cyclo converter 1

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  GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND TECHNOLOGY (Approved by A.I.C.T.E and Affiliated to JNTU) (Bachupally,Kukatpally ,Hyderabad 500 072.) SINGLE PHASE CYCLOCONVETER By B. ANUSHA (07241A0260) P.SARAJA (07241A02A1) SAMA SWETHA (07241A02B2) 1 

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Power electronics converter

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  • GOKARAJURANGARAJU

    INSTITUTEOFENGINEERINGANDTECHNOLOGY(ApprovedbyA.I.C.T.EandAffiliatedtoJNTU)(Bachupally,Kukatpally,Hyderabad500072.)

    SINGLE PHASE CYCLOCONVETER

    By B. ANUSHA (07241A0260)

    P.SARAJA (07241A02A1)

    SAMA SWETHA (07241A02B2)

    1

  • 1. INTRODUCTION TO CYCLOCONVERTERS

    CYCLOCONVERTERS:

    In industrial applications, two forms of electrical energy are used: direct current (dc) and alternating current (ac). Usually constant voltage constant frequency single-phase or three-phase ac is readily available. However, for different applications, different forms, magnitudes and/or frequencies are required. There are four different conversions between dc and ac power sources. These conversions are done by circuits called power converters. The converters are classified as: 1. rectifiers: from single-phase or three-phase ac to variable voltage dc 2. choppers: from dc to variable voltage dc 3.inverters: from dc to variable magnitude and variable frequency, single-phase or threephase ac 4.cycloconverters: from single-phase or three-phase ac to variable magnitude and variable frequency, single-phase or three-phase ac

    Traditionally, ac-ac conversion using semiconductor switches is done in two different ways: 1- in two stages (ac-dc and then dc-ac) as in dc link converters or 2- in one stage (ac-ac) cycloconverters (Fig. 1). Cycloconverters are used in high power applications driving induction and synchronous motors. They are usually phase-controlled and they traditionally use thyristors due to their ease of phase commutation.

    Fig.1 Block diagram of a cycloconverter

    Power conversion systems such as those used for large propulsion motor drives will often utilize cycloconverter technology in order to take advantage of the power handling capability of modern thyristors. Because cycloconverters produce non-integer harmonics of the input power frequency, both input current and output voltage require detailed study in order to insure trouble-free operation of both the supplying power system and the connected motor load. The cycloconverter is a device which converts input AC power at one frequency to output AC power at a different frequency with a one stage conversion. The frequency conversion is achieved using a phase control method. Phase controlled cycloconverters have the ability to operate in all 4 quadrants in the V-I plane. Thus, cycloconverters are capable of

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  • providing a variable frequency power supply to AC machines. Due to its 4-quadrant operation, the cycloconverter can handle loads of any power factor. The cycloconverter also allows power to flow freely in either direction. Over most of its range, the cycloconverter produces a reasonable sine wave output that leads to good output performance, particularly at lower frequencies. Traditionally, satisfactory performance has been understood to be available when the output frequency is up to approximately 40 percent of the input frequency (e.g. 24 hertz output from a 60 hertz supply). Above this output frequency, the waveforms become more distorted due to interaction between the mains and output frequencies. The cycloconverter performance deteriorates progressively as output frequency increases.

    A cycloconverter or a cycloinverter converts an AC waveform, such as the mains supply, to another AC waveform of a lower frequency, synthesizing the output waveform from segments of the AC supply without an intermediate direct-current link. They are most commonly used in three phase applications. In most power systems, the amplitude and the frequency of input voltage to a cycloconverter tend to be fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter tend to be variable. The output frequency of a three-phase cycloconverter must be less than about one-third to one-half the input frequency. The quality of the output waveform improves if more switching devices are used (a higher pulse number). Cycloverters are used in very large variable frequency drives, with ratings of several megawatts.

