Difference between Voltage Source Inverter & Current Source Inverter

The voltage source inverter (VSI) and the current source inverter (CSI) represent two distinct categories of inverters, both designed for converting direct current (DC) to alternating current (AC). Despite their shared purpose, they exhibit notable operational differences and cater to distinct application requirements.

Power electronics centers on the study and implementation of various power converters—devices or electronic circuits that transform one form of electrical energy into another suitable for a specific load. These converters are classified into multiple types, including AC-to-AC, AC-to-DC, DC-to-AC, and DC-to-DC, each tailored to different energy conversion needs.

An inverter is a specialized power converter designed to transform direct current (DC) into alternating current (AC). The input DC features a steady, fixed voltage, while the output AC can have its amplitude and frequency tailored to specific requirements. This versatility makes inverters indispensable for generating backup power from batteries, facilitating high-voltage direct current (HVDC) transmission, and enabling variable frequency drives (VFDs) that adjust motor speeds by controlling output frequency.

An inverter serves solely to convert electrical energy from one form to another, without generating power independently. It typically consists of transistors like MOSFETs or IGBTs to facilitate this conversion.

There are two primary types of inverters: voltage source inverters (VSIs) and current source inverters (CSIs), each with distinct advantages and limitations.

Voltage Source Inverter (VSI)

A VSI is designed such that its input DC voltage remains constant, unaffected by load variations. While the input current fluctuates in response to the load, the DC source exhibits negligible internal impedance. This characteristic makes VSIs suitable for purely resistive or lightly inductive loads, including lighting systems, AC motors, and heaters.

A large capacitor is connected in parallel with the input DC source to maintain a constant voltage, ensuring minimal variation even as the input DC current adapts to load changes. VSIs typically utilize MOSFETs or IGBTs paired with feedback diodes (freewheeling diodes), which are essential for managing reactive power flow in inductive circuits.

Current Source Inverter (CSI)

In a CSI, the input DC current remains constant (referred to as the DC-link current), while the voltage fluctuates with load changes. The DC source exhibits high internal impedance, making CSIs ideal for highly inductive loads like induction motors. Compared to VSIs, CSIs offer enhanced resilience against overloading and short-circuiting, a key operational advantage in robust industrial setups.

A large inductor is connected in series with the DC source to establish a constant current source, as the inductor inherently resists changes in current flow. This design ensures that in a CSI, the input current remains stable while the voltage adapts to load variations.

CSIs typically employ thyristors in their configuration and do not require freewheeling diodes, distinguishing them from VSIs in both component design and operational mechanics.

Main Differences Between Voltage Source and Current Source Inverter

The table below outlines the key comparisons between VSIs and CSIs: