Choosing an optimal kv w/o killing ESC

[swbluto]

Hello all. I'm extending beyond the original "ideal kv" post in the ebike technical section for its application to RC ESCs. The original post is in quotes below.

So, it comes time to decide what an optimal kv is. The relationship between kv, torque and heating has been discussed at viewtopic.php?f=30&t=13265. Now, I want to discuss the decision making process between choosing the proper type of motor.

Going from that thread, it was found that a higher kV results in a higher power potential. However, for the same motor, the resistance is reduced and the current increases. Eventually, you get to the point where the motor's resistance is too low and the controller has no chance of surviving. So, there's some point between too low a kv and too high a kv that's optimal. But what is that point?

Let's assume we want the most power possible. Let's assume that a given voltage that applies to one kv can be implied to another. But, while we still want as much power as possible, we still want things to survive.

So, if one wants the most power as possible, one would naturally choose the highest voltage. If the thing that determines whether our controller will survive or die is the phase current and there's some known limit for the phase current that decides whether it will die, then let's restrict ourselves to that phase current. IF, at some motor RPM, the steady state current at full throttle is low enough such that it won't kill the throttle, you could wring out more power from the motor. But, the controller would have to know the motor's RPM, kv and resistance in order to predict whether the full throttle current is acceptable without risking the mosfets. If by some lucky combination the full throttle current never exceeds the current ratings of the FETs, then there'd be no need for intelligent calculations. Just full current whenever desired, while the phase currents would remain below the limit during PWM.

With those conditions, a lower wind would naturally have more torque at a given RPM assuming the phase currents are limited meaning a lower wind would have more power. However, at some higher RPM, the current from the lower kV motor will start to drop below the phase limit and eventually the power output of the lower kv will be be lower than the higher kV and the higher kV may utlimately have greater peak power.

So, what to do? Well, I would want the lower kV motor to not reach that "crossover" until some far higher RPM, some RPM that exceeds the mechanical limits of the motor. If that's the case, then the best case would be the lowest wind that still meets the phase current limit at the fastest "critical rpm", then it would have the most power while also keeping the controller relatively safe.

However, if we used such a motor, than that would imply the "stall current" would probably be pretty high. Does this matter? Well, yes, kind of. The amount the peak-to-peak current rises to inside the controller during PWM depends on the steady state current of 100% duty cycle at a given voltage. If the stall current at full throttle is too high, then the peak to peak currents may counteract the intentions of phase limiting as the peak current might far exceed phase limiting. Now, one can ameliorate this by providing additional inductance in the controller OR motor, in which case, we get to the following conclusion.

In summary:
With a phase limited controller, use the highest voltage possible, get the lowest kV that you can that will generate the phase limit current in the motor at the mechanical RPM limits, and add inductance to the motor / controller if needed. This will help get the highest power possible while ensuring the controller's survival. Predicting motor currents would be tedious by hand, but it's made fairly easy and quick using my simulator.

Now, as for adding inductance, you might be thinking... "what?? But controllers with built in inductance don't exist! What can I do about that?". Well, I'm thinking you *can* do you something! I haven't experimented with this to know how well anything I'm going to think of will work, but I'm sure there's something relatively simple that can be done.

Ok, so now let's discuss inductance. When adding inductance to a motor to reduce the peak-to-peak current, we also reduce the average current during the phase period at higher motor rpms when the phase period is lower. But, if the phase current is being limited... that's not a problem? Eh... I'm getting confused thinking about - I'll need to draw some graphics later on.



Adding inductance

For now, though, let's discuss how to add inductance. Inductance mainly comes from loops in wire; the bigger the loop, the more the inductance (It's proportional to the loop area); the more loops there are, the greater the inductance(It's also proportional to the number of loops). I don't know if the "core material" will enhance inductance or not, so maybe someone else can fill me in on that.

So, basically, we want to add a lot of inductance while adding minimal resistance. The logical consequence would seem to be a big fat wire (or lots of smaller parallel wires) coiled into a large loop and/or with many loops. Attaching one of these in series with each lead of the motor will increase inductance while hopefully adding minimal resistance.

So, let me know what you think of my hare-brained idea. Obviously, for this be implemented, you'd need a controller with proper phase limiting. As far I as I know, none of the RC ESCs do this, sadly.

As noted, the ideal is phase current limiting but this isn't possible with the RC ESCs as they are. So, the next best thing is to be sure to choose your voltage and motor winding such that the "critical phase current" isn't exceeded. However, if you're doing that, then it seems like you really aren't exploiting the power potential of your motor which doesn't do well with extracting as much power as possible ideal, albeit safely. If you were to choose a low resistance motor for the increased power potential, then you'd need to pinky promise that you'd never use partial throttle for extended periods of time. :lol: (For most "real bikes", that's a joke. Because, in the real world, there's no single speed that you'd use ALL the time.)

If you're advanced, you could potentially use partial throttle at the steady state or near steady state speed, but there's risks when under load (Such as sudden gusts of wind, hills) and during acceleration with low resistance motors. With adequately higher resistance motors, this isn't a concern.

Today, while cruising at 15 mph at partial throttle and consuming like 10 amps of current, a sudden gust of high wind came through and my current jumped to 40 amps! Now, if I were going 25 mph under partial throttle and was consuming 45 amps (Typical, actually), than that implies that under the same gust of high wind, the current could've jumped to 100 amps or more, which means something like 200 amps or more of motor current. So, yes, wind itself can impose a particularly high load.