POWER ELECTRONICS PO POST GRAD POS UATE 2010 AC Ch AC o Ch p o per Prepare Prep d are by: d Dr. Gamal Gam SOwilam SOwila 11 December 2016 ١

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1 POWER ELECTRONICS POST GRADUATE 2010 AC Chopper Prepared by: Dr. Gamal SOwilam 11 December 2016 ١

2 1. Introduction AC Chopper is An AC to AC Converter employs to vary the rms voltage across the load at constant frequency. The AC Chopper voltage control by one of two methods: Phase control AC CHOPPER under natural commutation using pairs of silicon- controlled rectifiers (SCRs) or Triacs On-off control under forced commutation using fully controlled self- commutated switches such as Gate Turn-off Thyristors (GTOs), power transistors, Insulated Gate Bipolar Transistors (IGBTs), MOS-controlled Thyristors (MCTs) 11 December 2016 ٢

3 2. Single - phase AC-AC voltage controller Composed of a pair of SCRs connected back-to-back to operate as a) full wave controller b) half wave controller (a) full-wave, two SCRs in inverse parallel (b) full-wave with Triac 11 December 2016 ٣

4 (c) full wave with two SCRs and two diodes (d) full wave with four diodes and one SCR (e) half wave with one SCR and one diode in ant parallel 11 December 2016 ٤

5 2.1 Phase-Controlled Single-Phase AC Voltage Controller In this method the AC Chopper switches are employed to connect the load circuit to the source for a chosen portion of each cycle of the source voltage. 11 December 2016 ٥

6 2.1.1 AC Chopper connected to R load Waveforms for single-phase ac full-wave voltage controller with R-load 11 December 2016 ٦

7 The source voltage AC CHOPPER Then the rms output voltage The rms value of load current The input power factor The rms current in each SCR 11 December 2016 ٧

8 2.1.2 AC Chopper connected to R-L load Typical wave forms of single-phase ac voltage controller with an RL-load 11 December 2016 ٨

9 Due to the inductance, the current carried by the SCR T1 may not fall to zero at ωt = π when the input voltage goes negative and may continue until ωt= β, the extinction angle, as shown. The conduction angle is: The conduction angle of the SCR depends on the firing delay angle (α) and the load impedance angle ( ) For α > Φ.. produces discontinuous AC output (AC Chopper) For α = Φ.. produces continuous output (V orms = V srms For α < Φ.. represent single phase half wave rectifier and the output is a discontinuous DC. srms ) 11 December 2016 ٩

10 AC CHOPPER The expression for the load current I o (ωt) in a AC discontinuous mode: where Load impedance = Load impedance angle The r m s output voltage The r m s SCR current The average value of SCR current The r m s load current 11 December 2016 ١٠

11 2.1.3 Sinusoidal back E.M.F with L in series the circuit and wave form of the L-V b load connected to AC Chopper 11 December 2016 ١١

12 The input voltage The sinusoidal back voltage The phasor rms value of the current i o is I M and it lags by 90 0 o M α is measured from zero point of supply voltage V, α' is measured from point of V=V b, so that α' = α + Ø' 11 December 2016 ١٢

13 2.1.4 Parallel operation of phase angle controlled load the circuit and the wave form of the parallel operation of AC Chopper 11 December 2016 ١٣

14 Firing angle for T1 Firing angle for T3 For < ωt < π For < ωt < π For < ωt < 2I cb sin t for b t 11 December 2016 ١٤

15 2.2 Single-Phase AC-AC Voltage Controller with ON-OFF OFF Control Integral Cycle Control : the switch is turned on for a time tn with n integral cycles and turned off for a time tm with m integral cycles, (by forced commutation). The source voltage Then the r m s output voltage Where : K is the duty cycle = n/(n+m) Vs is rms phase voltage. The power factor is typical load-voltage waveforms for integral cycle control 11 December 2016 ١٥

16 3. Three phase AC/AC voltage controller - AC Chopper Circuit Configuration There are several circuit configuration used for the three phase AC Chopper; the three phase loads (balanced) are connected in star or in delta connection, two thyristors back to back or Triac is used for each phase. Herein under the study of one configuration for resistive load in star an one in delta will be shown. 11 December 2016 ١٦

17 3.1 AC CHOPPER Three phase with balanced resistive load (star connection) 11 December 2016 ١٧

18 Notes: -Resistance connected in all three phases is equal. -Two thyristors connected back to back are used per phase, thus needing, a total of six thyristors. 11 December 2016 ١٨

19 The wave forms AC CHOPPER 11 December 2016 ١٩

20 3.2 Three phase with balanced resistive load (Delta connection) 11 December 2016 ٢٠

21 The wave forms for AC CHOPPER 11 December 2016 ٢١

22 4. Some applications of AC Chopper For rms voltage control : AC CHOPPER in speed control of fan, pump or hoist drives soft starting of induction motors For rms current control : in lighting control, domestic and industrial heating Or as static ac switches (on=off control) : in transformer tap changing temperature control 11 December 2016 ٢٢

6. Explain control characteristics of GTO, MCT, SITH with the help of waveforms and circuit diagrams.

6. Explain control characteristics of GTO, MCT, SITH with the help of waveforms and circuit diagrams. POWER ELECTRONICS QUESTION BANK Unit 1: Introduction 1. Explain the control characteristics of SCR and GTO with circuit diagrams, and waveforms of control signal and output voltage. 2. Explain the different