Attempting to decipher the foreign motor tag. It appears to be a 2 speed motor. Two speed motors operate at 2 different horse powers as well. If I'm looking at it correctly, the high speed is 4.8 hp.
Running any kind of converter, you need a larger convertor than the actual hp of the working motor. And that applies to rpc, static, vfd or whatever. In this example, for 4.8hp motor, I'd want a bare minimum of a 7.5hp convertor. But the best operation comes when the convertor is roughly twice the size of the load motor. So if conditions permit, I'd really like a 10hp rpc, to run a 5hp motor.
The voltage of the wild leg of an rpc, unloaded is usually higher than the two utility legs. But as load is applied, the voltage on that wild leg begins to drop. I'm not seeing your voltage and amp numbers, but maybe because the your are trying to start a 4.8hp motor with a 5hp rpc, that the voltage drop on the wild leg is too much.
If you hook a meter up, and check voltage on all the legs, how does it look unloaded. Then read the voltage on all 3 legs as you attempt to start up. If you happen to have an amp clamp for your meter, might check amps on all 3 legs, but individually as you try to start also.
Now could be something totally different. Maybe a switch, contact, or relay. One test you could do is if you have breaker or kill box between rpc and machine. Kill the breaker. Disconnect the wires to the motor from the machine. Run the power directly, and only to the motor. Bump the breaker on and see if it spins motor. If it does, maybe you got another issue.
Edit: Hooking the 3 hot legs to three contacts of a 3 phase motor will spin the motor. But will the motor spin the correct direction ? Don't know till you try. But you need to know which direction the motor should be spinning. If the direction is wrong, swap any two hot legs, and the motor will spin the opposite way.
Also your 2 speed motor will take 2 sets of 3 hot wires. One for high speed, another for low speed. Can't power them both at the same time, or bad things happen.