“The thyristor has the advantages of high power, high efficiency, small size, light weight, no noise, and sensitive control. It also has the characteristics of using small current and low power to control large current and high power. Its application range and prospects are very wide. .Using a single-chip microcomputer to realize the zero-crossing speed regulation of the thyristor, compared with the frequency conversion speed regulation, not only can the conduction angle control of the thyristor be completed by software, but also the trigger circuit has a simple structure, flexible control, and the accuracy can be compensated by software and adjusted arbitrarily. Speed and other characteristics, but also to avoid similar phase-shift speed regulation, pulse width speed regulation (PWM), sinusoidal pulse width speed regulation (SPWM) and so on in the process of speed regulation to generate a lot of noise and high-order harmonics, the circuit
The thyristor has the advantages of high power, high efficiency, small size, light weight, no noise, and sensitive control. It also has the characteristics of using small current and low power to control large current and high power. Its application range and prospects are very wide. . Using a single-chip microcomputer to realize the zero-crossing speed regulation of the thyristor, compared with the frequency conversion speed regulation, not only can the conduction angle control of the thyristor be completed by software, but also the trigger circuit has a simple structure, flexible control, and the accuracy can be compensated by software and adjusted arbitrarily. Speed and other characteristics can also avoid similar phase-shift speed regulation, pulse width speed regulation (PWM), sinusoidal pulse width speed regulation (SPWM), etc. in the process of speed regulation to generate a lot of noise and high-order harmonics. higher requirements.
1. The method of thyristor zero-crossing detection speed control control
The schematic diagram of digitally realizing the zero-crossing control of thyristor is shown in Figure 1. It can be seen that the working voltage passed through the zero-crossing speed regulation is a complete sinusoidal waveform, which is turned on at the zero-crossing and turned off at the zero-crossing. During speed regulation, the speed of the motor is adjusted by changing the number of AC sine waves applied to the load within a given time. Since the thyristor is triggered and turned on when the voltage (current) crosses zero, and the waveform during conduction is a complete sine wave or half-wave, there are no disadvantages of the thyristor’s phase-shift voltage regulation and speed regulation. Such as: generate large radio frequency interference, high harmonics, etc. In this way, the first problem is solved, and the safety factor of the circuit device is also improved.
The digital realization of thyristor zero-crossing speed control needs to solve two problems: realize the positive and negative zero-crossing detection of the power frequency voltage, and generate a pulse signal when the zero-crossing; the zero-crossing pulse signal must be controlled by the output information of the single-chip microcomputer, so that the control can be Triac zero-crossing trigger time.
The number and time interval of the zero-crossing pulse signal can be solved through the coordination of software and hardware. It is enough to directly control the number of ON pulses and the number of OFF pulses, and the speed regulation range can be from zero to the highest speed (speed when a complete power frequency voltage is added). Suppose the maximum speed is n0, the number of on-pulses is k, the number of off-pulses is s, and the speed is n. Theoretically:
In actual work, the speed range should be set according to the drive load.
2. Hardware circuit design
The block diagram of the hardware circuit design is shown in Figure 2. The single-chip microcomputer is the core component of the controller. Its main job is to receive the zero-crossing signal of 220 V alternating current, and control the conduction time of the thyristor according to the zero-crossing detection signal; receive the power-down signal sent by the power-down detection circuit; detect the motor speed level and displayed on the nixie tube.
The ultimate goal of the zero-crossing detection circuit is to extract the pulse of the AC voltage when it passes through the zero point. Its working process is as follows: when the positive half cycle voltage is higher, the photocells D1 and T1 turn on Vo to a low level. When the positive half cycle voltage is close to the zero point in reverse, D1 does not reach the value of the turn-on voltage and turns off, so that the T1 cut-off Vo is high level; also when the negative half-cycle high voltage is passed, photocell D2, T2 turn-on Vo is low level, when the negative half-cycle voltage is close to zero, D2 cannot reach the value of the turn-on voltage and cut off, so that the T2 cutoff Vo is high. Send an external interrupt INT0 to the microcontroller 89C2051 through the positive pulse signal when the positive and negative cross the zero point, and the microcontroller controls the thyristor to turn on after a certain delay according to this signal. Its circuit is shown in Figure 3.
The power-down detection circuit is that when the rectified power supply voltage is less than a certain value, it is considered that the power supply is turned off, and a power-down signal is generated at this time.
The conduction control circuit drives the thyristor through the bidirectional thyristor driver MOC3052 with optical isolation to realize the conduction control of the thyristor by the single-chip microcomputer, so as to achieve the purpose of speed control. The thyristor trigger circuit is shown in Figure 4.
3. Key points of software design
The triac zero-crossing trigger mode is adopted, and the on-off of the triac is controlled by the single-chip microcomputer. By changing the number of the complete full-wave (or half-wave) signal of the thyristor that is turned on and off in each control cycle Adjust the load power to achieve the purpose of speed regulation. Since the INT0 signal reflects the zero-crossing time of the power frequency voltage, as long as the opening and closing of the control gate are completed in the interrupt service routine of the external interrupt O, and the number of interrupt services is used to control the quantity N (the thyristor is turned on in each control cycle) The number of sine waves) is counted and judged, that is, for each interruption, N is counted down by 1. If N≠0, keep the control level at “1”, and continue to open the control gate; if N=0, reset the control level to “0”, close the control gate, and make the zero-crossing trigger pulse of the thyristor no longer pass through . In this way, the zero-crossing control of the thyristor can be realized according to the requirements of the control amount obtained by the control process, so as to achieve the effect of control according to the control amount and realize the adjustable speed.
In the process of laboratory test and debugging, the speed regulation of the DC motor appears to be relatively stable, and the speed regulation range is also very wide; but in the process of the AC motor speed regulation, the speed regulation of the medium and high speed sections is relatively stable; There is a jitter phenomenon, and the lower the speed, the more serious the jitter, which is also a follow-up problem to be solved in this design.
The zero-crossing detection circuit is widely used in power Electronic rectifier circuits. It can not only realize motor speed regulation, optocoupler trigger circuit, but also realize power control adjustment, so as to achieve constant power output of the circuit. Zero-crossing detection is the key to our use of triac (TRIAC) to achieve constant power control. In fact, to achieve constant power control, we must accurately control the conduction angle of the AC power supply in every half cycle, and then combine the PID To achieve constant power control.
There are various ways of zero-crossing detection, and the implementation is different, but many people are not very familiar with this field, so based on this, the following live class is set up to explain the key to realizing constant power control based on TRIAC. : Zero-crossing detection.
Live theme: TRIAC four-quadrant & zero-crossing detection & constant power control in-depth analysis
Live broadcast time: 20:00 on June 4th
Live guest: Bai Jilong
Live broadcast outline (knowledge points):
1) Review the process of TRIAC realizing constant power control
2) The main role of zero-crossing detection
3) Detailed circuit design of zero-crossing detection
4) Simulation analysis of zero-crossing detection circuit