A frequency converter is a control device that uses the switching action of power semiconductor devices to convert mains frequency power into electrical energy of another frequency. It can achieve functions such as soft starting of AC asynchronous motors, variable frequency speed regulation, improved operating accuracy, power factor correction, and overcurrent/overvoltage/overload protection. So, what issues should be considered when using a frequency converter?
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., when a central air conditioner is cooling or when controlling pressure, flow, and temperature). However, it can also be negative-acting, meaning a large input signal results in a small output (e.g., when a central air conditioner is heating or when a heating station's hot water pump is operating).
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.
