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What Is a Permanent Magnet Synchronous Generator?

By Paul Reed
Updated May 17, 2024
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A permanent magnet synchronous generator (PMSG) is an electrical generator that uses permanent magnets instead of electrical wire coils to produce magnetic fields. Magnets that keep their magnetic fields for long periods in varying conditions are called permanent magnets. Standard electrical generators require motor windings called excitation coils to create the magnetic field used to produce electrical current. Replacing the coils with permanent magnets eliminates the power required to run the coils, reduces the motor size, and allows power to be generated at different speeds.

Standard coil electrical generators powered by steam or water normally operate at a constant speed to produce electricity with a steady voltage and frequency. In water-powered or hydroelectric generation, varying water levels in dams and supply lakes can make adequate flow to the generator difficult to maintain. A permanent magnet synchronous generator can operate at a wide variety of speeds, allowing better operation with seasonal water levels. They also do not need speed-controlling gearboxes.

Low speed PMSGs have resulted in development of efficient wind generators. The variable nature of wind creates problems for standard electrical generators that need to run at steady conditions. Permanent magnet generators can operate at different speeds, have reduced weight by operating as a direct-drive unit without gears, and are very durable.

These systems have been incorporated into automotive hybrid technology. A hybrid automobile uses battery banks to power one or more electric drives. The electric drive can use a permanent magnet synchronous generator that will operate as a motor or a generator. Battery power sent to the PMSG can help move the automobile by acting as a motor, and when the vehicle slows down the generator helps recharge the batteries.

A good market for PMSG systems is in electricity co-generation. Natural gas or coal generation produces a large quantity of combustion gases. Fuel-fired generation typically has efficiencies around 30 percent. The combustion gases can be fed to a high-speed turbine connected to a synchronous generator. Producing electricity from the waste gas stream can raise the total generation efficiency to around 70 percent.

Permanent magnets are typically not made of iron, which can lose magnetic strength over time. A type of magnet called a rare-earth magnet can be formed into shapes needed to fit inside a PMSG. Rare-earth magnets can be made from samarium/cobalt, or neodymium/iron/boron combinations. These magnets are referred to as rare-earth because samarium and neodymium are listed in the rare-earth section of the periodic table, a table of all known natural and synthetic elements.

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