Permanent magnet materials such as rare earth cobalt permanent magnets and neodymium iron boron permanent magnets were developed in the late 20th century. They offer excellent properties such as high remanent magnetic density, high coercivity, high magnetic energy product, and linear demagnetization characteristics, making them ideal for use in permanent magnet synchronous generators. Since then,
permanent magnet synchronous generators have entered an era of rapid development. Compared with traditional electrically excited synchronous generators, permanent magnet synchronous generators offer the following advantages:
① Simple structure. Permanent magnet synchronous generators eliminate the excitation winding, slip rings, and brushes that are prone to failure, making their structure simpler and reducing processing and assembly costs.
② Small size. The use of rare earth permanent magnets increases the magnetic flux density in the air gap and increases the generator speed to an optimal level, significantly reducing the size of the motor and improving the power-to-volume ratio.
③ High efficiency. Because the excitation power is eliminated, there is no excitation loss or friction and contact loss between the brushes and slip rings. Furthermore, when the tightening ring is installed, the rotor surface is very smooth, resulting in minimal windage. Compared to salient-pole AC-excited synchronous generators of the same power, the overall losses of a permanent magnet synchronous generator are approximately 15% lower.
④ Low voltage regulation. In the direct-axis magnetic circuit, the magnetic permeability of the permanent magnets is low, so the reactance of the direct-axis armature reaction is much lower than that of an electrically excited synchronous generator. This results in a lower voltage regulation than that of an electrically excited synchronous generator.
⑤ High reliability. Permanent magnet synchronous generators lack excitation windings on their rotors, and slip rings are not required on the rotor shaft. Therefore, they are free from the range of faults common in electrically excited generators, such as excitation short circuits, open circuits, insulation damage, and poor contact between brushes and slip rings. Furthermore, due to the use of permanent magnet excitation, permanent magnet synchronous generators have fewer components than conventional electrically excited synchronous generators, resulting in a simpler structure and more reliable operation.
Current practical applications still have certain limitations, hindering widespread promotion and use. The main reason is that permanent magnet synchronous generators use permanent magnet excitation. The high coercivity of permanent magnets makes it extremely difficult to externally adjust the generator's magnetic field. Since the excitation is not adjustable, changes in speed and load current will cause output voltage fluctuations.
