Difference between 4600kV, 5700kV, 6900kV and 7700kV motors?

[woodsxr]

Please bear with me I'm new to the RC crawler world.
I'm searching for an upgrade for my stock Venom motor and esc'
I started looking at a couple on line hobby shops and kept finding the different KV numbers and am trying to understand their meanings.
Any help would be great.
Thanks

 

[wyant50]

KV is rpm per volt. Higher the KV the more rpm or speed you can get.

 

[holycaveman]

Please bear with me I'm new to the RC crawler world.
I'm searching for an upgrade for my stock Venom motor and esc'
I started looking at a couple on line hobby shops and kept finding the different KV numbers and am trying to understand their meanings.
Any help would be great.
Thanks

In a nutshell. Kv ratings are rpms per volt.

So a 4600kv motor will turn 4600 times per volt.

So the higher the kv, the faster the motor is.

That's the 50cent tour. Now it gets complicated.

A high kv motor say 7700 is ment to run on a 2s lipo, usually no more.
So 7700 x 7.4volts = 56980 Rpms

A 5700kv motor will run on a 3s lipo or 11.1 volts.
So 5700 x 11.1 = 63270rpms.

So in this case the slower motor is actually faster.

Then it even gets more complicated. From heat to amp draw, etc....

Hope I didn't confuse things.

The key is first desciding what is it exactly you want your truck to do. Rather than just the fastest motor.

 

[K.O.M.]

holycaveman beat me to it, but I was just going to make a point that brushless motors for 1/8th scale can be rated lower at say 2650 kv, but handle up to 6s, which is 22.2v, which gives you 58,830 rpm :shock:

 

[woodsxr]

Thanks for all the knowledge.
Now I know why what I was reading was sounding confusing.
All above motors will run on 2S Lipos? and can you adjust the rmp range on all brushless?

I'm running a 35 turn motor and would like to stay in that range with having the option of tweaking the motor at times.

 

[woodsxr]

So it there a chart showing comparisons to brushed motors since they are badged different?
Any suggestion on what KV motor to run that would compare to a 35T?
any help would be great.
Thanks

 

[K.O.M.]

For an unloaded motor, my estimate for a brushed 35T would be 1200 to 1500 rpm/volt. My electrical motor theory is very rusty, so this is based solely on my experience with dyno tuning some spec motors way back in the day (FYI, we were using anywhere from 25T to 30T motors and running 7.2 and 4.8 volts nominally).

However, that being said, it's not really a direct comparison:

A brushed motor has to deal with frictional losses at the brushes and that affects how it works as the rpm changes and the load to the motor changes. Typically, at the top of the rpm range, brush friction increases reducing the useful torque. You also have to consider the voltage drop from the brushes to the commutator. Also, heat is going to be more of an issue since the armature, where the current is flowing, is rotating and can't dissipate the heat through the can like on a brushless setup.