Variable Frequency Drive vs. Servo Drive

[tomizzo]

I'm wondering what the difference is between a VFD and servo drive are. I'm pretty familiar with VFDs but not so much with servo drives. From what I read, would the following assumptions be correct?

A variable frequency drive without an encoder module will adjust the frequency and amplitude of a 3 phase voltage signal to control a motor.

A servo drive will essentially do the same, but is more accurate because an encoder is built into the motor which then will make requests back to the drive to make changes to the voltage signal to meet certain requirements such as speed or position.


Are these the main differences or is there more?

 

[milesyoung]

A variable frequency drive without an encoder module will adjust the frequency and amplitude of a 3 phase voltage signal to control a motor.

A VFD is, usually, just an inverter with variable output (with a rectifier stage if it's grid connected). As you say, there's no feedback involved, so you have to know the specifics of the motor if you want to control its angular velocity, e.g. is it self-starting etc. It's an open-loop control system, and as such, it's really only viable for certain types of loads (steady HVAC systems like fans, pumps and so on).

A servo drive will essentially do the same, but is more accurate because an encoder is built into the motor which then will make requests back to the drive to make changes to the voltage signal to meet certain requirements such as speed or position.

A servomechanism is synonymous with 'feedback control of something'. A servo drive for an electric motor might come as a controller for a VFD or it might come as a stand-alone unit with a built-in inverter. The latter is more common for servo drives that employ field-oriented control, as it needs direct access to switching signals and current measurements. It's a closed-loop control system.

Are these the main differences or is there more?

I think you highlighted what's important. It amounts to open-loop vs. closed-loop control. You can find a lot more on that subject with a couple of searches on Google or PF.

 

[Baluncore]

A VFD is used to “control the speed” of a high inertia AC motor.
A servo motor has low inertia and is used to “quickly position” something, often with feedback control.

 

[milesyoung]

A VFD is used to “control the speed” of a high inertia AC motor.

It seems to me that it's the type of load, not the inertia of the rotor assembly, that determines what type of drive to use. If you're just interested in the steady-state performance of, for instance, a slowly varying HVAC load, then a VFD feeding an induction motor in open-loop will probably do just fine. It won't do for a highly variable load, though, where you might be more interested in its transient characteristics.

A servo motor has low inertia and is used to “quickly position” something, often with feedback control.

I don't think that's necessarily true. I often work with speed and torque servos where the rotor assembly of the motor has a high inertia relative to that of the load. It depends on the dynamics of the loading.

The very definition of a servomechanism has to do with feedback control. I've seen the term used in vendor material for drives that employ open-loop scalar (V/Hz) control, but that's IMO the work of a misguided marketing department.