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View Full Version : So how does gearing affect FET-safety?


Skv012a
2009.04.29, 12:06 AM
The specific case I have in mind is Atomic Chili w/ stock 2.4 board. Am I going to be limited to any ratios on MR/MA or not? Also, whats the general rule of thumb regarding this matter?

EMU
2009.04.29, 02:39 AM
The motor is a fast motor. I cant see why you would want to gear it higher than a 44/8, or 52/10... I wouldnt overgear it too much. A gear ratio under 4.50 is going to seriously lower the low end acceleration, and may cause fet issues. It is a ballpark figure, but I wouldnt run anything lower than 5.00 on a chili, even on a big track. It has more than enough speed.

This is on an MR02, I havent used a chili in an AWD with 2.4.

Skv012a
2009.04.29, 12:10 PM
So what happens if the ratio is too tall when using boarderline safe motor? Also what happens if ratio is short?

MikeL
2009.04.29, 12:16 PM
So what happens if the ratio is too tall when using boarderline safe motor? Also what happens if ratio is short?

To low of a ratio will increase amp draw which will increase heat which can lead to failure. Too high of a ratio will not cause damage to the fets, the car will just top out very quickly.

Skv012a
2009.04.29, 12:19 PM
Failure like burning out or shutting down?

MikeL
2009.04.29, 12:30 PM
Failure like burning out or shutting down?

2.4 is susposed to shutdown before it fails, but I'm sure you can still kill it if you push it too hard. I've run on a big track with a chili and an 8t pinion with no issues, like it was stated, it was plenty fast enough and I did not feel the need to try the 9t.

EMU
2009.04.29, 12:35 PM
I have fried the fets off an ASF MR02 at the PN regional last year with an AnimaII... So, the shutdown doesnt allways work.

Skv012a
2009.04.29, 01:45 PM
Thanks guys. To sum this up: whats the lowest turn motor we can run on 2.4 boards and whats the max ratio/gear #s we can run with that motor safely?

cowboysir
2009.04.29, 10:41 PM
There are too many variables to be such a cut and dried case of " lowest turn/gear ratio/etc". Your best bet is to follow test information of motors to find out if it's safe to use for certain fet combinations.

As for fet amperage, if I recall (which I'm probably wrong) a 3010 fet had a 6amp rating. I guess that would mean that you'd have a 12 amp leeway for motor draw under load.

I probably haven't helped...:o

EMU
2009.04.29, 10:57 PM
Take in mind, that most modified motors pull about 1.5A at 5v with no load... once you add the load, that will increase a lot more.

It is not as simple as judging the motor by the turns. I wish it was that simple. There are many variables besides the amount of turns that make a huge difference in how the motor performs. An armature with longer poles will draw less amps at the same amount of winds as an armature with short poles. There is less wire on the winds for the short armature, therefore more amps and more speed.

It is better to judge by amp draw rather than turns.