I am sure this is correct. I never really looked into the internal circuits of a VFD, nor designed one from scratch, but the following is a picture of the workings as I have perceived them. So take the following for what it is worth.
During initial start up, an induction motor basically looks like a dead short (just the copper winding's resistance, no significant inductive load yet) to the sourcing supply, pulling huge currents until it has sufficient rpm to provide the back emf (inductive load) to limit the current. During this start up the current is limited by the supplying voltage, by the resistance in the wires bringing the current from the breaker to the motor. So during this initial start up the voltage appearing at the motor terminals is less than coming into the breaker box. Also, when a motor stalls the copper windings dissipate this current and so can heat excessively very rapidly and so burn out. So starting up a motor too slowly (very heavy load) without limiting the current can destroy the motor.
Circuit breakers are basically thermal devices. As the current exceeds their rating they head up and reach a temperature at which they trip. This heating takes time (delay time to trip) and so the motor is suppose to reach an RPM during this time so that the back-emf limits the current ... allowing the breaker to cool back down and not trip. In the case of the VFD (my simplified picture), current from the 60Hz source is rectified to DC and used to store charge on internal capacitors. This DC power is converted to AC at the desired frequency and provided to the motor. Current (and voltage) is then supplied to the motor at controlled rates provided there is sufficient charge on the capacitors to supply the desired amounts. Hence, for the non-turning motor (dead short), the current (and to a lesser extent voltage) must be controlled to cause the planned acceleration ramp. (I think the control of this current must be via pulse width modulation. Other techniques would require large energy dissipation inside the VFD.)
The Hitachi manual says to tune the VFD with the motor unloaded (only the load of the armature, bearings, etc.). Hence, the VFD tuned parameters would by definition be incorrect for a motor that is under a heavy load. I think this means that the VFD output is basically operated open loop WRT the motor's actual RPM. There is no reason that the VFD electronics could not be measuring the current during startup and making adjustments to compensate for this additional load. Surely some of this is happening, but to what extent is probably dependent upon the VFD hardware and software designs. Which raises the question, if you are going to start up with the motor under a load would it not be better to tune the VFD under conditions similar to those that the motor is going to operate under, i.e. keep the motor connected to a load during the tuning process that is similar to what is to be expected during typical operation?
By the way, the manual also says that the VFD capacitors can wear out ... after considerable operation time. But there is nothing saying that they cannot go bad early. There appears to be considerable stress put on them during operation. So the VFD life time is not infinite. Keeping it cool should extend the component life time. Replacing motor starter capacitors, especially in HVAC equipment, is common. They wear out/die.
So, @Cubmadein49 , you might also try tuning the VFD with the motor under some load, say with a chuck, spindle and gears at a normal setting, to see what effect this has on being able to operate on the higher gear settings (greater load). Of course the external gears, gear box, saddle etc further increased the load when they are engaged, but you usually do not have them all engaged during startup. If I recall correctly, there is a way, in the VFD programing data, to see the motor parameters that were determined during tuning. So you could get both before and after readings so see if they changed (much).
Lastly, induction motors rotation rates are not synchronized to the line frequency. They lag in phase and so are constantly out of phase. As a load is increased the motor RPM slows down and more energy is expended inside the motor (warming it more). At some point they stop operating completely.
Dave L.
