Recently tried to drive an IE3 15kW induction motor via a soft start. Although current rating is 29.5 amps for continuous running, the inrush for these high efficiency motors is much higher than that of lower efficiency motors. Motor stalled on first attempt, then tripped breaker. Ok - easy to think this was an overload as it was from a 32 amp mains breaker. Upon checking windings it could be seen that U1-U2, V1-V2 and W1 to W2 were all balanced at 0.1 ohms. Yes 0.1 ohms. Not exactly the resistances I would have thought. But all windings were the same. Tried running 22kw motor (91% eff), ran no problem at all. After further investigation, it was found that this motor can only be driven from an inverter. Seems odd to me but that's what I have been told. Resistance 0.1 rang alarm bells initially but for all windings to be the same (each winding cores tied up with manufacturers manual color coding).
We have run a 22kw motor through a soft start from this supply so it has nothing to do with inrush current or even continual running current. The motor requires 150 Hz to run and has rpm rating of 3000 rpm, is 3 phase 6 pole, internal Helix Geared motor which is probably why the high frequency is required because of the gearing. This now lies with our customer whether they, buy a new motor fit for soft start and application, or buy an inverter. I think inverter as they had this motor specially made for a reason so can't see them undoing the engineering that has went into specifying the motor in the first place.
Here are a couple of things to consider with regard to your "special" motor design.
The original rating is 22 kW (or so) at 3000 rpm and 150 Hz. That means it's good for about 7 kW at 50 Hz, all else being equal. No wonder the unit stalled on the first across-line start attempt! Connecting through a soft starter - which will do a great job of limiting inrush current by dropping voltage - will also drop the available starting torque even further. It is entirely possible that even if the low torque from the soft start can accelerate the load, the time to reach synchronism will be too long and the motor will essentially burn out the windings.
It is not uncommon to see a "high efficiency" design require more than the usual amount of inrush current (even when it is designed for an across-line start). This is because of the options available to increase efficiency: basically, reduce motor losses. One of many ways most manufacturers go about this is to reduce the resistance in the winding(s); when the resistance goes down, inrush current goes up.
One other thing - shaft speed is proportional to applied frequency and inversely proportion to number of poles. So a 6 pole design will never reach 3000 rpm ... unless frequency is much higher than the typical line value of 50Hz & 60Hz. If the process is trying to drive toward that kind of speed, the power factor will always be miserably low and varying as the machine cannot achieve synchronism (with the process) ... resulting in the observed protective trips.