How to & Guides

What is a Motor Generator Set and How Does it Work?


A motor generator set, also known as an M-G set, is a device used to convert electrical power from one form to another, typically from AC to DC. It consists of a motor and a generator that are connected mechanically on the same shaft. The motor is supplied with electrical power, and its mechanical output is used to rotate the generator. The type of generator used depends on the specific application.

Construction and Working Principle

The motor generator set comprises a three-phase AC motor and a DC generator, usually a compound DC generator. These two machines are mechanically coupled together. The motor is connected to the AC supply, while the generator provides DC voltage as output. The output of the generator is regulated using a field regulator. Alternatively, a synchronous motor can be used instead of an induction motor, which can also improve the power factor.

Motor Generator Set

The main purpose of using a motor generator set is to isolate the electrical load from the supply line. It can also be used to change the supply frequency or voltage and to eliminate spikes and variations in the industrial power supply. However, it should be noted that a motor generator set is not designed to generate electrical energy.


Motor generator sets find applications in various fields. Some of the common applications include:

  • Electrical load isolation
  • Supply frequency or voltage conversion
  • Elimination of power supply fluctuations
  • Conversion of one type of electrical energy to another

Working Process of Motor-Generator Set

A typical motor generator set consists of two electrical machines connected on the same shaft. One machine, designated as machine-1, receives the electrical supply and functions as a motor. As machine-1 rotates at its rated speed, the mechanical energy it produces is then used to rotate the other machine, designated as machine-2. Machine-2 acts as a generator and behaves as a load to the motor.

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The mechanical energy produced by the motor is converted back into electrical energy by machine-2. The output of machine-2 is selected based on the desired application for the electrical energy. In some cases, the motor and generator can be combined into a single unit.

Working Principle of Motor-Generator Set

Conversions Process

Motor generator sets are used for various electrical energy conversions. Here are a couple of examples:

  1. Converting DC power to AC power: If the load requires AC power but the available supply is DC, a motor generator set that includes a DC motor and an AC generator can be used. The DC power is supplied to the motor, which then rotates the AC generator to generate AC power.

DC to AC Conversion

  1. Frequency conversion: Suppose the load operates efficiently at a different frequency than the available AC power supply. In this case, an AC motor and generator are used, with the motor designed for the supply frequency and the generator designed for the desired frequency. By supplying power to the motor-generator set at one frequency, it generates electrical energy at the desired frequency.

Advantages and Disadvantages


  • Self-starting device
  • Can be used for power factor improvement
  • Reliable with a simple design
  • Allows a wide range of DC voltage regulation
  • No risk of reverse polarity or flash-over
  • DC output is independent of AC supply voltage
  • DC output can be easily controlled using the shunt field regulator
  • Synchronous motor can be directly connected to the supply voltage
  • No separate transformer installation required


  • Higher losses compared to other techniques due to multiple machines
  • Noisy and requires regular maintenance
  • Output voltage and frequency may be affected during interruptions
  • Costly, heavy, and space-consuming
  • Lower efficiency compared to other devices
  • Low power factor due to induction motors, requiring compensation
  • Limited to converting AC into DC and vice versa
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For more information on motor generator sets and related topics, visit OnSpec Electronic, Inc.


  • Hopkinsonā€™s Test based on Motor Generator Set – Circuit Diagram, Working, and Applications
  • Speed Control of DC Motor – Voltage, Rheostatic & Flux Control Methods
  • Cable Size Calculation for LT & HT Motors
  • Motor Starter – Types of Motor Starters and Motor Starting Methods
  • Direct Online Starter – DOL Starter Wiring Diagram for Motors
  • Voltage And Power Equations of a DC Motor
  • AC Drive – Working and Types of Electrical Drives & VFD
  • DC Drive – Working and Types of DC Drives

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