    A typical application of a cycloconverter is for use in controlling the speed of an AC traction motor and starting of synchronous motor. Most of these cycloconverters have a high power output in the order a few megawatts and silicon-controlled rectifiers (SCRs) are used in these circuits. By contrast, low cost, low-power cycloconverters for low-power AC motors are also in use, and many such circuits tend to use TRIACs in place of SCRs. Unlike an SCR which conducts in only one direction, a TRIAC is capable of conducting in either direction, but it is also a three terminal device. It may be noted that the use of a cycloconverter is not as common as that of an inverter and a cycloinverter is rarely used. However, it is common in very high power applications such as for ball mills in ore processing, cement kilns, and also for azimuth thrusters in large ships.

    The switching of the AC waveform creates noise, or harmonics, in the system that depend mostly on the frequency of the input waveform. These harmonics can damage sensitive electronic equipment. If the relative difference between the input and output waveforms is small, then the converter can produce subharmonics. Subharmonic noise occurs at a frequency below the output frequency, and cannot be filtered by load inductance. This limits the output frequency relative to the input. These limitations make cycloconverters often inferior to a DC link converter system for most applications.

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  • BASIC PRINCIPLE OF OPERATION OF CYCLOCONVERTERS:

    The basic principle of operation of a cycloconverter can be explained with reference to an equivalent circuit shown above. Each two-quadrant converter is now represented as an alternating voltage source, which corresponds to the fundamental or wanted voltage component generated at its output terminals. The basic diodes connected in series with each voltage source shows the unidirectional conduction of each two-quadrant converter. If the ripple in the output voltage of each converter is neglected, then it becomes ideal and represents the desired output voltage. The basic control principle of an ideal cycloconverter is to continuously modulate the firing angles of the individual converters, so that each produces the same sinusoidal a.c. voltage at its output terminals. Thus, the voltages of the two generators have the same amplitude, frequency and phase and, the voltage at the output terminals of the cycloconverter 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 cycloconverter is inherently capable of operation with loads of any phase angle, within a complete spectrum of 360. Because of the unidirectional 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, each two-quadrant converter operates both in its rectifying and in its inverting region during the period of its associated half cycle of the current.

    CLASSIFICATION OF CYCLOCONVERTERS:

    Generally, cycloconverters are classified according to the no of phase of input and output AC voltages. The tree diagram of the classification of cycloconveters is given below:

    CYCLOCONVERTER

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  • 1- to 1- 3- to 1- 3- to 3-

    Mid point Bridge 1- , 3 pulse 1- , 6 pulse 3-,3 Pulse 3- , 6 pulse

    Output Output Output Output

    Square output Sinusoidal output

    Voltage voltage

    SINGLE PHASE MIDPOINT TYPE CYCLOCONVERTER:

    A centre tapped power transformer is required in the mid-pint cycloconverter. It has 2 groups of thyristors. The thyristors Tp1 & Tp2 form positive group, which produces positive half cycle of the output. Similarly, Tn1 & Tn2 represent the negative group, which produces the

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  • negative half-cycle of the output. The line commutation process is used to turn OFF a conducting thyristor. By turning ON a thyristor of a particular group, a reverse voltage appears across the conducting thyristor, so by turning Tp1 ON Tp2 gets commutated and vice-versa. Let during positive half clce of the input voltage Vi, polarity of voltage at point A is positive and at point B is negative with respect to O. Similarly during negative half cycle of input voltage, voltage at point A is negative and at point B is positive with respect to the point O.

