s in a 1/8 vehicle (for example) will almost always result in a reduction in overall temperatures (if you choose motor kV and gearing to keep the same top speed, and ideally choose motor kV such that the motor still stays in its ideal efficiency range as discussed by Bry195). The opposite is also true: if you try to run a 1/8 buggy on 3s, you'll very quickly run into overheating issues, and notice that connectors will desolder themselves - this is because the reduced voltage (V) means higher current (I) through all conductors for a given level of power (P).

This is the exact reason why St. George's initial question / plan of switching from 2s to 3s to reduce temperatures is correct, even if he chooses the 2200kV and doesn't take a hit in top speed. As billdelong explains, you can also then use a lower capacity pack to get the same runtime, which will (somewhat / mostly) offset the increased mass from adding a cell. Of course at the same time it's useful to be aware of Bry195's research and comments in relation to motor efficiency vs. rpm / gearing, and relating that to what speed / rpm you spend most time driving at, to make a practical decision in regard to motor kV and gearing.

Finally, I think it's important to consider masses of parts you are adding, since heavier motors and packs will reverse any benefits - hence my comments on making sure you don't have to go to a physically larger motor to achieve your upgrade. For the same reason (mass) it's important to understand billdelong's comments on the ability to reduce capacity of pack with increased voltage.

billdelong

iTrader: (33)

A couple more things worth noting:

1) While both my 3S and 2S packs produce about the same amount of work in Wh( watt hours), the 3S pack weighs 36g more which actually improved the performance of my 4WD SCT on the track, I would improve my fast lap by 0.3 sec on a 40 sec hot lap. I do want to caution that there is a point of diminishing return, I also have noticed that my truck tends to bottom out a little more easily now and I probably need to add some more pack in my shocks to accommodate the extra weight. This is a very recent change I made and will require me to adjust my car setup accordingly as I monitor my lap times to make sure I'm not making anything worse.

2) The 2S pack was a 100C pack which cost about $150, definitely a very high quality pack is necessary for 2S when pulling extremely high current you need a higher C rating on the battery to sustain that much load. When I switched to 3S I was able to buy 50C pack that was only $50 where it's not necessary to have a high C rated pack when you are pulling less current. I did check my battery pack this last week and temps remained the same ambient temp at around 100°F after each run, ESC was around 115°F as well which tells me that my gearing was also in the optimal range, my lap times are right on par with most 4S powered 1/8 Buggies as well at the local club.

Bry195

I hope that was understood because its spot on. I dont do a great job of simplifying as hard as i try. There are allot of terms and curves that make some of these concepts difficult to absorb quickly. electrical power that goes into a motor as rpm goes up is converted into more and more and then less and less mechanical power. when its not converted into mechanical power its creating heat and it’s exactly opposite. allot of heat a little heat and then allot of heat. in the middle is a conversion around 60-70% efficient if you havent messed it up with timing and gearing. the trick is to keep out of the hot zones while staying in the power zones with gearing and then setting the timing to the average rpm you will run.

anyways I started going to deep again but look at any 2 motor curves. a power curve and an efficiency curve and remember that high efficiency is how well the electrical power is converted to mechanical power. when its not efficient its making heat. efficiency falls on its face as you reach the theoretical Kv. thats not more power its just less heating at higher rpm.