Unit-3-A. AC to AC Voltage Converters

Transcription

1 Unit-3-A AC to AC Voltage Converters

2 AC to AC Voltage Converters This lesson provides the reader the following: AC-AC power conversion topologies at fixed frequency Power converter options available for the conversion to formulate equations describing the current waveform for the PAC sketch the current waveform by observation of the circuit the performance of the converter of the topologies 1.1 Introduction AC to AC voltage converters operates on the AC mains essentially to regulate the output voltage. Portions of the supply sinusoid appear at the load while the semiconductor switches block the remaining portions. Several topologies have emerged along with voltage regulation methods, most of which are linked to the development of the semiconductor devices. Fig 26.1 Some single phase AC-AC voltage regulator topologies. (a) Back-to -back SCR; (b) One SCR in (a) replaced by a four-diode full wave diode bridge; (c) A bi-directionally conducting TRIAC; (d) The SCR in (b) replaced by a transistor. The regulators in Fig 26.1 (a), (b) and (c) perform quite similarly. They are called Phase Angle Controlled (PAC) AC-AC converters or AC-AC choppers. The TRIAC based converter may be considered as the basic topology. Being bi-directionally conducting devices, they act on both polarities of the applied voltage. However, dv dt re applied their ratings being poor, they tend to turn-on in the opposite direction just subsequent to their turn-off with an inductive load. The 'Alternistor' was developed with improved features but was not popular. The TRIAC is common only at the low power ranges. The (a) and (b) options are improvements on (c) mostly regarding current handling and turn-off-able current rating. A transistorised AC-AC regulator is a PWM regulator similar to the DC-DC converters. It also requires a freewheeling path across the inductive load, which has also got to be bi-

3 directional. Consequently, only controlled freewheeling devices can be used.

4 1.2 Operation with resistive loads Fig illustrates the operation of the PAC converter with a resistive load. The device(s) is triggered at a phase-angle 'α' in each cycle. The current follows the voltage wave shape in each half and extinguishes itself at the zero crossings of the supply voltage. In the two-scr topology, one SCR is positively biased in each half of the supply voltage. There is no scope for conduction overlap of the devices. A single pulse is sufficient to trigger the controlled devices with a resistive load. In the diode-scr topology, two diodes are forward biased in each half. The SCR always receives a DC voltage and does not distinguish the polarity of the supply. It is thus always forward biased. The bi-directional TRIAC is also forward biased for both polarities of the supply voltage. Fig Operation of a Phase Angle Controlled AC-AC converter with a resistive load The rms voltage V rms decides the power supplied to the load. It can be computed as V rms = π π 1 α 2V 2 sin 2 ωt dωt =V 1 α π + sin 2π 2 α

5 Fig The rms output voltage and the most important harmonics versus triggering angle. As is evident from the current waveforms, the PAC introduces significant harmonics both into the load and the supply. This is one of the main reasons why such controllers are today not acceptable. The ideal waveform as shown in Fig 26.2 is half wave symmetric. However it is to be achieved by the trigger circuits. The controller in Fig ensures this for the TRIAC based circuit. While the TRIAC has a differing characteristic for the two polarities of biasing with the 32V DIAC - a two terminal device- triggering is effected when the capacitor voltage reaches 32 V. This ensures elimination of DC and even components in the output voltage. Fig DIAC based trigger circuit for a TRIAC to ensure symmetrical triggering in the two halves of the supply. For the SCR based controllers, identical comparators for the two halves of the AC supply, which generates pulses for the two SCRs ensures DC and even harmonic free operation. The PAC operates with a resistive load for all values of α ranging from o The fundamental current, i f can be represented as 2V α sin 2α 1 cos 2α i f = ( π + )sin ωt ( )cosωt 26.1 Rπ In machine drives it is only the fundamental component, which is useful. However, in resistance heating type of application all harmonics are of no consequence. The corrupted supply current nevertheless is undesirable. Power Factor

6 '. The power factor of a nonlinear deserves a special discussion. Fig shows the supply voltage and the non-sinusoidal load current. The fundamental load/supply current lags the supply voltage by the φ1, 'Fundamental Power Factor' angle. Cosφ1 is also called the 'Displacement Factor'. However this does not account for the total reactive power drawn by the system. This power factor is inspite of the actual load being resistive! The reactive power is drawn also y the trigger-angle dependent harmonics. Now average power P power factor = = 26.2 apparent voltamperes VI L VI cosφ = L VI L I L1 distortion factor = I L The Average Power, P drawn by the resistive load is 2 P = 2 1 π 2π vi L dω t = π 1 α π 2V R sin 2 ωt dwt V 2 α sin 2α = π Rπ 2 2 The portion within square brackets in Eq is identical to the first part of the expression within brackets in Eq. 26.1, which is called the Fourier coefficient 'B 1 B The rms load voltage can also be similarly obtained by integrating between α and π and the result can be combined with Eq to give power factor = per unit rms load current = per unit load power = B p.u.. 1 Fig Variation of various performance parameters with triggering angle

7 1.3 Operation with inductive loads With inductive loads the operation of the PAC is illustrated in Fig The current builds up from zero in each cycle. It quenches not at the zero crossing of the applied voltage as with the resistive load but after that instant. The supply voltage thus continues to be impressed on the load till the load current returns to zero. A single-pulse trigger for the TRIAC 26.1 (c) or the antiparallel SCR (b) has no effect on the devices if it (or the anti-parallel device) is already in conduction in the reverse direction. The devices would fail to conduct when they are intended to, as they do not have the supply voltage forward biasing them when the trigger pulse arrives. A single pulse trigger will work till the trigger angle α >, where φ is the power factor angle of the inductive load. A train of pulses is required here. The output voltage is controllable only between triggering angles φ and 18 o. The load current waveform is further explained in Fig The current is composed of two components. The first is the steady state component of the load current, i ss and the second, i tr is the transient component. Fig Operation of a single phase PAC with an inductive load Fig 26.7 Load current for a single phase AC-AC converter with a R_L load. V s - supply voltage, i ss -steady state current component, i tr - transient current component and i load - load current (= i ss + i tr ).

8 ( t ) With an inductance in the load the distinguishing feature of the load current is that it must always start from zero. However, if the switch could have permanently kept the load connected to the supply the current would have become a sinusoidal one phase shifted from the voltage by the phase angle of the load, φ. This current restricted to the half periods of conduction is called the 'steady-state component' of load current i ss. The 'transient component' of load current i tr, again in each half cycle, must add up to zero with this i ss to start from zero. This condition sets the initial value of the transient component to that of the steady state at the instant that the SCR/TRIAC is triggered. Fig illustrates these relations. When a device is in conduction, the load current is governed by the equation i L di dt + Ri =v s load = Z 2V [ sin (ωt φ)+ sin (α φ)e L ω ] Since at t =, i load = and supply voltage v s = 2Vsinωt the solution is of the form The instant when the load current extinguishes is called the extinction angle β. It can be inferred that there would be no transients in the load current if the devices are triggered at the power factor angle of the load. The load current I that case is perfectly sinusoidal. 1.4 AC-AC Chopper R α Fig A complete Transitorised AC-AC chopper topology of the version shown in Fig and the corresponding load voltage and current waveforms for an inductive load. The output voltage is shown to be about 5% for a.5 Duty Ratio chopping. The AC-AC converter shown in Fig 26.1 has to be augmented with two additional controlled devices clamping the load as indicated in Fig A large capacitor across the supply terminals is also to be inserted. These devices which are mostly transistors of the same variety as used for the chopper are necessary to clamp the voltages generated by the switching-off of the current carrying inductors in the load while the input capacitor takes care of the line inductances. The harmonics in the line current and load voltage waveforms are significantly different from those with the PACs. Mostly

