KV means, at which rpm backEMF (induced voltage) equates battery voltage, means does not allow to push current anymore.

It means the KV is determines by the wire lenghth, wire speed and magnetic field strength. Means it is number of turns, strenght of magnets, gap, dimmensions and rpm.

The only usefull rule from all of that is (withou knowing the constuction), that if you have motor of some construction (dimmensions, materialls etc), then kv is relative to 1/number of turns ... or ... half the turns means twice the KV compared to the same motor difering only in number of turns.

I think this is more question for Alan, but I think that it does not affect too motor much.

There are two effects:

1/ delay after commutation (or better phase shift) of current in winding. This does not affect motor too much, because we can compensate it by advance angle.

2/ PWM efectivity. Induction makes current in coil flat. PWM are rectangles of voltage, but induction makes current more flat. It makes motor more effective in PWM mode, because the current goes via winding also after closing of transistors, it goes via diodes back to battery as a charging current and then back to coil when transistor is opened again. But in any case, 3 phase leads should be as short as possible, because that induction converts voltage ov battery to lower voltage needed for proper RPM like a transformer, and it also means that the current in 3 phase leads is higher that the current from battery. But the reason is its resistance, not inductance.

Leads between battery and ESC can be longer. ESC should have quick and large capacitors to suppress charging current to battery, it should catch all spikes from winding and give it back to motor without going to battery, so long leads does not make troubles, but in reality also those leads have some inductance, so if you use longer leads, you should use capacitors every 10" or so.

But you can figure it out yourself ... for example winding of Axi 2826/10 has ~7uH if I remember well.

Guys,
Just a quick question on how our motors relate to their construction. My understanding is that the KV value is determined by the number of turns of wire on the core stator poles. More turns lower KV. Is this because the number of turns determines the field strength produced by the electric pulse sent by the ESC and the time required to build the field then collapes the field? The more turns the stronger the field created, more torque but lower rpm? Seem that the things that then are important in motor construction are the purity of the wire and the iron core materials to allow faster cycling and the tolerance on the air gap between the magnets and the stator core. Have I got this right?

Best,        DennisT

Permanent-magnet motors have a constant that relates the torque to current, and one that relates the back EMF to speed.  In an ideal motor (100% efficient), and using the right units for speed, these are the same number.  In a halfway decent motor they'll be very close.  The Kv rating in our motors is the reciprocal of that back EMF to speed relationship.

All else being equal the torque/current constant (and the back EMF/speed constant) scale with the turns -- change from 10 turns to 5, and you'll need twice the current at half the voltage to get the motor to do the same thing.  Industrial servo motors come in lines where there's one mechanical and magnetics design with a whole bunch of different windings to work at all sorts of different voltages.  Our motors do this, too, to some extent, but the selection isn't usually as wide (and the motors are cheaper, and you can go buy them at your LHS!).

So, take a motor, rip out the wire that's in it, and replace it with twice as many turns of wire that has half the cross-sectional area.  You'll fill roughly the same space with copper, your wire will be twice as long and half as big.  The current needed to generate a given amount of torque will go down by a factor of two, the back EMF at a particular motor speed will go up by that same factor.  The amount of power you put in to the motor will stay the same, and the amount of power dissipated by the motor winding will stay the same, too.

I hope this is relevant and not too far off in technical la-la land for you.

"In an ideal motor (100% efficient), and using the right units for speed, these are the same number."

Ahhh, if only Thomas Jefferson had won that argument, they would be THE SAME number!

definitely far off in la-la-land!
Dean P.

I'm not sure if it doesn't make sense to you or if you're disagreeing.

At any rate, if the motor is lossless then basic physics constrains the torque/current constant to being equal to the voltage/speed constant.