However, the center tapped primary on the 3-phase side of a Scott connected transformer has higher KVA than the secondary of that transformer that is on the 2-phase side. That is, 1 of the transformer winding operates at less than 100% internal power factor much like how an open delta vee transformer operates at 58% internal power factor. A better arrangement is to use a balance coil that provides what is effectively 4-wire delta 3-phase so that the 2 main transformers operate at 100% internal power factor.
At one place where I worked they had 2 induction melters for aluminum investment casting that had dedicated 3-phase 3-wire to 2-phase 4-wire transformers to run the induction coils.
Another way to get 2-phase power from a 3-phase wye system is to connect 1 transformer primary A to B on the 3-phase ystem and connect the other transformer C to neutral. For say a 7,200Y12,470 volt system you would use a 7,200 volt transformer and a 2 primary bushing 14,400 volt transformer which has ANSI taps for 13,800; 13,200; 12,500; and possibly also 12,000 and 11,500 volts.
The 2-phase system proved to be not as efficient because you either needed an extra wire or in the case of 2-phase 3-wire the neutral carries 141% of the phase current. Also turns out that iron nonlinearities in motors cause 2 phase motors to have multiple torque dips in the accelaration curve. These nonlinearities are much easier to balance out in a 3-phase motor.
Also, a lot of older railway power such as on the Reading Railroad ( which operates off of a hydroelectric plant ) was 2-phase 25 Hertz for a number of reasons. One of them was balnacing a 2-phase system is easier given that a mainline is 2 tracks or 4 tracks. Single phase generators over a certain size do not work because single phase power has pulsating instantaneous power and really does produce a counterrotating electromagnetic field in generators. The reasons for 25 Hertz are that larger series wound motors have a maximum frequency of 25 Hertz.
Actually, as machine size and torque go up and machine speed goes down, the optimal frequency for an induction motor is on the order of 15 to 30 Hertz. Steel mills produce their own 2-phase 4-wire low frequency power using cycloconverters for each large motor. Using a dedicated cycloconverter for each motor the speed of each motor on a rolling line can be tweaked for the required speed of the actual rolling dies. For larger drives a cycloconverter is a more efficient drive that a pulse width modulated variable frequency drive.