I think I have mentioned a couple of times (at least!) that I have been winding some motors. Yesterday and today I got a chance to do a some testing. I learned a little, and got confused a lot.
In these examples, I was looking at the motors I've been using for my E-Nobler, specifically the Scorpion line. They are characterized by a numbering scheme similar to the AXI. In my case, the motors are the 3020 and 3014. The "30" is the diameter of the armature, and the "20" (or "14") is the magnet length. Finally the number of turns are appended at the end. What I tested was the stock wound 3020-12, my own wound 3020-16 and 3014-16. Except for the magnet and armature length, the 3020 and 3014 are mechanically identical.
What I have been trying to figure out is how to select a motor from among the myriad options one is confronted with. Most manufacturer's have "lines", based primarily on diameter (stator or sometimes the outer can), then lengths (8 mm to 32 mm in the case of Scorpion) and finally the number of wire winds. In any diameter and length category,the "kV" of the motor is set from the number of winds, the lower "kV" having more turns of finer wire (or the equivalent). As a note, within a diameter class, a longer motor will have a lower "kV" than a shorter motor with the same number of winds. So my 3020-16 has a "raw" kV of 880 while my 3014-16 has a raw kV of 1265. By raw kV I mean it is just the max rpm I measure with no prop divided by the applied voltage as measured by my Astro Whattmeter.
So how do you choose? First it depends on what your battery is going to be. I make that decision based on space and weight availability. In an ARF you more or less have to go with what you have, unless you want to do some major surgery. Also batteries are really the major expense, each one costing abut what the motor costs (or more). I will also mention that these decisions are not linear, since at this stage of the game, we are all going a little on other's experiences and more than a little guessing. It is certainly better to error on the over-capacity than under, since "over" means that you may be carrying around a little weight penalty, but "under" means you can't make the full pattern--either by length or by power. I admit I cut a little close to the limit, and have been burned at least once (so far). I also decided to try and fix on the 2100mAHr cell since it really seems to be one of the most popular and least expensive sizes. So once I choose the cell size, the next issue is the number of cells and how you arrange them. What ultimately matters is the total energy that the battery pack contains--it doesn't matter if you arrange things in series or parallel, until you chose the motor that is.
So on one of the other threads, a reasonable rule of thumb is to have a kV so that the target rpm is ~70% of the no load rpm for a given battery pack. The other thing to remember is that the pack voltage drops over the run, so don't choose the max voltage of the pack at the start, but rather the "safe" voltage near the end of a run. Another rule of thumb is that you should never use more than 80% of a pack's capacity (this is for Lipo packs) in a run. For a lipo, this "safe" voltage is 3.7V per cell. For a 4s pack, that means the "safe" voltage is 14.8V, and for a 3s pack 11.1V.
For the props I use, the target rpm's are "typically" in the 8000-9000 rpm range for props with diameters from 10" to 12" and pitches from 5" to 7" (although I might start trying higher pitches). So with a 4s pack, that implies a kV greater than
770 rpm/Volt (for rpm target at 8000). The 70% number gives some overhead so that when more power is needed in the overheads, you can still get it from the system.
For a 3s battery, the target kV would be ~
1030 rpm/Volt.
For the ENobler I started out with the 3020-12 stock motor (kV=1088 rpm/V) and a 3s battery---two 3s2100mAhr packs hooked up in parallel (~3s2p4200 mAHr). This was originally a guess--I was sure it would be enough to power the plane. And it was, but I discovered that I was only using about half the total capacity, but carrying around the full weight of 6 cells. So I thought that I could replace the setup with a single pack made up of 4 2100mAHr cells (a 4s1p 2100mAHr for 2/3 the total weight -6.7oz). I should mention that the 2100mAhr cells have a continuous "C" rating of 18 (~38A). Therefore the 4s pack has a 38A max continuos draw while the two 3s1p 2100 packs in parallel can supply twice that. In flight I am drawing about 18A in level flight, and up to 30A in the overheads from the 4s pack.
However I was now wondering if the original 1088 rpm/V was using my precious "Joules" efficiently, since it wasn't well matched to the 4s battery. I know from previous experience that my CC 35A ESC seemed to be pretty good in efficiently running at partial throttle, but there may be some room for improvement by going to a lower kV motor---but how much?
Actually I already had a Scorpion Motor kit that I bought for the ENobler, even before I bought the 3020-12 motor (I was in a hurry to get the ENobler flying before winter set in last year). So I thought I would wind it for a lower kV. I thought a 16 wind motor would get me a kV in the 800 rpm/Volt range, and I also saw that I could get more copper in the wind than the stock 16 wind motor (using a lower gauge wire--19 AWG). So that is what I did. It turned out that the raw kV ended up as noted above 880 rpm/V (showing it all isn't quite a science).
So as mentioned before, I finally tested things out yesterday and today, and I will post the results such as they are in the next post--but need a rest right now!!