--- title: "Single-phase current source inverter" subject: "Power Electronics" chapter: 5 key: "semester_7/power_electronics:single-phase-csi" source: https://www.bctengineer.com/q/semester_7/power_electronics/single-phase-csi site: BCT Engineering --- # Single-phase current source inverter Power Electronics · Chapter 5: Inverter - Asked in: 2075 Chaitra, 8 marks - Video: [Current Source Inverter (CSI)](https://www.youtube.com/watch?v=AIFH3g0KNIQ) — Anet Jose - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/single-phase-csi - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-5 ## Question Explain the operating principle of single phase current source inverter with necessary waveforms. ## Short answer A CSI is fed by constant current through a large series inductor Ld. Thyristors steer Id alternately, producing a square-wave load current of plus or minus Id, while a commutating capacitor reverse biases the outgoing thyristors. Output voltage depends on the load, and fundamental rms current is 0.9Id. ## Answer A current source inverter (CSI) is fed from a dc source of constant current. A large inductor $L_d$ in series with the dc supply keeps the input current $I_d$ nearly constant, so the inverter switches only steer this current. The output current is a square wave of $\pm I_d$, and the output voltage is decided by the load. #### Circuit (single-phase, capacitor commutated, parallel type) ```text Ld Id +-----mmm-----+---------+ | | | | T1 T3 Vdc | | | A---+-C---B | | | | | +-[Load]--+ | T4 T2 | | | +-------------+---------+ ``` The commutating capacitor C is connected across the load. #### Operation 1. **T1, T2 ON:** $I_d$ flows A → B through the load (and charges C with A positive): $i_0 = +I_d$. 2. **T3, T4 fired at $T/2$:** C (charged A positive) is placed across T1 and T2, reverse biasing them, so they turn off. Current $I_d$ now flows B → A: $i_0 = -I_d$. C discharges and recharges with B positive, ready for the next commutation. 3. **T1, T2 fired again at $T$:** C reverse biases T3, T4, and the cycle repeats. The thyristors must withstand reverse voltage (no anti-parallel diodes are used). #### Waveforms (resistive load) ```text i0 +Id |________ | | 0 |--------+--------- t | |________ -Id T1,T2 T3,T4 v0 | ___ | / '--. 0 |/--------\--------- t | \ .--' | '-' 0 T/2 T ``` - Load current: square wave of amplitude $I_d$. - Load voltage: exponential (the current divides between R and C after each reversal; C charges towards $I_dR$ with time constant $RC$). - Fundamental output current: $I_{1,rms} = \dfrac{4I_d}{\sqrt2\,\pi} = 0.9I_d$. #### Features of CSI | Point | CSI | |---|---| | DC source | Constant current (large $L_d$) | | Output current | Square wave, load-independent | | Output voltage | Depends on load | | Feedback diodes | Not needed | | Short-circuit protection | Inherent (current limited) | | Devices | Must block reverse voltage | Uses: large induction motor drives, induction heating, synchronous motor starting. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What keeps the input current of a CSI nearly constant? A. A large capacitor across the source B. A large series inductor Ld C. Feedback diodes D. Dead time Answer: B. The large Ld in series with the dc source smooths the current so switches only steer it. (Key point: Constant current) 2. How are T1 and T2 turned off when T3, T4 are fired? A. The load current falls to zero B. Gate reverse pulse C. The capacitor C, charged A positive, is placed across them and reverse biases them D. Anti-parallel diodes divert current Answer: C. Firing T3, T4 connects the charged commutating capacitor across T1, T2 with reverse polarity. (Key point: Commutation) 3. Which statement about a CSI is correct? A. Feedback diodes are essential B. Output voltage is fixed by the source C. Inherent short-circuit protection since current is limited D. Devices need not block reverse voltage Answer: C. Current is limited by Ld; no feedback diodes are used and devices must withstand reverse voltage. 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