Basic control scheme for VFD's

[freecrowder]

I was under the impression that all VFD's follow a basic control scheme in V/F mode and other modes of control also. That is that you have a speed or velocity control loop that cascades into a inner current loop. Outer loop Speed into inner control loop current, then output to the motor. Does anyone know if this is correct? I have been told by others that in V/F there is no inner current control loop.

 

[Jraef]

I was under the impression that all VFD's follow a basic control scheme in V/F mode and other modes of control also. That is that you have a speed or velocity control loop that cascades into a inner current loop. Outer loop Speed into inner control loop current, then output to the motor. Does anyone know if this is correct? I have been told by others that in V/F there is no inner current control loop.

In V/Hz there is no inner control loop, that is correct. The VFD puts out a V/Hz ratio commensurate with the commanded speed, but has no idea if the motor responded, or in many cases if the motor is even connected. That doesn't mean the VFD is not MONITORING current, it is just not doing anything with it other than motor OL protection or any specific trip / warning functions that you enable (if any).

Even in Vector Control, you don't necessarily have an internal current control loop, there is what is called "Velocity Vector Control" wherein the "inner control loop" is not one that is looking at current / torque as a control point, it is only looking at it in order to close the velocity control loop because without an encoder feedback, rotor shaft position is determined by watching the very small current anomalies created by the rotor bars passing through the stator fields. That then give the velocity control loop the feedback it needs to determine how the motor responded to the output command.

 

[synchro]

Here's what I know (or think I know) about your VFD questions:

Simpler VFDs use just V/f control (aka scalar control) without "current loop" control, they just allow some slip to occur between the rotor and stator magnetic fields.

More sophisticated vector control modulates not only the frequency and amplitude of the drive to the stator field coils like V/f does, but also their precise phase by using current loop control techniques. The objective is to dynamically rotate the field of the stator so that its angle tracks the angle of the rotor's magnetic field so that optimal torque is achieved. Basically it synchronizes the magnetic fields between the rotor and stator like a commutator does in a DC motor, but in a typical DC motor the field is stationary and the commutator activates only those coils in the armature that are appropriately oriented at a given moment to provide torque.

Some articles that may be of interest:

https://www.motioncontroltips.com/what-are-methods-for-vfd-control-of-ac-motors/
https://www.renesas.com/eu/en/solutions/key-technology/motor-control/motor-algorithms/ac.html
https://www.digikey.com/en/articles/techzone/2017/sep/an-easier-approach-to-high-performance-current-loop-control-design

I just saw Jraef's post, so much of this is redundant info along the same lines as what he said.