VFD single phase 50% derate, does rpm count?

I recently got a good deal on a 10hp Baldor H2 drive for my Hannifin 12 ton hydraulic press. The press has a 10hp three phase motor and I have been wanting to slow down the ram speed, as well as getting weaned off my 30hp rotary converter. The motor fla current is listed at 25 amps. My testing so far has been running the drive at 30hz for 1/2 motor (and ram) speed. The drive has not had any complaints so far, even with peaking near 30 amps at full stalled ram pressure. The drive is rated for 150% over current for 3 minutes. Should this setup be within the 50% derate for going single phase input? The press motor has shown no signs of getting hot either. So in theory, the motor at 50% speed would be a 5hp motor?

The basis of derating is current. Whatever draws full input amps is the same as full load to the VFD. It has to do with the current rating of the rectifiers, and the "ripple current" rating of the capacitors, both of which depend on input current.

Single phase produces pulses of input current, while 3 phase has a nearly continuous flow of current. For the same average, obviously single phase must have a much higher peak pulse current, which causes heating, etc.

There is the added consideration that single phase will not recharge the capacitors as often, so the average power supply voltage goes down, and that raises output current for the same power level.

I have found on the units we have installed so far, that Baldor has standardized capacitor and igbt size relative to frame size. Example: the aa sized units, rated 1, 2, 3, 5 & 7-1/2 hp use the same 60 amp igbt unit, and caps. If that holds true with the larger models, I may be fine using the 10 hp size b frame, as the size b frame is also used for the 15 & 20 hp (240volt) models. Thanks!

In direct answer to your first question though, it's not that RPM matters directly, it's LOAD that matters. So indirectly yes, your assessment is correct in that running your motor at 50% speed means the load is, at most, 50% (if it were a centrifugal pump or fan, it's going to be only 12.5%). So the current through the rectifier is reduced and there is less drain on the caps allowing them to handle the excess ripple better. One way to ensure this never exceeds the load capacity by the way is to limit the maximum speed to 30Hz and /or if possible, program a Current (torque) Limit setting to 50% of the unit rating. When you set a Current /Torque Limit in a VFD, it accomplishes that by reducing the output frequency, overriding the speed setting.

As to the subsequent observation you made, I would not count on that. While I know that happens a lot at 3HP 230V and below, the caps may be the same part numbers but a different configuration. Also, the IGBT module is not the issue, it's the diode bridge rectifier ahead of it.