Synchronous means it has no slip. The rotor magnetic field is fed in from an external source (in the motor but not induced in the rotor) with brushes and slip rings is possible. Rotor follows (or generates) the field exactly in synch, 1800 rpm for a 4 pole machine and 60 Hz.
The state university, in the basement of Castleman, Engr I, probably built in the 1930's, they had a DC lab in one quarter and a DC powerplant in the middle section. All surface mounted exposed busbar with exposed surface old 1930's or so breakers and knife switches, two 50 kW DC mg sets. like 8 ft tall 40 ft of black slate with everything front mounted. It's all gone now, replaced with debris dry storage. Could have made great antiques or historical artifacts
I did two semesters as student labor job there, had the E2 a few weeks after starting the semester. My boss's office was across the hall from the DC lab. It was unused for years for no maintenance and things did not work but I worked on it and got things working. Two engine dynomometers and a wind tunnel. At that time no one cared the exposed busbar was not grandfathered.
Starting the 50 kW mg sets with the 1930's equipment was a full two handed throw. Two 1930's handles almost as big as axe handles with interlocks. First was the start winding, current would come up to 180 or so, then the other handle was the run winding. There were interlocks and you had to synch the mechanical throw.
That started on induction only and when the run winding was in you could throw a small lever for the (rotor) synch field. And there was another knob to adjust current flow through the rotor synch field to adjust power factor, with a meter that showed the power factor.
I knew the hazards but that was before clothing became a thing. My EE boss gave me a copy of Chapman Electric Machinery Fundamentals and put me to work. Should have been a 3 credit course at least.