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A permanent magnet synchronous generator (PMSG) uses magnets instead of coils to produce magnetic fields, reducing power and allowing for operation at different speeds. PMSGs are used in wind generators, hybrid cars, and electric cogeneration. Rare earth magnets are used in PMSGs for their durability.
A permanent magnet synchronous generator (PMSG) is an electrical generator that uses permanent magnets instead of coils of electric wire to produce magnetic fields. Magnets that maintain their magnetic fields for long periods under varying conditions are called permanent magnets. Standard electric generators require motor windings called exciter coils to create the magnetic field used to produce electric current. Replacing the coils with permanent magnets eliminates the power required to operate the coils, reduces motor size, and allows power to be generated at different speeds.
Standard coil-operated electric generators powered by steam or water normally operate at a constant speed to produce electricity with a constant voltage and frequency. In water or hydroelectric generation, fluctuating water levels in dams and feed lakes can make it difficult to maintain adequate flow to the generator. A permanent magnet synchronous generator can operate at a wide variety of speeds, allowing for better operation with seasonal water levels. Furthermore, they do not require speed reducers.
Low speed PMSGs have led to the development of efficient wind generators. The variable nature of the wind creates problems for standard electric generators which need to operate under stable conditions. Permanent magnet generators can operate at different speeds, are light in weight by operating as a gearless direct drive unit, and are very durable.
These systems have been incorporated into automotive hybrid technology. A hybrid car uses battery banks to power one or more electric drives. The electric drive can use a permanent magnet synchronous generator which will function as a motor or generator. Battery power sent to the PMSG can help move the car by acting as a motor, and when the vehicle slows down the generator helps recharge the batteries.
A good market for PMSG systems is electric cogeneration. The production of natural gas or coal produces a large amount of flue gas. Fuel-fired generation typically has efficiencies around 30 percent. The flue gases can be fed to a high-speed turbine connected to a synchronous generator. Generating electricity from the flue gas stream can increase the total generation efficiency to around 70%.
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 the shapes needed to fit inside a PMSG. Rare earth magnets can be made with samarium/cobalt or neodymium/iron/boron combinations. These magnets are referred to as rare earths 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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