Side-by-side comparison
| Parameter | VSI | CSI Inverter |
|---|---|---|
| DC Link Element | Large capacitor C_dc — maintains stiff DC voltage | Large inductor L_dc — maintains stiff DC current |
| Output Quantity Controlled | Output voltage is impressed on load | Output current is impressed on load |
| Output Voltage Waveform | Quasi-square or PWM voltage; current depends on load | Quasi-square current; voltage depends on load |
| Short Circuit on Output | Dangerous — large capacitor discharges; needs fast protection | Tolerates output short; current is limited by L_dc |
| Open Circuit on Output | Safe — capacitor holds voltage | Dangerous — inductor forces high voltage spike |
| Switch Type | IGBT with anti-parallel diode (e.g. FGL60N100) | Gate Turn-Off (GTO) or symmetrical IGBT (no diode); must block reverse voltage |
| Reactive Power Handling | Easy — diodes provide reactive current path | Difficult — requires capacitors across motor terminals |
| Input PF / Harmonics | Diode rectifier front-end: PF ≈ 0.95 with front-end correction | Thyristor front-end: PF varies with firing angle |
| Typical Power Range | 0.1 kW to several MW (Siemens SINAMICS, ABB ACS880) | 1 MW to hundreds of MW (GTO-CSI for large motors) |
| Application Example | HVAC fan drives, conveyor, pump VFDs | Large synchronous motor drives in steel mills, ship propulsion |
Key differences
The DC link capacitor of a VSI acts as a stiff voltage source; if the output is short-circuited, the capacitor dumps energy into the fault — fast IGBT protection is mandatory. The DC link inductor of a CSI acts as a stiff current source; an output open circuit causes a dangerous voltage spike across L_dc, so the output must never be opened without a freewheeling path. VSI switches (IGBTs) need anti-parallel diodes for reactive current; CSI switches must block reverse voltage, so GTOs or special symmetric IGBTs without body diodes are used. Modern drive manufacturers have all shifted to VSI with IGBT because PWM VSI gives far better output waveform quality and the motor sees near-sinusoidal current.
When to use VSI
Use a VSI for standard industrial motor drives where PWM output quality, compact size, and wide power range (0.1 kW – 2 MW) are needed. Example: an ABB ACS880 VFD uses a IGBT-based VSI with SVM PWM at 4 kHz to drive a 75 kW induction motor on a centrifugal pump with < 5% THD.
When to use CSI Inverter
Use a CSI for very large motor drives (above 1 MW) where fault-tolerant current control and high-voltage synchronous motor loads are used. Example: a GTO-based CSI drive in a 5 MW compressor application in a petrochemical plant naturally limits short-circuit current and suits a synchronous motor's voltage-source load character.
Recommendation
For any drive application below 1 MW, choose VSI with IGBT — better waveform quality, lower harmonics, and mature protection make it the default. Above 1 MW with synchronous motor loads, CSI may be justified. In practice, modern MLI (multilevel inverters) and VSI topologies now handle even large drives; pure CSI is a legacy or niche choice.
Exam tip: Examiners ask students to explain why a VSI needs a freewheeling diode in parallel with each IGBT while a CSI does not — the answer is that VSI handles inductive load reactive current through the diodes, whereas CSI uses its DC inductor and output capacitors instead.
Interview tip: A placement interviewer at ABB or Siemens drives division will ask what happens if the output terminals of a VSI are short-circuited versus open-circuited — state that short circuit is hazardous (capacitor discharge) and open circuit is safe, while for CSI it is exactly the opposite.