9 switching frequency harmonics are present in both the waveforms. 1.5 PAC as a static switch Both single phase and three phase PACs are often used as static switches for applications like switching on of highly inductive loads without transients or for regulating output AC voltages by switching in tapings of a transformer. Such sequence control PACs while controlling the output voltage also permit improvement of the power factor as seen by the source. Sequence control can be two or multiple phase depending upon the application. Typical load voltage and current waveforms are shown in Fig The outer TRIACs connected to thwe higher voltage leads of the input transformer are triffered at the desired angle α, to realize the required load voltage. Obviously this voltage is greater than that available at the low voltage terminal of the transformer. This device continues conduction into the next half of the supply voltage till the load current falls to zero. The inner TR 2 starts conduction subsequently, requiring a wide pulse or a train of pulses. TR 1 can be however triggered by a single pulse. Fig Load voltage and current control with a two-stage sequence control 2. Three-phase AC Regulators Objectives We will be able to know The circuits used for the three-phase ac regulators (ac to ac voltage converters) The operation of the above circuits with three-phase balanced resistive (R) load, along with the waveforms

10 The important points of comparison of the performance with different types of circuits 2.1 Introduction In the last lesson first one in the first half of this module, various circuits of the single-phase ac regulators, also termed as ac to ac voltage converters, are described. In the basic circuit, one Triac, or two thyristors, connected back to back, are used. The operation of the above circuits with different types of loads resistive (R) and inductive (R-L), along with the waveforms, is then discussed. Lastly, the output voltage waveform is analysed. In this lesson the second one in the first half, firstly, the circuits of the three-phase ac regulators, also termed as ac to ac voltage converters, are described. The operation of the above circuits with three-phase balanced resistive (R) load, along with the waveforms, is then discussed. The two basic circuits are three-phase three-wire type with load connected in star and three-phase delta-connected one. Lastly, the important points of comparison of the performance with different types of circuits, including the above two, are presented. Keywords: Three-phase ac regulator circuits, AC to AC voltage converter, balanced three-phase star- and delta-connected loads 2.2 Three-phase AC Regulators There are many types of circuits used for the three-phase ac regulators (ac to ac voltage converters), unlike single-phase ones. The three-phase loads (balanced) are connected in star or delta. Two thyristors connected back to back, or a triac, is used for each phase in most of the circuits as described. Two circuits are first taken up, both with balanced resistive (R) load 2.3 Three-phase, Three-wire AC Regulator with Balanced Resistive Load The circuit of a three-phase, three-wire ac regulator (termed as ac to ac voltage converter) with balanced resistive (star-connected) load is shown in Fig It may be noted that the resistance connected in all three phases are equal. Two thyristors connected back to back are used per phase, thus needing a total of six thyristors. Please note the numbering scheme, which is same as that used in a three- phase full-wave bridge converter or inverter, described in module 2 or 5. The thyristors are fired in sequence (Fig. 27.2), starting from 1 in ascending order, with the angle between the triggering of thyristors 1 & 2 being 6 (one-sixth of the time period (T ) of a complete cycle). The line frequency is 5 Hz, with T =1/ f =2 ms. The thyristors are fired or triggered after a delay of α from the natural commutation point. The natural commutation point is the starting of a cycle with period, ( 6 = T / 6 ) of output voltage waveform, if six thyristors are replaced by diodes. Note that the output voltage is similar to phase-controlled waveform for a converter,

11 with the difference that it is an ac waveform in this case. The current flow is bidirectional, with the current in one direction in the positive half, and then, in other (opposite) direction in the negative half. So, two thyristors connected back to back are needed in each phase. The turning off of a thyristor occurs, if its current falls to zero. To turn the thyristor on, the anode voltage must be higher that the cathode voltage, and also, a triggering signal must be applied at its gate. A I L T 1 i a a + T EAN E L Ean - R - - B i b b - + T3 R T 6 + n - EBN - Ebn ECN R + Ecn T 5 i c + c C T 2 Fig Three-phase, three-wire ac regulator The procedure for obtaining the expression of the rms value of the output voltage per phase for balanced star-connected resistive load, which depends on range of firing angle, as shown later, is described. If E s is the rms value of the input voltage per phase, and assuming the voltage, E AN as the reference, the instantaneous input voltages per phase are, e AN = 2 E s sinωt, e BN = 2 E s sin (ω t 12 ) and e CN = 2 E s sin (ω t +12 ) Then, the instantaneous input line voltages are, e AB = 6 E s sin (ω t + 3 ), e BC = 6 E s sin (ω t 9 ) and e CA = 6 E s sin (ω t +15 )

12 E EAB EBC ECA EAB EBC EAB EBC ECA EAB E π/6 π 2 ωt π ωt E EAN EBN ECN EAN E EAN EBN ECN π 2π 3 ωt ωt 2π I g 1 I g 3 I g 5 I g 2 I g 4 I g Ean E EAB EBC ECA EAB EBC α.5 E AB.5 E AC ωt ωt ωt ωt ωt ωt ωt I g 1 ωt I g 3 I g 5 I g 2 I g 4 I g 6 Ean ωt ωt ωt ωt ωt ωt α 1.5 E AB.5 E AC.5 E AB (a) For α = 6

13 (b) For α = 12 Fig Waveforms for three-phase three-wire ac regulator The waveforms of the input voltages, the conduction angles of thyristors and the output voltage of one phase, for firing delay angles (α ) of (a) 6 and (b) 12 are shown in Fig For α 6 (π / 6), immediately before triggering of thyristor 1, two thyristors (5 & 6) conduct. Once thyristor 1 is triggered, three thyristors (1, 5 & 6) conduct. As stated earlier, a thyristor turns off, when the current through it goes to zero. The conditions alternate between two and three conducting thyristors. At any time only two thyristors conduct for 6 α 9.Although two thyristors conduct at any time for 9 α 15, there are periods, when no thyristors are on. For α 15, there is no period for which two thyristors are on, and the output voltage becomes zero at α =15 (5π / 6). The range of delay angle is α 15. The expressions of the rms value of the output voltage per phase for balanced starconnected resistive load are as follows. Please note that θ =ωt. For α 6 : 1 2π E = 2π (e AN ) dθ π / 3 π / 2+α 2π / 3 π / 2+α π 2 sin θ sin θ sin θ sin θ sin θ = 6 E s dθ + dθ + dθ + dθ + dθ 2π α 3 π / 2 4 π / 3+α 3 π / 2 4 2π / 3+α 3 1 π α sin 2α = 6 E s + π For 6 α 9 : π / 6 π / 3+α 2 5π / 6 π / 3+α sin θ sin θ 6 E = E s dθ + dθ 2π π / 2 π / 3+α 4 π / 2 π / 3+α π 3sin 2α 3 cos 2α 2 1 π 3 sin (2α = 6 E + = 6 E + s s π π 12 8 For 9 α 15 : E = 6 π 2 π sin θ sin θ E dθ + dθ s 2π 4 4 π / 2 π / 3+α π / 2 π / 3+α π α sin 2α 3 cos 2α 2 1 5π α sin (2α + 6 ) 2 = 6 E s + + = 6 E s + π π

14 Three-phase Delta-connected AC Regulator with Balanced Resistive Load The circuit of a three-phase, delta-connected ac regulator (termed as ac to ac voltage converter) with balanced resistive load is shown in Fig It may be noted that the resistance connected in all three phases are equal. Two thyristors connected back to back are used per phase, thus needing a total of six thyristors. As stated earlier, the numbering scheme may be noted. It may be observed that one phase of the balanced circuit is similar to that used for single-phase ac regulator described in the previous lesson (26) of the module. Since the phase current in a balanced three-phase system is only (1/ 3 ) of the line current, the current rating of the thyristors would be lower than that if the thyristors are placed in the line. A I L iab i a a R T 2 T 5 EAB E L B C T 4 ECA T 1 R - - ica - - R c + i b b ibc T 3 EBC T 6 i c - + Fig Delta connected three-phase ac regulator Assuming the line voltage E AB as the reference, the instantaneous input line voltages are, e AB = 2 E s sinωt, e BC = 2 E s sin (ω t 12 ) and e CA = 2 E s sin (ω t +12 ) It may be noted that E s is the rms value of the line voltage in this case. The waveforms of the input line voltages, phase and line currents, and the thyristor gating signals, for α =12 are shown in Fig

15 E EAB EBC ECA EAB EBC E m ωt π 2 3π I g 1 I g 2 I g 3 I g 4 I g 5 I g 6 iab ibc ica ωt ωt ωt ωt ωt ωt ωt ωt ωt i a ωt i b ωt i c π ωt 3π 2π For α = 12 Fig Waveforms for three-phase delta-connected ac regulator The rms value of the output phase voltage is obtained as 2π 1 π

