Rotary Frequency Converters & Motor Generators

What is a Rotary Frequency Converter?

Rotary frequency converters (also called "Motor-Generators" or MG Sets") convert incoming AC Power into rotary mechanical power (spinning motor), which transmits its rotary power to a generator, which converts its mechanical power into Electric AC output power. Rotary power is often described in terms of Horsepower, while Electrical power is described in Kilowatts (kW) or Kilovolt-Amperes (KVA). The conversion of frequency (hertz – Hz), voltage, and/or phase (3 phase, 1 phase) are inherent in this process.

Types of Rotary Converters & Motor Generators


What powers a Rotary Frequency Converter?

Motor Generator sets use several methods of coupling the drive motor to the generator. The simplest and least expensive method is belt coupled converters that use drive belts and pulleys to not only transfer the energy from motor to generator, but also change the frequency by pulley ratio. Some clients are concerned with the longevity of drive belts, but in practice, drive belts do not fail when properly designed and installed.  Georator has a flawless record in that regard.

Another method is direct coupled converters that directly couple the motor shaft to the generator shaft using a mechanical coupling, and adjusting the speed of the drive motor to vary the rotational speed of the generator, thus varying the output frequency. An adjustable speed electronic drive (ASD) is used for this purpose, instead of a conventional motor starter.

Finally, the most sophisticated and most costly method is to build the motor and generator onto a single common shaft, called common shaft frequency converters. In this case, the frequency change is made by winding the motor with a different number of electrical poles than the generator. For example, a 12 pole motor and 10 pole generator will yield a 60-to-50 Hz conversion.

In certain applications, only power line isolation (complete electrical discontinuity, input to output) or power line conditioning (poor incoming electrical power converted to good output power), is required. In these power line isolators, an insulated flexible coupling is used between motor and generator, to transmit the power from motor to generator, and completely isolate the input from the output. Typically, no change is made to the frequencies, although phase or voltage conversion may be required.

What are Common applications for Rotary Converters?

Rotary frequency converters are very good at starting and running typical factory loads. They have the ability to produce high starting surge currents for short periods, making them ideal for motor loads. These converters are very robust, and can tolerate harsh environments. While they are susceptible to driving rain, with the proper enclosures these units can be placed outdoors, and tolerate a wide range of operating temperature environments.

Typical Characteristics of Rotary Frequency Converters

  • More attuned to larger applications 10 KVA plus
  • Much better at starting motor loads
  • Rugged floor mount construction
  • Generally, fixed output frequency
  • Costs do not increase linearly with power; e.g., 3x power = 1.5x cost
  • Harmonic distortion and noise on input power is not passed to the output
  • Can source heavy overload currents 2-4X for short periods of time
  • Full load efficiency up to 90+% on large units 

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