Constant Torque vs Constant Power VFD

[Its_Ali]

For the past couple of months, I've been working with Siemens G120X drives for compressor applications. The setup is pretty straight forward, a 4-pole induction motor connected to a G120X framesize FSH VFD.
I mention the framesize because it's related to the question I want to ask. In Siemens manual it mentions that up to framesize FSG (250kW) the drives are constant torque drives meaning constant output current regardless of the supply voltage, while above 250kW (FSH drives) the drives are constant power drives, meaning variable output current depending on the drive supply voltage.
basically, if you consider a 400kW drive, the drive delivers an output current between 720-600A depending on the supply voltage 380V-480V.
Tbh, I have never worked with such drives before. At first, I thought this constant power drives are basically current source drives, but it doesn't seem to have the characteristics of a current source drive and I cannot find any references to this in the manuals or certifications. I'm determined to find an answer for this to answer another question on the motor side.
Compressor application is somewhat constant torque application. The variation in torque with respect to motor RPM is very limited (up to 4% torque variation for 65% RPM variation). But for my work and for a specific reason I need to find the current drop/variation with respect to the motor RPM. I have this variation but the issue is that this variation varies with supply voltage to the drive.
lets say at 380V supply voltage, my motor current at 2370RPM is 678A and by dropping the speed to 2190 my motor current also drops to 608A. But if the supply is 440V for the same RPM drop the current drop is from 580A to 550A. I believe/assume this variation in drop is somewhat related to motor design and the FWP point definition for different voltages. but tbh I'm not sure.
Any suggestion or explanation is highly appreciated.

 

[FacEngrPE]

Positive displacement compressors all have input torque proportional to compressor pressure rise. Speed only creates a small change in additional torque requirement, probably from friction effects.
For motors current is proportional to torque, and voltage is proportional to speed. For AC motors speed is also proportional to frequency.

So the drive must now support the load applied, IE constant current (assuming constant compressor presser rise), voltage and frequency proportional to compressor flow rate.

[ul]
[li]Constant torque, able to output constant torque from minimum to synchronous speed. Usual selection for PD compressor.[/li]
[li]Constant power, Torque capability decreases as speed increases, power capability remains constant. OK for PD compressor providing selection is made for torque at maximum speed.[/li]
[/ul]

https://iecmotores.com/what-do-cons...orque-mean-in-the-inverter-of-electric-motor/