16 E = (eab ) dθ = 2(E s ) sin θ dθ =E s π α + sin 2α 2π 2π α α π 2 When α =, the maximum value of the output voltage is obtained, and the control range of delay angle is α 18 (π). The line currents, which can be determined from the phase current, are, i a = i ab i ca, i b = i bc i ab and i c = i ca i bc. From Fig. 27.4, it can be observed that the line currents depend on the delay angle, and may be discontinuous. The rms value of line and phase currents in this case can be determined by numerical solution or Fourier analysis. If I n is the rms value of the n th harmonic component of a phase current, the rms value of the phase current is obtained from I = [I 2 + I 2 + I 2 + I 2 + I 2 + I I 2 ] 1 2 ab n For delta connection, the triplen harmonic components (i.e., those of order, n = 3m, where m is an odd integer) of the phase currents flow around the delta, and would not appear in the line. This is due to the fact that these harmonic currents are like the zero sequence component, being in phase in all three phases of the load. So, the rms value of the line current is, I = 3 [I 2 + I 2 + I 2 + I I 2 ] 1 2 a n As a result, the rms value of the line current would not follow the normal relationship of a three-phase system such that I a < 3 I ab. 2.5 Comparison of the Different Circuits used for Three-phase AC Regulators Besides the two circuits shown in figures 27.1 & 27.3, other circuits used for three-phase ac regulators (ac to ac voltage converters), are shown in Fig. 27.5a-c. As given in point #6, the balanced load for the circuits (Fig & 27.3) is taken as inductive (R-L) one, not resistive as shown. The important points of comparison between the circuits are stated. A T 1 T 4 B T 3 T 6 C T 5 T 2 Fig. 27.5(a) 3-wire delta load Z L, R L Z L, R L Z L, R L N A Z L, R L

17 T 1 T 4 B T 3 T 6 C T 5 T 2 Fig. 27.5(b) 4-wire star load Z L, R L Z L, R L A ZL, RL T 1 T 4 B ZL, RL T 3 T 6 C ZL, RL T 5 T 2 Fig. 27.5(c) Control in delta 1. In two circuits (Fig & 27.5b), the individual phase controllers control their own loads independently of the other. As stated earlier, they can, therefore be studied as three single-phase controllers. 2. In other circuits, the individual phase controllers affect the other phase loads also, and they have to be studied as complete three-phase circuits, as stated earlier in one case (Fig. 27.1). 3. The peak voltages occur across thyristors at or near the fully off state. In case of two circuits (Fig & 27.5c), the maximum thyristor voltage is the peak of the line voltage, whereas in the circuit (Fig. 27.5b), it is the peak of the phase voltage; in two circuits (Fig & 27.5a), the maximum thyristor voltage will be somewhere between the peak of the phase and line voltages depending on the leakage currents of the thyristors, the method of firing and the presence of voltage-sharing resistors across the thyristors. 4. All the five circuits can be used under phase control. 5. The range of phase angle required to achieve full output range from zero to maximum varies between the circuits, and are given in Table The maximum current in the thyristors is decided from the fully on condition, and the size of the thyristors to be used should be chosen from this condition. The peak, mean and rms values of the thyristor currents (Table 27.1) are related to the rms value of the input (ac) current, which should be

18 found by applying the full supply voltage to the load circuit. The load impedance per phase is equal in magnitude (Z) and angle (ϕ ), which is taken as positive, as it is mostly inductive (R-L). 7. The difference between the two circuits (Fig & 27.5b) is that in the second one, the neutral point is available, making it a 4-wire one. 8. The difference between the two circuits (Fig & 27.5a) is that in the second one, the three-phase balanced loads are connected in delta, which can be converted into its equivalent star, making it identical to the first circuit (Fig. 27.1). Also, the current in the thyristors in the second case (Fig. 27.5a) are the line currents, which are higher than the phase currents, which are flowing in the thyristors in the first one. 9. The difference between the two circuits (Fig & 27.5c) is that, in the second one, the thyristors connected back to back, are in delta, with the load connected in the three lines. Also, the current in the thyristors in the second case (Fig. 27.c) are the phase currents, which are lower than the line currents, which are flowing in the thyristors in the first one. Table 27.1 summarises the current and voltage rating parameters associated with all these circuits used as three-phase ac controllers. E ac and I ac are the rms values of the line voltage and line current respectively. It may be mentioned that other types of circuits for three-phase ac regulator can be used, but either the circuits are not bidirectional, i.e. unidirectional, or if they are bidirectional, in one half, only diode connected back to back per phase, instead of thyristor, is used. In the second case, only in one half with the thyristor per phase, controlled output voltage as shown earlier is obtained, but in the other one, uncontrolled output voltage, same as input one, is obtained. In the first case, where only one thyristor per phase is used, in one half, controlled output voltage is obtained, but in the second half, output voltage is zero, as only one device, but not two devices, is used. The readers are requested to refer to text books. In this lesson the second one in the first half, firstly the study of two basic circuits one with star connection and the other with delta connection, for three-phase ac regulator (ac to ac voltage converter) are taken up. The operation with three-phase balanced resistive load, along with waveforms, is then described. Lastly, the important points of comparison of the performance with different types of circuits, including the above two, are presented. In the next, i.e. third and final lesson in the first half, the control circuit for ac regulators will be described in detail. Rating of the Parameters used inthree-phase AC Regulators Circuits Delay Maximum Maximum Thyristor Thyristor (Fig. No.) angle, α input line load power current for full current dissipation V RWm control ( I Peak Mean RMS ac - rms) V I I I (degrees) ac ac ac ac E ac 3 I ac R Z E ac 2 I ac R a 15 Z 2 3 E ac I ac R Z E b 18 ac 3 I ac R c 15 3 Z E 2 ac 3 I ac R Z

19 3. Phase Angle Control in Triac-based Singlephase AC Regulators Objectives We will able to know The circuit used for the phase angle control in triac-based single-phase ac regulators (ac to ac voltage converters) The operation of the various blocks used in the circuit, along with the waveforms The harmonic analysis of the output voltage of a single-phase ac regulator with resistive load 3.1 Introduction In the last lesson second one in the first half of this module, various circuits of the three-phase ac regulators, also termed as ac to ac voltage converters, are described. Two basic circuits star-connected and delta-connected, are first taken up. The operation of the two circuits with three-phase balanced resistive (R) load, along with the waveforms, is then discussed. Lastly, the important points of comparison of the performance with different types of circuits, including the above two, are presented. In this case, the load is balanced inductive (R-L) one. In this lesson the third and final one in the first half, firstly, the circuit used for the phase angle control in triac-based single-phase ac regulator, also termed as ac to ac voltage converter, is presented. Then, the operation of the various blocks used in the above circuit, along with the waveforms, is described. Finally, the harmonic analysis of the output voltage of a single-phase ac regulator with resistive load is, briefly discussed. Keywords: Phase angle controller circuit, Triac-based single-phase ac regulator, or ac to ac voltage converter, harmonic analysis of the output voltage waveform. Phase Angle Controller Circuit for Triac-based Single-phase AC Regulator The phase angle controller circuit for a triac -based single-phase ac regulator (ac to ac voltage converter), is shown in Fig The power circuit (also shown in Fig. 26.1c (lesson #26)) consists of a Triac in series with inductive (R-L) load, fed from a single phase supply, with rated voltage of, say 22 V(rms), having rated frequency ( f =5 Hz). Before going into the operation of the phase angle controller circuit, some important points of the bidirectional controlled device (TRIAC), used in the ac circuit, having already been introduced in lesson #4 (module 1), is briefly presented, as it is not frequently used. Similarly, for the same reasons, DIAC (may have been introduced earlier), being used here as an uncontrolled bidirectional device, is also briefly described. TRIAC A Triac is equivalent to two thyristors connected back to back as shown in Fig. 26.1a. Thus, it is a bidirectional switching device, in contrast to the thyristor, which is a unidirectional device, having reverse blocking characteristic, preventing the flow of current from Cathode to Anode. So, when it (triac) is in conduction mode, current flows in both directions (forward and reverse). This switching device is called as TRIAC (TRIode AC switch), with the circuit symbol shown in Fig The three terminals of the triac are designated as MT 1, MT 2 and gate, G, shown in the same

