My BL Revo project - Page 8 - RC-Monster Forums

HotnCold · 04.07.2006, 05:32 AM

Yep - sounds spot on...

BrianG · 10:01 PM

Well, I took it out for a while today and had a blast. Runtime is still right around 25-30 minutes and I can't figure out why it's so long. The truck is heavy and is using GP3300 cells. :032: That calculates to 6.6A average current draw. And I'm not really easy on the throttle either...

I got a wheelie bar and it was working good until I landed on it from a jump and bent it. Oh well. Now I have to find a new one that will last more than one bashing session. Is there a good one that someone can recommend? I also bent the roll bar I made all to hell. Hmmph. That'll teach me to run without the body.

Man, the thing is hard to control! The power is almost too much. I ended up reducing the throttle to 85% and the brake/reverse to 60% on the transmitter. Otherwise the tires break loose when I goose it or it does forward flips when I brake. Seems like a waste to keep the EPA so conservative. Kinda makes me think I should have gone with a 9XL or 10XL instead of an 8XL? Is there a way to program the ESC so it will take longer to go full throttle if I "floor" it? According to the manual, it says there is a pre-programmed 0.3 second ramp-up time for the throttle. I was thinking maybe a 0.5-0.75 second ramp-up might help.

maxxdude1234 · 04.10.2006, 03:34 AM

I believe you can adjust the throttle curve on some transmitters, so that there will be a smooth progression when you floor the throttle.

Sylvester · 04.10.2006, 05:44 AM

I think you need the programming cable for that? Ask mike, he should know.

BrianG · 08:40 AM

Maxxdude: I have the Spektrum DX3, which I don't think has that feature.

Sylvester: I thought I heard there was a programming cable, but when I looked, I only saw the Castle link. I just went there again and saw the one for BK in the controllers section. I must have looked in the "motors" section before for some reason, which only lists the Castle link. Doh.

Something else I noticed, but not sure if it's the batteries. I also have two 6-cell packs that I was using and it seemed to reduce the speed to about what I'm looking for, but runtime was half of what I had with two 7-cell packs. All four packs are GP 3300 cells, but the two 6-cell packs are packaged differently. Does it make sense that runtime is reduced using a lower voltage? I would think that a lower voltage on a relatively constant motor resistance (impedance not included) would create less current, increasing runtime.

Assuming 1.2v/cell (I know they start out a little higher), what is the minimum number of cells recommended to use with the 8XL? For max cell count, it seems like 18-20 cells will keep it in the 45k rpm range, but that would be wayyy too fast for me. 14 cells equates to 31,500 rpm.

squeeforever · 04.10.2006, 08:26 AM

not sure about your transmitter but im pretty sure the m8 has a feature like this. ill try it out and let you know later. i kinda had to be a little easy on the trigger last night. i have the 10xl on 14 cells for now (soon 16 or 6s lipo). i ran it last night for th first time on my new ib 4200s and it was INSANE. top speed was about 40 to 45ish with 20/51 gearing. lets put it this way...i had no problem ketching up with the cars going by at 35 to 40 mph....

coolhandcountry · 04.10.2006, 08:54 AM

Well you have a tendency to run the motor with more throttle on the 12 cells than the 14. You have more power to use so you accually are easier on the trigger so you get more run time. If you launched it the same every time the 6 cells would or should last longer.

squeeforever · 09:01 AM

i was under the impression the higher the voltage the better the runtime up to a certain point when the efficiency falls? maybe im loosing it :confused:

BrianG · 04.10.2006, 11:56 AM

CHC, what you say makes sense, but then Squee's response sounds right too.

04.10.2006, 01:30 PM

i also think that what squee said makes sense and sounds right

maxxdude1234 · 04.10.2006, 01:36 PM

In short, they are both right! CHC's is right from a scientific view. In that a higher voltage will increase current. However, in the context on rc cars it is slightly different. Because when we add voltage we 'gear down' (ie. use a smaller pinion) the motor has to work less hard, and hence we draw less current. This is how squee is also right.

BrianG · 02:08 PM

Ah, but I didn't gear down...

There must be a point where the ESC sends an "ideal" signal of pulese where the BL motor operates with the most effieciency, and this ideal range is based on voltage and wind. I'm sure the AC reactive properties of the coils must have something to do with it, but without inductive ratings, and the frequency of the ESC pulese, it's pretty hard to tell.

coolhandcountry · 04.10.2006, 03:39 PM

Well if you think about it. If it takes say 50 watts of power to get the truck to move. With more voltage it will take less amps to move the truck. 12 cells it may wheelie 14 cells it will do a back flip.

BrianG · 04.10.2006, 04:36 PM

Quote:

Originally Posted by coolhandcountry

Well if you think about it. If it takes say 50 watts of power to get the truck to move. With more voltage it will take less amps to move the truck. 12 cells it may wheelie 14 cells it will do a back flip.

If what you say is true, then it is a function of power, not voltage or current individually. Meaning, for a given power, as voltage goes up, current goes down and vice-versa. In order for that to happen, the motor resistance (actually impedance) must change. Generally speaking, when you have a certain voltage applied to a fixed load, which produces a current. If voltage goes up, current goes up, and so does power. What I thought would happen is that at 12cells it would wheelie and 14cells it would flip (like you said) but because the higher voltage generated a proportionally higher current (and higher power). Following that thought, 12 cells should draw less current and run longer.

It could be the fact that I'm using the throttle more aggressively at 12 cells to make up for the loss like you said. The more I think about it, it seems to have more to do with the three-phase AC inductive reactance coupled with the pulsewidth and frequency of the signal. Maybe it's time to drag out some of my old electronics books that deal with AC theory.

So, what cell count would give me the best runtime? 16 cells? 18 cells? Or stick with 14? Of course I'd have to reduce my throttle EPA even further if I went UP in cell count!

By the way, I'm not intending to argue, I'm just trying to get it straight in my head.

Sylvester · 04.10.2006, 07:10 PM

Id say stay 14, or you can even step it up to 16, you'd get similar runtimes, due to the extra power ( you'd keep your finger alot farther away from full :D )