Motor-Generator Set

Motor-Generator Set

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A motor generator (M-G) set refers to a composite device consisting of a motor and a generator mechanically coupled through the common shaft. Practically a motor generator set is a system where a motor and a generator are connected or rather placed in a single circuit. It is a device used to convert electrical power from one form to another. That is mainly it converts electrical power to any other type of power.

Motor generator set are also used to convert voltage, phase and frequency of power. They also find application in isolation of electrical loads from the supply line. Here is a pictorial representation of a MG set.

Motor-Generator Set

Here a motor and a generator are coupled together using a single shaft; they are wound around a single rotor. Necessary condition for coupling is that rated speed of both motor and generator should be same.

The diagram of a motor generator set is shown below,

motor generator set

Working Principle of Motor Generator Set

From the above diagram we can see that in a typical motor generator set, the power is given externally to a motor and as a result the shaft of the motor rotates the rotor of the generator. That means, motor receives electrical energy input from the supply line.

Its shaft rotates and since the generator shaft is mechanically coupled with it, the generator also receives its mechanical input through shaft. Thus generator also creates electrical output power or in other words generator converts the mechanical energy into electrical energy.

Thus while the power at the input as well as output side is electrical in nature, the power flowing between the machines is in the form of mechanical torque. This provides isolation of the electrical system as well as some buffering of power between the two electrical systems.


The motor generator set can be used for conversions of electrical power in various forms

    1. AC to DC – This is possible by using an AC motor (induction motor or synchronous motor) and a DC generator.
    2. DC to AC – This can be accomplished by using a DC motor and an AC generator.
    3. DC at some voltage level to DC at another voltage level.
    4. Alternating power at one frequency to Alternating power at another frequency.
    5. Fixed AC voltage to variable or regulated AC voltage.
    6. Single phase AC voltage to 3 phase AC voltage.

Now days motor generator sets have been upgraded in many ways. Earlier these were used in the places where speed regulation is required highly like elevators, factories etc.

Now a day’s semiconductor devices are working as the motor generator set i.e. with the introduction of power semiconductors devices like thyristor or SCR, GTO, MOSFET etc. motor generator set are replaced by them.

These conversions of power from one form to another can be brought about very easily. They are small and compact in size, losses are less and control is very easy.

What is Type of Motor Used in a Motor-Generator Set?

A motor generator set converts from one form of electricity to another using an electric motor to drive the generator or alternator.

  • Convert single-phase AC to Multi-phase AC.
  • Convert multi-phase AC to single-phase AC.
  • Convert one AC frequency to another.
  • Convert AC to DC or from DC to AC.
  • Produce a different voltage than what is available.

Only the first two examples given are common. The last three had historical applications, but to my knowledge are no longer commonly used.

In one example, a single-phase AC induction motor could drive a three-phase alternator (called a rotary phase converter) to operate three-phase equipment in a location where only single-phase is available.

Because AC current is widely available almost everywhere, the most common motor used is an AC induction motor. The rated horsepower, often given in kilowatts, must exceed the desired output rating to overcome losses in the alternator and the motor. Also, the AC Induction motor’s rated full load speed must match the alternator rated speed.

The most common use of this arrangement operates three phase motors. The phase converter output rating should allow for the total of all motors used at once plus the starting requirement of the largest motor. In this case, only one motor can start at a time.

You can find rotary phase converters on farms, in small manufacturing shops, and anywhere single-phase is the only option when three-phase current is required. I personally have a friend whose hobby is woodworking. He uses machines with three-phase motors that run off a phase converter.

Motor Generator Set

After going through the above portion of motor generator set we can now establish what type of motor is used in a motor-generator set. I hope you enjoy when reading this article, thank you.