20 figure. These are similar to the terminals A (Anode), K (Cathode) and G (Gate), of the thyristor The terminal, MT 1 is taken as the reference point for the measurement of the voltages and currents at other two terminals, G (gate) and MT 2. The gate (G) is near to the terminal, MT 1. The thyristor conducts with the current direction from Anode to Cathode (positive), when a positive pulse is fed at the Gate terminal with respect to Cathode, and at that time, with positive voltage applied between Anode and Cathode terminals, being connected in series with the load. The triac conducts in the positive direction from MT 2 to MT 1, when a positive pulse is applied at the gate (G) terminal with respect to MT 1 and at the same time, the positive voltage is applied between two terminals, MT 2 (+) and MT 1 ( ). Similarly, the triac conducts in negative direction from MT 1 to MT 2, when a negative pulse is applied at the gate (G) terminal with respect to MT 1 and at the same time, the positive voltage is applied between two terminals, MT 1 (+) and MT 2 ( ). Please note that the voltage between two terminals, MT 2 and MT 1, is negative, in this case. So, the triac can conduct in both directions (positive and negative) as given here, whereas the thyristor conducts in one (positive) direction only. Only one triac is needed, whereas it is to be replaced by two thyristors, with consequent change in the control circuit. The V- I characteristics of both thyristor and triac, have been discussed in lesson #4 (module 1). A thyristor turns off (non-conducting mode), if the current through it, falls below holding current. Similarly, a triac turns off (non-conducting mode), if the magnitude of the current, irrespective of its direction, falls below holding current. As a triac is connected in an ac circuit, and if the load in the circuit is resistive, the triac turns off at the zero crossing points of the voltage in each half (the supply (input) voltage reaches zero at the end of each half cycle). This will be nearly valid, if the load inductance is small, though the triac in that case turns off, as the current though it goes to zero, after the zero crossing point is reached in each half. The case of higher inductance in the load has been discussed in detail in lesson #26 (module 3). The triac is a low power device, used in voltage control circuits, used as light dimmers, speed control for fan motors (single-phase), etc. Some of the advantages and disadvantages of the triac vis-a-vis thyristor are given. Advantages b Triacs are triggered by positive or negative polarity voltages applied at the gate terminal. c A triac needs a single heat sink of slightly larger size, whereas anti-parallel thyristor pair needs two heat sinks of slightly smaller sizes, but due to the clearance total space required is more for thyristors. Disadvantages 1. Triacs have low dv / dt rating as compared to thyristors. 2. Triacs are available in lower rating as compared to thyristors. 3. Since a triac can be triggered in either direction, a trigger circuit for triac needs careful consideration. 4. The reliability of triacs is lower than that of thyristors. DIAC A Diac is equivalent to two diodes connected back to back. Also, it is a bidirectional device, in contrast to the diode, which is a unidirectional device, having reverse blocking characteristic, preventing the flow of current from Cathode to Anode. So, when it (diac) is in conduction mode, current flows in both directions (forward and reverse). This switching device is called as DIAC (DIode AC switch), with the circuit symbol shown in Fig The two terminals of the diac are designated as T 1 and T 2, shown in the same figure.

21 These are similar to the terminals, A (Anode) and K (Cathode), of the diode. The diac conducts, when the break-over voltage is reached in either polarity across its two terminals. When T 1 is positive with respect to T 2, and if at that time if the voltage, V 12 exceeds V BO1 (break-over voltage), the diac conducts in positive direction from T 1 to T 2. Similarly, when T 2 is positive with respect to T 1, and if at that time if the voltage, V 21 exceeds V BO2 (breakover voltage), the diac conducts in negative direction from T 2 to T 1. So, a diac can conduct in both directions (positive and negative), whereas a diode conducts only in positive direction from Anode (A) to Cathode (K), if, at that time, the voltage, V AK exceeds V BO (break-over voltage). A diode does not conduct in the negative direction, if the voltage, V AK is negative. A diode turns off (non-conducting mode), if the current through it, falls below holding current. Similarly, a diac turns off (non-conducting mode), if the magnitude of the current, irrespective of its direction, falls below holding current. If the V-I characteristic of diode is known, as given in lesson #2 (module 1), the V-I characteristic of diac, on the lines of the triac can be developed. The students are requested to study the characteristic of diac from a text book, as it is not included here for obvious reason. Now, the operation of the phase angle controller circuit (Fig. 28.1) is presented, with the waveforms at various points shown in Fig The power circuit, the main component of which is the triac, has been described earlier. The diac is symmetrical, unlike the triac, as described earlier. So, the diac (Fig. 28.1) can be connected in opposite direction, with T 1 in place of T 2, and vice versa, i.e., T 2, T 1 in place of T 1. But the operation here is described with the connection as in the figure. The triac is not symmetrical, though it conducts in both directions like diac. Two reasons are: the presence of third terminal, Gate (G), and the gate signal to be fed between G & MT 1 (reference) for triggering. The snubber part ( R s & C s ), shown in the figure, is used for the protection of the triac the power switching device. The remaining part, including the diac used for triggering of the triac, is the controller for the triac. R S C S Snubber A MT 2 TRIAC G MT 1 D + l 1-phase DIAC o supply ac Rpot. R 1 a - T 1 T 2 d v c + C - Control Circuit

22 B Fig. 28.1: Phase angle controller circuit for a single-phase ac regulator using TRIAC V m v i π/2 π 3π/2 2π 5π/2 3π θ(ωt) -V m (a) v c π+α1 π+α 2 2π α 1 α 2 π 2π+α1 2π+α 2 3π DIAC breaks θ(ver

23 down (b) V m v L π+α 1 5π/2 2π α 1 π/2 π 3π/2 2π+α 1 3π θ -V m (c) V m v L π+α 2 α 2 π 2π 2π+α 2 3π θ -V m (d) Fig. 28.2: Waveforms at various points of the controller circuit (a) Input (source) voltage, v AB (b) Voltage across capacitor, c(vc) (c) Output (load) voltage, v DB with R put = R 2 (lower) (d) Output (load) voltage, v DB with R put = R 3 (higher) As soon the input (supply) voltage is given to the circuit, the capacitor, C starts getting charged through the potentiometer resistance, R pot = R 2, the value of which is low and the load resistance. The polarity of the input voltage is important. The start of the input voltage is taken as the positive zero-crossing point (Fig. 28.2a), when the voltage changes from negative to positive. The point, A is now positive with respect to B (Fig. 28.1). The polarity of the voltage across the capacitor, C is that the left hand side is positive, with the right hand side as negative. The capacitor voltage ( v C ) is shown in Fig. 28.2b. As soon as the capacitor voltage, v C reaches the break-over voltage (V BO ) of the diac (about 3 V), the diac starts to conduct in the positive direction from T 1 to T 2. At this point, the triac gets a positive pulse at its gate (G is now positive with respect to MT 1 ) and also MT 2 is at a higher potential than MT 1. So, the triac is turned on at the angle, θ =α 1 = ω t 1 = 2π f t 1. The current through the triac is in the positive direction from MT 2 to MT 1. Please note that the time constant of the charging circuit is related to the potentiometer resistance ( R 2 ), which is low. So, the time needed for the capacitor voltage to

