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WaxyChicken 01-29-2008 07:37 PM

AC Induction Motor used as Brushless DC?
Can it be done?

From the ever flaky Wikipedia:


Although BLDC [brushless DC] motors are practically identical to permanent magnet AC motors, the controller implementation is what makes them DC. While AC motors feed sinusoidal current simultaneously to each of the legs (with an equal phase distribution), DC controllers only approximate this by feeding full positive and negative current to two of the legs at a time. The major advantage of this is that both the logic controllers and battery power sources operate on DC, such as in computers and electric cars.
Does this mean i could get an AC current motor and force it to act as BLDC motor without any major drawbacks in performance or cost?

(when it doubt, i ask. besides - Wiki has been proven wrong many times in my experience)

Brushless DC Motor (BLDC motor)

AC motor (AKA induction motor)

An electronics expert on another forum:

A popular myth that I fell victim to myself. Turns out that even though the ESC does switch the polarity back and forth on the various phases of the motor to make it turn, it's still putting out DC voltage. It's similar to AC, but the wave is a "square" type. Unlike AC, where the voltage gradually changes from maximum + to minimum -, producing a "sine" wave, this switched DC changes from full + to full - instantly.

The picture below kind of shows what I mean. This is what you'd see on the screen of an oscilliscope if you compared true AC to what the brushless ESC is doing, sort of, kinda, maybe

The ESC is a square wave generator. It produces 3 seperate square 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 of the square wave on the 3 phases the ESC causes the motor to spin faster and slower. Amp draw is a direct result of the work load caused at the faster speed.
So can an Induction motor be used as a substitute for a BLDC motor allowing you to substitute cheaper parts for the controller, etc (some minor modifications may be required, also)?

MetroMPG 01-29-2008 08:55 PM

Sorry can't help here - beyond my limited level of electric motors.

WaxyChicken 01-29-2008 09:05 PM

The main thing i notice on the exterior of these motors is the Brushless motor usally uses 3 wires. While the AC motors use 4 (apparently depending on how many poles? they have)

Gone4 01-29-2008 09:06 PM

There is generally a difference in rotors but the primary problem with using an AC induction motor as a BLDC is that a BLDC has integrated controls that include an absolute position sensor, required for the digital controller to properly power the motor so it spins. Also, for many reasons, BLDC motors uses permanent magnets, which you lose some power over by not being able to manipulate the magnetic (B) field whereas AC motors have a magnetic (B) field proportional to voltage and frequency. You can take away from this vague response that this also correctly implies that BLDC are more efficient for smaller motors but experience more losses as you scale the design up, so AC induction becomes more efficient for higher performance applications.

The reasons for these differences get complex, so I assume this will suffice. They may seem the same, but they are not easily interchangeable.

Gone4 01-29-2008 09:07 PM


Originally Posted by WaxyChicken (Post 7444)
The main thing i notice on the exterior of these motors is the Brushless motor usally uses 3 wires. While the AC motors use 4 (apparently depending on how many poles? they have)

AC motors tend to have an even number of poles so that the peaks and troughs of the voltage wave are being applied properly whereas a DC motor cannot start with an even number of poles. You will definitely find motors with more poles.

I hope this helps :)

WaxyChicken 01-29-2008 09:26 PM

Very well explained. Thanks, GenK. :)
(drat. for a moment i thought i might be able to get away with using the large AirCond motors.)

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