Voltage and current question
 

Only if you gear down...

If you do go up in voltage, does the amperage increase? (keeping the same gearing)

Yes.

If you gear so that the motor produces the same output (wattage) on both 4 and 6 cells you should produce the same top speed. The higher voltage setup will be more effecient, due to the higher voltage drawing less current (amps) to produce the same output (volts x amps = watts), in turn the lower draw produces less heat in the wires, motors, esc, etc. Small gain but still there, plus stuff runs cooler, so it should last longer too...

There's one simple law that explains it all: Ohm's Law, which states that V=IR (voltage = current x resistance). If you don't change the gearing, then R (resistance) remains the same. So by Ohm's Law, current will increase proportionally with voltage. This doesn't always hold true, however, because you could change your driving style.

Changing the gearing has no effect on the electrical resistance.



If you go up from 4s to 6s, your motor will draw less current if the total motor rpm is the same, for example a 3000kv motor on 4s (= 44400rpm) is drawing more amps than a 2000kv motor on 6s (also 44400rpm).

Right, changing the gearing affects the mechanical resistance. But since a motor converts electrical energy into mechanical energy that mechanical resistance is a factor here.

We're talking about increasing the voltage on the same motor, and its effects.

Oh no, here we go again! Griffin and I had this discussion a while back.

The vast majority of a motor's "resistance" comes from the back-EMF. A slower turning motor under heavy mechanical load reduces the BEMF, thus lowering the "resistance" and increasing current flow. By reducing the gearing, you are allowing the motor to spin faster doing less work for each rev. Since the motor is spinning faster with less load, more BEMF is generated, thus increasing resistance and lowering the current.

I remember it well; talk about clash of the titans... :whistle:

And even though it looked like we disagreed, we were actually saying the same basic thing; it was the terminology that was getting in the way. :smile:

I'm actually loving my 8S 60A or so spikes truggy set up now :lol:. I still haven't charged my 2 4S packs and tomorrow I'm going to run some more testing my 6 pole 36 x 60 1300Kv motor. This will be good to see if a smaller motor does actually need more AMPs to get the truggy moving then a larger motor.

Thanks for all the info guys. I am an electrician so I am familiar with ohms law which makes this hobby a little easier, but very new to brushless and did not know if you could use the same theorys with the esc and brushless motor.

There seems to be many people here with some incredible knowledge and can always count on an intelligent answer!!! Thanks again. I will post pics when I am done.

Are their any good reference sites I could visit????

Depends what you want to reference exactly- BrianG's website is great for electrical setup and gearing info (in his sig), and I have a nice FAQ in my sig, though its aimed more at the noobs on the traxxas forum- still some good links in there however.

Been to your site....AWESOME!!!!!!!!!!!!!!!!!!

So is I am reading all the right, if rpm stays the same which is controlled by gearing,increasing the voltage should drop the amps.

Also, am going to buy a new 5-6cell batt tomorrow. Looking at the max-maps 6000 lipo, sound good?

Noooooooooooooooo! to the maxamps. There are so many better options, take a look around and see what the pros use (thats the guys on this forum :wink:). I would suggest anything from rclipos.com basically.

RPMs are determined by motor kv (rpms per volt rating) and input voltage; gearing only alters wheel rpms. if you use a higher voltage with the same motor, you must gear down in order to reduce the load on the motor, and therefore maintain the same overall speed- efficiency will increase usually, and temps will decrease (longer runtime is also possible usually).

another option is to run a motor with a lower kv value

+1. Lower kv motor, plus that higher voltage and the same gearing will yield the same result, better even perhaps :yes:

I will check out the rclipo.com. I have fried 2 MMM already and there is not going to be a third. After talking to several people, it seems my batt did not have enough amps and the gearing was too tall.

Who makes a good onboard amperage monitor. I want to see what is going on in my truck.

Also, do you guys recomend balance charging or after charging with a blinky?

i dont really see any point balancing AFTER charging. if they are out of balance youve already damaged them. charge AND balance would be my opinion

I balance at the end of the charge, and let it continue after the charge.

Differences in cells show up when there is less of a charge in them. So if you balance them early on in the charging process you could make the balancer work harder at the end of the charge than it would otherwise need to.

This is my theory anyways, and has proved successful for me.

yes that makes sense. theres no point in balancing when they are less than 75% full. the cells will usually sort themselves out near the end, but i just plug them in and let the charger do its own thing :)

<<plain and simple>> as long as you don't have your voltage so high that you are revving into your motors inefficiency rpm range, the following statement is almost always true. In a truck with two available setups, if it's geared to go the same speed with each setup, the higher voltage setup will draw less current.

I agree fully with the lower kv motor option. A much better solution than just gearing down a motor spinning way faster than it needs too. No need to spin the motor much more than 30K rpm in my opinion.

Adding voltage and just gearing down to attain the same top speed can help a little if you are having ESC temp problems, but really wont do anything for you as far a motor temp problems in my experience. It may actually cause motor temp problems from what I have seen.

Getting a motor of the proper kv range for the voltage you plan to run is by far the best solution.

Higher voltage, lower motor kv, and similar gearing will product the best results for lowering current (amp) draw.

HV rules :lol:.