The reason why harmonic currents are generated when the frequency converter is working is that the impedance of the rectifier circuit at the input terminal of the frequency converter is not a constant value. Its impedance changes with the change of the applied voltage, which causes the current drawn by the rectifier from the power grid to be a non-sinusoidal current.
A single-phase rectifier consists of a rectifier bridge and a smoothing capacitor. Normally, the load current is supplied by the smoothing capacitor. Current only flows into the capacitor and the load when the voltage of the sine wave is higher than the voltage of the smoothing capacitor. Therefore, a pulse-like current is generated only at the voltage peak, and this pulsed current contains abundant harmonic components. Similarly, a three-phase rectifier also generates harmonic currents, but in this case, for each peak, there are two pulsed currents instead of one.
Whether it's a single-phase or three-phase rectifier, the current waveform is distorted and no longer sinusoidal, thus containing harmonic components. Loads that generate harmonic currents are called nonlinear loads, while loads that do not generate harmonic currents are called linear loads. The impedance of a linear load does not change with the voltage applied to it. In this case, the current flowing through the load is I = U/R , meaning that the current I and voltage U have a linear relationship, hence the name linear load. When the voltage is sinusoidal, the current flowing through a linear load is still sinusoidal, therefore no harmonic current components are generated.
Ideal resistors, inductors, and capacitors are all linear loads. However, practical inductors can be nonlinear loads. For example, an inductor with an iron core has an inductance that changes with the applied voltage (and consequently, its impedance), making it a nonlinear negative load. This is why transformers generate harmonic currents.
A nonlinear load is defined as a load whose impedance changes with the voltage applied to it . In this case, the current flowing through it is not linearly related to the voltage applied, hence the name nonlinear load. When a sinusoidal voltage is applied to such a load, the current flowing through it is no longer sinusoidal but contains harmonic components.
Rectifiers with smoothing capacitors are common nonlinear loads, and the harmonic currents they generate are related to the circuit structure. The rectifier draws pulse current from the power grid; the number of pulses rectified per AC cycle is called the rectifier's pulse count. For example, a single-phase rectifier circuit outputs two DC pulses per cycle, hence it is called a two-pulse rectifier ; a three-phase rectifier circuit outputs six pulses per cycle , hence it is called a six- pulse rectifier. Besides these, there are also 12- pulse rectifiers, 13- pulse rectifiers, etc.
In summary, the harmonic current of a frequency converter is caused by the converter's rectifier input circuit. Rectifiers with different pulse numbers produce different harmonic components; the harmonic current generated by a three-phase six-pulse rectifier is mainly of the 5th , 7th , 11th , and 13th orders. Increasing the pulse number of the input rectifier of the frequency converter can reduce the harmonic current.
