Application of solid state relay to realize automatic on-load voltage regulation in agricultural electricity

Solid-state relay SSR, also known as solid-state switch, is a new type of electronic switching device composed of solid-state electronic components. It integrates optocoupler, high-power bidirectional thyristor and its trigger circuit, resistance-capacitance absorption circuit, etc., and has the advantages of safe and reliable operation, no contact, no spark, low trigger current, fast switching speed, and matching with digital circuit. Zero-crossing solid-state relay adopts zero-crossing turn-off, can obtain a complete sinusoidal waveform on the load, can reduce the inductive load (such as fan, three-phase motor, etc.) back potential and can significantly reduce the surge when driving resistive load, etc., has been widely used.
Solid-state relay SSR, also known as solid-state switch, is a new type of electronic switching device composed of solid-state electronic components. It integrates optocoupler, high-power bidirectional thyristor and its trigger circuit, resistance-capacitance absorption circuit, etc., and has the advantages of safe and reliable operation, no contact, no spark, low trigger current, fast switching speed, and matching with digital circuit. Zero-crossing solid-state relay adopts zero-crossing turn-off, can obtain a complete sinusoidal waveform on the load, can reduce the inductive load (such as fan, three-phase motor, etc.) back potential and can significantly reduce the surge when driving resistive load, etc., has been widely used. The forward and reverse control principle of a one-way AC motor is shown in Figure 3. When the switch S in the circuit is placed in the position of "I" and "II" respectively, the motor is positive and reversed respectively. In the figure, M is a unidirectional AC motor, RC is connected in series to form an external suction circuit, RC is a zinc oxide varistor, which plays an overvoltage protection role. The key to the working effect of this circuit is the choice of AC-SSR rated voltage. From the operating characteristics of the C circuit of the running capacitor in the circuit, it can be seen that when either AC-SSR1 or AC-SSR2 is turned off, the voltage at the output end of the turned off SSR is about twice the voltage of the AC power supply. Therefore, when selecting the rated voltage of the SSR, it should be greater than 2 times the power supply voltage, and the commutation time must also have a delay of more than 30ms. The control circuit of positive and negative rotation of three-phase AC motor constituted by AC-SSR is shown in Figure 4. When switch S is connected to "I", the motor is turning forward; When S is connected with "II", the motor is reversed due to the phase reversal of A and B, and its working waveform is shown in Figure 5. It can be seen from the waveform diagram that when the input signal is added to the input end, each AC-SSR is switched on when the voltage of each phase is zero, so that the voltage on the load is synchronized with the voltage of the three-phase power supply. When the input signal voltage is zero, the load current is not immediately zero, if the reverse trigger signal voltage is immediately added at this time, it will cause the main circuit to short-circuit, which is the problem that must be paid attention to to control the positive and negative turn-relay of the three-phase asynchronous motor.
Therefore, when designing the actual control circuit, the reverse delay circuit must be set up to ensure that the reversal trigger signal of the switched SSR can be answered after the AC-SSR current crosses zero. For inductive loads such as motors, the delay is generally about 60 to 70ms. In order to ensure that the motor has enough delay in the positive and negative conversion, the electronic switch or an additional delay circuit is used at the DC control signal end, and the delay length of the single-phase and three-phase control-relay circuit is different, which can be adjusted by adjusting the component parameters. With the advantages that the traditional relay can not compare, it can realize the control of the motor, which has a good application prospect in the petrochemical industry, instruments and equipment, a variety of machinery, digital control machine tools, entertainment facilities and other wide fields.