(08-Apr-2016, 12:51 AM)SonicCopter Wrote: Thank you for the answer that's good to know... but about the number of Poles and eRPMs, it's still possible to max out on a micro motor on 4S i think...
if you do the math, a 4000kv motor at 4S and 12 poles will still max out both an F330 and the Atmel MCU.
14.8v x 6 magnetic phases x 4000kv x 70% efficiency is still almost 250,000 commutations per minute (eRPM), which is right at the limit of F330s, and above the limit for Atmels.
I stumbled on this thread lookin to learn something about this matter so it's safe to say I'm no expert when it comes to BLHeli parameters or chip specs but as an engineer I do have a fair bit of understanding of the technical factors and a whole lot of experience with brushless motors.
I recently started winding my own brushless outrunners and I'm currently working on a set of cheapo-chinese 1306's that I use as a mechanical basis but i'm completely overhauling them after a full out melt-down of all four motors.
(in case anyone is left wondering how
(if not, skip to next paragraph)
the hair thin windings of at least one phase group in every motor had just melted into solid blobs of molten copper/insulation gun during an intense maiden that ended in a reversed failsafe situation. All ESCs (i think they were littlebees 16A BLH) burnt out after a full out throttle punch fly-away deal on 4S 65C in just a couple of seconds. I had Gemfan GRP3030 straight tip bi-blades on the originally 3300kv 1306's. I did the new windings with thicker wire gauge and fewer turns on each pole, dropping my KV value and allowing me to go 4" on 4S for the rebuild of that 160 quad. Aaaanyways what im trying to say is that i have some experience lol
But, reading this thread raises a few questions about what you doing with the math there so I was hoping to pick ur brain a lil on that...
In your calculation you are determining eRPM for your ESC's correct? But from what i understand the concept of eRPM is basically the actual mechanical RPM (revolutions (=angular displacement around center axis until fully returning to starting position) per minute) but when considering the electrical "fully returned" situation, this has occured at a number of degrees that is just a fraction of a full revolution of the rotor. (north and south poles of the magnets correspond to the same coil phases magnetic fields from the stator, just a few theeth over) Now, pls bear with me as i sum up everything i can think of that may be of influence on the math:
- The eRPM value is a unit of is electrical "cycles" per minute. (this is a fixed ratio to the mech. revolution)
- Commutations are the steps that an ESC uses to time it's back-EMF readings over the length of 1 eRPM cycle. it is used in a unit of frequency
, noted as Hertz (= occuring intervals per second
) The purpose of commutations is to keep the activated magnetic field at a approx 90deg angle to the closest magnetic counterpole moving around the coils as the bell spins.
- Cogging steps are all the possible rotor-stator positions relative to each other that cause a direct radial orientated magnetic force which either applies no rotational displacement or max displacement. (in other words, the mag field is directly in front of the active coil, or right in between the little gap between 2 magnets in the bell.) when a motor is not powered it will immediately try to snap to any cogging position and when hand turned you can feel the clicks)
- kv rating is simply the RPM increase per applied Volt. (this will happen without any load and needs time to accelerate to that speed) When load is also considered in the equation, thats where it becomes tricky as you can imagine that the inertia of the prop's mass is in fact dragging the motion of the rotor somewhat dynamically under acceleration/decelleration conditions and your ESC's will start to "fire" the phase groups slightly ahead its optimal moment. This is what you can detect as performance decrease and often mentioned as de-sync effect. This is inherit to a brushless motor and good ESC's will quickly make changes to the timing to adjust itself back to the sweetspot. However, too much of this "error" for too long and ur motors will get so hot that it causes a chain reaction of effects that kill your precious motors, often resulting in a short and when that happens you get to say goodbye to your ESCs as well.
- A 4S Lipo when fully charged will put out approx 16.7V, not its nominal rating of 14.8V
- To my knowledge, all 1306 outrunners are built as 9n12p configurations, meaning they have 9n / 3phases =3 groups of three teeth, typically wired in Delta.
- with a stator diameter of 13mm to devide over 9 teeth and assuming your 1306's also have 12 magnets, that would give you 36 cogging steps.
My question to you is,
What is "the math" you used to get to the 250000 number and is a chip rated in commutations like that?
by magnetic phases, are you referring to magnet pairs? or coil fields?
what is the 70% efficiency for and how does that work?