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Power Electronics (5XT2)

Power Electronics (5XT2). Prof. V. N. Bhonge. Dept. of Electronics & Telecomm. Shri Sant Gajanan Maharaj College of Engg,. Shegaon – 444203. vnbhonge@gmail.com. SSGMCE Shegaon. Unit III

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Power Electronics (5XT2)

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  1. Power Electronics (5XT2) Prof. V. N. Bhonge Dept. of Electronics & Telecomm. Shri Sant Gajanan Maharaj College of Engg, Shegaon – 444203 vnbhonge@gmail.com

  2. SSGMCE Shegaon Unit III Classification of ckt. for forced commutation, series inverter, improved series inverter, parallel inverter, output voltage and waveform control, principle of operation for three phase bridge inverter in 120 deg. and 180 deg. mode, single phase transistorised bridge inverter, current source inverter, harmonics reduction techniques. Dept. of E & T Prof. V. N. Bhonge

  3. SSGMCE Shegaon • What is Commutation? The process of turning off an SCR is called commutation. It is achieved by • Reducing anode current below holding current • Make anode negative with respect to cathode • Types of commutation are: • Natural or line commutation • Forced commutation Dept. of E & T Prof. V. N. Bhonge

  4. SCR Turnoff Methods SSGMCE Shegaon • Diverting the anode current to an alternate path • Shorting the SCR from anode to cathode • Applying a reverse voltage (by making the cathode positive with respect to the anode) across the SCR • Forcing the anode current to zero for a brief period • Opening the external path from its anode supply voltage • Momentarily reducing supply voltage to zero Dept. of E & T Prof. V. N. Bhonge

  5. (1) AC line commutation SSGMCE Shegaon Dept. of E & T Prof. V. N. Bhonge

  6. a) Natural Commutation: Becauseof the load current varies sinusoidally, the thyristor should be turned –off when the load current falls below the holding value: ILoad<IH . Furthermore, in the negative half cycle, the applied source voltage being negative with respect to anode-cathode terminals, causing reverse biasing of the device. Principle electrical circuit is shown below:

  7. b) Forced Commutation: In this case, because of no alternating character of the current “ DC “, therefore it must force decreases by applying the following approaches: - the load current must reduced below the holding value: ILoad<IH -by applying negative voltage across the thyristor, causing forced removing of internal charge, therefore the load current falls below the holding value IH . Several techniques realized these approaches: • 1) Class A : Resonant Commutation • 2) Class B : Self Commutation • 3) Class C : Complementary Commutation • 4) Class D : Auxiliary Commutation • 5) Class E : External Pulse commutation • 6) Class F : Line- commutation

  8. Commutation by Resonance SSGMCE Shegaon Series resonant turnoff circuit Dept. of E & T Prof. V. N. Bhonge

  9. Parallel resonant turnoff circuit SSGMCE Shegaon Dept. of E & T Prof. V. N. Bhonge

  10. SCR turnoff circuit using commutation capacitor • Value of capacitance is determined by: C>=tOFF 0.693RL SSGMCE Shegaon Dept. of E & T Prof. V. N. Bhonge

  11. Commutation By External Source SSGMCE Shegaon Dept. of E & T Prof. V. N. Bhonge

  12. Series-Resonant Inverter ECE 442 Power Electronics

  13. Operation T1 fired, resonant pulse of current flows through the load. The current falls to zero at t = t1m and T1 is “self – commutated”. T2 fired, reverse resonant current flows through the load and T2 is also “self-commutated”. The series resonant circuit must be underdamped, R2 < (4L/C)

  14. Operation in Mode 1 – Fire T1 ECE 442 Power Electronics

  15. To find the time when the current is maximum, set the first derivative = 0

  16. To find the capacitor voltage, integrate the current The current i1 becomes = 0 @ t=t1m

  17. Operation in Mode 2 – T1, T2 Both OFF

  18. t2m

  19. Operation in Mode 3 – Fire T2

  20. Summary -- Series Resonant Inverter

  21. To avoid a short-circuit across the main dc supply, T1 must be turned OFF before T2 is turned ON, resulting in a “dead zone”. This “off-time” must be longer than the turn-off time of the thyristors, tq. The maximum possible output frequency is

  22. Series Resonant Inverter Coupled Inductors

  23. Improvement in performance • When T1 turned ON, voltage @ L1 is as shown, voltage @ L2 in same direction, adding to the voltage @ C • This turns T2 OFF before the load current falls to 0.

  24. Half-Bridge Series Resonant Inverter Note: L1 = L2 C1 = C2

  25. This configuration reduces the high-pulsed current from the dc supply • Power drawn from the source during both half-cycles of the output. • Half of the current is supplied from the associated capacitor, half of the current is supplied from the source.

  26. SSGMCE Shegaon Thank You Dept. of E & T Prof. V. N. Bhonge

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