Are the terms BLDC motor and "brushless AC servomotor" exactly the same thing? I cannot find information so far whether there is difference.

For instance, at this site they are treated as if the same thing.

And at the moment I have this brushless motor where it is called at page 8 as "AC servo motor". And here is the photos of its stator and rotor:


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It seems very similar to BLDC but they don't call it as such and they call it brushless AC servo motor. What are the differences if there is any?


1 Answer 1


BLDCs are a type of PM (permanent magnet) synchronous motors, as are AC synchronous motors without brushes. They are very, very, similar.

A BLDC always has armature magnets and not armature windings.

AC synchronous motors come in two types:

  • Permanent magnets on the rotor and windings on the stator. These requires no brushes because the permanent magnets are always placed on the rotor. If you placed the permanent magnets on the stator and windings on the rotor you would need brushes so no one does that.

  • Field windings replace the permanent magnets. This is where my understanding gets a bit murky. In theory, you could have either the field winding or the main winding on the stator with the other on the rotor. But I think the field winding always goes on the rotor just like the permanent magnets do.

    The reason is because the field windings are DC so already needs brushes anyways to rectify AC to DC. By sticking the field winding on the rotor you can design the brushes to do double-duty to also transmit current to the spinning rotor. If you placed the field windings on the stator and the main winding on the rotor then you would now need two sets of brushes: one to commutate AC to DC for the field winding and another set of brushes to transmit power to the main winding spinning on the rotor. Since the field winding just replaces the permanent magnets, the brushes for the field winding are lower power than those you would find in other motors that transmit "load current" to the main winding.

You asked specifically about brushless though so if it is brushless, it must have magnets instead armature windings. That makes the two almost identical, only really differing in ratings and intended usage. (someone correct me if I'm wrong).

For comparison, a brushed DC motor has armature windings, while the field could be either a magnet or winding, which is the opposite which is interesting.

A BLDC motor actually does run on a form of AC when you think about it. It is just not expected to be run straight from an AC source, but a DC source with an inverter in between to synthesize the AC (though you could argue that's just a VFD and VFDs are used with AC motors all the time).

The BLDC motor is more likely to have a trapezoidal BEMF while the AC brushless motor is more likely to have a sinusoidal BEMF to optimize operation with the expected drive waveforms, but not always.

The "servo" part means it's not just a motor. It probably has a rotor position sensor like an encoder included and potentially an entire electronic motor driver and control package.

  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. Any conclusions reached should be edited back into the question and/or any answer(s). \$\endgroup\$
    – Dave Tweed
    Commented Jan 15, 2020 at 22:13
  • \$\begingroup\$ Well, a synchronous motor with a field winding would use sliprings, not brushes, I believe. Also I think an AC servomotor would probably include a high-resolution shaft position encoder. \$\endgroup\$
    – user57037
    Commented Jan 22, 2022 at 5:40
  • \$\begingroup\$ @mkeith Don't slip rings go hand-in-hand with brushes? I mean...what's the point of a slip ring that has no brush? Or a brush that has no slip ring? Well, I guess there are commutator rings. I guess those don't count as slip rings. But commutator rings still need brushes too. \$\endgroup\$
    – DKNguyen
    Commented Jan 22, 2022 at 5:41
  • 1
    \$\begingroup\$ @mkeith Ah, I see what you mean. I can edit that in. \$\endgroup\$
    – DKNguyen
    Commented Jan 22, 2022 at 6:32
  • 1
    \$\begingroup\$ @mkeith Check modifications. I'm pretty sure I have it right now about where everything is in an AC synch motor. \$\endgroup\$
    – DKNguyen
    Commented Jan 22, 2022 at 7:01

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