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12 practical tips when applying frequency converter

Oct 16, 2020

12 practical tips when applying frequency converter

        1. Signal wires and control wires should be shielded wires, which are beneficial to prevent interference. When the line is long, for example, the distance jumps 100 m, the cross section of the wire should be enlarged. Signal wires and control wires should not be placed in the same cable trench or bridge frame as power wires to avoid mutual interference. It is better to put them through pipes, which is more appropriate.

        2. The transmission signal is mainly based on the selection of the current signal, because the current signal is not easy to attenuate, and it is not easy to be interfered. In practical applications, the output signal of the sensor is a voltage signal, which can be converted into a current signal by a converter.

        3. The closed-loop control of the frequency converter 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 the general pressure, flow, temperature, etc.) are controlled. But there is also a negative effect, that is, the input signal is large, but the output is small (for example, when the central air conditioner is heating and the heating water pump of the heating station).

        4. If pressure signal can be used in closed-loop control, do not use flow signal. This is because the pressure signal sensor is low in price, easy to install, small in workload, and convenient to debug. However, when the process requires flow ratio and accuracy, then a flow controller must be selected, and a suitable flow meter (such as electromagnetic type, target type) must be selected according to the actual pressure, flow, 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 signal fluctuations and relatively stable. However, because the built-in PLC and PID functions only adjust the time constant when working, it is difficult to obtain a more satisfactory transition process requirement, and debugging is time-consuming.

        In addition, this kind of adjustment is not intelligent, so it is generally not used often, but an external intelligent PID regulator is used. For example, Fuji PXD series, Xiamen Anton, etc. are very convenient. When using, only need to set SV (upper limit value), there is PV (operating value) indication when working, and it is intelligent to ensure 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 the control quantity, number of points, digital quantity, analog quantity, signal processing and other requirements.

        6. The signal converter is also often used in the peripheral circuit of the frequency converter, which is 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 input and multiple output, multiple input and one output, signal superposition, signal shunt, etc. Various converters. For example, Shenzhen Shengsier CE-T series electrical isolation sensor/transmitter, application is very convenient. There are many similar products in China, and users can choose their applications according to their needs.

        7. When the inverter is used, it is often equipped with peripheral circuits, and the methods are often:

        (1) Logic function circuit composed of self-made relays and other control components;

        (2) Buy a ready-made unit external circuit;

        (3) Choose simple programmable controller LOGO;

        (4) When using different functions of the inverter, a function card can be selected;

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

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

Solution (1) Save the initial investment, but the energy saving effect is poor. When starting, first start the inverter to 50 Hz, then start the power frequency, and then switch to energy-saving control. In the water supply system, only the pump driven by the frequency converter has a slightly lower pressure, and the system has turbulence and loss.

Solution (2) has a large investment, but saves 20% more energy than solution (1). The pressure of the Yuantai pump is the same, 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 in signal series connection. The advantages are as follows.

        (1) Cost saving. Just a set of sensors and PID.

        (2) Because there is only one 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) During constant pressure water supply, when the flow rate changes, the number of pumps that can be activated is controlled by PLC. The minimum is 1 unit, the medium quantity is 2 units, and the larger quantity is 3 units. When the inverter stops working, the circuit (current) signal is open (signal flows in, no output voltage and frequency).

        (4) What is more advantageous is that because the system has only one control signal, even if the three pumps are put in differently, the working frequency is the same (ie synchronization) and the pressure is the same, so that the turbulence loss is zero, that is, the loss is the smallest, so it saves electricity Best results.

        10. Reducing the base (basic frequency) is the most effective way to increase the starting torque.

        Why is it the most effective to reduce the base frequency and increase the starting torque? The details are listed in Table 1.

        Because the starting torque is greatly improved, some difficult-to-start equipment, such as extruders, washing machines, spin dryers, mixers, paint machines, mixers, large fans, water pumps, roots blowers, etc. can be started smoothly Up. This is more effective than usual increasing the starting frequency for starting. Using this method, combined with the measures from heavy load to light load, increase the current protection to the maximum value, and almost all equipment can be started. Therefore, reducing the base frequency to increase the starting torque is the most effective and the most convenient way.

        When this condition is applied, the base frequency reduction does not necessarily have to be reduced to 30 Hz. It can be used to gradually decrease every 5 Hz, and the frequency reached by the decrease is sufficient to start the system.

       The lower limit of the base frequency should not be lower than 30 Hz. In terms of torque, the lower the lower limit, the greater the torque. However, it must also be considered that the voltage rises too fast and the dynamic du/dt is too large to damage the IGBT. The actual use result is that when 50 Hz drops to 30 Hz, this method of increasing torque can be used safely and securely.

        Some people worry that, for example, the voltage has reached 380 V when the base frequency is reduced to 30 Hz. Then when the normal operation may need to reach 50 Hz, whether the output voltage jumps to 380 V, so that the motor can't stand it, the answer is that such a phenomenon will not happen.

        Some people worry that if the drop base is 30 Hz, the voltage has reached 380 V. Then the normal operation may need to reach 50 Hz, whether the output frequency can reach the rated frequency 50 Hz, the answer is that the output frequency can of course reach 50 Hz.

        11) The relationship among dynamic pressure, static pressure and total pressure is as follows.

        The static pressure is the required pressure (head) when the pressure at the outlet of the water pump reaches the highest point, generally 1 kg water pressure per 10 m water column.

        Dynamic pressure is the pressure drop caused by the resistance caused by the difference in flow velocity between the liquid and the pipe wall, valves (regulating valve, check valve, pressure reducing valve, etc.), and different layers of the same section during the flow of water. This part of the calculation is very important. Difficult, according to actual experience, the dynamic pressure is assumed to be 20% (at maximum) the static pressure value.

Total pressure = (static pressure + dynamic pressure) = 1.2 static pressure.

        The lower limit frequency of the water pump must be set at about 30 Hz, otherwise the water in the closed pipe will be easily pumped out. Because a large amount of air dissolves into the water, when the water pump is started, an air chamber is easily generated, which creates a high pressure hazard.

        12) The experience value and economic value are introduced as follows.

        It is feasible to use frequency converters to save electricity for various devices, which has been confirmed by many real successful cases.

        The empirical value is relatively conservative, and has a greater degree of wealth, not the most economical, and has potential to be tapped. When using empirical values, it should be arranged according to the actual site, and the working condition parameters should be changed to a certain extent. The lower limit condition is not to affect the normal use. This is a prerequisite for the possibility of energy saving.

The economic value is based on the principle of meeting the lower limit of the system, and the experience value is moderately reduced to tap the potential to achieve energy-saving effects. If the operating conditions and parameters remain unchanged, where do you start to save energy? Moreover, the inverter itself is not an energy generating device (generator, battery, solar energy). It has a high efficiency of 97% to 98%, but there is always a loss of 2% to 3%.