A permanent magnet synchronous generator is a generator where the excitation field is provided by a permanent magnet instead of a coil.

Synchronous generators are the majority source of commercial electrical energy. They are 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 the speed of the rotor must always match the supply frequency.

In a permanent magnet generator, the magnetic field of the rotor is produced by permanent magnets. Other types of generator use electromagnets to produce a magnetic field in a rotor winding. The direct current in the rotor field winding is fed through a slip-ring assembly or provided by a brushless exciter on the same shaft.

Advantages of permanent magnets in synchronous generator

• 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.

• Condensers are not required for maintaining the power factor in synchronous generators, as it is required in induction generators.

Disadvantages of permanent magnets in synchronous generator

• Large permanent magnets are costly.

• Unlike MMF
  Magnetomotive force
  Magnetomotive force is any physical driving force that produces magnetic flux. In this context, the expression "driving force" is used in a general sense of "work potential", and is analogous, but distinct from force measured in newtons...
  
  -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.

• High performance permanent magnets, themselves, have structural and thermal issues.