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Some precautions for the peripheral circuit when the inverter is applied
1. The methods are:
(1) Logic function circuit composed of control components such as self-made relays
(2) Directly buy off-the-shelf unit external circuits
(3) Choose a simple programmable controller LOGO
(4) When using different functions of the inverter, the function card can be selected
(5) Selection of small and medium-sized programmable logic controllers
two,
1) The signal line and control line should be shielded, which is beneficial to prevent interference. When the line is long, for example, the distance jumps 100m, the cross section of the wire should be enlarged. Signal lines and control lines should not be placed in the same cable trench or bridge as the power lines to avoid mutual interference. It is better to place them through pipes, which is more suitable.


2) The transmission signal is mainly based on the current signal, because the current signal is not easy to be attenuated, and it is not easy to be disturbed. In practical applications, the signal output by the sensor is a voltage signal, and the voltage signal can be converted into a current signal by a converter.


3) The closed-loop control of the inverter is generally positive, that is, the input signal is large, and the output is also large (for example, when the central air-conditioning refrigeration works and when the general pressure, flow, temperature, etc. are controlled). But there are also adverse effects, that is, the input signal is large and the output is small (for example, when the central air conditioner is in heating operation and the heating hot water pump in the heating station).


4) If the pressure signal can be used in closed-loop control, the flow signal should not be used. This is because the pressure signal sensor has low price, easy installation, small workload and convenient debugging. However, when the process has flow ratio requirements and requires accuracy, a flow controller must be selected, and a suitable flowmeter (such as electromagnetic type, target type, etc.) must be selected according to the actual pressure, flow rate, temperature, medium, speed, etc. , vortex street type, orifice plate type, etc.).


5) The built-in PLC and PID functions of the inverter are suitable for systems with small and stable signal fluctuations. However, because the built-in PLC and PID functions only adjust the time constant during operation, it is difficult to obtain satisfactory excessive process requirements, and debugging is time-consuming.


In addition, this kind of adjustment is not intelligent, so it is not often used, but an external intelligent PID regulator is selected. For example, Japan's Fuji PXD series, Xiamen Anton, etc., are very convenient. As long as the SV (upper limit value) is set during use, there is a PV (operation value) indication during operation, and it is intelligent, ensuring the best transition process conditions, and it is ideal for use. Regarding PLC, various brands of external PLC can be selected according to the requirements of the nature, number of points, digital quantity, analog quantity and signal processing of the control quantity, such as Siemens S7-400, S7-300, S7-200 and so on.


6) Signal converters are also often used in the peripheral circuits of frequency converters, and are generally composed of Hall elements and electronic circuits. According to the signal conversion and processing methods, it can be divided into voltage-to-current, current-to-voltage, DC-to-AC, AC-to-DC, voltage-to-frequency, current-to-frequency, one-in-multiple-out, multiple-in-one-out, signal superposition, signal branching, etc. various converters. There are many similar products in China, and users can choose the application according to their needs.