KV and runtimes

[BrianG]

There should be a sticky about this type of topic since I know this has been covered numerous times, but here goes:

For a given voltage and speed, it does not matter if you use high kv and gear down or use low kv and gear up; the amount of work being done is the same. Low kv motors geared up will pull roughly the same current as a high kv geared down.

The advantage to using low kv is you have a little headroom to run with higher voltage if you want to go faster. If extended runtime is the goal, use higher voltage and lower gearing. Every motor has an efficiency curve, and most of the good models are pretty good all the way up to 50k. GriffinRU's program demonstrates this.

The advantage to using higher kv and low gearing is that there are more rpms per mph, so at really slow speeds, the chance of cogging is reduced since the ESC can get more back-EMF pulses for efficient commutation. You may be physically limited by the size/availablitity of the spur/pinion too.

There are limits to each setup. You can gear up a low kv system so much that the batteries and ESC run hot trying to deliver the acceleration current. You can gear down a high kv system so much that the motor is running over its peak efficiency.

Personally, I prefer higher kv and lower gearing for lots of start/stop style of driving. Starts and acceleration seems smoother. I shoot for ~40k-45k rpm on the good motors (Medusa/Neu). If I was doing just speed runs, I would probably go the other route since starts aren't as important.

BTW: mjderstine, I'm not trying to be short with you, I know sometimes it's hard to search when you don't know the exact search phrasing...

[mjderstine]

brianG: no need for the apology, i have been on this site long enough that i realize this info is here somewhere, weeding through the searches was making my eyes cross. maybe we can get a sticky on this thread so that its easier to find!

i understand the goods and bads about each one, both high and low KV motors will have their specific applications. i suppose the best thing for me is to just buy something, and start my own testing.

my track is a pretty small windy track=, with lots of humps and bumps to keep the speeds relatively slow. slow enough that stock slashes are about equal when driven correctly (except for the straights).

i can then deduce from your statement

The advantage to using higher kv and low gearing is that there are more rpms per mph, so at really slow speeds, the chance of cogging is reduced since the ESC can get more back-EMF pulses for efficient commutation. You may be physically limited by the size/availability of the spur/pinion too.

that since my track is rather small and windy, a higher KV motor might have less change of cogging at low speeds.
but what are these speeds? under 5 mph? because i am sure there will be lots of wheel spin that can limit the cogging effect.

i am still coming down to be ing torn between the 3 motors. the 2300 will work best for a standard 4s pack but may stress the ESC too much, MM.

1600 may be limited by the physical limitations of the spur and pinion to gear it to a higher speed, and may be prone to more cogging.

the 2000 is in the middle of the road, and this may seem like the best bet, but the 1600 also has the added advantage of being able to perform at much 6s levels "better" than the other two higher KV motors.

maybe i should just buy all 3...

[littlegiant]

Originally Posted by BrianG

The advantage to using low kv is you have a little headroom to run with higher voltage if you want to go faster. If extended runtime is the goal, use higher voltage and lower gearing. Every motor has an efficiency curve, and most of the good models are pretty good all the way up to 50k. GriffinRU's program demonstrates this.

U forgot to mention that heat loss from I^2*R is lesser in a low KV high voltage system since the voltage makes up a higher part of the wattage as compared to higher kv systems. :D And i totally agree with u on that high KV systems run smoother.