My 2c
"Brushless" motors are indeed 3-phase AC synchronous motors. They are synchronous because they use permanent magnets and not induced magnetism (like a squirrel cage 3-phase AC industrial motor)
Like all synchronous motors, the speed is determined solely by the frequency of the supply and the number of poles on the motor. Synchronous motors of any size cannot run with significant "slip" - that is, difference between applied frequency and rotor speed.
The speed controller senses the zero-crossing voltage in the unused winding every cycle to determine when to power the next winding. There is a very good explanation with diagrams in the Microchip Application Note 00857a. But it is important to understand that the speed on a synchronous motor is determined only by the frequency applied to it, and the number of poles.
Now, what does the voltage do? Basically it is the voltage that drives the current through the motor. What determines the maximum current? This is where it gets a little (but not a lot) more complicated. Two things limit the current:
1) The maximum allowable power dissipation - heat / thermal limit
2) Magnetic saturation limit
Now, if you run an AC motor at low speed (low frequency) you have to limit the voltage applied in order to limit the current. It is the
current that causes the heating, remember? As the frequency (speed) rises you can increase the voltage in order to drive current through the motor. At constant current, the applied voltage can rise lineraly with frequency. Therefore, for higher speed, you need a higher voltage if you want to run at maximum current. And that of course gives higher power.
The way that the voltage (and therefore current) applied to the motor is varied is by Pulse Width Modulation (PWM). If it is only applied for 50% of the time, the voltage on the motor will only be 50% of the battery voltage. And so on.
It is unfortunate that electrics for models came "via" DC brushed motors, because terms that are applicable to DC brushed motors are being used for brushless DC (ie AC) motors and controllers. There is really no such thing as k/v for an AC motor, for instance. Yet the term is widely used.
Terms applicable to AC motors are:
Number of poles (more poles =
slower speed)
Maximum current (determined by thermal considerations)
Maximum speed (above which it flies apart!)
Maximum voltage (usually of academic interest only, as its insulation dependent)
Like all electric motors, the speed / power curve is a straight line graph because at constant torque power=speed x torque. The implications of this are that one can get very high power out of a very small motor - if you run it fast enough!
That is to say, if you can get 500 watts out of a motor at 10,000 rpm then you can get 1,000 watts out of the same motor run at 20,000 rpm! Which is why pretty well all the small RC electric power trains use geared motors.
To put it another way; if you need 500W at a prop speed of 10,000 rpm you could use a motor rated at 250W at 10,000 rpm with a 2:1 gearbox and run the motor at 20,000 rpm. It would then produce (and draw from the battery) 500W
I hope I have tried to teach anyone to suck eggs? But I do wish that brushless DC motors and controllers were defined using more suitable (and correct) terms because it would be easier for all to understand, in the end.
Charles