Motor-generator sets consist of a motor driving an AC generator or alternator so that the load is completely electrically isolated from the power line. In the past, in some cases DC or induction drive motors were used requiring close speed control to maintain stable frequency to the load. The tendency nowadays is to use synduction or synchronous motors. The synduction motor resembles an induction motor but runs at synchronous speed. With either of these unit types, alternator speed and thus frequency to the load is as stable as power line frequency. Motor-generator sets have been widely used to supply 415 Hz power to the many mainframe computers which require this frequency, but recently there has been a substantial shift to the use of solid-state inverters.
Motor-generator sets shield the load from impulses and from voltage sags and surges. For substantial power line voltage changes of 20 percent or more, voltage to the load is still maintained at 1 percent of nominal. The unique feature of the motor generator is its ability to bridge severe short-term sags or voltage dropouts. 
The rotational momentum of the rotating elements permits the motor-generator to span dropouts of up to about 300 milliseconds or 0.3 seconds, the type that cause lights to flicker and plays havoc with digital circuitry. This period can be extended by adding inertia via a flywheel.
The problems with motor generators are mostly on the output or load side. Very high alternator output impedance can cause substantial voltage dips in response to sudden load changes such as result from large inrush motor starting current, and response to load changes is sluggish in the range of 0.25 to 0.5 seconds. Also the drive motor may overheat under long term brownout or low line voltage conditions. The cost of a motor-generator set is somewhat more than for a functionally equivalent line conditioner. Also motor-generator efficiency is relatively low at about 85%, so that electrical energy costs over its lifetime may be substantial. Heat dissipation, weight and bulk, and the potential for annoying audible noise are factors which must be considered in motor-generator installation. The use of rotating field exciters has eliminated the need for slip-rings, with the consequent brush inspection and replacement, but as with any rotating equipment, bearings must be inspected and periodically replaced.
