Our transformer designer can design transformers for 150Hz or any desired frequency RANGE

An inverter, as standard, is designed to run a 3 phase AC motor and will start a a low frequency and gradually ramp up to the desired frequency. It also includes some fancy DSP circuitry to see the "phase error" in order to make sure the motor "follows". They can be controlled externally by resistive or voltage means.

With the above, I do not think an inverter for AC motors will serve your purpose. Also, Ac motors can not run at 3* the nominal frequency, just 110-125%. It seems that what you want is a frequency converter. In designing one such, you can gain a lot of knowledge from inverter design, but the startup and the "is the motor with me" circuitry must be removed. It is also possible that you, for purposes of welding, can get by with a lot less circuitry to make the facsimile of a sinusodial output.

Just one word: when you have calculated how much voltage the IGBTs must be capable of handling, increase it! (see above post).

Erik