The main differences between voltage-source frequency converters and current-source frequency converters are as follows:
1. The load of a frequency converter that buffers reactive power is an asynchronous motor, which is an inductive load. In addition to transferring active power, reactive power is also exchanged between the intermediate DC link and the motor. Since power electronic switching devices cannot store energy, reactive power can only be buffered by energy storage elements (filters). The difference between the two types of frequency converters lies in the type of energy storage element used to buffer reactive power.
2. Regenerative braking and regenerative braking are very different.
3. Dynamic Response During Speed Regulation: The DC voltage Ua of a current-source inverter can change rapidly in both magnitude and direction, resulting in a faster dynamic response for speed regulation systems powered by it. In contrast, the Ud of a voltage-source inverter changes slowly due to the charging and discharging of the dry filter capacitor, thus its dynamic response is also slower.
4. Scope of application: Voltage-type frequency converters are suitable for irreversible speed regulation systems and applications where frequent acceleration and deceleration are not required, and are also suitable for multi-motor drives. Current-type frequency converters are suitable for applications requiring rapid braking and reversible operation.
For voltage-source inverters, the output voltage can be filtered using a large-capacity capacitor. The structure of the rectifier is exactly the same as that of the inverter. Its working principle is: using PWM control, the input current of the rectifier is controlled to make the input current waveform approximately sinusoidal and in phase with the input voltage. When it works as a rectifier, it converts AC power into DC power to supply the load; it can also work as an active inverter, converting the energy fed back from the load into AC power to be sent to the grid. Here, we call it a chopper rectifier or a PWM rectifier.
By controlling the magnitude and phase of the alternating current, it brings the input AC current close to a sine wave and the system's power factor close to 1, thereby significantly reducing reactive power components and harmonic interference in the power grid. Simultaneously, when the motor decelerates and returns regenerative power from the inverter, causing the DC voltage to rise, the phase of the AC input current can be made opposite to the phase of the power supply voltage, achieving regenerative operation and feeding the regenerative power back to the AC grid, maintaining the DC voltage at a constant value. In this case, the chopper rectifier operates in active inverter mode.
The main function of the inductor at the rectifier input is to filter out high-frequency current. Its inductance value is relatively small, resulting in a very small voltage drop across the inductor relative to the grid voltage due to the fundamental current. When the load changes, during power regulation, the rectifier input fundamental voltage and the grid voltage remain very close, and the modulation ratio changes very little. Therefore, for load changes, a constant DC terminal voltage can be achieved by maintaining a constant modulation ratio and adjusting the phase angle to regulate the input power.
For voltage-source inverters, the output voltage can be filtered using large-capacity capacitors. Due to the large capacitance, electrolytic capacitors are generally used. To obtain the required voltage rating and capacitance, the capacitors are often connected in series and parallel according to the inverter's capacity requirements. For current-source inverters, the DC intermediate circuit filters the output current using an inductor.
