Turnigy Trackstar 150A GenII ESC

[Serum]

At some point the wire thickness and resistance plays a part. No matter how hard you cool it. :)

That optimum point; I think it might be a bit more complicated and setup-dependent. Artur could elaborate on that much better than I ever can.

It also depends on timing. These new controllers with a timing-boost should be able help to get a higher top-end as well (I mean the ones that up their timing towards the end of the end-rpm) Never found a movie on youtube to back this theory though

[Dr_T]

Originally Posted by Serum

At some point the wire thickness and resistance plays a part. No matter how hard you cool it. :)

Yeah, I will have to add that to my lessons learnt now :).

Maybe I should have looked into it a bit earlier, but after burning up the Turnigy, I got my crayons out and made some graphs to get a better understanding of how much Power is actually required to obtain a certain speed, within a certain time, or distance; thought I’d share it here for later reference and for anyone interested. Basically there are 3 types of Forces at play when accelerating to a certain speed:

• Acceleration Force, proportional to weight and acceleration;
• Aerodynamic Drag Force, proportional to air density, frontal surface area, and quadratically dependent on speed;
• Friction and rolling resistance Forces, proportional to speed.

Power = Force * Velocity, so from these Forces, we can get an idea of the Power requirements. The first graph below shows the Force and Power components as function of speed for a 5 kg RC, with 30cmx15cm frontal area (yes, my GT2 :)), accelerating at 10 m/s^2 (based on Lizard’s 1717 Slash: 0-150km/h in 4 seconds) through standard airmass. Drag coefficient, Cd, is guessed to be 0.35, same as the default in BrianG’s online calculator. Combined friction and rolling resistance coefficients are the biggest ‘fudge factor’ in the model. I chose them to have the Drag and friction/resistive Forces cross-over at ~50 km/h - just a shot from the hip, no idea how valid that is. The model is just a very simple and straightforward approximation, based on constant acceleration to top-speed (been way too long since I bothered with differential equations :D). In reality, the maximum traction Force the tires can put on the road is limited obviously, so the net longitudinal Force that is available for acceleration, will be limited and, in addition, will reduce with increasing speed due to the growing of the velocity dependent opposing Forces.

Dashed lines in the graphs are the individual components (acceleration: a, Drag: D, friction and rolling risistance: f&r), solid cyan lines are the totals; solid magenta lines are totals minus acceleration, so representing the constant velocity Force and Power requirements. You can see how much the acceleration part actually contributes to the total Power requirements, and this model does not even take into account the (acceleration of) rotational masses (just needed a quick fix, might look at that later… well, probably not :D). Accelerating slower would lower Power demands, but running-space would become an issue then quickly. Even with 10 m/s^2 (about 1g), it takes about 100 m to accelerate from 0-160 km/h (100 mph). 0-200 km/h would take ~155 m… so accelerating slower would leave very little time for driving at top-speed (and getting a good reading of it out of a flimsy 1 Hz GPS logger to brag with on the interwebz :D).



Next step is taking into account system efficiency, also a bit of a ‘fudge factor’. After some tweaking and comparison with results of BrianG’s calculator model, I put efficiency at 69%. Main difference with the scriptasylum model is that that model seems to levy the friction and rolling resistance into the 'overall efficiency' (from input to usable output Power). If I put the friction and rolling resistance coefficients in my model on zero and take an overall efficiency of 63.7%, it seems to produce the same results as the online tool with electrical efficiency set to 85% and 4WD drive configuration - at least for the couple of speeds I checked, in the range that I’m interested in.

The graph below shows the electrical Power and Current (@6S) requirements as function of speed. Based on these approximations, at 6S, it would take about 217 A to get the 5 kg example car with 10 m/^2 to 160 km/h; sustaining that speed would only take 72 A. Continuing up to 200 km/h would take 315 A , plus/minus ~30 A for a kilo more or less and 135 A in steady state. Seems that beyond 160 km/h, things are becoming a bit more tricky.



Long story short: I guess it takes at least a 220-250A motor to be able to do 160 km/h (100 mph) comfortably and reliably with a low-profile 5 kg car on 6S, which also explains the premature death of my 106A Turnigy. I think gearing the Turnigy in your X0-1 for 100 km/h might even be on the heavy side for it Lizard, unless traction sucks and you can’t get near 10 m/s^2 acceleration... good thing it was only 10 bucks :).

Anyway, cheers!