Design and application of single chip microcomputer in thyristor trigger circuit

Design and application of single chip microcomputer in thyristor trigger circuit

This article introduces a trigger control system composed of 8031 ​​single chip microcomputer, which can realize high-resolution digital triggering. In conventional control, electronic control devices are mainly used to trigger thyristors. Due to the limitations of electronic components, this method has low resolution and sometimes causes false triggering.

In electric traction systems and electric furnace control systems, thyristor (silicon controlled rectifier) ​​components are now widely used as adjustable power supplies to power motors or electric furnaces. This control system composed of thyristors mainly adjusts the supply voltage by changing the control angle θ of the thyristor.

1 Hardware composition and principle

The system hardware composition is shown in Figure 1. It only needs to add a 16-bit timer/counter 8253 and a crystal oscillator circuit to the 8031 ​​minimum system, and a programmable RAM/IO expander 8155 with a 14-bit timer/counter to form the system circuit of the microcontroller.

1.1 θ angle timing

The control angle θ is the electrical angle that lags the natural commutation point. Under power frequency conditions, it has the following linear relationship with time tθ:

Where T is the power supply period, and θ is the control angle.

From the above formula, we can know the corresponding timing time tθ from the electrical angle θ, and the timing of the angle θ can be realized by using the timer/counter. This method of hardware timing can greatly save the online working time of the CPU.

8031 itself has two 16-bit timer/counters T0 and T1. If they are used for timing and mode 1 is selected, it is a 16-bit timer/counter mode. Because one machine cycle of the 8031 ​​microcontroller consists of 12 oscillation cycles, when working in the timing state, the counting frequency is 1/12 of the oscillation frequency, and when working in the counting state, the counting frequency is 1/24 of the oscillation frequency. Therefore, when the crystal oscillator frequency is 6MHz and mode 1 is selected for timing working state, it can be obtained:

Where T is the power frequency period, T = 20ms.

Since the maximum timing time of a 16-bit timer/counter is 65536, the maximum timing angle is:

It can be seen that the phase shift range is large when using 8031 ​​single-chip microcomputer T0 and T1 for timing, but the resolution is limited by the machine cycle of the local machine. In addition, two timers/counters are not enough for three-phase timing. Therefore, it is finally determined to use NEC8253C-2 timer/counter to realize θ angle timing. 8253 is a three-channel 16-bit timer/counter that works in minus 1 counting mode. The three channels just meet the three-phase timing, and the counting frequency is provided by the external crystal oscillator, which is not limited by the system frequency. The counting frequency is selected as 4MHz, and the resolution and maximum timing angle are:

From the above, we can see that both the resolution and the phase shift range can achieve satisfactory results.

1.2 Synchronous signal input and trigger pulse output

This system uses a three-phase synchronous circuit. The three-phase AC synchronous power supply is taken from the secondary winding of the synchronous transformer. After RC phase shift, its zero crossing point is aligned with the six natural commutation points. Then, three voltage comparators output a three-phase square wave synchronous signal with a period of 20ms and send it to P1.3~P1.5 of the single-chip microcomputer P1. Since the jump of the synchronous signal is the natural commutation point, the single-chip microcomputer detects these three status words, and can perform software phase recognition and make ±A, ±B, ±C signs for θ angle timing and output (trigger) and control.

In order to simplify the circuit, reduce the size of the pulse transformer, and enhance the anti-interference ability of the circuit, this system adopts the pulse train triggering method. Six trigger signals are sent out through the 8155 A port, modulated by the external circuit into a trigger pulse train with a frequency of 2kHz, and added to the gates of the six thyristors through the power amplifier circuit.

1.3 Interrupt Arrangement

The 8031 ​​MCU has 5 interrupt sources, one of which is used for the serial port, two are T0 and T1 timer/counter interrupts, and only and can be used directly for external interrupts . In order to leave more hardware resources for the control system, the trigger only occupies interrupt sources, and a combination of hardware and query is adopted. As shown in Figure 1, the three-phase interrupts A, B, and C are connected to through the NOT gate and are connected to P1.0~P1.2 respectively. In this way, as long as there is application, the 8031 ​​MCU can determine the interrupt source by detecting the status of these three bits.

3 Conclusion

This paper uses 8031 ​​single-chip microcomputer to realize digital triggering of thyristors, forming a high-resolution digital trigger, and realizes smooth regulation in the control system. Its performance is unmatched by electronic triggering devices. The system has been applied in actual production and achieved satisfactory results.