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How to adjust the voltage of PMG/PMA

Views: 283     Author: Site Editor     Publish Time: 2019-12-20      Origin: Site

    Permanent magnet synchronous generators use permanent magnets instead of electric excitation. There is no excitation loss during operation. There is no need to draw lagging excitation current from the grid, which greatly saves reactive power. The permanent magnet synchronous generator has many advantages such as simple structure, reliable operation, small size and light weight. In recent years, automotive power supplies and other mobile power applications have more advantages than traditional electric excitation generators. In the field of mobile power, permanent magnet synchronous generators have increasingly replaced traditional synchronous generators. However, as the use time becomes longer, there is always a demagnetization phenomenon in the permanent magnet, and the magnetic field becomes weak, resulting in the output voltage not meeting the requirements.


    At present, permanent magnet synchronous generators mainly use the electric excitation part in hybrid excitation to adjust the magnetic field inside the motor, thereby adjusting the output voltage of the generator to meet the voltage regulation requirements. However, the hybrid excitation generator has a complicated structure and high cost.


    This research uses the transformer principle to perform voltage regulation outside the generator [3]. When the output voltage of the permanent magnet synchronous generator deviates from the rated requirements, the control device adjusts the transformation ratio of the transformer and adjusts the load voltage so that the output voltage from the generator to the load meets the requirements.


    2, Permanent Magnet Synchronous Generator System Structure with Voltage Adjustment

    The system structure of a permanent magnet synchronous generator using a dual winding transformer is shown in Figure 1. It is mainly composed of a prime mover, a permanent magnet synchronous generator, a multi-tap dual winding transformer, and a ratio controller. The tap switch is turned off using a thyristor. It mainly uses thyristors to turn off large rated currents, high rated voltages, no arcing, and many advantages of high-frequency conversion.


    3. Working principle of voltage regulation

    As shown in the figure, the high-voltage side of the three-phase dual-winding transformer is connected to the output of the permanent magnet generator, and the different taps of the high-voltage winding are connected to the output through a control switch. When the output voltage of the generator is higher than the rated voltage of the load, the high-voltage side winding of this transformer increases the number of winding turns through a control switch; otherwise, it reduces the number of winding turns through a control switch. Thereby, adjustment of different transformation ratios is achieved.

    Each tap of the transformer's high-voltage winding is controlled by a thyristor. The voltage detection device feeds back the output voltage to the controller. The controller provides the corresponding thyristor gate voltage signal under different voltage ranges. The thyristor triggers conduction and the tap is realized. Switching, using the transformer ratio to achieve voltage regulation, so that the voltage changes within an acceptable range.

    When the detected output voltage U is in the following range, the corresponding transformer transformation ratio K is as follows:

    Through the above control device, when the voltage of the generator is 0.87 to 1.25 times the rated voltage, the secondary side voltage is stabilized in the Un2 (1 ± 2.5%) range by using the adjustment function of the switching of the transformer high-voltage winding taps, that is, Realize the adjustment of the rate output voltage.


    4. System simulation experiment

    In order to verify the feasibility of the solution in this paper, in accordance with the above ideas, a mathematical model of the system was established using the Simulink toolbox under the matlab platform, and simulation experiments were performed. Due to the symmetry of the three-phase voltage, this simulation only performed simulation experiments on one-phase voltage. In order to simplify the model, an AC voltage source is used instead of the permanent magnet synchronous generator. In the experiment, the output voltage of the permanent magnet synchronous generator due to the change of the magnetic field is simulated by adjusting the effective value of the voltage of the AC voltage source.

    In practice, after the permanent magnet synchronous generator is used for a long time, the magnetic field of the permanent magnet will weaken, resulting in a smaller output voltage. However, it is also possible that the magnetic field will be enhanced due to interference from external magnetic fields during use. There will be two kinds of results in the end: As time goes by, the effective value of the output voltage will gradually decrease; as time goes by, the effective value of the output voltage will gradually increase. All these changes are the result of a long-term accumulation. As far as a permanent magnet machine is used for a short period of time, its output voltage will generally not change, but the output voltage will deviate greatly from the rated value. The two graphs respectively reflect the change trend of the effective value of the load voltage in both cases with and without a regulating device. In the simulation experiment, we use 380V as the rated voltage.

    As the output voltage changes, when there is a regulating device, different transformation ratios are achieved through the regulating device, so that the difference between the rated voltage and 380V is very small, and the large difference is less than 30V (the difference is less than 10%). Without a regulating device, the output voltage changes greatly over time. In this case, the load often cannot work because the voltage is too high or too low.

    Through comparison, it can be known that the voltage regulation device designed in this paper can make the output voltage adjustment within a small deviation range from the rated voltage, which plays a role of voltage regulation.


      5. Conclusion

    This paper proposes a method for externally adjusting and controlling the output voltage of a permanent magnet generator. A multi-tap transformer is used in series with the generator. The output voltage is changed by adjusting the transformer's tap method, which avoids the use of expensive and complicated hybrid excitation machine. The use of thyristors to achieve non-contact switching taps, so that there is no danger of arcing during switching, and high-frequency changes can be achieved. The method was simulated later to verify the feasibility of the method.


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