brushless generators.
Alternators, to be precise. When silicon diodes became available to rectify AC and produce DC, brushed machines disappeared. For some time, alternators were used for larger AC loads and motor-generator sets (an AC motor driving a brush commutated DC generator) supplied smaller DC loads.
Slip ring connection of field coils also disappeared by producing alternators with a small excitation generator on the same rotor shaft. Basically, an inside-out alternator, the field is on the outside (fixed) producing and controlling a small amount of AC power on the stator, which is attached to the shaft and rotates. A small rectifier takes this and produces DC, which feeds the main generator field on the rotor. So you now have a system with only magnetic field connections between the spinning and non-spinning parts for low maintenance.