M/G Set vs Rotary Converter

An M-G set can convert single phase to 3 phase or even DC to AC and vice versa. The motor and the generator are not electrically connected, they are only mechanically connected.

An RPC just allows the use of a single phase source to add the missing phase so that you can run a 3 phase motor by using another motor (or motors). Two of the phases for the 3 phase circuit are coming from the single phase source (assuming the same voltage), so the two parts ARE electrically connected. 3 phase motors will not spin when given a single phase source, but it will KEEP spinning if already started.

So in an RPC, there are two types; one with separate single phase motor belted to a "pony motor" that is the 3 phase portion for creating the phantom phase, or a direct RPC, which has just the 3 phase motor fed by a single phase source, and therefore needs a STATIC way to get it to start spinning in the first place. So can you just use that second type and directly couple the load to it? Yes, absolutely. But because the power getting to it is single phase, the mechanical output power of that motor is reduced to 58% on the rated motor power, or looked at the other way, you have to essentially double the size of the motor for the load that you want to run.