motor choice.

[ash12321]

so, i see a lot of people running low rpm and high torque motors over high rpm motors. can any 1 explain why as the average motor alone destroys a nitro in regards to power to weight ratio and only high end race engines are close to rivalling most motor's so I don't see that it is the power side of the motor that makes everyone go low rpm (1800kv) why not a 2650kv? i am sure i will get a logical explanaion and i think i already know but i would just like some 2nd opinions

 

sorry i mean to add that this is aimed at 1/8 buggy and truggy

Edited August 30, 2013 by ash12321

[DreXeL]

Running a lower KV with higher voltage (say, 1800kv on 5/6s opposed to 2650kv on 4s) is more efficient, so you get longer run times and less heat. Obviously gearing plays a part too.

Edited August 30, 2013 by DreXeL

[jordan_bench10]

an electric doesnt kill a nitro completely just accelerates faster which is where people love it a lower kv gives mmore torgue but still plenty of speed 

[NuttyProfessor]

I use a 2650kv in my bugger, I love it. 30 mins runtime on a full pack and still plenty of torque.

[ash12321]

I use a 2650kv in my bugger, I love it. 30 mins runtime on a full pack and still plenty of torque.

thats what i mean. i have a MM 2200kv and even at 10% punch it is insane, even with 4s !. 

[Noj]

Try a 780kv on 10s.... that is quite fast 

[ash12321]

haha!

[Jacksprogis]

Believe me it's nutty dude. ......

Jack

[Randum]

I know it's been a little while but this was right up my street

It's nothing to do with torque. It's to do with revolutions per volt. Theoretically a 3300kv and 5700kv brushless motor built the same to an extent would have the same torque. Brushed was different.

It's to do with what Drexel said

If you had a motor that did 40,000rpm on 4s that produced 3hp and an equal motor that did 40,000rpm on 6s and produced 3hp then both would be using the same required power.

Power is watts

Now I am gonna use some numbers as an EXAMPLE not true numbers I will use nominal lipo ratings for this example as voltage will fluctuate during use and quality of used batteries (which is a main reason for high c rated batteries, voltage drop under higher loads)

The 4s setup is using 2000watts (power) on 4s 14.8v. So to create 2000watts of power we can figure out how many amps will be drawn, watts divided by volts equals amps. So 2000 divide by 14.8 = 135amps (rounded)

Now on 6s to create the same amount of power we will divide 2000watts by 22.2volts = 90amps

Now as you can see. To get the same 2000watt (2kw) power for a 4s setup we need 135amps of current as opposed to 90amps of current on 6s.

What does that mean? Ok so think of it as pipes. The wire is the pipe. We all know thicker wire allows greater current. And bigger bore pipes allow more water.

The current (amps) is the volume of water

The voltage is the amount of energy in the water

The watts is the energy coming out the end of the pipe.

On 4s example we need 135amps now we'll use litre instead of Amps. We need 135 litres to pass through a small pipe in an hour. And 6s 90litres to go through same pipe in an hour. So the water needs to move faster on 4s creating frictional resistance therefore losing the initial force pushing it through pipe. The longer the pipe the more is lost, every joint is a resistance. The same is happening with electric. The resistance of your esc, wires, connectors and then the lost power is turned into heat. So if you can get the same power only using 90amps then why not?

Also I would like to add that it puts less stress on every electrical component in your car, the built in bec is not being robbed of power as the motor pulls all the available current, allowing a more consistent power supply to receiver and servo. The batteries aren't pushed so hard trying to deliver as high amp draws.

But I'll need a massive battery won't I? Not if you are only going up voltage for more efficient and cooler running,

Using the above number. If you took a 5000mah 4s battery. It will contain (5amp x 14.4v=) 72watt-hours of power

A 3300mah 6s battery contains (3.3 x 22.2=) 73.2watt-hours

So a 3300 6s battery will actually last the same as a 5000 4s battery providing you are setup to be more efficient and not setup using the 6s 2200kv setup everyone thinks of in the savage flux and erevo.

Ideally if you went from 2200kv on 4s gives you 14.4v x 2200 = 31,680

So 31,680 / 22.2v = 1,427. So a 1450 kv motor on 6s would be as fast as a 2200kv on 4s but looking at what we learned above is more efficient. Are batteries are similar in price as we need a smaller capacity battery as our voltage is making up the power not amps.

[Cod head]

Ouch my head hurts reading that

[ninjagaiden]

To keep it simple, energy loss through heat and the resistance of your electrical system depends on the amp draw ( energy loss = current squared times resistance)

 

To to put down a certain amount of power, going to a higher voltage reduces the current you need. So you have less energy lost to heat, and you are more efficient.

 

That's why the National grid 'transports' electricity at high voltage.

 

Lower Kv motors can run at higher voltages before they hit their maximum RPM rating.