Trigger circuit synchronization signal selection

In the three-phase controlled rectifier circuit, selecting the correct synchronization power supply for the triggering circuit is essential. Only when the synchronization power supply is properly chosen can each thyristor trigger in sequence at a specific time. The selection of the synchronization power supply depends on three key factors: (1) the configuration of the rectifier circuit and the connection of the transformer windings; (2) the winding connection of the synchronization transformer; and (3) the phase relationship between the synchronization voltage and the trigger pulse in the circuit. To illustrate this, we'll take the example of a three-phase full-bridge rectifier circuit. In the three-phase full-bridge rectifier circuit shown in Figure 1(a), when the control angle α is 0°, the trigger pulses for the six thyristors T1 to T6 must be arranged in order as shown in Figure 1(b). This ensures that the trigger pulses are synchronized with the main power supply. Synchronization is achieved by matching the phase of the synchronization voltage with the main circuit, provided both have the same frequency. Since different trigger circuits require different phases of the synchronization power supply, let's take the example of a sawtooth wave trigger circuit that uses a synchronized voltage. In such a circuit, the zero crossing of the synchronization voltage uT marks the start of the sawtooth wave. To ensure the pulse phase shift falls within the range where the thyristor can conduct, the start of the sawtooth wave must align with the α = 0° control angle. Therefore, the synchronization voltage uT should be set at 180° to match α = 0°. For the thyristor T1 connected to the main circuit’s ua phase, the synchronization voltage should be 150° ahead of the ua voltage. This allows the trigger pulse for T1 to appear at 30° of the ua waveform. Based on this relationship, once the main transformer connection is determined, the synchronous transformer connections and the synchronization power supply for each phase trigger circuit can be further selected. By analyzing the voltage vector diagram of the rectifier and synchronization transformers, it can be determined that the synchronization signal for the T1 trigger circuit leads the ua voltage by 150°. If the main transformer connection or the trigger circuit changes, the synchronous transformer connections and synchronization power supply must also be adjusted accordingly to ensure the trigger circuit starts at the natural commutation point of the main voltage phase. Determining the initial phase is crucial for the control system. When the main circuit and control circuit are powered on, the drive system should be at zero. At zero control voltage, the output voltage of the controlled rectifier device should also be zero. This phase of the trigger pulse is known as the initial phase. The initial phase angle depends on the main circuit configuration and the load type. For example, in a three-phase half-controlled bridge rectifier, Ud = 0 when α = 180°. In a three-phase full-bridge rectifier with resistive load, Ud = 0 when α = 120°. Under large inductance load conditions, Ud = 0 when α = 90°. If the inductive load is insufficient, the initial phase angle will fall between 90° and 120°. To achieve the desired initial phase when the control voltage UK = 0, an offset voltage is added to the trigger circuit. By adjusting the magnitude of the offset voltage, the initial phase of the trigger pulse can be fine-tuned. The polarity of the offset voltage is always opposite to that of the sawtooth voltage, ensuring proper operation of the circuit.

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