- For the other meanings of MG related elevators, please refer to MG.
A motor-generator set (or M-G set) is a machine commonly found on older traction elevator installations. Its primary purpose is to convert a building's alternating current (AC) electricity supply into direct current (DC) for powering the DC hoisting machine and controller of the elevator.
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.
The DC hoist machine powered by an M-G set was an industry standard for higher speed elevators up until the 1980s. The key disadvantage is higher energy consumption since the M-G set has to run continuously whenever the elevator is in use, even when the car is stationary. A timer circuit within the controller will however shut down the M-G set after the elevator has been idle for a set period of time.
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. This makes for cost effective modernization in older buildings.
- As "hkelev.com" stated, the motor-generator are nicknamed as "piglet" (also known as 「豬仔」 in Chinese) by the elevator mechanics in Hong Kong, China.