Why do folks not understand ohm's law?
 

I've run across this a few times here and on other forums. I keep seeing folks comment in various forums that running a higher voltage setup will yield lower current draw. This is only true when you drop the kv of the motor!

Ohm's law is quite simple and proves this easily:

P = V^2/R or P = I^2*R

Assume a motor with R = 0.2 ohm on 4S:
P = 14.8^2/0.2
P = 1095W
Now solve for I.
1095 = I^2*R
I = 74A

Now take the same motor on 6S:
P = 22.2^2/0.2
P = 2464W
Again, solve for I.
2464 = I^2*R
I = 111A

Now let's make an assumption that you run a lower kv motor with R = 0.3 on 6S:

P = 22.2^2/0.3
P = 1642W
Solve for I.
1642 = I^2*R
I = 74A

Now, this is simplified a bit assuming a purely resistive load in the functions. However, it proves that it is possible to get more power with maintaining the same current (or even less if you further increase resistance and voltage input). But, it also proves that if you think current will be lower because you increase voltage on the same motor, you're quite incorrect.

That is all. I'll step off my soapbox now.

You will have lower current at full tilt if you gear down too, but acceleration currents may be higher since you will have more overall power on tap.

Well, when I say that increasing the voltage yields lower current, I assume people know that is true [i]for a given overall rpm]/i] (which means kv has to be lower).

However, you are forgetting a valid point there in your calculations: gearing. Reducing or increasing the gearing effectively changes the resistance for a given kv. Geared high, a motor will pull more current, especially at startup, than a motor geared lower. Really, ohm's law still works, just not in such an obvious manner.

Quite true. My math is quite simplified being a purely resistive load and neglecting any changes in current draw based on the gearing. I primarily did this as a rant because I've seen an awful lot of people stating they'll draw less current by going to 6S from 4S on the same motor. Some folks seem to think that it's a constant power device and you get the same power out no matter what voltage you put in.

I do agree that gearing is critical as it changes the load on the motor. However, please show me someone that runs 6S on a motor that wants to go the same speed they did on 4S. That's the primary reason I didn't factor it in.

You mean Trx noobs don't gear down when running 6s instead of 4s on the same motor? :lol:

*raises hand*

That's exactly what I did with my T4. In all of my messing around, I've settled in on 40 mph as being my "happy top speed". May be slow for some, but it's fine for me. Gets the job done and breaks less parts. So yes, when I add cells, I gear down. I add cells to get the efficiency advantage of running higher voltages. I ran a 2S A123 in there, but wasn't happy with it. I put a 3S A123 in there, and then geared down to something crazy like a 13T. I'm keeping the load (work) the same and increasing voltage, so yes, my amp draws went down.

Eventually I went to a 3300kv motor, because the 5700kv really limited my choices, and I got tired of hearing the car sound like a dentist's drill as it went by. I still geared for the same speed, so I'd assume my amp draws would be very close to the same, since I would be doing the same work.

IMHO, if you have a ESC capable of running 6S, you're throwing away efficiency if you run at 4S. That's why I run all of my rigs at the highest voltage I can. Otherwise it's like buying a 10MP camera, and then shooting everything in 640x480 because you don't want to buy a big memory card. :lol:

I do agree that if you add cells and do not change the gearing, you amp draws will absolutely NOT go down, they will do exactly as you show.

I completely understand your rationale. I'd probably do the same if I didn't already have a ton of 4S packs already.

As already mentioned gearing and choosing the right motor kv is important otherwise your amp draw will run out of hand especially with high voltage.

I also gear for 38-40mph and can achieve this with 8-11s A123 using a Neu 1515/2Y (1100kv) in my truggy. As i can not influence startup power with my ESC, gearing and cellcount are the methods of choice to influence speed, amp draw and thus power output.

A lot of people don't have the usual and nice common sense most do. Sometimes even simple ways of explaining something will not click on those people. I hope this and many other threads like this will teach people some common sense. I hope manufacturers too can learn something as they like to advertise crazy numbers just to impress everyone, but forget the danger that is also involved.

I believe that the [incorrect] assumption is more that the power draw remains a constant. With that premise the current draw will indeed drop as voltage increases.

This 2d interpretation of Ohms law is....uhm....faulted LOL

To answer the original question; why people think going going up to 6s from 4s without changing anything else, is simple.

They are probably the same people who have no idea what 6s on the same motor would be like and end up driving very cautiously if they don't already break parts before something overheats. This perpetuates the assumption that moving up in voltage alone reduces current, as they go on and brag to their friends about their experience.

Ok I accept that if I go from a 4s to a 6s and not change anything else Amp draw will increase as will top speed (naturally) however as I gear for 40mph if I go to 6s and then gear back down to be around the 40mph mark, my amp draw will drop and as such my motor will run cooler. (I can also choose a lower kv motor which will give me more room to move and not run the risk of undergearing a high kv motor to stay around the 40mph mark)

Or am I WAAAY off the mark.

That's pretty much the idea. The problem that you may run into in that scenario though is being able to find adequate gearing choices to gear down far enough on 6S to run the same speed as 4S. Also, IMO, it is much better from the get go to run a motor that will be in the desired RPM range on the voltage you're running.

This is the exact situation I ran into when running my 5700 on 3S vs 2S. Going to 3300kv made the gearing choices much more reasonable. I think I ended up with 23/87. I could run that thing all day like a bat out of hell and it would stay cool enough that you could keep your finger on it as long as you liked. Speed was around 38 - 40 mph (radar verified)