I’ve helped a few clients in the past determine energy savings on an AC drive based on the changing in speed to combat disturbances in the plant. It’s mostly geared towards tuning the PID loop associated with the drive (if it’s controlled by a PLC) and better tuning typically yields less speed changes. Typically, the client will know it’s a critical loop and know the energy cost before/after we optimize it, however, it can be a bit of a pain to separate your impact trying to improve the process and the impact of other systems interacting with the variable frequency drive – just something to think about.
If really want to calculate the drive’s energy saving, you should take the energy consumption before changing the system or adding anything to it. More accurate you do this work out more you will be satisfying. Most of the old system (5 years or more in running) including Motor, Best, gear consume more energy than currently available energy saving motor, gear or AC drive. But you must evaluate how much you can save by replacing all or part or adding AC drives to the system. Specially for the AC drive on a constant torque load does not give satisfactory saving.