24 reach the break-over voltage (V BO ) is t 1 α 1. The triac is turned off at θ =π, when the input voltage reaches the negative zero-crossing point. So, the conduction period (angle in rad) is from α 1 to π in the positive half. The output (load) voltage ( v L = v DB ) waveform (Fig. 28.2c) is nearly same as the input voltage ( v i = v AB ), neglecting the voltage drop across the triac. The capacitor voltage (Fig. 28.2b) starts decreasing at t = t 1, and reaches zero after some time, the time being small. The discharge path is through diac, the resistance R 1, and the gate, G & MT 1 terminals of the triac, the total resistance is quite low. So, the time constant during discharge is quite low, as compared to that during charging. The resistance, R 1 is used to decrease the capacitor current during discharge. The pattern is repeated in the negative half of the input voltage, which is briefly described. The capacitor, C starts charging in the opposite direction through the same path as given earlier. The charging starts from the negative zero -crossing of the input voltage (Fig. 28.2a). The polarity of the input voltage is now opposite, with the point, B being positive with respect to A. The polarity of the capacitor voltage (Fig. 28.2b) is also opposite, with the right hand side as positive, and the left hand side as negative. The charging time constant remains same (low), as it was earlier. The capacitor voltage, v C (in magnitude) reaches the break-over voltage (V BO ) of the diac after time t 1 α 1,measured from the negative zero-crossing of the input voltage (θ =π ). The diac now starts to conduct in the negative direction from T 2 to T 1. At this point, the triac gets a negative pulse at its gate (G is now negative with respect to MT 1 ) and also MT 1 is at a higher potential than MT 2. So, the triac is turned on at the angle, (θ = π +α 1 ). The current through the triac is in the negative direction from MT 1 to MT 2. The triac is turned off at the next positive zero-crossing point (θ = 2π ). The conduction period (Fig. 28.2c) is from (π +α 1 ) to ( 2π ) in the negative half, the total conduction time (π α 1 ) being same in both half. The output voltage waveform is identical, but it is opposite in this (negative) half. As in the earlier case, the capacitor voltage (Fig. 29.2b) starts decreasing, and reaches zero after some time, the discharge path remaining same. Thus, the diac helps in the turning on of the triac in both directions, making the control circuit simple with few components only (Fig. 28.1). Though the function of the diac could have been performed by using two diodes connected back to back, the control circuit would have to be modified. To change the conduction period, or the start of conduction of the triac, the potentiometer resistance is to be increased from R 2 to R 3, which is higher. The capacitor voltage waveform for this case is shown in Fig. 28.2b as dotted line, as the time constant of the charging circuit also increases. So, the time needed for the capacitor voltage (in magnitude, as both halves are considered) to reach the break-over voltage (V BO ) of the diac is now ( t 2 α 2 ). The conduction period in the positive half (Fig. 28.2d) is from α 2 >α 1 to π, the total time in both half is (π α 2 ). The conduction period decreases. The rms value of the output voltage also decreases. Other conditions, say during discharge of the capacitor voltage remaining same, is not described. The range of phase angle delay, in the ideal case, is <α <π. But normally, the lower limit is higher than, while the upper limit is lower than π (18 ). The input voltage (Fig. 28.2a) is zero at the two limits ( & 18 ) in the ideal case. As the input voltage has to exceed at least the voltage drop in the triac, and the capacitor voltage (Fig. 28.2b) also has to reach the break-over voltage of the diac as given earlier, the normal range of phase angle delay is to be used, not the ideal ones. Also, if the load is inductive, the current in the triac has to exceed a threshold value, before the gate pulse can be withdrawn. Otherwise, the triac may not be triggered, returning to off state again. This point may have been described in the case of phase-controlled single-phase (bridge) converters (ac-dc), with inductive load in series with battery or back emf, in lessons

25 #1-11 (module 2). Objectives Cyclo-converters We will be able to know The cyclo-converter circuits basic principle of operation The circuit for the single-phase to single-phase cyclo-converter using thyristors The operation of the above cyclo-converter circuit, along with the voltage waveforms Introduction Earlier in the last three ( ) lessons (first half) of this module, the circuit and operation of ac to ac voltage controllers both single-phase and three-phase, were described in detail. The devices used are either triac, or thyristors connected back to back. In this lesson (4.4) first one in the second half of this module, the cyclo-converter is introduced as a type of power controller, where an alternating voltage at supply frequency is converted directly to an alternating voltage at load frequency (normally lower), without any intermediate dc stage. As will be shown in the last (fifth) module, an alternating voltage at any frequency (output) is obtained using an inverter as a power controller from a dc voltage fed at its input. This input, i.e. dc voltage, is again obtained using a rectifier (converter) with ac voltage (normally at supply frequency) fed at its input. This type has been described in module 2. Note that this is a two -stage process with an intermediate dc stage. Now-a-days, the power switching devices used in the inverter circuit belong to transistor family (termed as self-commutated ones), starting with power transistors, whereas thyristors are still being used in the converter (rectifier) circuits. These devices are called force-commutated ones, when used in dc chopper circuits (described in module 3), but in this case, i.e. converter circuits, line commutation takes place. As stated earlier, the output frequency of the cyclo-converter is limited to about one-third of supply (line) frequency of 5 Hz. Initially, the basic principle of operation used in a cyclo-converter is discussed. Then, the circuit of a single-phase to single-phase cyclo-converter using thyristors is presented. This is followed by describing the operation of the above cyclo-converter circuit, along with voltage waveforms. The readers at this stage, have gone through the following lessons single-phase fully controlled converter using thyristors, for obtaining dc output voltage from ac supply (#2.2), and ac to ac voltage controllers both single-phase and three-phase, using either triac, or thyristors connected back to back (# ). In the above cases, the output voltage obtained is, in the form of phase-controlled one, as can be observed from the waveforms shown in the above lessons. In the present case, the output voltage of the cyclo-converter circuit (singlephase) using thyristors, is synthesized from the above phase-controlled voltage waveforms, so as to obtain an ac waveform (output) of low frequency, with the input being an ac voltage of higher frequency, say line. The angle, at which the thyristors are triggered, is controlled to obtain the desired waveform. Keywords: Single-phase to single-phase cyclo-converter using thyristors, Voltage waveforms. Cyclo-converter Basic Principle of Operation The basic principle of operation of a cyclo-converter is explained with reference to an equivalent circuit shown in Fig Each two-quadrant converter (phase-controlled) is

26 represented as an alternating voltage source, which corresponds to the fundamental voltage component obtained at its output terminals. The diodes connected in series with each voltage source, show the unidirectional conduction of each converter, whose output voltage can be either positive or negative, being a two-quadrant one, but the direction of current is in the direction as shown in the circuit, as only thyristors unidirectional switching devices, are used in the two converters. Normally, the ripple content in the output voltage is neglected. i P i N + i O e P = E m sin ω o t e o ac load - e N = E m sin ω o t Positive (P) converter Negative (N) converter Control Circuit e r = E r sin ω o t Fig. 29.1: Equivalent circuit of cyclovonverter The control principle used in an ideal cyclo-converter is to continuously modulate the firing angles of the individual converters, so that each produces the same sinusoidal (ac) voltage at its output terminals. Thus, the voltages of the two generators (Fig. 29.1) have the same amplitude, frequency and phase, and the voltage of the cyclo-converter is equal to the voltage of either of these generators. It is possible for the mean power to flow either to or from the output terminals, and the cyclo-converter is inherently capable of operation with loads of any phase angle inductive or capacitive. Because of the uni-directional current carrying property of the individual converters, it is inherent that the positive half-cycle of load current must always be carried by the positive converter, and the negative half-cycle by the negative converter, regardless of the phase of the current with respect to the voltage. This means that each twoquadrant converter operates both in its rectifying (converting) and in its inverting region during the period of its associated half-cycle of current. The output voltage and current waveforms, illustrating the operation of an ideal cycloconverter circuit with loads of various displacement angles, are shown in Fig The displacement angle of the load (current) is (Fig. 29.2a). In this case, each converter carries the load current only, when it operates in its rectifying region, and it remains idle throughout the whole period in which its terminal voltage is in the inverting region of operation. In Fig. 29.2b, the displacement angle of the load is 6 lagging. During the first 12 period of each half-cycle of load current, the associated converter operates in its rectifying region, and delivers power to the load. During the latter 6 period in the half-cycle, the associated converter

27 operates in its inverting region, and under this condition, the load is regenerating power back into the cyclo-converter output terminals, and hence, into the ac system at the input side. These two are illustrative cases only. Any other case, say capacitive load, with the displacement angle as leading, the operation changes with inverting region in the first period of the half-cycle as per displacement angle, and the latter period operating in rectifying region. This is not shown in Fig. 29.2, which can be studied from a standard text book.

