My point was that rotor inertia is a value. It is what it is.
And (another thread in mind<g>) does it depend on unstable operation of the motor?
NO, that is not my question ... just teasing.
I'm having a discussion with another (totally non electrical) engineering friend that some of this would contribute to understanding ...
A "typical" hydraulic pressure compensated piston pump requires approximately (per the manufacturer) 3 revolutions to reach stability. I am trying to determine how long a "typical standard NEMA or IEC" unloaded squirrel motor requires to accelerate from 0 to "nominal" speed started across the line. I'm not really concerned with whether the speed is 0.9PU or 0.999PU. For the sake of discussion (if it matters), let's assume a motor in the 7.5kW (10HP) to 75kW (100HP) range, 4 pole.
The moment of inertia is published for these pumps, and is usually (from visual guestimation) significantly less than that of the motor rotor ...
To furnish, perhaps, TMI (too much information), I am interested in determining whether starting load current would be lower with the pump outlet vented (low outlet pressure at full flow) or blocked (displacement will go from full to "zero" during that 3 revolutions) with the compensator vented (stable pressure reached after, WE THINK, those 3 revolutions, but that pressure will be on the order of 5x the vented pressure. OK, real numbers as example ... 100cc pump, vented at 3 bar, compensated at 15 bar, say 1500 synchronous RPM. To expand, this pump MIGHT operate at 220 bar, 140 liter/minute and consume 60kW. Oh, hard numbers ... for this 100cc pump, J is on the order of 0.02 kgm^2.
What I'd like is some thought of what the 1st second current might look like ... NO PUMP LOAD.
I've looked at things like this before with a CT into a signal conditioner, but accepting the time constant built in all I've used, things are so smoothed that I don't trust the data; it is great in operating modes, though.
To put this in perspective ... 50Hz, motor turning 25 revolutions/second synchronous ... it is shorter than the first 0.2 seconds where my HYDRAULIC loads are of interest.
My friend's employer has actually commissioned a local university to do a research project to evaluate energy savings possible via VFDs, but their equipment (and inherent slow acceleration via the VFD) won't show any more than I can do, and probably less, in startup.