The bias frequency and frequency gain of a high-voltage frequency converter are used when the external given signal is inconsistent with the default given signal of the frequency converter. This article introduces the concepts and setting methods of bias frequency and frequency gain.
Most frequency converters use a basic frequency setpoint, meaning that when the converter's setpoint frequency is 0-50Hz, it corresponds one-to-one with the external voltage or current signal. For any other setpoint frequency, such as 5-40Hz instead of 0-50Hz, and 0-5VDC instead of 0-10VDC, the basic setpoint frequency cannot be used. Instead, the bias frequency and frequency gain parameters must be used. This is a feature provided by the frequency converter manufacturer for user convenience.
bias frequency
The bias frequency, sometimes called the deviation frequency or frequency deviation setting, is used to adjust the output frequency when the frequency is set by an external analog signal (voltage or current), as shown in Figure 1. Some inverters define the frequency corresponding to a "0" frequency setting signal as the bias frequency. That is, the deviation value can operate within the range of 0-fmax. Some inverters (such as Meidensha and Sanko) also allow setting the bias polarity. For example, during commissioning, if the inverter output frequency is not 0Hz but any frequency value when the frequency setting signal is 0%, setting the bias frequency to a negative value will make the inverter output frequency 0Hz. The bias frequency is expressed as a frequency value, or sometimes as a percentage of the frequency.
Frequency gain
Frequency gain refers to the gain of the frequency setting signal. This function is only effective when using an external analog signal to set the frequency. It is used to compensate for the inconsistency between the external setting signal voltage and the inverter's internal voltage (+10V); it also facilitates the selection of the analog setting signal voltage. When setting, when the analog input signal is at its maximum (e.g., 10V, 5V, or 20mA), calculate the percentage of frequencies that can be output as a U/f graph, and use this as the parameter for setting, as shown in Figure 2. For example, if the voltage setpoint signal terminal of a certain inverter requires an output frequency of 0-50Hz+ corresponding to 0-10V, but the external setting signal is only 0-5V, in order to maintain the inverter's output frequency at 0-50Hz, the frequency setting signal gain can be set to 200%.
Inverter bias frequency and frequency gain setting method
For example, a frequency inverter requires an output frequency of 0-50Hz when the frequency command signal is 0-5V or 4-20mA. For inverters with selectable command signals, simply set the command signal to 0-5V or 4-20mA, so that 0V or 4mA corresponds to a frequency of 0Hz, and 5V or 20mA corresponds to a frequency of 50Hz. In this example, the bias frequency is 0Hz or 0%.
However, some frequency converters only offer a 0-10VDC frequency input. If a 1-5V voltage signal is used as the input, then the bias frequency and frequency gain parameters are needed. The basic frequency input line can be drawn first, as shown in Figure 3. From the given voltage Vx=1V, fx=0Hz, the starting point of frequency input line II can be determined as A. From Vx=5V, fx=50Hz, the ending point of frequency input line II can be determined as B. From the extension of input line II, we know that the frequency fx corresponding to Vx=0 is -12.5Hz, which is -25% of the highest frequency. The frequency fx corresponding to Vx=10V is 112.5Hz, which is 225% of the highest frequency. Therefore, when setting the frequency, the highest frequency is set to 50Hz, the bias frequency is set to -12.5Hz, and the frequency gain is set to 225%. This way, the 1-5V voltage input corresponds to a frequency of 0-50Hz.
Since the setting parameters of different types of frequency inverters are not entirely the same, the corresponding instruction manual should be consulted to select the setpoint signal to avoid mismatch. For example, using a 1-5V voltage to match the frequency setpoint of the frequency inverter (0-10V) will cause problems for the 5000G11S frequency inverter, because the maximum setpoint gain of the frequency setting signal for this type of inverter is 200%, while the required gain for the frequency setting signal in this example is 225%. Therefore, an alternative frequency setpoint signal must be selected.
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