28 Single-phase to Single-phase Cyclo-converter The circuit of a single-phase to single-phase cyclo-converter is shown in Fig Two full-wave fully controlled bridge converter circuits, using four thyristors for each bridge, are connected in opposite direction (back to back), with both bridges being fed from ac supply (5 Hz). Bridge 1 (P positive) supplies load current in the positive half of the output cycle, while bridge 2 (N negative) supplies load current in the negative half. The two bridges should not conduct together as this will produce short -circuit at the input. In this case, two thyristors come in series with each voltage source. When the load current is positive, the firing pulses to the thyristors of bridge 2 are inhibited, while the thyristors of bridge 1 are triggered by giving pulses at their gates at that time. Similarly, when the load current is negative, the thyristors of bridge 2 are triggered by giving pulses at their gates, while the firing pulses to the thyristors of bridge 1 are inhibited at that time. This is the circulating-current free mode of operation. Thus, the firing angle control scheme must be such that only one converter conduct at a time, and the change over of firing pulses from one converter to the other, should be periodic according to the output frequency. However, the firing angles the thyristors in both converters should be the same to produce a symmetrical output. i P i N + i O P 1 P 2 l N 1 N 2 o 1-phase e O a 1-phase ac ac d supply supply - P 4 P 3 N 4 N 3 Bridge 1 (Positive) Bridge 2 (Negative) Fig. 29.3: Single-phase to single-phase cycloconverter (using thyristor bridges) When a cyclo-converter operates in the non-circulating current mode, the control scheme is complicated, if the load current is discontinuous. The control is somewhat simplified, if some amount of circulating current is allowed to flow between them. In this case, a circulating current limiting reactor is connected between the positive and negative converters, as is the case with dual converter, i.e. two fully controlled bridge converters connected back to back, in circulating-current mode. The readers are requested to refer to any standard text book. This circulating current by itself keeps both converters in virtually continuous conduction over the whole control range. This type of operation is termed as the circulating-current mode of operation. The operation of the cyclo-converter circuit with both purely resistive (R), and inductive (R-L) loads is explained. Resistive (R) Load: For this load, the load current (instantaneous) goes to zero, as the input voltage at the end of each half cycle (both positive and negative) reaches zero ().

29 Thus, theconducting thyristor pair in one of the bridges turns off at that time, i.e. the thyristors undergo natural commutation. So, operation with discontinuous current (Fig. 29.4) takes place, as current flows in the load, only when the next thyristor pair in that bridge is triggered, or pulses are fed at respective gates. Taking first bridge 1 (positive), and assuming the top point of the ac supply as positive with the bottom point as negative in the positive half cycle of ac input, the odd-numbered thyristor pair, P 1 & P 3 is triggered after phase delay (α 1 ), such that current starts flowing through the load in this half cycle. In the next (negative) half cycle, the other thyristor pair (even- numbered), P 2 & P 4 in that bridge conducts, by triggering them after suitable phase delay from the start of zero-crossing. The current flows through the load in the same direction, with the output voltage also remaining positive. This process continues for one more half cycle (making a total of three) of input voltage ( f 1 =5 Hz). From three waveforms, one combined positive half cycle of output voltage is produced across the load resistance, with its frequency being one-third of input frequency ( f 2 = f 1 / 3 = Hz). The following points may be noted. The firing angle (α) of the converter is first decreased, in this case for second cycle only, and then again increased in the next (third) cycle, as shown in Fig. 29.4b. This is, because only three cycles for each half cycle is used. If the output frequency needed is lower, the number of cycles is to be increased, with the firing angle decreasing for some cycles, and then again increasing in the subsequent cycles, as described earlier. e s 5π π 2π 3π 4π 6π e Mean output voltage (a) α 2 α 3 4π 5π 6π α 1 π 2π 3π α 5 (b) α 4 α 6 Fig. 29.4: Input (a) and output (b) voltage waveforms of a cycloconverter with an output frequency of Hz for resistive (R) load To obtain negative output voltage, in the next three half cycles of input voltage, bridge 2 is used. Following same logic, if the bottom point of the ac supply is taken as positive with the top point as negative in the negative half of ac input, the odd-numbered thyristor pair, N 1 & N 3 conducts, by triggering them after suitable phase delay from the zero -crossing. Similarly, the even-numbered thyristor pair, N 2 & N 4 conducts in the next half cycle. Both the output voltage and current are now negative. As in the previous case, the above process also continues for three consecutive half cycles of input voltage. From three waveforms, one combined negative half cycle of output voltage is produced, having same frequency as given earlier. The pattern of firing angle first decreasing and the increasing, is also followed in the negative half cycle. One positive half cycle, along with one negative half cycle, constitute one complete cycle of output

30 (load) voltage waveform, its frequency being Hz as stated earlier. The ripple frequency of the output voltage/ current for single phase full-wave converter is 1 Hz, i.e., double of the input frequency. It may be noted that the load (output) current is discontinuous (Fig. 29.4c), as also load (output) voltage (Fig. 29.4b). The supply (input) voltage is shown in Fig. 29.4a. Only one of two thyristor bridges (positive or negative) conducts at a time, giving non-circulating current mode of operation in this circuit. Inductive (R-L) Load: For this load, the load current may be continuous or discontinuous depending on the firing angle and load power factor. The load voltage and current waveforms are shown for continuous and discontinuous load current in Fig and 29.6 respectively. e s π 2π 3π 4π 5π 6π 7 8π e Mean o/p output voltage 5π 6π 7π π 2π 3π 4π 8π (a) Mean o/p output voltage (b) i α1 α 2 α 3 Fig. 29.5: Input (a) and output (b) voltage, and current (c) waveforms for a cyclo-converter with discontinuous (c)

31 e s π 2π 3π 4π 5π 6π 7 8π e Mean o/p output voltage π 2π 3π 4 π (a) 5π 6π 7 8π α 1 α 2 α 3 α 4 α 5 α 6 α 7 (b) α 8 i (c) i (d) Fig. 29.6: Input (a) and output (b) voltage, and current (c, d) waveforms for a cyclo-converter with continuous load current.

32 (a) Discontinuous load current The load current in this case is discontinuous, as the inductance, L in series with the resistance, R, is low. This is somewhat similar to the previous case, but difference also exists as described. Here, also non-circulating mode of operation takes place, with only one of the bridges #1 (positive), or #2 (negative), conducting at a time, but two bridges do not conduct at the same time, as this will result in a short circuit. In this case, the output frequency is assumed as ( f 2 =12.5 Hz), the input frequency being same as ( f 1 =5 Hz), i.e., f 1 = 4 f 2, or f 2 = f 1 / 4. So, four positive half cycles, or two full cycles of the input to the full-wave bridge converter (#1), are required to produce one positive half cycle of the output waveform, as the output frequency is one-fourth of the input frequency as given earlier. As in the previous case with resistive load, taking bridge 1, and assuming the top point of the ac supply as positive, in the positive half cycle of ac input, the odd-numbered thyristor pair, P 1 & P 3, is triggered after phase delay (θ = ωt =α 1 ), such that current starts flowing the inductive load in this half cycle. But here, the current flows even after the input voltage has reversed (after θ =π ), till it reaches zero at (θ = β 1 ) with (π +α 2 ) > β 1 > π, due to inductance being present in series with resistance, its value being low. It may be noted that the thyristor pair is, thus, naturally commutated. In the next (negative) half cycle, the other thyristor pair (even-numbered), P 2 & P 4, is triggered at (π +α 2 ). The current flows through the load in the same direction, with the output voltage also remaining positive. The current goes to zero at (π +β 2 ), with (π +α 3 ) > β 2 >π. This procedure continues for the next two half cycles, making a total of four positive half cycles. From these four waveforms, one combined positive half cycle of output voltage is produced across the inductive load. The firing angle (α) of the converter is first decreased, in this case for second half cycle only, kept nearly same in the third one, and finally increased in the last (fourth) one, as shown in Fig. 29.5b. To obtain negative output voltage, in the next four half cycles of output voltage, bridge 2 is used. Following same logic, if the bottom point of the ac supply is taken as positive in the negative half of ac input, the odd-numbered thyristor pair, N 1 & N 3 conducts, by triggering them after phase delay (θ = 4 π +α 1 ). The current flows now in the opposite (negative) direction through the inductive load, with the output voltage being also negative. The current goes to zero at ( 4 π + β 1 ), due to load being inductive as given earlier. Similarly, the even-numbered thyristor pair, N 2 & N 4 conducts in the next half cycle, after they are triggered at ( 5 π +α 2 ). The current goes to zero at ( 5 π + β 2 ). Both the output voltage and current are now negative. As in the previous case, the above process also continues for two more half cycles of input voltage, making a total of four. From these four waveforms, one combined negative half cycle of output voltage is produced with same output frequency. The pattern of firing angle first decreasing and then increasing, is also followed in the negative half cycle. It may be noted that the load (output) current is discontinuous (Fig. 29.5c), as also load (output) voltage (Fig. 29.5b). The supply (input) voltage is shown in Fig. 29.5a. One positive half cycle, along with one negative half cycle, constitute one complete cycle of output (load) voltage waveform, its frequency being 12.5 Hz as stated earlier. The ripple frequency remains also same at 1 Hz, with the ripple in load current being filtered by the inductance present in the load. (b) Continuous load current As given above, the load current is discontinuous, as the inductance of the load is low. If the inductance is increased, the current will be continuous. Most of the points given earlier are

