What is the difference between a DC brushless servomotor and an AC synchronous servomotor?

 

I do not know if there is a formal definition, but I have read that a servo is considered to be AC if it is commutated sinusoidally. Less sophisticated drives use trapezoidal commutation and are referred to as DC brushless.

Bill Sturm

 

Rather than just explain the basics of servo design let me refer you to Motorola's tutorial explaining the fundimentals of most modern motor designs. It not only answers your question clearly with text there is anamation to show the phasing that really bring the principals home. This tutorial has been so helpful I have included it on the additional resource page of my web site (just below Nerd's In Control!)ww.motioncontrolled.com.

 

A DC brushless servo motor is in the majority of cases, a special synchronous motor. The DC brushless motor (BLDC) has no slip rings to allow for commutation of phase currents. The commutation is provided by adding a position transducer (resolver, etc.) to the motor shaft and taking the position signal to the servo amplifier for solid state phase current commutation purposes. The position signal is also differentiated to produce a synthetic velocity servo loop for the servo drive. Induction motors can also be used as servo motors with Vector control concepts. These drives are usually found in the larger HP ranges say up to 300 HP, where most BLDC drives are in the lower HP ranges say up to about 15 HP. I hope this will be of some help in understanding the features of different kinds of AC servo drives.

 

See my comments from virtually this same question asked earlier this year:

http://www.control.com/1026172925/index_html

 

AC servo and DC servo same type.

There are not DC servo motor, This is new given name for the DC bus opearted and electronics commutated AC servo motors.

 

A good question because the two seems to blur
nowadays.

See another tutorial: http://www.microchip.com/1010/suppdoc/design/mtrcntrl/menufaq/mtrtypes/

Basically, similarities are both brushless, stators (fixed part) have electromagnets (coils).
Differences: the rotor for brushless is permanent magnet and the AC is made of iron laminate stacks called squirrel cage.
The speed control are implemented differently. AC speed is varied by frequency, DC brushless is by pwm duty cycle which effectively reduces current/voltage. The power topology looks the same e.g. 3 half bridges of transistors.

oj

 

No, oj, you are describing an AC induction motor, which is an asynchronous motor, not a synchronous motor. A "DC brushless motor" is really an AC synchronous motor. In a synchronous motor, the electric frequency matches the mechanical frequency (that is what it means to be synchronous). "DC brushless motor" is a marketing term, not a technical term, to indicate that this motor and its drive can be a drop-in replacement for a DC brush motor. The motor itself is AC -- it needs alternating inputs to maintain motion in one direction -- an it is synchronous.

On Sept 16 OJ wrote:
A good question because the two seems to blur
nowadays.

See another tutorial: http://www.microchip.com/1010/suppdoc/design/mtrcntrl/menufaq/mtrtypes/

Basically, similarities are both brushless, stators (fixed part) have electromagnets (coils).
Differences: the rotor for brushless is permanent magnet and the AC is made of iron laminate stacks called squirrel cage.
The speed control are implemented differently. AC speed is varied by frequency, DC brushless is by pwm duty cycle which effectively reduces current/voltage. The power topology looks the same e.g. 3 half bridges of transistors.

( Complete Thread: http://www.control.com/1026181016 )

 

Where do these people come from?

What is the difference between a DC brushless servomotor and an AC synchronous servomotor?

NONE! There is no difference.