Medusa kv problem - RC-Monster Forums

E-Revonut · **Medusa kv problem -** 01.25.2010, 12:39 AM

Awhile back I had doubted that the 36-70-1600 I have was actually 1600kv, I thought it might have been mis-labeled and was really a 2000kv version. Well I finally got around to hooking it up to my Eagletree with it on my E-maxx. I did this quickly inside with all the wheels off the ground just to get this reading. I used 2x 7cell nimh packs and the logger is hooked up to one pack so the voltage needs to be doubled. I have a peak RPM of 34,090 and at that RPM the aprox voltage was at 17.6 which equates to 1937kv. I would have to say that this confirms that I have a 2000kv motor which means that my sense of speed was correct and that I have never been geared correctly, I have always been way faster than what I was geared for which has put a tremendous amount of extra load on my setup. Currently my gearing should put me at the same speed as the stock brushed motors, but I'm def going faster. Now that I know I really need to gear for a 2000kv motor, I will be much better off.

sikeston34m · 01.25.2010, 12:48 AM

Interesting!

I'm not so sure if you just got a motor that was labeled wrong OR if their kv rating system is messed up.

The reason why I say that is:

I'm running a Medusa 36-70-2000kv motor in my E Revo.

I also have the data logger with the brushless rpm sensor. On 6S, gearing 14/52, it says I'm spinning up to 65,000 rpms.

Now, I haven't quite figured out how to calibrate the sensor yet. I just got it. I don't think it's completely accurate on my setup, because it rarely goes back to 0.

There are times that I know the truck is stopped and it's still showing 7500 rpms.

If it's just a matter of zeroing it out to get the calibration right, this still leaves me spinning up to around 57,500 rpms.

This still leaves me at around 2500kv. hmmmm..............

I know this truck really screams with this setup. Gearing is for 40mph on 6S, but it's way faster than that.

E-Revonut · 01.25.2010, 01:02 AM

I Know that roughly the stock Titans are about 1700kv, even though brushed motors aren't rated in kv thats roughly what they work out to be. With the stock gearing (68/19) and 4s lipo a stock E-maxx 3905 is geared to do aprox 25MPH. With a 1600kv motor and 68/20 gearing the speed should be about the same but it's no where close, I'm much closer to 30MPH which if I plug in a 2000kv motor it works out. When I had this motor on my RC8T I always thought I was way faster than what I was geared for and I tried putting in 2000kv then and the speeds I was coming up with seemed much closer to what I was actually doing. This just confirmed it.

I never calibrated the rpm sensor, at least I don't remember doing it, but it started at 0 and came back to zero in live mode

sikeston34m · 01.25.2010, 01:07 AM

I'm gonna have to play around with live mode and the rpm sensor some more.

What kinda amps are you pulling with your setup?

I'm showing 100 amp peaks on 6S. Right at 2100 watts.

E-Revonut · 01.25.2010, 01:12 AM

I just did this as a quick test with the wheels off the ground so amps didn't come into play. If it stops raining tomorrow I will do a run with the logger.

01.26.2010, 10:10 AM

So how does the fact that it was pretty much unloaded for the test , come into play? It was my understanding that the KV should be taken at load. But mine always did end up being a little over the calc, so maybe they do something different, like take the KV at the full rated load. Then we put a much lower load, and the kv rises.

E-Revonut · 01.26.2010, 10:27 AM

Af far as I'm aware kv values are for an unloaded motor, at least that's what I beleived the "U" after the value in Brian's calculator meant. I haven't posted up the graph yet but I did do a short run yesterday and I have still concluded that I'm much closer to 2000kv. Everytime I do the math I come up between 1925-1960kv. It's tough as the only way to figure it out is at full throttle and figuring out where I hit full throttle can be tough, that's why I had started with a quick test just holding the truck in the air, I could grab full throttle and hold it there. I'm gonna do another test when I can have a friend help. I want to hold full throttle for a bit and see where everything stabilizes.

BrianG · 01.26.2010, 10:51 AM

Quote:

Originally Posted by E-Revonut

Af far as I'm aware kv values are for an unloaded motor, at least that's what I beleived the "U" after the value in Brian's calculator meant...

Yes, you are right; "U" means the kv is unloaded, "L" means the kv is loaded, "?" or blank means it is unclear.

Don't forget, because some ESCs have active timing of sorts, the final motor speed can be more than kv*voltage. Patrick has even stated this.