01
Shielded cables should be used for signal and control lines to prevent interference. For longer lines, such as distances exceeding 100m, the conductor cross-section should be increased. Signal and control lines should not be placed in the same cable trench or tray as power lines to avoid mutual interference; it is best to run them in conduits.
02
The primary signal used for transmission is current signal, as it is less prone to attenuation and interference. In practical applications, the sensor outputs a voltage signal, which can be converted into a current signal using a converter.
03
Inverter closed-loop control is generally positive-acting, meaning a large input signal results in a large output (e.g., during central air conditioning cooling operation and general pressure, flow, and temperature control). However, it can also be negative-acting, meaning a large input signal results in a small output (e.g., during central air conditioning heating operation and heating hot water pumps in heating stations). Closed-loop control is shown in Figure 1.
04
In closed-loop control, pressure signals should be used whenever possible, rather than flow signals. This is because pressure signal sensors are inexpensive, easy to install, require less work, and are convenient to debug. However, if the process requires precise flow ratios, a flow controller must be used, and a suitable flow meter (e.g., electromagnetic, target, vortex, orifice plate, etc.) should be selected based on the actual pressure, flow rate, temperature, medium, velocity, etc.
05
The built-in PLC and PID functions of frequency converters are suitable for systems with small and stable signal fluctuations. However, since the built-in PLC and PID functions only adjust the time constant during operation, it is difficult to obtain satisfactory transient response, and the debugging process is relatively time-consuming.
In addition, this type of adjustment is not intelligent, so it is not commonly used. Instead, an external intelligent PID controller is preferred.
06
Signal converters are also frequently 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 method, they can be divided into various types of converters such as voltage to current, current to voltage, DC to AC, AC to DC, voltage to frequency, current to frequency, one input to multiple outputs, multiple inputs to one output, signal superposition, and signal splitting.
07
Inverters often require external circuitry in their applications, and these circuits typically take the following forms:
(1) A logic function circuit composed of self-made relays and other control components;
(2) Purchase ready-made external circuitry for the unit;
(3) Select a simplified programmable controller logo (this product is available both domestically and internationally);
(4) When using different functions of the frequency converter, a function card can be selected;
(5) Select small and medium-sized programmable controllers.
08
The following are two common variable frequency technology retrofit solutions for multiple water pumps connected in parallel for constant pressure water supply (such as clean water pumps in urban waterworks, medium and large-sized water pumping stations, hot water supply centers, etc.).
Based on experience, scheme (1) saves initial investment, but has poor energy-saving effect. During startup, the frequency converter is started up to 50Hz first, then the power frequency is started, and then the energy-saving control is switched. In the water supply system, only the water pump driven by the frequency converter has slightly lower pressure, and there is turbulence in the system, resulting in losses.
Option (2) requires a larger investment, but it saves 20% more energy than Option (1), maintains consistent pressure with the Yuantai pump, has no turbulence loss, and is more effective.
09
When multiple water pumps are connected in parallel for constant pressure water supply, a signal series connection method is used, which only requires one sensor. The advantages are as follows.
(1) Cost saving. Only one set of sensor and PID are needed.
(2) Since there is only one control signal, the output frequency is consistent, that is, the same frequency, so the pressure is also consistent and there is no turbulence loss.
(3) During constant pressure water supply, the number of pumps started changes according to the flow rate through PLC control. The minimum is 1 pump, the medium is 2 pumps, and the large is 3 pumps. When the frequency converter stops working, the circuit (current) signal is still connected (signal flows in, but there is no output voltage or frequency).
(4) Even more advantageously, because the system has only one control signal, even if the three pumps are put into operation at different times, their operating frequencies are the same (i.e., synchronized) and their pressures are consistent. Thus, the turbulence loss is zero, which means the loss is minimal, so the energy-saving effect is the best.
10
(1) Reducing the base frequency (fundamental frequency) is the most effective way to increase starting torque. The principle is analyzed as follows.
(2) Why is reducing the base frequency the most effective way to increase starting torque? See Table 1 for details.
As shown in Table 1, due to the significant increase in starting torque, some difficult-to-start equipment, such as extruders, washing machines, centrifuges, mixers, coating machines, blenders, large fans, water pumps, and Roots blowers, can now be started smoothly. This is significantly more effective than simply increasing the starting frequency. Using this method in conjunction with measures to reduce heavy load to light load and maximizing current protection, almost all equipment can be started. Therefore, reducing the base frequency to increase starting torque is the most effective and convenient method.
(3) When applying this condition, the fundamental frequency does not necessarily have to be reduced to 30Hz all at once. It can be reduced gradually in 5Hz increments, and the frequency reached only needs to be sufficient to start the system.
(4) The lower limit of the base frequency should not be lower than 30Hz. From a torque perspective, the lower the lower limit, the greater the torque. However, it should also be considered that excessively rapid voltage rise and excessively large dynamic du/dt can damage the IGBT. Practical experience shows that this torque-boosting measure can be safely and reliably used when the frequency drops from 50Hz to 30Hz.
(5) Some people worry that, for example, when the base frequency drops to 30Hz, the voltage has already reached 380V. So when normal operation may require reaching 50Hz, will the output voltage jump to 380V, which the motor cannot withstand? The answer is that such a phenomenon will not occur.
(6) Some people worry that if the voltage reaches 380V when the base frequency is 30Hz, then the normal operation may require 50Hz. Will the output frequency be able to reach the rated frequency of 50Hz? The answer is that the output frequency can certainly reach 50Hz.
(7) The above two points (5) and (6) are determined by the software development process. The usage process has confirmed this, so you can rest assured about these two points.
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