That's true for a VFD too. Both Danfoss and Mitshbishi suggested
derating by 50% for single phase use.

Robert Swinney wrote:
If you opt for a rotary phase converter just remember the idler motor /
total load ratio of 1.5 X is on the low end. A reasonably larger sized
idler won't be a problem but you will probably have to consider a pony motor
for starting. It is usu. recommemded that idlers of 10 HP and larger be
started with a pony to reduce "start-up" current surge.