TAILIEUCHUNG - Electronic Power Units phần 2

Tuy nhiên, trong nhiều quá trình này cung cấp năng lượng trong các khối lớn sẽ gây ra các biến thể quan trọng trong quá trình sản lượng. Ví dụ, nó sẽ không thể kiểm soát mức độ ánh sáng chỉ đơn giản bằng cách sử dụng như hai nhà nước bật / tắt các yếu tố chuyển đổi. Không điều khiển nhiệt độ tốt có thể được thực hiện theo cách này, | 1 Thyristor power units Fig. 3 shows the relationships for a firing angle a 45 . When a 0 the output is at a maximum . the supply voltage is applied to the load without interruptions. On the other end when a 180 the voltage is continually blocked by the thyristor through the half-cycle. The dotted line shows the voltage waveform while the thyristor is in the blocking off state at a firing angle of 45 . This mode of operation is suitable for resistive inductive and resistive-inductive loads. In the first case the load voltage and current are in phase but in the other cases the current lags behind the voltage. Thyristor power units from JUMO have a built-in soft-start circuit for transformer loads and a current limiting function to ensure that no excessively high current flows when the load is switched on for the first time. The phase angle starts at a 180 completely cut back and is then gradually advanced to the required control angle. The advantages of phase-angle control are the fine control of the power output and the fast response time which makes it possible to use it for very fast control loops. Current limiting can also be implemented in this way. The disadvantage of this mode of operation is the generation of harmonics by the fast transitions of the cut-back half-cycles of the supply at the firing point and the HF interference that this produces. Another disadvantage is that a reactive power component appears even when driving a resistive load. With resistive loads this is entirely due to the phase-angle control and it is therefore known as phase control reactive power. The generation of the phase control reactive power can be understood if one studies the Fourier analysis of the cut-back half-cycles of current. These can be represented by sinewaves of various harmonic frequencies superimposed upon the fundamental frequency. The phase shift of the fundamental frequency of the current with respect to the load voltage is responsible for the abovementioned .

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