--- title: "Natural vs forced commutation" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:natural-vs-forced-commutation" source: https://www.bctengineer.com/q/semester_7/power_electronics/natural-vs-forced-commutation site: BCT Engineering --- # Natural vs forced commutation Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2080 Baishakh, 8 marks - Video: [#43 Difference Between Natural Commutation And Force Commutation | SCR Commutation Techniques.](https://www.youtube.com/watch?v=cmjvBJFLuyw) — Quick Learn - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/natural-vs-forced-commutation - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Define Commutation Techniques. Differentiate between Natural and Forced Commutation with suitable circuit examples. ## Short answer Commutation turns off a thyristor by reducing current below holding and applying reverse voltage. Natural commutation occurs in AC circuits as supply reversal forces current zero. Forced commutation uses external LC and auxiliary thyristor to force zero current in DC circuits, employed in choppers and inverters. ## Answer **Commutation** is the process of turning OFF a conducting thyristor. The anode current is reduced below the holding current and a reverse voltage is kept across the device for longer than its turn-off time $t_q$, so that it regains its forward blocking ability. #### Natural (line) commutation ```text vs ~ ---[SCR]---+ | R (load) | ----------------+ ``` In an a.c. circuit, the supply voltage passes through zero and reverses every half cycle. The load current falls to zero and the SCR is reverse biased by the supply itself, so it turns OFF without any extra components. Examples: phase-controlled rectifiers, AC voltage controllers, cycloconverters. #### Forced commutation ```text +---- C ------[TA]---+ | (- +) | +Vdc --+--------[T1]--------+--- load ---+ | GND -------------------------------------+ ``` When the auxiliary SCR $T_A$ is fired, the pre-charged capacitor $C$ is placed across $T_1$ with reverse polarity, forcing its current to zero and reverse biasing it. In d.c. circuits the current never reaches zero naturally, so an external commutation circuit (usually a charged capacitor and an inductor, switched by an auxiliary thyristor $T_A$) forces the current to zero or applies reverse voltage. Classes: - **Class A** (self/load commutation by resonant LC load) - **Class B** (resonant pulse, LC across SCR) - **Class C** (complementary, two SCRs with a capacitor) - **Class D** (auxiliary SCR, impulse commutation) - **Class E** (external pulse source) Examples: DC choppers and inverters. #### Comparison | Point | Natural commutation | Forced commutation | |---|---|---| | Supply | A.C. | D.C. (or a.c. where turn-off before zero is needed) | | How current goes to zero | Supply reversal | External L-C circuit | | Extra components | None | Capacitor, inductor, auxiliary SCR | | Cost and losses | Low | Higher | | Switching frequency | Fixed by supply (50 Hz) | Can be high, set by control | | Uses | Rectifiers, AC voltage controllers | Choppers, inverters | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What must be achieved for a thyristor to be successfully commutated (turned OFF)? A. Anode current must be raised above latching current B. Anode current reduced below holding current and reverse voltage kept for longer than tq C. Gate current reversed for any short time D. Forward voltage increased above VBO Answer: B. The current must fall below IH and a reverse voltage must be maintained longer than the turn-off time tq so the SCR regains forward blocking. (Key point: Definition of commutation) 2. Why does natural (line) commutation need no extra components? A. The gate circuit supplies a negative pulse every cycle B. The a.c. supply voltage reverses every half cycle, taking the current to zero and reverse biasing the SCR C. A capacitor is always charged across the SCR D. The load inductance forces current to zero Answer: B. The supply itself passes through zero and reverses, so the SCR is reverse biased automatically. (Key point: Why natural commutation works) 3. Why is forced commutation needed in d.c. circuits? A. The d.c. supply voltage is too high B. The current never falls to zero naturally, so an external L-C circuit must force it off C. The SCR cannot be triggered with d.c. D. Gate current is not available in d.c. Answer: B. A d.c. source does not reverse, so external components (charged capacitor, inductor, auxiliary SCR) must force the current to zero. (Key point: Need for forced commutation) 4. In the circuit with auxiliary SCR TA and pre-charged capacitor C, what happens when TA is fired? A. C is placed across T1 with reverse polarity, forcing T1 current to zero B. C is short circuited and the load is disconnected C. T1 gets extra gate current D. The supply is reversed across the load Answer: A. Firing TA connects the charged capacitor across T1 with reverse polarity, which turns T1 OFF. (Key point: Impulse commutation action) ## Related in Power Electronics - [Thyristor turn-on and forced commutation](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-turn-on-forced-commutation) — Power Electronics, chapter 1 - [Self and line commutation; minimum gate pulse width](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Aself-line-commutation-gate-pulse) — Power Electronics, chapter 1 ## Related in other subjects - [Natural and forced convection compared](https://www.bctengineer.com/question.md?key=semester_5%2Fheat_transfer%3Afree-versus-forced-convection) — Heat Transfer, chapter 3 - [Forced response depends on input, natural does not](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_circuit_theory%3Aforced-vs-natural-response-example) — Electrical Circuit Theory, chapter 3 - [Damped vs forced LCR oscillation](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Aforced-lcr-differential-compare) — Engineering Physics, chapter 1 - [Natural vs artificial convection; heating curve](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Anatural-artificial-convection) — Electric Machine Design, chapter 2