    For keeping the frequency of output voltage to 1/3 of the frequency of the input voltage, three positive pulses are being obtained across the load to produce one combined positive half-cycle of the load voltage. Similarly, the three negative half cycles are being obtained across the load to produce one combined negative half cycle of the load voltage. During the first positive half cycle of the input, the point A is positive, therefore, thyristor Tp1 is forward biased and it is fired at firing angle to conduct for remaining half cycle. The load current io flows through point A thyristor Tp1 load and the midpoint O of the transformer. Due to resistive load, the load current falls to zero at the end of this half-cycle and thyristor Tp1 turns OFF. In the negative cycle of the input voltage the point B is positive and point A is negative. The forward biased thyristor Tp2 is turned ON at firing angle + of input voltage by the applied gating signal. At present, the load current flows through point B, thyristor Tp2, load and midpoint O of the transformer. In the next positive half cycle of the input, Tp1 becomes in the forward biased and Tp2 becomes in the reverse biased voltage. Tp2 is automatically line commutated at about 2 and Tp1 is conducted at firing angle 2+ of the input voltage. The load current flows again in the same path as in the previous cases when Tp1 or Tp2 was conducting. In this way, the three positive pulses are being obtained at the output to produce one combined positive half cycle of output voltage. The amplitude of the output voltage can be controlled by controlling the firing angle of the thyrsitors. Now for getting one combined negative half cycle in the period of three input half cycle, we aplly the firing pulses to the negative group thyristors Tn1 and Tn2. In the next negative half cycle of the input, the thyristor Tp1 is automatically turned OFF at about 3 due to the reverse biasing and zero current. The forward biased thyristor Tn1 is gated at firing angle . The load current flows through the midpoint O, load, Tn1 and point A. So the load current direction is reversed. In the next positive half cycle of the input voltage, the gated and forward biased thyristor Tn2 is triggered ON with delay angle and Tn1 is automatically line commutated at about 4. The load current flows through midpoint O, load, Tn2 and point B. In the next half cycle of the input voltage, Tn2 is automatically turned OFF at about 5. The gated and forward biased thyristor Tn1 is turned ON at firing angle . The load current path and its direction again remain same as in the previous case when Tn1 or Tn2 was conducting. In this circuit, the thyristors conduct only when they are fired at a firing angle at the gate of the thyristor. The magnitude of the output voltage and current can be controlled by controlling the firing angle of the thyristors. Thyristors are conducted at firng angle and therefore average output voltage has about square waveform contains significant harmonic about square waveform, which leads to excessive temperature rise in AC motors.For producing sinusoidally controlled output voltage, thyristors T4 and T3 are fired at zero degree of the

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  • input voltage and T1 and T2 are fired at firing angle for controlling the output voltage. This scheme can be called as a modulating firing scheme.

    SINGLE PHASE BRIDGE TYPE CYCLOCONVERTER:

    The circuit of a single-phase bridge type cycloconverter is shown in below figure. 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 (50 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.

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  • 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 (0). Thus, the conductingthyristorpairinoneofthebridgesturnsoffatthattime,i.e.thethyristorsundergonaturalcommutation.So,operationwithdiscontinuouscurrenttakesplace,ascurrentflowsintheload,onlywhenthenextthyristorpairinthatbridgeistriggered,orpulsesarefedatrespectivegates.Takingfirstbridge1(positive),andassumingthetoppointoftheacsupplyaspositivewiththebottompointasnegativeinthepositivehalfcycleofacinput,theoddnumberedthyristorpair,P

    1&P

    3istriggeredafterphasedelay(1),suchthatcurrentstarts

    flowingthroughtheloadinthishalfcycle.Inthenext(negative)halfcycle,theotherthyristorpair(evennumbered),P

    2&P

    4inthatbridgeconducts,bytriggeringthemaftersuitablephase

    delayfromthestartofzerocrossing.Thecurrentflowsthroughtheloadinthesamedirection,withtheoutputvoltagealsoremainingpositive.Thisprocesscontinuesforonemorehalfcycle(makingatotalofthree)ofinputvoltage(f1=50Hz).Fromthreewaveforms,onecombinedpositivehalfcycleofoutputvoltageisproducedacrosstheloadresistance,withitsfrequencybeingonethirdofinputfrequency(f2=f1/3=50/3Hz).Thefollowingpointsmaybenoted.Thefiringangle()oftheconverterisfirstdecreased,inthiscaseforsecondcycleonly,andthen

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  • againincreasedinthenext(third)cycle,asshowninfigure.Thisis,becauseonlythreecyclesforeachhalfcycleisused.Iftheoutputfrequencyneededislower,thenumberofcyclesistobeincreased,withthefiringangledecreasingforsomecycles,andthenagainincreasinginthesubsequentcycles,asdescribedearlier.

