Quote:
Originally Posted by sikeston34m
The traditional definition of AC as it pertains to electricity is "Alternating Current". Household single phase current is 60hz, or 60 cycles per second. In other words, it reverses line voltage polarity 60 times a second.
DC current, of course, only flows in one direction.
With the method that brushless ESC's use, I think a more precise term would be "chopped DC".
There are tricks from several different electrical types at work here. Yes on the 3 phase, but it's not AC current.
With Industrial 3 phase, the motors are AC AND 3 phase. Industrial 3 phase has three hot leads. They fire in a 1-2, 2-3, 3-1 pattern, much the way that your ESC sends power pulses to your brushless motor. The difference is, the frequency of the pulses doesn't change AND the power being fed is cycling AC at 60 times per second.
I hope this helps.
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Well, technically, an AC motor (3-phase motors), each phase is firing ALL the time. 3-Phase AC machines have what is called a 'rotating magnetic field', created by the 3 individual, constant sine waves (phases) being 60degrees out-of-phase of one another, thus creating what appears a rotating field.
And, for example, the AC Induction motor (the standard industrial workhorse motor type)... to control the speed and torque of one of these motors, you must change the 'frequency' of the 3 phases (Brian mentions the 'variable frequency drive'). These motors are A-synchronous, meaning that there is NO commutation (electronic (BL motors), or brushed motors) going on....
First, the speed is proportional to the 3-phase frequency being fed to the motor by the drive, or 3 phase power lines if it is not running from a motor controller (or 'inverter' is the standard term).
Second, torque is proportional to the magnetic 'slip' between the 'stator' (stationary) and the rotor... this magnetic slip is the difference between the rotating magnetic field frequency, and the rotors physical rotating fequency... think, the faster the stators 'rotating magnetic field' is turning, the more force (torque) is created at the rotors surface, and ultimately the output shaft.
I can see advanced types of AC Induction replacing brushless DC motors that we use in the near future (if is utilized), such as RaserTech's new AC induction technologies. Reason being is that these types of motors will, and do NOT COG; they can spin at ultra low speeds, and they have 'glass smooth' torque, unlike the BL motors which have alot of torque 'ripple'.