Quite interesting indeed. And certainly antique motors.I recall working on turning the slip rings on some very old WORTHINGTON 400 HP centrifugal compressor drive motors many long years ago. I do not recall the voltage of those motors, But there were three slip rings, as I recall. Now I am wondering about what kind they were. When they were started it took many long seconds to spin up.

Going only by your description, it appears it was a wound-rotor induction motor. Were also commonly used for elevator and crane service.

https://en.m.wikipedia.org/wiki/Wound_rotor_motor

 

They were on the early Worthington compressor drives, and they used a gearbox to increase the speed for the centrifugal compressors. They were only started by the "Stationary Engineer", because overall it was a rather complex system, and a lot of the safety things were manual, requiring a fair amount of skill. And the motors were rather large, as 400 HP motors tend to be.

 

Going only by your description, it appears it was a wound-rotor induction motor. Were also commonly used for elevator and crane service.

https://en.m.wikipedia.org/wiki/Wound_rotor_motor

That was the ones I described in #32.