Some tips when using the inverter

- Jan 21, 2019-

1. Frequency conversion circuit laying and signal processing

1) The signal line and control line should be shielded, which is good for preventing interference. When the line is long, for example, the distance jumps 100 m, the wire cross section should be enlarged. Do not place the signal wires and control wires in the same cable trench or bridge as the power cables to avoid mutual interference. It is better to place them in the tube. This is more appropriate.

2) The transmission signal is mainly based on the selection of the current signal, because the current signal is not easily attenuated and is not susceptible to interference. 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 the converter; if the current signal fluctuates, it can be solved by a current conditioner.

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 is working and the general pressure, flow, temperature, etc. are controlled). However, it is also counterproductive, that is, the input signal is large, and the output is relatively small (for example, the central air conditioner is used for heating and the heating water pump of the heating station).

4) If the pressure signal can be selected during closed-loop control, do not select the flow signal. This is because the pressure signal sensor is low in price, easy to install, small in workload, and easy to debug. However, if the process has flow ratio requirements and the requirements are accurate, then the flow controller must be selected, and the appropriate flow meter (such as electromagnetic type, target type) should be selected according to actual pressure, flow, temperature, medium, speed, etc. , vortex street, orifice plate, etc.).

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

In addition, this adjustment is not intelligent, so it is not often used, but an external intelligent PID regulator is used. There is PV (running value) indication during work, and it is intelligent, ensuring the best transition process conditions, and it is ideal to use. Regarding PLC, various brands of external PLC can be selected according to the nature of control quantity, number of points, digital quantity, analog quantity, signal processing, etc.

6) The signal converter is also frequently used in the peripheral circuit of the inverter, and is generally composed of a Hall element plus an electronic circuit. According to the signal conversion and processing methods can be divided into voltage variable current, current variable voltage, DC variable AC, AC to DC, voltage variable frequency, current variable frequency, one input and multiple output, multiple input and output, signal superposition, signal splitting, etc. Various converters.

2. When the inverter is applied, it must be equipped with peripheral circuits to meet the needs of users. The methods are often as follows:

(1) A logic function circuit composed of control elements such as self-made relays;

(2) buy off-the-shelf unit external circuit

(3) Select simple programmable controller LOGO

(4) Optional function card when using different functions of the inverter

(5) Select small and medium programmable controllers.

3. Some methods of using the inverter in constant pressure water supply:

At present, water is generally used in a constant pressure water supply method: multiple pumps are connected in parallel with constant pressure water supply, and the common conversion frequency constant pressure water supply technology transformation schemes are commonly used in the following two types.

According to the experience of use, the scheme (1) saves the initial investment, but the energy saving effect is poor. Start the inverter to 50 Hz after starting, then start the power frequency, then switch to energy-saving control. In the water supply system, only the water pump driven by the frequency converter is used, the pressure is slightly smaller, and the system has turbulence and loss.

The scheme (2) has a large investment, but it is 20% more energy efficient than the scheme (1). The pressure of the pump is the same, and there is no turbulence loss, and the effect is better.

When multiple pumps are connected in parallel with constant pressure water supply, only one sensor is used in the signal series mode, and the advantages are as follows.

(1) Saving costs. As long as a set of sensors and PIDs,

(2) Since there is only one control signal, the output frequency is the same, that is, the same frequency, so the pressure is also consistent, and there is no turbulence loss.

(3) During constant pressure water supply, when the flow rate changes, the number of pumps that are started is changed by PLC control. At least one at a minimum, two at a moderate volume, and three at a larger volume. When the inverter is not working, the circuit (current) signal is in the path (the signal flows in, there is no output voltage, frequency).

(4) It is more advantageous because the system has only one control signal, even if the three pumps are different, but the operating frequency is the same (ie, synchronous), the pressure is also the same, so the turbulence loss is zero, that is, the loss is the smallest, so the power saving The best results.

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