How to identify a stepper motor and AC servo motor? (by its appearance)

Hi guys,

I'm working on the motors during this semester, and I have an assignment like this:

How to identify whether a motor is a stepper or AC servo motor w/o open the inner parts (recognize it through its shaft, cover, fan, base... only), because they look almost the same for newbies?

From what I have found out so far, the only difference is the diameter of the shaft, and stepper motor doesn't have an encoder while AC servo always has one. .


Are you allowed to read the model number from the label and look it up on Google?

I can't imagine that shaft diameter is a universal discriminator.

I am at a disadvantage as I have never seen an AC servo motor.

Beware - it may be a trick question. Maybe it is not possible to distinguish them visually. I don't think any professional would use a motor without having access to its technical datasheet.


Servo motors have 3-phase winding, bipolar steppers have 4-wire (2 separate windings) or sometimes
6 or 8 wires.
Servo motors generally have a 3 wire winding connector (possibly with extra connections for
temperature sensor), and a another connector for the encoder/resolver - usually 8 to 20 wires.

Steppers always have lots of cogging force every few degrees.