33 applicable to this case, as described. To repeat, non-circulating mode of operation is used, i.e., only one of the bridges #1 (positive), or #2 (negative), conducts at a time, but two bridges do not conduct at the same time, as this will result in a short circuit. Also, the ripple frequency in the voltage and current waveforms remains same at 1 Hz. The output frequency is one-fourth of input frequency (5 Hz), i.e., 12.5 Hz. So, for each half-cycle of output voltage waveform, four half cycles of input supply are required. Taking bridge 1, and assuming the top point of the ac supply as positive, in the positive half cycle of ac input, the odd-numbered thyristor pair, P 1 & P 3, is triggered after phase delay (θ = ωt =α 1 ), such that current starts flowing the inductive load in this half cycle. But here, the current flows for about one complete half cycle, i.e., up to the angle, (π +α 1 ) or (π +α 2 ), whichever is higher, even after the input voltage has reversed, due to the high value of load inductance. In the next (negative) half cycle, the other thyristor pair (evennumbered), P 2 & P 4, is triggered at (π +α 2 ). At that time, reverse voltage is applied across each of the conducting thyristors, P 1 /P 3, and the thyristors turn off. The current flows through the load in the same direction, with the output voltage also remaining positive. Also, the current flows for about one complete half cycle, i.e., up to the angle, (π +α 2 ) or (π +α 3 ), whichever is higher. This procedure continues for the next two half cycles, making a total of four positive half cycles. From these four waveforms, one combined positive half cycle of output voltage is produced across the inductive load. The firing angle (α) of the converter is first decreased, in this case for second half cycle only, kept nearly same in the third one, and finally increased in the last (fourth) one, as shown in Fig. 29.6b. To obtain negative output voltage, in the next four half cycles of output voltage, bridge 2 is used. Following same logic, if the bottom point of the ac supply is taken as positive in the negative half of ac input, the odd-numbered thyristor pair, N 1 & N 3 conducts, by triggering them after phase delay (θ = 4 π +α 1 ). The current flows now in the opposite (negative) direction through the inductive load, with the output voltage being also negative. The current flows for about one complete half cycle, i.e., up to the angle, ( 5.π +α 1 ) or ( 5 π +α 2 ), whichever is higher, as the load is inductive. Similarly, the even-numbered thyristor pair, N 2 & N 4 conducts in the next half cycle, after they are triggered at ( 5 π +α 2 ). As described earlier, both the conducting thyristors turn off, as reverse voltage is applied across each of them. Both the output voltage and current are now negative. Also, the current flows for about one complete half cycle, i.e. up to the angle, ( 5 π +α 2 ) or ( 5 π +α 3 ), whichever is higher. As in the previous case, the above process also continues for two more half cycles of input voltage, making a total of four. From these four waveforms, one combined negative half cycle of output voltage is produced with same output frequency of 12.5 Hz. The pattern of firing angle first decreasing and then increasing, is also followed in the negative half cycle. It may be observed that the load (output) current is continuous (Fig. 29.6c), as also load (output) voltage (Fig. 29.6b). The load (output) current is redrawn in Fig. 29.6d, under steady state condition, while the supply (input) voltage is shown in Fig. 29.6a. One positive half cycle, along with one negative half cycle, constitute one complete cycle of output (load) voltage waveform. Advantages and Disadvantages of Cyclo-converter Advantages d In a cyclo-converter, ac power at one frequency is converted directly to a lower frequency in a single conversion stage. e Cyclo-converter functions by means of phase commutation, without auxiliary forced commutation circuits. The power circuit is more compact, eliminating circuit losses

34 associated with forced commutation. f Cyclo-converter is inherently capable of power transfer in either direction between source and load. It can supply power to loads at any power factor, and is also capable of regeneration over the complete speed range, down to standstill. This feature makes it preferable for large reversing drives requiring rapid acceleration and deceleration, thus suited for metal rolling application. g Commutation failure causes a short circuit of ac supply. But, if an individual fuse blows off, a complete shutdown is not necessary, and cyclo-converter continues to function with somewhat distorted waveforms. A balanced load is presented to the ac supply with unbalanced output conditions. 5. Cyclo-converter delivers a high quality sinusoidal waveform at low output fre-quencies, since it is fabricated from a large number of segments of the supply waveform. This is often preferable for very low speed applications. 6. Cyclo-converter is extremely attractive for large power, low speed drives. Disadvantages Large number of thyristors is required in a cyclo-converter, and its control circuitry becomes more complex. It is not justified to use it for small installations, but is economical for units above 2 kva. For reasonable power output and efficiency, the output frequency is limited to one-third of the input frequency. The power factor is low particularly at reduced output voltages, as phase control is used with high firing delay angle. 3-phase ac supply Converter + Inverter - 3-phase Vdc output Fig. 29.7: DC link converter The cyclo-converter is normally compared with dc link converter (Fig. 29.7), where two power controllers, first one for converting from ac input at line frequency to dc output, and the second one as inverter to obtain ac output at any frequency from the above dc input fed to it. The thyristors, or switching devices of transistor family, which are termed as self-commutated ones, usually the former, which in this case is naturally commutated, are used in controlled converters (rectifiers). The diodes, whose cost is low, are used in uncontrolled ones. But now-a -days, switching devices of transistor family are used in inverters, though thyristors using force commutation are also used. A diode, connected back to back with the switching device, may be a power transistor (BJT), is needed for each device. The number of switching devices in dc link converter depends upon the number of phases used at both input and output. The number of devices, such as thyristors, used in cyclo-converters depends on the types of connection, and also the number of phases at both input and output. It may be noted that all features of a cycloconverter may not be available in a dc link converter. Similarly, certain features, like Pulse Width Modulation (PWM) techniques as used in inverters and also converters, to reduce the harmonics in voltage waveforms, are not applied in cyclo-converters. The various circuits used load

35 and their operational aspects are discussed in detail in the next (last) module (#5) on DC to AC Converters termed as Inverters. Advantages and Disadvantages of DC Link Converter Advantages 1. The output frequency can be varied from zero to rated value, with the upper frequency limit, being decided by the turn-off time of the switching devices, which is quite low due to the use of transistors in recent time. 2..The control circuit here is simpler, as compared to that used in cyclo-converter. 3. It has high input power factor, if diode rectifier is used in the first stage. If phase-controlled thyristor converter is used, power factor depends upon phase angle delay. 4. It is suitable for higher frequencies, as given earlier. Disadvantages 1. The conversion is in two stages, using two power controllers one as converter and other as inverter, as stated earlier. 2. Forced commutation is required for the inverter, if thyristors are used, even though phase control is used in converter, where natural commutation takes place. 3. The feature of regeneration is somewhat difficult, and also is involved to incorporate in a dc link converter. 4. The output waveform of the inverter is normally a stepped one, which may cause nonuniform rotation of an ac motor at very low frequencies (< 1 Hz). The distorted waveform also causes system instability at low frequencies. This can be reduced by using PWM technique as given earlier. In this lesson, the first one in the second half of this module (#4), the cyclo-converter is first introduced, along with the basic principle of operation. The circuit and the operation of singlephase to single-phase cyclo-converter, with both resistive and inductive loads, are described in detail, with voltage and current waveforms. The current is discontinuous, with resistive and inductive (with low value of inductance) loads, but can be continuous, if the inductance is higher. In the next lesson, the circuit and operation of three-phase to single-phase cyclo-converter, followed by three-phase to three-phase one, will be described in detail. Three-phase to Single-phase Cyclo-converters Objectives We will be able to know The three-phase to single-phase cyclo-converter circuit, using two three-phase fullwave thyristorised bridge converters The operation of the above cyclo-converter circuit, along with the voltage waveforms Introduction