    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 &

    N3 conducts, by triggering them after suitable phase delay from the zero-crossing. Similarly,

    the even-numbered thyristor pair, N2 & 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 (load) voltage waveform, its frequency being 3216Hz as stated earlier. The ripple frequency of the output voltage/ current for singlephase full-wave converter is 100 Hz, i.e., double of the input frequency. It may be noted that the load (output) current is discontinuous, as also load (output) voltage. The supply (input) voltage is shown in figure. Only one of two thyristor bridges (positive or negative) conducts at a time, giving non-circulating current mode of operation in this circuit.

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  • ADVANTAGES OF CYCLOCONVERTER:

    1. In a cyclo-converter, ac power at one frequency is converted directly to a lower frequency

    in a single conversion stage. 2. Cyclo-converter functions by means of phase commutation, without auxiliary forced

    commutation circuits. The power circuit is more compact, eliminating circuit losses associated with forced commutation.

    3. 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.

    4. 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 OF CYCLOCONVERTER:

    1. 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 20 kVA.

    2. For reasonable power output and efficiency, the output frequency is limited to one-third of the input frequency.

    3. The power factor is low particularly at reduced output voltages, as phase control is used with high firing delay angle.

    The cyclo-converter is normally compared with dc link converter , 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-

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  • 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 cyclo-converter 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.

    APPLICATIONS OF CYCLOCONVERTERS:

    1. Speed Control AC motors.

    2. To interconnect two power grids operating at different frequencies

    3. Controlled induction heating

    4. In variable frequency input and constant frequency output (VSCF) system, which is suitable for aircraft power supplies. In VSCF system, a rigged, compact and low weightage solid state cycloconverter produce a constant frequency AC voltage from a variable speed alternator.

    5. Cement mill drives

    6. Ship propulsion drives 7. Rolling mill drives

    8. Scherbius drives

    9. Ore grinding mills

    10. Mine winders

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  • HARDWARE IMPLEMENTATION OF

    CYLOCONVERTERS:

    BLOCK DIAGRAM OF THE CONTROL CIRCUIT FOR CYCLOCONVERTER:

    CONTROL CIRCUIT:

    The block diagram of a type of triggering circuit for the midpoint cycloconverter is shown above. These circuits are very simple and require less and easily available components. The 6V to 12V AC synchronizing signal can get from 230V : 6V to12V(100mA) transformer. The zero crossover detector (ZCD) which uses an operational amplifier transforms the synchronizing signal into a square wave signal (A) as shown in figure. The negative pulses are eliminated by using diodes at the output stage of the op-amp.

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  • The 1st EXCLUSIVE OR gate (Pin Nos 11, 12 ,13 of IC 74C86) is used as inverter. The waveform is obtained at the output of the inverter. The waveform is given to the delay circuit(R-C integrator circuit) and waveform P is obtained across the capacitor C of the delay circuit. The waveforms P and are fed to the input terminals of an other EX-OR gate and the waveforms C is obtained at the output of the EX-OR gate.

    This C waveform is used for resetting the ramp generator. The comparator compares the ramp(D waveform) with a DC voltage Vc which varies between 0 about +10V(when Vcc= +12V) and is controlled by the 47K Pot. The output of the comparator(waveform E) is used for triggering IC 555 pulse shaper whose output produces waveform G which is used for triggering thyristors.

