Motor–generator

A motor–generator (an M–G set) is a device for converting electrical power to another form. Motor–generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electrical power supply line. Large motor–generators were widely used to convert industrial amounts of power while smaller motor–generators (such as the one shown in the picture) were used to convert battery power to higher DC voltages. These motor-generators should not be confused with "motor generator", which is a term occasionally used to describe a portable generator powered by an internal combustion engine.

Low-powered devices such as vacuum tube mobile radio receivers did not use expensive and bulky motor–generators. Instead, they used an inverter circuit consisting of a vibrator (a self-exciting relay) and a transformer to produce the higher voltages required for the vacuum tubes from a 6 or 12V car battery.

While a motor–generator set may consist of distinct motor and generator machines coupled together, a single unit dynamotor (for dynamo–motor) has coils both to drive the motor and to generate the output wound around a single rotor; both coils share the same outer field coils or magnets. Typically the motor coils are driven from a commutator on one end of the shaft, when the generator coils output to another commutator on the other end of the shaft. The entire rotor and shaft assembly is smaller, lighter, and cheaper than a pair of machines, and does not require exposed drive shafts.

In the context of electric power generation and large fixed electrical power systems, a motor–generator consists of an electric motor mechanically coupled to an electric generator (or alternator). The motor runs on the electrical input current while the generator creates the electrical output current, with power flowing between the two machines as a mechanical torque; this provides electrical isolation and some buffering of the power between the two electrical systems.

...
Wikipedia