36 In the last lesson first one in the second half of this module, firstly, the basic principle of operation of the cyclo-converter circuits has been presented. This followed by the discussion of the circuit, and the operation of the single-phase to single-phase cyclo-converter circuit with both resistive and inductive loads, in detail. Two full-wave bridge converters (rectifiers) connected back to back, with four thyristors as power switching device in each bridge, are used. Also described are the advantages and disadvantages of the cyclo-converter. The dc link converter is introduced briefly, along with its advantages and disadvantages. In this lesson the second one in the second half, firstly, the three-phase to single-phase cyclo-converter circuit, using two three-phase full-wave thyristorised bridge converters, is presented. Then, the operation of the above cyclo-converter circuit, with both resistive and inductive loads, is described in detail, along with voltage waveforms. The mode of operation used is the non-circulating current one. The following are discussed in brief.the circulating current mode of operation for the above, and also the cyclo-converter circuit, using two threephase half-wave converters. Keywords: Three-phase to single-phase cyclo-converter, Voltage waveforms, Non-circulating current, and Circulating current modes of operation, Three-phase full-wave bridge, and halfwave converters. Three-phase to Single-phase Cyclo-converter The circuit of a three-phase to single-phase cyclo-converter is shown in Fig Two threephase full-wave (six-pulse) bridge converters (rectifier) connected back to back, with six thyristors for each bridge, are used. The ripple frequency here is 3 Hz, six times the input frequency of 5 Hz. So, low value of load inductance is needed to make the current continuous, as compared to one using single-phase bridge converters described in the previous lesson (#4.4) with ripple frequency of 1 Hz. Also, the non-circulating current mode of operation is used, where only one converter bridge 1 (positive) or bridge 2 (negative), conducts at a time, but both converters do not conduct at the same time. It may be noted that each thyristor conducts for about 12 (π / 3), i.e., one-third of one complete cycle, whereas a particular thyristor pair, say 1& 2 conduct for about 6 (π / 6), i.e., one-sixth of a cycle. The thyristors conduct in pairs as stated, one (odd-numbered) thyristor in the top half and the other (even-numbered) one in the bottom half in two different legs. Two thyristors in one leg are not allowed to conduct at a time, which will result in short circuit at the output terminals. The sequence of conduction of the thyristors is 1 & 6, 1 & 2, 3 & 2, and so on. When thyristor 1 is triggered, the conducting thyristor (#5) in top half, being reverse biased at that time, turns off. Similarly, when thyristor 2 is triggered, the conducting thyristor (#6) in bottom half, being reverse biased at that time turns off. This sequence is repeated in cyclic order. So, natural or line commutation takes place in this case. Otherwise, the procedure is similar to the one as discussed in the previous lesson. i P i N P 1 P 3 P 5 + N 1 N 3 N 5 l A 3-phase A 3-p o ac B a B ac i O supply C d C su

37 e Fabricated output voltage α = 9 α = Mean output voltage α = 9 R - P 4 P 6 P 2 N 4 N 6 N 2 Fig. 3.1: Three-phase to single-phase cycloconverter The procedure to be followed in the triggering of the thyristors in sequence in the two bridge converters has been briefly given earlier. The readers are requested to go through two lessons (# ) in module 2 (AC-DC Converters), or any standard text book. As given in the earlier lesson (#4.4), the firing angle (α) of two converters is first decreased starting from the initial value of 9 to the final value of, and then again increased to the final value of 9, as shown in Fig Also, for positive hcycle of the output voltage waveform, bridge 1 is used, while bridge 2 is used for negative half cycle. The two half cycles are combined to form one complete cycle of the output voltage, the frequency being decided by the number of half cycles of input voltage waveform used for each half cycle of the output. As more no. of segments of near 6 (π / 6) is used, the output voltage waveform becomes near sinusoidal, with its frequency also being reduced. The initial value of firing angle delay is kept at α 1 9, such the average value (dc) of the output voltage in this interval of near 6 (π / 6) [V av cos α 1 = cos 9 =. ], is zero. It may be noted that the next thyristor in sequence is triggered at α 2 < 9, as the firing angle is decreased for each segment, to obtain higher voltage V av cos α 2 = +ve, to form the sine wave at the output. This can be observed from the points, M, N, O, P, Q, R & S, shown in Fig From these segments, the first quarter cycle of the output voltage waveform from to 9, is obtained. The second quarter cycle of the above waveform from 9 to18, is obtained, using the segments starting from the points, T, U, V, W, X &Y (fig. 3.2). It may be noted that the firing angle delay at the point, Y is α = 9, and also the firing angle is increased from (T) to 9 (Y) in this interval. When the firing angle delay is, the average value of the segment is V av cos α = cos =1.. The two quarter cycles form the positive half cycle of the output voltage waveform. In this region, the bridge 1 (positive) is used. To obtain the negative half cycle of the output voltage waveform (18 36 ), the other bridge converter (#2) termed negative (N) is used in the same manner as given earlier, i.e. its firing angle delay (α) is first decreased starting from the initial value of 9 to the final value of, and then again increased to the final value of 9, as given earlier. The two half cycles (positive and negative) together give one complete cycle ( 36 ) of the output voltage waveform.

38 F i g. T S U Q P V W 3. 2 M N X θ = ωt Y O u tput voltage waveforms for a three-phase to single phase cycloconverter. The load on the output of the cyclo-converter is assumed to be inductive (R-L). The load can also be capacitive. For inductive load, the output current (Fig. 3.3) lags behind the voltage by its phase angle, φ (assumed to be positive). The load power factor is also +ve ( cos φ ). It may be noted that the current is unidirectional in a thyristor converter. As the current, being alternating in nature, flows in both directions in a complete cycle, two converters are connected in antiparallel. The positive (P) converter carries current during positive half cycle of output current, while the other, i.e. negative (N) one carries current in the negative half cycle. As discussed in the previous lesson (#29), P-converter acts as a rectifier, when the output voltage is positive, and as an inverter, when the output voltage is negative (Fig. 3.3). Similarly, N-converter acts as a rectifier, when the output voltage is negative, and as an inverter, when the output voltage is positive. It can thus be inferred, in general, that one of two converters would operate as rectifier, if its output voltage and current have the same polarity, and as an inverter, if these are of opposite polarity.

39 Fabricated output voltage Mean output voltage α = 9 α = α = 9 e θ = ωt (a) Inversion Inversion Inversion i Rectification Rectification Current in positive group Current in negative group Angle of load impedance (φ) (b) Fig. 3.3 Voltage (a) and current (b) waveforms for a three phase full-wave (six-pulse) cycloconverter. Circulating Current Mode of Operation In all the cases described earlier (Lesson 29 and current one (#3)), both for single-phase to single-phase and for single-phase to three-phase cyclo-converters, circulating current-free or non-circulating current mode of operation was described, wherein only one of two bridge converters conducts at a time, but not both, in which case the converters would be short-circuited. The positive (P) converter conducts, when the current is in the positive half of the cycle, whereas the negative one conducts with the current flowing in the negative half. But, in this case, i.e. circulating current mode of operation of the cyclo-converter, both the converters would conduct at a time, with an inter-group reactor (IGR) between the positive and negative groups as shown in Fig It may be noted that, though the output voltages of two converters in the same phase have the same average value, but their output voltage waveforms as a function of time are, however, different, and as a result, there is a net potential difference (voltage) across two converters. Due to this voltage, the reactor is inserted to limit the circulating current. As described, the main converter positive/negative, as the case may be, acts in the rectifier mode, and the other one acts in the inverter mode, with the average value along with the sign, of the output voltage being same. Thus, the sum of their firing delay angle must be 18 (π). In other words, if α p and α n are the firing angles for positive and negative group of converters, respectively, then these firing angles must be controlled so as to satisfy the condition (α p +α n ) =18 (π).