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This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2011) This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. The talk page may contain suggestions. (April 2009) Permanent Magnet Synchronous Generator is a type of Synchronous Generator where the excitation field is provided by a permanent magnet instead of a coil. Synchronous Generators are the primary source of all electrical energy and commonly used to convert the mechanical power output of steam turbines, gas turbines, reciprocating engines, hydro turbines and wind turbines into electrical power for the grid. They are known as synchronous generators because they operate at synchronous speed, which is the same principle of operation as a synchronous motor. The speed of the rotor with a constant magnetic field always matches supply frequency of the stationary winding. The constant magnetic field of the rotor is produced by the persistent magnetic field of a rotor permanent magnet assembly or by controlling direct current to a rotor field winding (i.e., electromagnet) fed through a slip-ring assembly or a brushless means. Advantages of permanent magnets in synchronous generator They are more stable and secure during normal operation and they do not require an additional DC supply for the excitation circuit. The permanent magnet synchronous generators avoid the use of slip rings, hence it is simpler and maintenance free. Higher power coefficient and efficiency. Synchronous generators are suitable for high capacities and asynchronous generators, which consume more reactive power, are suitable for smaller capacities. Voltage regulation is possible in synchronous generators where it is not possible in induction types. Condensers are not required for maintaining the power factor in synchronous generators, as it is required in induction generators. Because of high coercivity of high performance permanent magnet materials, such as neodymium, air-gap depth is more tolerable, which puts lower structural constraints on frame and bearing assemblies. Disadvantages of permanent magnets in synchronous generator Unlike MMF produced flux density in a winding, the flux density of high performance permanent magnets, such as derivatives of neodymium and samarium-cobalt, is limited regardless of high coercivity. After all, permanent magnets are magnetized with the higher flux density of an electromagnet. Furthermore, all electric machines are designed to the magnetic core saturation constraints. Torque current MMF vectorially combines with the persistent flux of permanent magnets, which leads to higher air-gap flux density and eventually, core saturation. Uncontrolled air-gap flux density leads to over voltage and poor electronic control reliability. A persistent magnetic field imposes safety issues during assembly, field service or repair, such as physical injury, electrocution, etc. In all cases, high performance permanent magnet materials are always expensive or virtually cartel controlled by a single country.[citation needed] The mining of high performance permanent magnet materials is environmentally demanding and as a result, the use of permanent magnets is by no means environmentally friendly. Air-gap depth tolerance improves only 20% over other electric machines before magnetic leakage becomes the same concern for any electric machine. High performance permanent magnets, themselves, have structural and thermal issues. See also Alternator