The main circuit of the thyristor AC power regulator is exactly the same as the AC voltage regulator. It consists of three sets of anti-parallel thyristors or three bidirectional thyristors. The 6 terminals of the three branches can be connected to external circuits in different forms. Figure1(a) shows that the regulator is connected to the three-phase load in a star shape; Figure1 (b) shows that the power regulator is connected to the three-phase load in a triangular shape.

Figure 1. Main circuit of AC Power Regulator

The difference between an AC power regulator and a phase-controlled AC voltage regulator is that the two control modes are different. The power regulator adopts on-off control mode, and the thyristors of each branch are continuously turned on for several cycles and then cut off for a period of time. When the thyristor is conducted, the load voltage is the same as the power supply voltage, and the load voltage of each phase when the thyristor is off is 0. And the power of the load can be adjusted by turning on and off the thyristor periodically. Set the period of the AC power supply voltage be T and the on-off period of the thyristor be TC, which contains N AC power periods, that is, TC = NT. In each TC, the thyristor has n alternating current cycles in the on-state, and the effective voltage value UO of the load is

When the TC is a certain value, we could adjust the effective value of the output voltage by changing the number of alternating current cycles n corresponding to the on-time of the thyristor in each working cycle. Figure 2 shows the waveform of the output voltage of the AC power regulator. uS is the AC power voltage, and uO is the output voltage of the power regulator. In the figure, the number of cycles of the alternating current in each on-off cycle TC of the thyristor is N = 7. The waveforms of the output voltage when n = 5, n = 4, and n = 3 are plotted.

Figure 2. Waveform of Output Voltage of the AC Power Regulator.

Compared with phase-controlled AC voltage regulators, AC power regulators do not generate additional phase shifts during operation, and can make the power factor be 1 during resistive loading. However, this mode of operation causes the power supply voltage to be intermittently applied to the load, which is only suitable for applications with a large time constant, such as temperature control systems, and cannot be used for loads such as lighting and electrical drive. The voltage and current of the power regulator cannot be measured with ordinary voltmeters and ammeters. In addition, the adjustment of the output voltage of the AC power regulator is achieved by changing the number of output sine waves in each cycle, that is, changing n. N can only be an integer, so the adjustment of the output voltage is actually not continuous. The calculation of the thyristor current in the AC power regulator should be based on the current during the conduction rather than the average current in a duty cycle (TC). If the effective value of the phase current of the load when the thyristor is on is IR, then the effective value of the current flowing through each thyristor is:

According to the principle of thyristor current calculation, the average current ITa of the thyristor in the on-state should satisfy:

In order to avoid the negative impact of the output voltage and current jumps on the load and other equipment in the same network during the conduction, the AC power regulator usually adopts the "zero crossing trigger" method. The thyristor is always triggered when the power voltage cross zero point, so that each occurrence of load current and voltage starts at 0. Figure 3 is a schematic diagram of a zero-crossing trigger control circuit composed of discrete components.

Figure 3. Schematic Diagram of a Zero-crossing Trigger Control Circuit

In the picture, voltage of the phase of the thyristor triggered by the primary side of the synchronous transformer is U1 , so the synchronous voltage U2 is in phase with the main circuit voltage. The synchronous voltage is connected to the input of the regulator composed of R1 and VD1 after bridge rectification, which is a trapezoidal wave synchronized with the main circuit power supply. For most of time of the half-wave, its voltage value is the regulated value of VD1. The voltage will drop only for a short period of time near the zero-crossing point of the sinusoidal voltage as U2 is very small and cannot reach the regulated value of VD1. The collector electrode of the transistor VT is the output of the zero-crossing pulse. The base electrode is connected to the trapezoidal wave voltage through resistors R2 and R3. When the instantaneous value of the trapezoidal wave is large, the transistor is turned on and the collector output voltage uO is 0 with no pulse output. Only when the trapezoidal wave voltage is small, , which means it is near the zero-crossing point of the power supply voltage, the transistor is turned off because the base electric potential is too low. At this time, the collector voltage uO is a high level, and a trigger pulse is output to the thyristor. Every half cycle of the main circuit power supply voltage, a trigger pulse appears near the zero-crossing point, thereby achieving "zero-crossing trigger".

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