FWIW:
Although BLDC 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.

Vector drives are DC controllers that take the extra step of converting back to AC for the motor. The DC-to-AC conversion circuitry is usually expensive and less efficient, but they have the advantage of being able to run smoothly at very low speeds or completely stop in a position not directly aligned with a pole. Motors used with a vector drive are typically called AC motors.

RC

AC servo has windings on both the rotor and stator.

Daniel

Well all the motors I have seen that are described as AC Servos have magnets on the rotor, so they should be described as BLDC?

Hood

RC, yes that seems to confirm what  I have gathered, basically its the drive that determines whether its BLDC or AC Servo, or am I reading you wrong?

Hood

Hood,
 That's my assumption as well.
All of my AB AC Servomotors have magnets.....as far as I know anyway.

Yes so do mine, also the indramat, the yaskawa, the PacSci etc, I have a nice wee collection

Hood

The BLDC and AC servo's are constructed practically identical, three star connected stator windings with P.M. rotor, the difference comes in the commutation, Brushless DC switches two windings at any one time, like a brushed motor turned inside out.
The AC is controlled with true three phase, sinusoidal in nature.
The BLDC requires three encoder pulses  or hall effect devices to commutate.
 BLDC pattern http://users.tinyworld.co.uk/flecc/4-pole-bldc-motor031102.swf
The AC use resolvers to commutate but some also use the hall effect for initial rotor detection and immediately switch over to encoder resolution pulses to synchronize the  three phases.
Nosmo.

Ok so they are one in the same and its the drive that determines what they are?

Hood

Well not quite, the drive AND the feedback device. IOW an AC motor with resolver will not run on a  BLDC drive without changing the motor resolver to equivalent hall effect device.
The Fanuc AC Red cap motors I mentioned earlier have a differential encoder and a 4 bit binary code which they use in their systems to produce a pseudo sinusoidal pattern from the 4 bit code, similar to what a resolver would produce.
Jon Elson is selling a unit where he  has found a way to convert this code for BLDC commutation.
I have generally changed the whole encoder if I get the right price.
Nosmo.

Once read somewhere (can't recall where exactly) :

Found where I read it. See bottom of page 26 and 27.

http://www.techno-isel.com/H835/835DownLoad.asp?Page=H835_Cat

Daniel

Found where I read it. See bottom of page 26 and 27.

Daniel

Daniel,  Whoever wrote that article is basing his information on very old technology, there has not been a wound rotor AC servo  motor used for many years, for a motor to be made synchronous to the applied frequency, it requires either a permanent magnet on the rotor or energized coils of some kind.
I think the author was mixing induction motor technology with his explanation, an ordinary AC induction motor can never be a synchronous motor, without some other tricks.
Apart from dismantling the motor to prove an AC servo motor has magnets on the rotor, is to back feed the motor shaft by some means and 'scope the three phase stator  leads, you will see a generated three phase, the magnitude of which is dependant on rpm.
Generation will not occur with a solid iron rotor, only by energizing rotor coils, IF it has them.
Nosmo.