The biggest difference between a permanent magnet generator and an excitation generator is that its excitation magnetic field is generated by the permanent magnet. A permanent magnet is not only a magnetic source but also a component of the magnetic circuit in the motor.
The magnetic properties of permanent magnets are related to the manufacturing process, the shape and size of permanent magnets, the capacity of magnetizers, and the method of magnetization. The specific performance data are very discrete. Moreover, the magnetic flux and magnetic potential provided by permanent magnets in motors also vary with the material properties, dimensions of the remaining parts of the magnetic circuit, and the running state of the motor.
Permanent magnet generator has a variety of magnetic circuit structures. The leakage circuit is very complex and the proportion of leakage flux is large. The ferromagnetic material part is easy to be saturated, and the permeance is non-linear. All these increase the complexity of electromagnetic calculation of permanent magnet generators and make the accuracy of calculation lower than that of electric excitation generators.
Like the alternator, the mechanical energy of the prime mover is converted into electric energy output by using the electromagnetic induction principle of the electric potential induced by the magnetic force line cut by the wire. It consists of a stator and rotor. The stator is the armature that generates electricity, and the rotor is the magnetic pole. The stator is composed of armature core, uniformly discharged three-phase windings, frame, and end cover.
The magnet rotor is usually a hidden pole type, consisting of excitation winding, iron core, and shaft, retaining ring, central ring, etc. The excitation winding of the rotor passes through the DC current and produces a nearly sinusoidal magnetic field (called the rotor magnetic field). Its effective excitation flux interacts with the stationary armature winding. When the rotor rotates, the magnetic field of the rotor rotates along with the same start and every turn. The magnetic wire cuts each phase winding of the stator sequentially and induces a three-phase alternating current potential in the three-phase stator winding.
When the generator operates with a symmetrical load, the three-phase armature current synthesizes to produce a rotating magnetic field with synchronous speed. The interaction between the stator magnetic field and the rotor magnetic field will produce braking torque. From the steam turbine/gas turbine, the mechanical torque input overcomes the braking torque coupling and works.
Advantages of Permanent Magnet Generator
I: It has a simple structure and high reliability. A permanent magnet generator eliminates excitation winding, carbon brush, and sliding ring structure of the excitation generator. The structure of the whole machine is simple. It avoids burning and breaking of excitation winding, carbon brush, and sliding ring structure. The structure of the whole machine is simple. It avoids burning and breaking of excitation winding, wearing and tearing of carbon brush, and sliding ring of excitation generator. The reliability is greatly improved.
II: Small size, lightweight, and high specific power. The use of a permanent magnet rotor structure makes the internal structure design of the generator arranged very compactly, and the volume and weight of the generator are greatly reduced. The simplification of the permanent magnet rotor structure also reduces the inertia of the rotor, increases the practical speed, and achieves a very high specific power.
III: Medium and low-speed power generation performance is good. Under the same power level, the output power of the permanent magnet generator is twice as high as that of the excitation generator at idle speed. That is to say, the actual equivalent power level of the permanent magnet generator is the excitation generator.
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