    FIRING CIRCUIT

    SUB CIRCUITS OF THE FIRING CIRCUIT:

    The firing circuit consists of the following circuits:

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  • 1. Zero Crossover Detector (ZCD)

    2. Ramp Generator

    3. Comparator

    4. Pulse Shaper

    ZERO CROSSOVER DETECTOR (ZCD):

    A zero crossing detector literally detects the transition of a signal waveform from positive and negative, ideally providing a narrow pulse that coincides exactly with the zero voltage condition. Zero crossing detector is used to convert sine wave or other signal into square-wave, the output should be low if the input is negative and high if the input if positive. Many zero crossing detector use split supply (symmetric supply), but this zero crossing detector circuit only need a single supply, thus suitable for battery-operated circuits. Here is the schematic diagram:

    Zero-crossing detector is an applied form of comparator. Either of the op-amp basic comparator circuits discussed can be employed as the zero-crossing detector provided the reference voltage Vref is made zero. Zero-crossing detector using inverting op-amp comparator is depicted in figure. The output voltage waveform shown in figure indicates when and in what direction an input signal vin crosses zero volt. In some applications the input signal may be low frequency one (i.e. input may be a slowly changing waveform). In such a case output voltage vOUT may not switch quickly from one saturation state to the other. Because of the noise at the input terminals of the op-amp, there may be fluctuation in output voltage between two saturation states (+ Vsat and Vsat voltages). Thus zero crossings may be detected for noise voltages as well as input signal vin. Both of these problems can be overcome, if we use regenerative or positive feeding causing the output voltage vout to change faster and eliminating the false output transitions that may be caused due to noise at the input of the op-amp.

    14

  • zero crossing detector waveform

    RAMP GENERATOR:

    A circuit that generates a sweep voltage which increases linearly in value during one cycle of sweep, then returns to zero suddenly to start the next cycle.If we apply a constantly changing input signal such as a square wave to the input of an Integrator Amplifier then the capacitor will charge and discharge in response to changes in the input signal. This results in the output signal being that of a sawtooth waveform whose frequency is dependant upon the RC time constant of the resistor/capacitor combination. This type of circuit is also known as a Ramp Generator and the transfer function is given below.

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  • COMPARATOR:

    In electronics, a comparator is a device which compares two voltages or currents and switches its output to indicate which is larger. The input voltages must not exceed the power voltage range:

    In the case of TTL/CMOS logic output comparators, negative inputs are not allowed:

    An operational amplifier has a well balanced difference input and a very high gain. The parallels in the characteristics allows the op-amps to serve as comparators in some functions.

    A standard op-amp operating in open loop configuration (without negative feedback) can be used as a comparator. When the non-inverting input (V+) is at a higher voltage than the inverting input (V-), the high gain of the op-amp causes it to output the most positive voltage it can. When the non-inverting input (V+) drops below the inverting input (V-), the op-amp outputs the most negative voltage it can. Since the output voltage is limited by the supply voltage, for an op-amp that uses a balanced, split supply, (powered by VS) this action can be written:

    where sgn(x) is the sign function. Generally, the positive and negative supplies VS will not match absolute value:

    when else when

    In practice, using an operational amplifier as a comparator presents several disadvantages as compared to using a dedicated comparator:

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  • 1. Opamps are designed to operate in the linear mode with negative feedback. Hence, an opamp typically has a lengthy recovery time from saturation. Almost all opamps have an internal compensation capacitor which imposes slew rate limitations for high frequency signals. Consequently an opamp makes a sloppy comparator with propagation delays that can be as slow as tens of microseconds.

    2. Since opamps do not have any internal hysteresis an external hysteresis network is always necessary for slow moving input signals.

    3. The quiescent current specification of an opamp is valid only when the feedback is active. Some opamps show an increased quiescent current when the inputs are not equal.

    4. A comparator is designed to produce well limited output voltages that easily interface with digital logic. Compatibility with digital logic must be verified while using an opamp as a comparator.

