Pierre, I wish I knew how to post graphics on this BBS but I don't so here goes with a word description.
First, you need to examine a NEMA design B motor speed/torque curve. You will see that the torque is zero at motor sync speed (for a four pole motor at 60Hz, that would be 1800rpm). At the motor begins to produce torque, the shaft speed starts to drop slightly until, at the point where the motor output reaches its nameplate hp, the speed will have dropped to its nameplate speed (using the example above, at 1.5hp output, the speed will have dropped to, say, 1750rpm). Using the formula hp = torque x rpm/5252 we calculate that the torque output at that point would be 4.5ft-lbs. Note that the torque speed curve is close to linear between no-load and the rated load point.
At this rated load point, the manufacturer calculates that, in a 40 degree C ambient, the motor will, over a long time period, rise in temperature to whatever insulation temperature class the nameplate lists.
If you were to superimpose over the NEMA B speed/torque curve a speed/current curve, you would find that, at no load, there would be, not zero current but about 25% of nameplate current (this is magnetizing amps and, for the motor we are discussing, would be about .65amps at 460V). This current also rises about linearly from that point to the full load current of about 2.6amp. If you scale and position the two curves so the full load torque and the full load
amps occur at the same point then you will see that the torque and current co-incide pretty well except that as you get close to no-load, the difference becomes noticeable (the torque goes to zero and the current goes to 25% of full load).
As the motor is pushed into the overload range, the torque continues to rise almost linearly (it develops a slight arch toward the torque axis) up to about the 220% of full load point. The current will track almost exactly with the overload torque up to about 190% where it starts diverging on a less-arching path upward.
At the 220% point in the torque curve, the rotor begins to loose its ability to stay synchronous with the stator field and the torque falls off rapidly. Unfortunately, the current does not fall off similarly but continues on up until, at the point where the rotor is completely stopped (as at starting across the AC line with a contactor) the current has risen to 6 to 8 times the nameplate full load current. This is the starting inrush current that happens whenever a NEMA B motor is started across the line.
It is important to understand that, while the above description is for a motor that is motoring, the very same curve and torque/current behavior occurs when the motor is expected to act as a brake by absorbing torque.
In view of the above, when an inverter is feeding an induction motor, there can be no sensible reason for sizing the drive output current for more than 220% nameplate since the current falls beyond that point rather than increasing further. So, for Pierre's example with a 1.5amp motor (2.6amp full load) and a 5hp drive (at least 9amps short term output current) it can be clearly seen why the larger drive didn't help as much as expected---9/2.6 = 346% !
Pierre simply didn't have enough motor. Even tho he had more than enough drive, the weakest link was the motor and having a drive that could absorb a lot of current still didn't get the job done.
And, for Rick, drives usually have a continuous current rating and a short-term rating (generally spec'ed at 1 minute). Some manufacturers including ABB now include a 5sec rating also which is handy for fast braking applications and fast accels.
These ratings are far below the max limits for the drive semi-conductors. Instead, they are based on heat sink capacity. That is why, if you are running a drive at 50% of its rated output, the short term max current is higher than if you are running at 95% of rated output (the heatsink is cooler at 50% than at 95% and therefore will withstand a higher current burst for a few seconds before it reaches its max rated temp). It's not uncommon in ABB's drives for the semiconductors to be rated at four times the max short term current just for safety sake.
While you can understand that I think the new ABB ACS550 is an excellent drive, Rick's success is due to the drive/motor system being sized properly to handle both motoring and braking currents. If the motor had been too small as we were discussing above, not even the miracle-working ACS550 (snicker!) would have been able to save the job.