Permanent Magnet Generator - More than just another Generator
The permanent magnet generator is a synchronous machine where the rotor windings have been replaced with permanent magnets. This eliminates the excitation losses in the rotor, which otherwise typically represent 20 to 30 percent of the total generator losses. The reduced losses also give a lower temperature rise in the generator, which means that a smaller and simpler cooling system can be used.
The temperature reduction in the rotor also reduces the temperature in the bearings, improving reliability by increasing the lifetime of the bearings and the bearing grease.
The high efficiency of the generator means better utilisation of the wind energy, producing more electrical power. The reduction in size and weight also makes the construction of the windmill easier as smaller lifting equipment can be used.
Recent developments in permanent magnet generator technology have been made possible by a significant improvement of the magnetic materials during the past decade. A piece of neodymium boron iron (NeFeB) material can have a magnetic force more than 10 times stronger than a traditional ferrite magnet. Advantages PMG download
Permanent magnet generators do not require a DC supply for the excitation circuit, nor do they have slip rings and contact brushes. However, large permanent magnets are costly which restricts the economic rating of the machine. The flux density of high performance permanent magnets is limited. The air gap flux is not controllable, so the voltage of the machine cannot be easily regulated. A persistent magnetic field imposes safety issues during assembly, field service or repair. High performance permanent magnets, themselves, have structural and thermal issues. Torque current MMF vectorially combines with the persistent flux of permanent magnets, which leads to higher air-gap flux density and eventually, core saturation. In this permanent magnet alternators the speed is directly proportional to the output voltage of thealternator. See generator range