Ok, I'm confused.  Is a model electric motor with three windings( like all the brushless outrunners) a 3 phase motor similar to an industrial one?  I'm getting an industrial motor and controller for my lathe, but this question came up.  The industrial controller changes the frequency of the phases to the motor to control speed.   Is that the same way a model controller works?  As opposed to changing the voltage or feeding the motor short pulses like many of the speed controllers for the old DC can motors did.

In the case of our sensorless model motors, the frequency control (the ESC changing the windings to keep the motor turning, like the commutator on a brushed motor) is separate from the voltage control. The voltage control is a pulse width modulation. Full voltage is 100% width (on all the time). Lower "effective" voltages  are made by shortening the width of the pulse. The current through the motor is limited by the inductance and circuit resistance (L/R). This L/R (which has units of seconds) should be long compared to the ESC Update frequency. So for example my CC Phoenix 10 has a pulse frequency of 12,500 Hz or 80 microseconds. This 12500 Hz is constant, not varying with the motor rpm. Anyway the voltage applied to the motor will be less than the full battery voltage (if the throttle is set less than full). I can reprogram in a higher frequency if I have a low inductance motor,which would make the L/R tme constant small.

Anyway the rpm of a particular motor is a combination of the battery voltage (and throttle setting) and the load. The switching of the winding polarities just follows the voltage and load.

I know this isn't the best explanation in the world, but it is the best I can do without using equations and showing a graph of what the waveforms look like.

Gentlemen,

There are classes of AC motors which use permanent magnet rotors like our brushless DC motors, where no "slip" is required and the motor must rotate at synchronous (line frequency) RPM.  AC motors usually have a "squirrel cage" rotor, which is made up of disklike iron laminations embedded in an aluminum casting which forms two end rings connected by many bars running axially.  If the casting could be viewed without the iron, it would look like a spinning exercise wheel for a squirrel.  As the AC field  increases and decreases from winding to winding, a current is induced (induction Motor) in the inductor bars and end rings of the rotor, and it is dragged around with the (apparently) rotating magnetic field in the stator winding.  There is slippage which increases with load.

Our brushless DC motors are similar to 3 phase synchronous AC motors.  The trick is applying power at the exact instant that the rotor is in the right position, via dedicated Hall effect sensors, or by reacting to signals from the motor coils themselves.

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 

>>> Is a model electric motor with three windings( like all the brushless outrunners) a 3 phase motor similar to an industrial one?<<<

While usual AC motor synchronizes to the feeding current and makes more or less power by phase slip of harmonic back EMF to input voltage, the BLDC motor has ESC and that ESC makes him synchronouse feeing like commutator of DC motor (means the logic is just opposite). Power of the BLDC motor changes at loading by value of back EMF volage compared to input voltage the same way as it happens with DC motor.