Precautions for using inverter - Sunland inverter

A frequency converter (VFD) is an electrical energy control device that converts a power supply to another frequency by using the on-off effect of power semiconductor equipment. It can realize the functions of AC asynchronous motor such as soft start, frequency conversion speed regulation, improved operation accuracy, power factor change, overcurrent/overvoltage/overload protection, etc. What do I need to pay attention to when using VFD?

Precautions for using inverter - Sunland inverter

Precautions for using inverter - Sunland inverter

1. The signal line and control line should be shielded wire, which is conducive to avoiding disturbance. When the line is long, e.g. with an interval of 100 meters, the conductor cross-section should be increased. Do not place the signal cable and control cable in the same cable trench or tray as the power cable to avoid disturbing each other. It is better to put it in through a pipe, which is more appropriate.

2. The transmission signal is mainly based on the current signal, because the current signal is not easy to attenuate and disturb. In practice, the signal output by the sensor is a voltage signal, which can be converted into a current signal by a converter.

3) VFD closed-loop control is generally positive, that is, the input signal is large, and the output is also large (for example, when central air conditioning is operated, general pressure, flow and temperature control). But it also has the opposite effect, that is, the input signal is large and the output is small (for example, when the central air conditioner is heating and the heating station is heating the hot water pump). The closed-loop operation is shown in Figure 1.

4. If the pressure signal can be used for closed-loop control, do not use the flow signal. This is because pressure signal sensors have the advantages of low price, simple installation, small amount of work, and easy commissioning. However, if the flow ratio and accuracy are required in the process, it is necessary to select the flow controller and the appropriate flow meter (such as electromagnetic, target, vortex street, orifice plate, etc.). It should be selected according to the practice pressure, flow rate, temperature, medium, and speed.

5. VFD has built-in PLC and PID functions, which is suitable for systems with small signal change and relatively stable functions. However, because the built-in PLC and PID functions only adjust the time constant during operation, it is difficult to obtain satisfactory requirements for too many processes, and it takes time to debug.

In addition, this conditioning is not intelligent, so it is not commonly used. Instead, an external smart PID conditioner is used.

6. The signal converter is often used in the peripheral circuit of VFD, which is generally composed of Hall elements and electronic circuits. According to the signal conversion and processing methods, it can be divided into voltage and current converters, current and voltage converters, DC AC converters, AC DC converters, voltage and frequency converters, current and frequency converters, one input and one output converters, multiple input and one output converters, signal superposition converters and signal shunt converters. For example, Shenzhen's CE-T series power barrier sensor/transmitter is very convenient to use. There are many similar products in China, and users can choose their own applications according to their needs.

7. In the application of VFD, peripheral circuits are often needed, and the methods are often:

(1) Logic utility circuit composed of self-denial relay and other control components;

(2) purchase ready-made external circuits of the unit;

(3) Choose a simple programmable controller.

(4) When using different functions of VFD, you can select function cards;

(5) Select small and medium-sized programmable controllers.

8. There are two common frequency conversion technology transformation plans for parallel constant pressure water supply of multiple pumps (such as clean water pumps, medium and large water pumping stations, central hot water stations, etc.) of urban waterworks. ).

As a rule of thumb,

Plan (1) saves initial investment, but the energy saving effect is poor. Start the VFD 50Hz first, then start again, and then switch to the energy-saving control. In the water supply system, only the VFD drag pump is used, the pressure is slightly lower, and the system has turbulence and loss.

Plan (2) has a large investment, but it is 20% more energy-efficient than plan (1). The pumping pressure of the platform is common, there is no turbulence loss, and the effect is better.

9. When multiple pumps are connected in parallel for constant pressure water supply, only one sensor is used for signal series connection, and its advantages are as follows.

(1) Cost savings. Only one set of sensors and PIDs are required, as shown.

(2) Because as long as there is a control signal, the output frequency is the same, that is, the same frequency, so the pressure is also the same, and there is no turbulence loss.

(3) When the constant pressure water supply is met, when the flow rate changes, the number of pumps to be started will change after PLC control. Minimum 1 set, appropriate amount of 2 sets, large number of 3 sets. When the VFD is out of operation, the circuit (current) signal is a channel (there is a signal flowing in, but there is no output voltage and frequency).

(4) What is more advantageous is that because the system only needs one control signal, even if the input of the three pumps is different, the operating frequency is the same (i.e., synchronous) and the pressure is the same, so the turbulent loss is zero, that is, the loss is the smallest, and the power saving effect is the best.

Precautions for using inverter - Sunland inverter

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