Steam or electric motors powered a line shaft which drove the entire line.
Electric motors and generators were combined into Motor-Generator (M-G) Sets. These M-G sets made a lot of noise and produced power for individual motors on the line. An M-G set is powered with one ac motor driving any number of dc generators.
Vacuum tube regulators controlled the excitation (magnetization) for the dc generators and motors on the line (1940’s).
Thyristors or SCR’s were invented (1956) and applied to generator and motor excitation. See the photo of the semiconductor in a Thyristor. We also find a Thyristor in a light dimmer switch.