Difference between Inverters VSI vs CSI

The main difference between VSI and CSI lies in their input characteristics, output control methods, and load compatibility. While VSIs have a constant voltage input and regulate output voltage by adjusting the switching patterns, CSIs have a constant current input and regulate output current by controlling the load impedance. The choice between VSI and CSI depends on the specific application requirements and the type of load to be connected to the inverter.

What Is Inverter?

An inverter, is a power electronics device which used to convert fixed DC (Direct Current) into controlled AC (Alternating Current)”. Here, Controlled AC means we controlled two parameter of AC signal Frequency and Amplitude.

Inverter

In other words, the inverter is a static device. It can convert one form of electrical energy into other forms of electrical energy. But it cannot generate electrical power.

Inverter takes DC input power from the batteries and converts into AC output power at the time of the power failure or power cutoff.

In inverter we use some power semiconductor switching devices like IGBT, MOSFET, GTO because these devices having self-commutation property.

A power inverter used in the power system network to convert bulk DC power to AC power. i.e., It used at the receiving end of HVDC transmission lines. This inverter is known as a grid-tie inverter.

Classification Of Inverter

Inverters are mainly classified into two types:

  1. VSI (Voltage Source Inverter)
  2. CSI (Current Source Inverter)

What Is VSI (Voltage Source Inverter)?

In voltage source inverter (VSI) input voltage is kept constant. VSI is fed from a DC voltage source having negligible or zero impedance.

So, to make a VSI we connect a DC source in parallel with large capacitor that maintained Input voltage constant. In case of VSI an output voltage is independent of load. Here, the waveform of the load current as well as its magnitude depends upon the nature of load impedance.

Voltage Source Inverter

A voltage source inverter is fed by a stiff DC voltage or constant DC voltage is called Voltage Source Inverter (VSI) or Voltage Fed Inverter (VFI).

In Voltage Source Inverter (VSI) we use power semiconductor switching devices in the circuit like Power BJT, Power MOSFET, IGBT having self-commutation properties.

VSI drives are more efficient than CSI designs. This is because in VSI drives we use IGBT switching devices that is more efficient than the GTO or SGCT devices used in CSI versions. 

1. Voltage Source Inverter (VSI): A Voltage Source Inverter is an inverter where the input DC voltage is considered as a constant voltage source. Here are some key characteristics and features of VSIs:

Summary points of VSI

  • Input Configuration: VSIs typically have a DC input source, such as a battery or a rectifier, supplying a constant voltage.
  • Output Current: The output current is controlled by varying the inverter’s switching pattern or modulation techniques like Pulse Width Modulation (PWM). The inverter regulates the output voltage by adjusting the duty cycle of the switching signals.
  • Output Voltage Control: VSIs can provide variable output voltage with a fixed frequency. By adjusting the pulse width, the RMS value of the output voltage can be controlled.
  • Load Type: VSIs are generally suitable for supplying voltage-controlled loads like motors, induction heaters, or certain types of AC drives.
  • High Input Impedance: The input impedance of a VSI is typically high, which means it draws less current from the input source.
  • Short Circuit Protection: VSIs may require additional protection circuitry to prevent damage due to short circuits at the output.

What Is CSI (Current Source Inverter)?

In current source inverter (CSI) input current is kept constant. CSI is fed with adjustable current source from a DC voltage source of high impedance.

VSI can be converted into CSI, by connecting large series inductance that maintained input current constant. In CSI output current is independent of load. Here, the magnitude of output voltage and its waveform depends upon the nature of the load impedance.

Current Source Inverter

A Current Source Inverter is fed by a stiff current source or constant current source is also called Current Source Inverter (CSI) or Current Fed Inverter (CFI).

In Current Source Inverter (CSI) we use Thyristor as a semiconductor switching device.

