- Motor Generator Set Definition: A motor generator (M-G) set is defined as a device that includes a motor and a generator connected through a common shaft to convert electrical power from one form to another.
- Applications: M-G sets convert voltage, phase, and frequency of power and isolate electrical loads from the supply line.
- Working Principle: The motor in the M-G set receives electrical energy, rotates its shaft, which is coupled to the generator, thereby converting mechanical energy back into electrical energy.
- Power Conversions: M-G sets can convert AC to DC, DC to AC, DC at one voltage level to another, different frequencies of alternating power, and single-phase AC voltage to three-phase AC voltage.
- Modern Alternatives: Semiconductor devices such as thyristors and MOSFETs now often replace M-G sets due to their smaller size, reduced losses, and easier control.
A motor generator (M-G) set contains an electric motor mechanically coupled to a separate generator. Direct-coupled machines commonly share a shaft, while other arrangements may use a coupling or gearbox. The set converts input electrical power to shaft power and then to an electrically distinct output with the required current type, voltage, phase or frequency.
Motor generator sets can provide voltage, phase and frequency conversion together with galvanic separation between input and output windings. A suitable M-G set also provides waveform filtering, ride-through from rotating inertia and fault-current separation, subject to its design.
In a direct-coupled set, motor and generator rotors turn at the same mechanical speed. They retain separate electromagnetic circuits and need compatible speed, torque and power ratings. A single-frame dynomotor may place both machine functions on one rotor, but that construction does not describe every set.
The diagram of a motor generator set is shown below.
Working Principle of Motor Generator Set
Input power drives the motor, which produces shaft torque. The coupled generator converts that shaft power into output electrical energy. Motor, coupling, bearing and generator losses make output power lower than input power.
Electrical energy exists at both terminals, while mechanical torque and speed carry power between the machines. Separate motor and generator windings provide galvanic isolation. Rotational inertia can buffer brief disturbances, but sustained ride-through needs enough stored kinetic energy or another source.
Conversions
Machine selection, excitation and speed control allow these power conversions:
- AC to DC: an AC motor, such as an induction motor or synchronous motor, drives a DC generator.
- DC to AC: a DC motor drives an AC generator whose excitation, pole count and shaft speed set the output.
- DC at one voltage level to regulated DC at another level.
- AC at one frequency to AC at another frequency, using generator pole count and shaft speed to set output frequency.
- Fixed AC voltage to variable or regulated AC voltage through generator excitation control.
- Single-phase AC input to three-phase AC output with a suitable motor and three-phase generator.
Static power converters have replaced many M-G sets in variable-speed drives and routine power conversion. Modern semiconductor systems use devices such as a thyristor, SCR, GTO or MOSFET to reduce size, rotating maintenance and conversion loss. Rotary sets remain useful where their isolation, inertia, overload behaviour, waveform conditioning or legacy-machine characteristics justify the added noise, maintenance and losses.
