Funny how the world works, I've had almost this exact conversation with two customers today, plus it came up in another forum. So I'm being lazy and copy-pasting my other response, which is slightly specific to that other thread, but has the info I think you want.
99& of the "Constant Torque" (which is what you would have on a machine) rated VFDs out there are going to be rated for the continuous FLC of the given HP based upon the NEC charts, and are capable of 150% current for 60 seconds and 200-250% current for 3 seconds. The overload rating allows for accelerating current. Since the VFD is modulating voltage and frequency together, the output torque of the motor is going to follow the current, so at 150% of full load torque, the drive will deliver 150% current. 150% FLT is roughly the Locked Rotor Torque of a Design B motor, so what most loads are designed around. The actual PEAK torque the motor can deliver is called Break Down Torque (BDT) and is between 200-220% of FLT (again, design B). So that's what is behind the overload ratings of the VFD; the VFD can allow the motor to deliver the same expected torque you would get from an across-the-line start, but do it at any time, not just on start-up.
In addition, the VFD can be programmed to LIMIT the torque to the FLT value and essentially use no more than FLA to do it. In many cases this is sufficient to accelerate a motor, as long as time is not a factor. In other words if I limit a motor to FLA with the VFD, it might take 3 hours, 3 days or 3 weeks for it to get to full speed, but it CAN be done and the VFD allows that to take place because if you never exceed FLA, the motor can do that forever. So if you have a 20HP 230V motor with a FLC of 54A, you can program the VFD to limit the current to 54A and it will do that forever. It limits current by artificially limiting speed / acceleration, so the accel time becomes an unknown, but if you can live with that, you can do it. I've done this countless times on centrifugal pumps and accelerated in 10 seconds or so. Ive done it a dozen times on refrigeration reciprocal compressors, works great but took a minute or so to get to full speed in one case. So you do have to watch out for things like mechanical oilers and such.
Not part of that other thread:
So if you WANT to accelerate as fast as possible, in theory if the BDT can accelerate the load in <3 seconds, the drive can do that and not trip, so you could say that the maximum current is 200-250% (depending on the VFD). But if you don't NEED the load to accelerate in 3 seconds or less, or the normal LRT of an across-the-line start was sufficient to accelerate it in a few seconds, then you could say that the VFD accelerating current is 150% max. Or if you NEED to accelerate it without going over FLC, then you can do that too, for as long as it takes.