Important points of CSI

A Current Source Inverter is an inverter where the input DC current is considered as a constant current source. Let’s examine the characteristics and features of CSIs:

  • Input Configuration: CSIs typically have a DC input source, such as a current-regulated rectifier or a current-controlled DC source.
  • Output Current Control: The output current is controlled by varying the inverter’s switching pattern or modulation techniques. The inverter regulates the output voltage by adjusting the load impedance to maintain a constant current.
  • Output Voltage: The output voltage of a CSI is determined by the load impedance and the output current. As the load impedance changes, the output voltage varies accordingly.
  • Load Type: CSIs are generally suitable for supplying current-controlled loads, such as electric arc furnaces, some types of motor drives, or applications where a constant current is desired.
  • Low Input Impedance: The input impedance of a CSI is typically low, which means it draws more current from the input source.
  • Self-Short Circuit Protection: CSIs have inherent short circuit protection since they are current-regulated. In case of a short circuit, the output current is automatically limited.

Below circuit diagram help you to understand about the difference between VSI and CSI.

Block diagram of inverter

Power circuit of CSI

Difference between Inverters VSI vs CSI

VSI (Voltage Source Inverter)CSI (Current Source Inverter)
1. In voltage source inverter input voltage is kept constant.1. In current source inverter input current is kept constant.
2. VSI is fed from a DC voltage source having small or negligible impedance.2. CSI is fed with adjustable current source from a DC voltage source of high impedance.
3. DC source in parallel with large capacitor.3. VSI can be converted into CSI, By connecting large series inductance.
4.Input voltage is maintained constant.4. The input current is constant but adjustable.
5. An output voltage is independent of load.5. An output current is independent of load
6. The waveform of the load current as well as its magnitude depends upon the nature of load impedance.6. The magnitude of output voltage and its waveform depends upon the nature of the load impedance.
7. VSI has slow response than CSI. 7. CSI has fast response than VSI.
8. VSI requires feedback diodes8. The CSI does not require any feedback diodes.
9. The commutation circuit is complicated. It uses current commutation.9. Commutation circuit is simple as it contains only capacitors. It uses voltage commutation.
10. Power BJT, Power MOSFET, IGBT with self commutation can be used in the circuit.10. Thyristors are Used.
11. In VSI diodes are connected in parallel.11. In CSI diodes are connected in series.
12. PWM technique is used.12. PWM technique is not used.

FAQ’s (Frequently Asked Questions)

What are the main differences between VSI and CSI?

In voltage source inverter (VSI) input voltage is kept constant. VSI is fed from a DC voltage source having negligible or zero impedance. In current source inverter (CSI) input current is kept constant. CSI is fed with adjustable current source from a DC voltage source of high impedance.
A VSI can also be operated in current-controlled mode, and similarly a CSI can also be operated in the voltage control mode.

What are the advantages of VSI?

The advantages of VSI are; (i) It has a small size. (ii) Its output voltage waveform does not depend on the type of load.

What are the advantages of VSI over CSI?

VSI drives are more efficient than CSI designs. This is because in VSI drives we use IGBT switching devices that is more efficient than the GTO or SGCT devices used in CSI versions. 

What is voltage source inverter?

In voltage source inverter (VSI) input voltage is kept constant. VSI is fed from a DC voltage source having negligible or zero impedance.
So, to make a VSI we connect a DC source in parallel with large capacitor that maintained Input voltage constant. In case of VSI an output voltage is independent of load. Here, the waveform of the load current as well as its magnitude depends upon the nature of load impedance.

What is current source inverter?

In current source inverter (CSI) input current is kept constant. CSI is fed with adjustable current source from a DC voltage source of high impedance.
VSI can be converted into CSI, by connecting large series inductance that maintained input current constant. In CSI output current is independent of load. Here, the magnitude of output voltage and its waveform depends upon the nature of the load impedance.

Read More on inverters

Classification of inverters click here

Three Phase Inverter click here

Parallel inverts click here

Single phase full inverter click here

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