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What is Permanent Magnet DC Generator?

Apr. 11, 2020

Definition of permanent magnet DC generator

Permanent magnet DC generator is a kind of power generation device which transforms mechanical energy into electrical energy. It is a kind of permanent magnet DC generator with double permanent magnet and double winding structure.


Principle of permanent magnet DC generator

It is same as AC generator, using the electromagnetic induction principle of the electric potential induced by the cutting of the magnetic line of force to transfer the mechanical energy of the prime mover into electrical energy, then output electrical energy. It is composed of stator and rotor. The stator is the armature that generates electricity, and the rotor is the pole. The stator is composed of armature core, three-phase winding with uniform discharge, base and end cover, etc.


The rotor is usually of hidden pole type, which consists of excitation winding, iron core and shaft, retaining ring, central ring, etc. The exciting winding of the rotor is connected with DC current, which produces a magnetic field close to sinusoidal distribution (called rotor magnetic field). Its effective exciting magnetic flux intersects the static armature winding. When the rotor rotates, the magnetic field of the rotor rotates together with each rotation for one cycle. The magnetic line of force cuts each phase winding of the stator in sequence and induces three-phase AC potential in the three-phase stator winding.


The biggest difference between permanent magnet generator and self excited generator is that its excitation field is generated by permanent magnet. Permanent magnet is not only the magnetic source, but also the component of the magnetic circuit.


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Advantages and disadvantages of permanent magnet DC generator

The permanent magnet DC generator has the advantages of small size, low loss and high efficiency. Today, the energy conservation and environmental protection are paid more and more attention to, so it is very necessary to study it. In many cases, it can be realized without brush, so it is mostly used by small and micro motors. When the variable frequency power supply is adopted, the permanent magnet motor can also be used in the speed regulating drive system. With the continuous improvement of the performance of permanent magnet materials, permanent magnet motors have been widely used in home appliances, medical devices, automobiles, aviation, national defense and other fields.


If using permanent magnet DC generator as a vehicle motor, permanent magnet DC generator has the advantages of high efficiency, energy saving, low carbon, large power and small size, but its output voltage is not easy to adjust, and has disadvantages of high frequency iron loss, high temperature of motor, large starting torque and other defects. In addition, the disadvantage of permanent magnet motor is that if it is not used properly, it may produce irreversible demagnetization under the armature reaction caused by impulse current or severe mechanical vibration when it works under high or low temperature, which may degrade the performance of motor and even make it impossible to use. Therefore, special attention should be paid when using permanent magnet motor.


The permanent magnet DC 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 is easy to saturate, and the magnetic conductivity is nonlinear. All of these increase the complexity of the electromagnetic calculation of the permanent magnet DC generator and make the accuracy of the calculation result lower than that of the excitation generator.


Three problems of permanent magnet generator

1.Control problem

The permanent magnet DC generator can maintain its magnetic field without external energy, but it is also very difficult to adjust and control its magnetic field from the outside. However, with the rapid development of the control technology of power electronic devices, the permanent magnet generator only controls the motor output without magnetic field control.

2.Irreversible demagnetization

In case of improper design and use, when the temperature of permanent magnet generator is too high (NdFeB permanent magnet) or too low (ferrite permanent magnet), it may produce irreversible demagnetization or demagnetization under the action of armature reaction generated by impulse current or severe mechanical vibration, which may reduce the performance of motor and even make it unable to use. 

Therefore, it is necessary to research and develop methods and devices suitable for the use of motor manufacturers to check the thermal stability of permanent magnet materials, as well as analyze the anti demagnetization ability of various structural forms, so that corresponding measures can be taken to ensure that the permanent magnet generator will not lose field in the design and manufacture.

3.Cost problem

Because the price of rare earth permanent magnet materials is still relatively expensive, the cost of rare earth permanent magnet generator is generally higher than that of electric excitation generator, but it will get better compensation in the high performance and operation of the motor. The cost price of the product under development is slightly higher than that of the current general generator, but we believe that with the further perfection of the product, the cost problem will be solved well.

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