The fundamental reason for the slightly lower power factor on the input side of the frequency converter is fundamentally different from the low power factor of the three-phase motor with variable frequency. Series resonance occurs because the three-phase motor is a rational and objective electrical load, and the phase of the operating current lags behind the working voltage. The fluctuation of the power factor depends on the phase difference relationship between the working current and the working voltage. However, the low power factor of the frequency converter is caused by its power supply circuit structure. The frequency converter is generally of an “AC-to-AC” type structure, that is, the three-phase circuit source is transformed into DC electricity through a three-phase rectifier bridge and filtering power capacitors, and then converted into adjustable-frequency alternating current through the control circuit and the inverter power transistor. During the rectification process, only when the instantaneous value of the AC power supply is greater than the DC voltage UD will the rectifier diode conduct, and there will be a charging current in the rectifier bridge. Obviously, the charging current always appears within a relatively limited period around the peak of the switching power supply and is in a non-continuous pulse wave form. Such a non-sinusoidal waveform has a strong component of higher harmonics. A small part of the instantaneous power is “+”, and the other part is “-”, which belongs to reactive power compensation. This reactive power compensation causes the power factor of the frequency conversion speed control system to be relatively low, approximately 0.7 to 0.75.
This is the fundamental reason for the low power factor on the input side of the frequency converter. It is not because the waveform of the working current lags behind the working voltage; rather, it is composed of high-order harmonic working currents. Therefore, it is impossible to increase the power factor by paralleling compensating capacitors. Instead, we should try to minimize the high-order harmonic working currents. The specific approach is to connect a series reactor. JT is a DC series reactor, connected between the rectifier bridge and the filtering power capacitor. During its application, this will have a remarkable practical effect. When both are used together, the power factor can be increased to above 0.95. In addition, the DC series reactor can also control the charging surge current that occurs in the instant of connecting the switching power supply. Moreover, it is not allowed to connect power capacitors in parallel at the output end of the frequency converter, that is, at the connection point with the three-phase motor.
The so-called sine wave output by the frequency converter is actually a pulse width modulation wave where the sizes of pulse width and PWM duty cycle are distributed according to the basic sine law. This pulse sequence is formed by the continuous and alternating conduction of the inverter power supply tubes in the frequency converter. If a power capacitor is connected at the output end, during the process of the inverter power supply tubes changing their conduction states, they not only need to provide working current to the three-phase motor, but also increase the charging current and discharging current of the power capacitor, which will lead to the damage of the inverter power supply tubes.
Series reactors are not standard equipment for most frequency conversion speed regulators; they are optional accessories. They should be selected based on the requirements. Sometimes, in order to better reduce the cost of equipment investment, AC series reactors are not connected, and the frequency conversion speed control system operates under a low power factor.