    PULSE SHAPER:

    Pulse shaping is the process of changing the waveform of transmitted pulses. The 555 Timer IC is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. Depending on the manufacturer, the standard 555 package includes over 20 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8).] Variants available include the 556 (a 14-pin DIP combining two 555s on one chip), and the 558 (a 16-pin DIP combining four slightly modified 555s with DIS & THR connected internally, and TR falling edge sensitive instead of level sensitive).

    Modes of operation: theres basically two of them: monostable & astable. Monostable: a circuit that produces an output pulse of fixed duration each time the input circuit is triggered. Monostables are useful in producing timing signals Astable Multivibrator: a circuit that produces an oscillatory output. The frequency and duty cycle of the oscillator can be varied through the choice of resistors, capacitors, and a control voltage.

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  • 4-Bit Programmable Binary Counter (74191):

    The 4-Bit Programmable Binary Counter (74191) is being used in down counting mode. It can count from 15 to 0 pulses(1111 to 0000 binary number). When DPDT (double pole double throw) switch is kept at stop position, the Load (Pin No 11) and En(Enable, Pin No 4) become low and high respectively. As soon as the DPDT switch is turned ON the Load and En become high and low respectively and the counter starts counting the pulses C in down count mode from a preset number N(the binary number of pins states PD, PC, PB and PA respectively)to the number (0000). The pins PD, PC, PB and PA can be made high (+5V or open) or low (ground) with the help of switches as shown in figure. The counter overflow signal (max/min) is processed to trigger Toggle Flip Flop. Here IC 74109 (J-K positive edge trigger F/F with preset and clear) is used for toggling the F/F. Thus, by varying the preset input of the counter one can control the numbers of the controlled half cycle pulses of the input voltage in a half cycle of the output of the cycloconverter.

    The modified Flip Flop controls the counter through En pin so that the counter can start counting when the positive half cycle of the input voltage is present(i.e. when waveform A is present)

    MODIFIED RS FLIP FLOP:

    Set input terminal (S) and reset input terminal (R) of the flip flop are connected with the output pulse A and +5V respectively through the DPDT switch. The S and R input terminals are kept at low by putting a DPDT switch at stop position. This makes the output terminal B as high. The output terminal B of the flip lop is connected with En(enable) of counter IC 74191. This high En stops counting. When the DPDT switch is turned ON and waveform A becomes high both the input terminals S and R are at high. It sets to 1 and B to 0. A low En of the counter starts counting the pulse C. Now till the input R of the flip flop is high the low or high value of the input S has no effect on the position of the output terminal B as low.

    TOGGLE FLIP FLOP:

    The IC 74109 toggle flip flop gives toggle output pulses T and at each input clock which receives from the output ( max/min) of the counter. The preset terminal (PR) gets zero input when DPDT switch is kept at stop position. The presence of zero at PR makes the output pulse T as high at the starting.

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  • DRIVER CIRCUIT FOR GETTING SINUSOIDAL WAVEFORMS FROM THE CYCLOCONVERTER:

    The following four separate driver circuits are required for getting triggering pulses vgp1, vgp2, vgn1 and vgn2 to rigger thyristors Tp1, Tp2, Tn1 and Tn2 respectively.

    a. For getting vgp1 firing pulses waveforms G, A, and T are ANDed and the ouput signal Gp1 is fed to the pulse amplification and isolation stage.

    b. For receiving vgp2 firing pulses waveforms , and T are ANDed and the ouput signal Gp2 is fed to the pulse amplification and isolation stage.

    c. For receiving vgn1 firing pulses waveforms G, , and are ANDed and the ouput signal Gn1 is fed to the pulse amplification and isolation stage.

    d. For getting vgn2 firing pulses waveforms A, and are ANDed and the ouput signal Gn2 is fed to the pulse amplification and isolation stage.

