A permanent magnet generator is a power generation device that converts mechanical energy, which is converted from heat energy, into electrical energy. It has the functions of starting an engine and charging a DC power source. In traditional motors, these two functions are achieved separately by the starter motor and the alternator. Compared to traditional motors, permanent magnet generators have many advantages, such as improved efficiency, reduced fuel consumption, and reduced noise. Since the advent of switched reluctance motors, the research and application of novel doubly salient pole motors based on switched reluctance motors have become increasingly widespread. In these new doubly salient pole motors, the windings or permanent magnets are only located on the stator; the rotor does not have coils or permanent magnets, and the rotor is directly formed by laminations.
The principle of a permanent magnet generator is based on electromagnetic induction. In an alternating current generator (AC) generator, a conductor cutting magnetic lines of force induces an electromotive force, thereby converting the mechanical energy of the prime mover into electrical energy output. In a permanent magnet generator, the stator and rotor constitute the generator. The stator is the armature that generates electricity, and the rotor is the magnetic pole. The stator consists of an armature core, evenly spaced three-phase windings, a frame, and end covers. The rotor is usually a salient-pole type, consisting of excitation windings, an iron core, a shaft, retaining rings, and a center ring.
A direct current is applied to the rotor's excitation winding, generating a near-sinusoidal magnetic field (called the rotor magnetic field). Its effective excitation flux links with the stationary armature winding. As the rotor rotates, the rotor magnetic field rotates along with it. With each revolution, the magnetic lines of force sequentially cut each phase of the stator winding, inducing a three-phase alternating electromotive force within the three-phase stator windings. When the generator operates with a symmetrical load, the three-phase armature currents combine to generate a rotating magnetic field at synchronous speed. The interaction between the stator and rotor magnetic fields produces a braking torque. The mechanical torque input from the turbine/water turbine/gas turbine overcomes this braking torque and performs work.
Permanent magnet generators have the following characteristics:
It has a compact and simple structure, high efficiency, and high reliability.
The low starting torque solves the problem of wind turbines being difficult to start at low speeds, thus improving the utilization rate of wind energy.
The elimination of the speed increaser improves the reliability, power generation efficiency, and reduces maintenance requirements of the power generation equipment.
It has no carbon brushes, no excitation winding, and no excitation control box. It has a simple structure, does not generate sparks, has high reliability, an IP54 protection rating, and a long service life.
The higher number of poles increases the power generation frequency and rectification/inverter efficiency of the wind turbine at low speeds, saving on the cost of subsequent processing equipment.
By eliminating the excitation winding, carbon brushes, and slip rings of a traditional excitation generator, it avoids common problems such as excitation winding burnout, wire breakage, and wear of carbon brushes and slip rings, thus greatly improving reliability. At the same time, by eliminating the power loss of the excitation winding as the excitation power source, it can reduce fuel consumption per kilowatt by 10%, lowering power generation costs and reducing environmental pollution.
The three major problems with permanent magnet generators include:
Demagnetization issues: This can occur if the design calculations are inaccurate and a lower grade magnet is selected. Overheating and demagnetization is a sensitive issue; a decrease in the magnetic properties of the magnet can also lead to overcurrent and overheating. When the motor's load current exceeds the magnet's demagnetizing resistance, irreversible demagnetization can occur.
The overlapping and deterioration of magnetic properties and current demagnetization: This is something that must be considered in the application of permanent magnet motors. If magnetic properties deteriorate during motor operation, the motor current will increase instantaneously, causing the motor to overheat severely. This further deteriorates the magnetic properties of the magnets, causing the current to increase again. The overlapping and deterioration caused by these two factors can lead to the motor collapsing in a very short time.
Magnet detachment issue: During actual assembly, adhesives are used to reinforce the magnets to the substrate. The purpose of filling the space between permanent magnets is to increase the adhesion between them and prevent them from flying off due to centrifugal force during high-speed rotation. When factors such as poor adhesive performance, insecure embedding, overheating, water ingress into the motor cavity, or dampness interact, magnet detachment may occur, leading to direct mechanical friction and loss of motor drive function.
A permanent magnet generator is an electric motor composed of permanent magnets. It is characterized by high efficiency, low noise, high reliability, low maintenance costs, long service life, and insensitivity to temperature, making it widely used in aerospace, automotive, elevator, water pump, air conditioning, and other consumer and industrial control fields. Compared to other generators, permanent magnet generators can adapt to a wider speed range, and their torque and power characteristics are better, enabling high-efficiency power generation.
