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This article discusses a three-phase half-controlled bridge rectifier trigger circuit used in an extruder from the 1980s. The system uses a 380V power supply, driven by a rectifier transformer (B: 380V/190V, 32kV•A) and a three-phase half-controlled bridge rectifier to power a DC motor (Z2-72, 220V/22kW). The original trigger circuit was composed of 13 discrete-component boards, including integral control, speed adjustment, current regulation, and protection units. It operated as a double closed-loop DC speed control system using current and speed feedback.
Due to long-term disrepair, the trigger circuit became complex and unreliable, leading to frequent failures. After thorough analysis, it was determined that the issue stemmed from a faulty triggering mechanism. To resolve this, the original complex circuit was replaced with a modern three-phase full-control bridge rectifier trigger board (SMC-IIIE) developed by a university in Shandong.
This new control board is designed for use in dual-loop thyristor systems, DC speed controllers, AC voltage regulators, and soft starters. It offers a simple structure, easy installation, and reliable performance. The board supports stepless adjustment of output DC voltage and can trigger thyristors up to 1500A. It includes built-in overcurrent, current limiting, and overvoltage protection, which cuts off the main circuit via a normally open contact (GDL) after a fault occurs.
The control board operates on a dual AC 18V/0.5A input, with GND connected to the center tap. A phase shift control voltage of 0–10V DC is used, along with 15V, UG, and GND for the reference signal. A potentiometer with a resistance of more than 2.2kΩ is used, with one end connected to GND and the other to 15V. TA, TB, and TC are connected to the secondary windings of the three-phase synchronous transformer, while the star point is connected to the intermediate tap.
The trigger pulses are modulated at 20kHz, and the DL and SD terminals receive current and speed feedback signals, respectively. During debugging, the power supply must be set to a positive phase sequence, and the phase sequence of the synchronous transformer and thyristor power supply must match. The specific wiring is shown in Figure 1, where the lower-left part represents the original trigger system, and the upper-left dotted area shows the improved circuit.
The updated trigger circuit is compact, cost-effective, and highly reliable, making it ideal for plastic extruders and similar applications. It also works well with other three-phase full or half-controlled bridge rectifier circuits that use current and speed feedback. This upgrade significantly reduces failure rates and maintenance challenges, delivering substantial economic benefits.
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