On higher speed elevators, the use of a two speed AC motor becomes impractical owing to the difficulties in decelerating the elevator car and accurately levelling it into the floor. Traditionally DC hoist machines were used to overcome this problem because they allowed for almost infinite control of their speed by varying the current or voltage supplied to the armature windings.
However the problem is that a building's electricity supply is usually AC. The M-G set consists of an AC motor which runs continuously, and drives a DC generator which is in turn connected to the hoist machine. The field windings of the generator are connected to resistor packs within the relay controller which are in turn switched in and out in sequence to vary the generator's output, thus varying the speed of the elevator. When running, they make a very distinctive sound, and can sometimes be heard from the elevator car.
M-G sets also demand a lot of maintenance, as the carbon brushes on the generator side regularly need replacing. The advent of solid state, variable frequency drives for AC hoist motors means that an AC hoist machine can have its speed controlled just as accurately, but without the complexity and higher energy requirements of the M-G set. Solid state drives can also be applied to existing DC hoist machines, meaning they can be used to upgrade old MG-based installations, without the added cost of replacing the entire elevator machine