The ESC controls the speed of an AC motor with frequency, not voltage. If you plug an 11.1 volt battery into your power system, you have 11.1 volts going to the motor with the full amperage potential of the battery backing that voltage.
The AC brushless motors we use are true 3-phase AC motors. The motors DO run on AC current. The ESC is a trapezoidal wave generator. It produces 3 separate waves (one for each wire to the motor). The speed of the motor has nothing to do with voltage or amps, but instead the timing of the current fed into it. By increasing and decreasing the wave length (frequency) of the trapezoidal wave on the 3 phases, the ESC causes the motor to spin faster and slower. The ESC switches the polarity of the phases to create the waves. This means that the voltage through any given winding flows 'Alternately' one direction then the other. This creates a push-pull effect in the magnetic field of each winding, making the motor more powerful for its size and weight. The motor and the load that is placed on it, is what determines the amp draw from the ESC and the battery.
In the below picture, we have 2 motors with 3 poles each. Their winding are labeled as poles ”A”, “B”, and “C”. The graph (under the 2 motors) shows the 3 separate waves that the ESC generates to drive a motor. The graph shows the signals time to voltage relationships. The black wave on the graph is the signal that is sent to winding “A”. The red signal goes to winding “B”, and the blue signal goes to winding “C”.
If you look at "AC Motor 1" and "AC Motor 2", and the signals shown on the graph that are sent to the windings; it is easy to see that when we swop any two motor connections, we change the order that the waves hit the windings, and that changes the direction of the motor
Shown in the above graph is how the signal would look on an oscilloscope; it shows the signal’s time to voltage relationship. On the graph, refer to "point A" at the start of the Black wave. One full wave length is: a wave that starts at 0 volts (point A), as it moves forward in time, it sweeps up to positive 11.1 volts (point B), then sweeps down past 0 volts(point C), to negative 11.1 volts (point D), then sweeps back up to 0 volts (point E). This is one cycle of this wave and it represents one turn of a 3 pole motor or one step in a motor with 6 or more poles. To increase the speed of the motor or increase the frequency, you must decreasing the time the wave length takes to travel from point A to point E.
How a Motor works
types of RC motors
An AC motor's ESC controls the speed (RPMs) of the motor by changing the frequency of the 3-phase trapezoidal waves; the higher the frequency, the faster the prop will turn.
The battery controls (provides) the voltage going to the ESC and the motor.
The motor's size and configuration, and the load that is placed on it, controls the amp draw.