This article details 12 tips for using frequency converters, hoping to provide you with more inspiration.
1) Shielded cables should be used for signal and control lines to prevent interference. When the line is long, such as more than 100m, the conductor cross-section should be increased. Signal and control lines should not be placed in the same cable trench or cable tray as power lines to avoid mutual interference. It is best to place them in conduits, which is more suitable.
2) Current signals are primarily used for transmission because they are 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.
3) Variable frequency drive (VFD) 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). A closed-loop control diagram is shown in Figure 1.
4) In closed-loop control, pressure signals should be used instead of flow signals if possible. 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.
5) The built-in PLC and PID functions of the frequency converter 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 a satisfactory transient process, and the debugging is relatively time-consuming.
Furthermore, this type of regulation is not intelligent, so it is generally not used frequently. Instead, an external intelligent PID controller is preferred. Examples include the Japanese Fuji PXD series and Xiamen Andong, which are very convenient. When using it, simply set the SV (upper limit value), and during operation, there is a PV (running value) indicator. Being intelligent, it ensures optimal transient conditions, making it a more ideal choice. Regarding the PLC, various brands of external PLCs can be selected based on the nature of the controlled quantity, the number of points, whether it is a digital or analog quantity, and the signal processing requirements. Examples include Siemens S7-400, S7-300, and S7-200.
6) Signal converters are also frequently used in the peripheral circuits of frequency converters, and are generally composed of Hall elements and electronic circuitry. According to the signal conversion and processing method, they can be divided into various types such as voltage-to-current, current-to-voltage, DC-to-AC, AC-to-DC, voltage-to-frequency, current-to-frequency, one-input-to-multiple-output, multiple-input-to-one-output, signal superposition, and signal splitting converters. For example, the Shenzhen-made Shengsil CE-T series power isolation sensors/transmitters are very convenient to use. Many similar products are available domestically; users can choose the appropriate one according to their needs.
7) Frequency converters often require external circuitry during application, and the common methods include:
(1) A logic function circuit composed of self-made relays and other control components;
(2) Purchase ready-made external circuitry for the unit (e.g., from Mitsubishi Corporation of Japan);
(3) Select a simple programmable controller logo (this product is available both domestically and internationally);
(4) When using different functions of the frequency converter, a function card (e.g., a Sanken frequency converter from Japan) can be selected.
(5) Select a small or medium-sized programmable controller.
8) The following two are 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. When starting, the frequency converter is started 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.
9) 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. Its advantages are as follows.
(1) Cost saving. Only one set of sensor and PID is needed, as shown in Figure 4.
(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) When the water supply is under constant pressure, the number of pumps started changes according to the flow rate through the 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 (there is a signal flowing in, but no output voltage or frequency).
(4) Even more advantageous is that, since 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., synchronous) 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) 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 the 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 the perspective of torque, the lower the lower limit, the greater the torque. However, it should also be considered that if the voltage rises too quickly and the dynamic du/dt is too large, it will damage the IGBT. The actual usage results show that this torque-enhancing measure can be used safely and reliably 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 drops to 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.
11) The relationship between dynamic pressure, static pressure, and total pressure is as follows:
(1) Static pressure is the pressure (head) required when the water pump outlet reaches its highest point. Generally, 1 kg of water pressure is required for every 10 m high water column.
(Circular) Dynamic pressure is the pressure drop caused by the resistance caused by the flow velocity difference between the liquid and the pipe wall, valves (regulating valves, return valves, pressure reducing valves, etc.), and different layers of the same cross section during the water flow process. This part is very difficult to calculate. According to practical experience, dynamic pressure is 20% (at maximum) of static pressure.
(Ape) Total pressure exceeds (static pressure + dynamic pressure) 1.2 static pressure.
The water pump must be set to a lower frequency of approximately 30Hz; otherwise, it may evacuate the water from the closed pipe. Because a large amount of air dissolves into the water, an air chamber may be generated when the pump is started, creating a high-pressure hazard.
12) Experience points and economic points are explained below:
It is feasible to save energy for various devices by using frequency converters, as has been proven by many successful real-world cases.
(1) Empirical values are relatively conservative and have a large margin of safety. They are not the most economical and have potential for improvement. When using empirical values, the actual layout and operating parameters should be adjusted according to the site conditions, with the minimum condition being that they do not affect normal use. This is a prerequisite for achieving energy conservation.
The (circle) economic value is based on meeting the lower limit conditions of the system, appropriately reducing the empirical value, and tapping the potential to achieve energy-saving effects. If the operating parameters remain unchanged, how can energy saving be mentioned? Moreover, the frequency converter itself is not an energy-generating device (generator, battery, solar energy). Its own efficiency is very high, at 97% to 98%, but there are still losses, at 2% to 3%.
Author: Zhang Xuanzheng (1935-), male, graduated from Shanghai Electric Power Institute in 1956 with a bachelor's degree. After graduation, he worked in the field of frequency conversion technology. After retiring in 1995, he served as a technical support and technical consultant for many domestic and foreign frequency converter application companies, working on frequency conversion energy saving.
