Methods for setting the frequency of an inverter

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The frequency converter is a new type of product for speed control and energy savings that has become popular in recent years. It represents a perfect combination of power electronics technology and computer application technology. Thanks to its high speed control accuracy, ease of operation, and energy-saving capabilities (when the output frequency is below 50Hz), it is now widely used in industries such as machinery, chemicals, metallurgy, and light industry. Depending on the requirements of practical applications, there are various methods for setting the frequency of frequency converters. Taking Mitsubishi Corporation’s FR-500 series of frequency converters as an example, the characteristics of several such frequency setting methods will be explained. 　　The methods for setting the frequency of an inverter can be divided into two main categories: the first is to set the frequency using the inverter’s operation panel, and the second is to do so by using the inverter’s control terminals. The first type uses the frequency controller’s operation panel to set the frequency; by simply using the up and down buttons on the panel, it is possible to adjust the frequency. This method does not require external wiring; it is simple, offers high precision in frequency setting, and belongs to digital frequency control, making it suitable for adjusting the frequency of a single frequency converter. The second category involves using the frequency control terminals of the inverter for frequency setting, which is further divided into two methods: the first method uses an external potentiometer for frequency setting; the second method makes use of the special function of the inverter’s control terminals to employ an electric potentiometer for frequency setting. 　　The first method involves using an external potentiometer for frequency setting; as shown in Figure 1, terminal 10 of the FR-500 series frequency converter provides a standard 10V DC voltage, terminal 2 is the input for frequency setting, and terminal 5 is the common terminal for analog inputs. By adjusting the output voltage at terminal 2 of the external potentiometer R, the input voltage at terminal 2 of the inverter is changed, which in turn alters the frequency setting of the inverter, thereby achieving the desired frequency setting. This method has the following advantages: (1) Simple wiring – it is sufficient to connect the three terminals of the potentiometer to the voltage input terminal, the voltage output terminal, and the common terminal of the inverter. 　　(2) The frequency setting is simple and easy to operate; simply turn the knob of the external potentiometer to adjust the frequency. 　　(3) It features flexible installation; the external potentiometer can be placed in any location as needed to enable remote operation. 　　However, this method also has the following disadvantages: (1) There is temperature drift, as the resistance value is affected by temperature; when the external temperature changes, the resistance value changes as well, and thus the frequency setting also changes. 　　(2) Low anti-interference capability. When there is strong electromagnetic interference in the surroundings, an induced voltage is generated in the connection cable between the inverter and the external potentiometer, causing the voltage applied to the input terminal of the inverter to change. This, in turn, alters the frequency setting, affecting the stability of the set frequency. 　　(3) The installation distance of the potentiometer is subject to certain limitations. Theoretically, the voltage range at terminal 2 of the inverter is 0–10V. However, if the external potentiometer is installed too far away, voltage drop will occur in the connection cables, and the voltage at terminal 2 of the inverter will not reach 10V; as a result, the output frequency cannot reach its maximum set value. 　　Therefore, this frequency setting method for inverters is generally used in applications where the speed control accuracy is low, there is little external interference, and environmental temperature changes are minimal; it belongs to analog control. 　　The second method makes use of the specific functions of the control terminals of the inverter; by setting the internal parameters of the inverter, the terminals RH and RM can be turned into electric potentiometers. That is, when RH is connected to the common terminal SD, the output frequency of the inverter increases, while when RM is connected to SD, the output frequency decreases, thereby achieving the desired frequency setting. As shown in Figure 2, compared to the first method, this approach has the following advantages: (1) High precision in frequency setting. The method using an external potentiometer is a analog-based approach, with a frequency variation range of within ±0.2% of the maximum output frequency, whereas using electric potentiometers allows for a frequency variation range of within 0.01% of the maximum output frequency. 　　(2) Strong anti-interference capability. Since it is merely a switch signal input, it is not affected by surrounding electromagnetic fields. 　　(3) No temperature drift. Since the external potentiometer has been removed, it is not affected by changes in ambient temperature. 　　(4) Flexible installation – buttons SB1 and SB2 can be installed in any position. 　　(5) It has good synchronization performance, enabling the increase and decrease of frequency for multiple inverters simultaneously. 　　In short, we should choose the frequency setting method appropriately based on actual needs to achieve the desired application results.