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 signal 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. However, it can also be negative-acting, meaning a large input signal results in a small output. A closed-loop control example is shown in Figure 1.
4. In closed-loop control, if pressure signals can be used, flow signals should not be used. 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 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 response, and the debugging is relatively time-consuming.
6. 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.
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;
(3) Select a simple programmable controller;
(4) When using different functions of the frequency converter, a function card 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.
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. Reducing the base frequency (fundamental frequency) is the most effective way to increase starting torque. The principle analysis is 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.
Disclaimer: This article is a reprint. If there are any copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
