The following uses the FR-500 series inverter from Mitsubishi Corporation of Japan as an example to illustrate the characteristics of several frequency settings.
There are two main categories of methods for setting the frequency of a frequency converter. The first is using the frequency converter's operation panel, and the second is using the frequency converter's control terminals. The first method, using the operation panel, only requires the up and down keys on the panel to set the frequency. This method requires no external wiring, is simple, offers high frequency setting accuracy, and is a digital frequency setting method suitable for setting the frequency of a single frequency converter. The second method, using the frequency converter's control terminals, is further divided into two approaches: one is using an external potentiometer, and the other is using the special function of the frequency converter's control terminals to set the frequency with an electric potentiometer.
The first method uses an external potentiometer for frequency setting. As shown in Figure 1, terminal 10 of the FR-500 series inverter provides a standard 10V DC voltage, terminal 2 is the frequency setting input terminal, and terminal 5 is the common analog input terminal. By adjusting the output voltage at terminal 2 of the external potentiometer R, the input voltage value at terminal 2 of the inverter is changed, thus changing the inverter's frequency setting value and achieving the purpose of frequency setting. This method has the following advantages :
(1) The wiring is simple. Just connect the three terminals of the potentiometer to the voltage input terminal, voltage output terminal and common terminal of the frequency converter.
(2) The frequency setting is simple and easy to operate. You can set the frequency by simply turning the knob of the external potentiometer.
(3) Flexible installation: the external potentiometer can be installed in any position according to actual needs for remote operation.
However, this method also has the following drawbacks :
(1) There is a temperature drift phenomenon. Since the resistance value is affected by temperature, when the external temperature changes, the resistance value also changes, and the frequency setting value also changes.
(2) Low anti-interference capability. When there is strong electromagnetic interference in the surrounding area, an induced voltage will be generated in the connecting cable between the frequency converter and the external potentiometer, which will cause the voltage value input to the 2nd terminal of the frequency converter to change, thus causing the frequency setting value to change and affecting the stability of the set frequency.
(3) The installation distance of the potentiometer is limited. Theoretically, the voltage variation range of the inverter terminals 2 is 0-10V. However, if the external potentiometer is installed too far away, the connecting cable will generate a voltage drop, and the voltage of the inverter terminals 2 will not reach 10V, thus the output frequency will not reach the maximum set value.
Therefore, this frequency setting method for frequency converters is generally used in situations where speed regulation accuracy is low, surrounding interference is small, and ambient temperature changes are small, and it belongs to analog quantity regulation.
The second method utilizes the specific functions of the inverter's control terminals. By setting the inverter's internal parameters, terminals RH and RM can be made into electric potentiometers. That is, when RH is connected to the common terminal SD, the inverter's output frequency increases; when RM is connected to SD, the inverter's output frequency decreases, achieving the purpose of frequency setting, as shown in Figure 2. Compared with the first method, this method has the following advantages :
(1) The frequency setting accuracy is high. The external potentiometer method is an analog setting method, and the frequency change range is within ±0.2% of the maximum output frequency . However, when the frequency is set by an electric potentiometer, the frequency change range is within 0.01 % of the maximum output frequency.
(2) Strong anti-interference capability. Since it is only a switch signal input, it is not affected by the surrounding electromagnetic field.
(3) No temperature drift. Since the external potentiometer is eliminated, it is not affected by changes in ambient temperature.
(4) Flexible installation: buttons SB1 and SB2 can be installed in any location.
(5) It has good synchronization performance and can simultaneously increase and decrease the frequency of multiple frequency converters.
In general, to achieve the desired application effect, the frequency setting method should be selected reasonably according to the actual needs.