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  • WAVEFORMS AT DIFFERENT POINTS OF THE DRIVER CIRCUIT:

    20

  • 21

  • 22

  • DATASHEET OF IC DM74191:

    23

  • 24

  • 25

  • 26

  • 27

  • 28

  • DATASHEET OF IC MM74C86:

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  • 30

  • 31

  • 32

  • 33

  • DATASHEET OF 74109

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  • 35

  • 36

  • 37

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  • ABOUT MULTISIM:

    Introduction to Multisim

    Multisim is the worlds most popular electronics simulator, a virtual electronics lab in a computer! The parts bin contains an unlimited supply of indestructible digital and analog electronics component that work just like the real devicesbut never burn out.

    Turn on Multisims virtual test instruments and see what is happening in your circuit. Change component values and see the instruments respond just as they do in a hands-on lab. You can build schematic circuits or circuits with photo images of real components if you wish. Thousands of ready-to-use teaching labs featuring Multisim circuits have been created to accompany dozens of leading textbooks.

    Multisim's user interface consists of the following basic elements:

    The following menus are found in Multisim.

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    Introduction to the Multisim Instruments:

    Multisim provides a number of virtual instruments. These instruments are used to measure the behavior of the circuits. These instruments are set, used and read just like their real-world equivalents.

    Virtual instruments have two views: the instrument icon you attach to your circuit, and the instrument face, where you set the instrument's controls. one can show or hide the face by double-clicking on the instrument's icon. The instrument faces will always be drawn on top of the main workspace so that they will not be hidden. one may place the instrument faces wherever you wish on your desktop. The faces are automatically hidden when you activate a different view. Upon returning to a view, the instruments are restored to their original visibility and placement. When you save your circuit, the instrument face locations and

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  • hide/show status are stored with the circuit. As well, any data contained in the instruments is saved, up to some maximum size (see Saving Simulation Data with Instruments).

    The instrument's icon indicates how the instrument is connected into the circuit. Once the instrument is connected to the circuit, a black dot appears inside the terminal input/output indicators on the instrument face.

    The Component toolbar contains buttons that let you select components from the Multisim databases for placement in your schematic. See Components Toolbar.

    Introduction to Simulation

    Simulation is a mathematical way of emulating the behavior of a circuit. With simulation, you can determine much of a circuit's performance without physically constructing the circuit or using actual test instruments. Although Multisim makes simulation intuitively easy-to-use, the technology underlying the speed and accuracy of the simulation, as well as its ease-of-use, is complex.

    Multisim incorporates SPICE3F5 and XSPICE at the core of its simulation engine, with customized enhancements designed by Electronics Workbench specifically for optimizing simulation performance with digital and mixed-mode simulation. Both SPICE3F5 and XSPICE are industry-accepted, public-domain standards.

    Using Multisim Simulation

    To view the results of your simulation, you will need to use either a virtual instrument or run an analysis to display the simulation output. This output will include the combined results of all Multisim simulation engines.

    When you use interactive simulation in Multisim (by clicking on the Run Simulation button), you see the simulation results instantly by viewing virtual instruments such as the oscilloscope. You can also view the effect of simulation on components like LED's and 7-segment digital displays.

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  • During simulation, you can change the values of "interactive" components (those whose behavior can be controlled through the keyboard) and view the effect immediately. Interactive components include such devices as the potentiometer, variable capacitor, variable inductor, and multiple switcher. For example, changing a 100 kohm resistor to the next smaller resistor may alter the results more than desired, but with Multisim, you could use a variable resistor, reducing its value gradually, all the time seeing the simulation result change, until you reach the desired result.

    Structure of the Component Database

    The Multisim component database is designed to hold the information necessary to describe any component. It contains all the details needed for schematic capture (symbols), simulation (models) and PCB layout (footprints), as well as other electrical information.

    There are three levels of database provided by Multisim. The master database is read only, and contains components supplied by Electronics Workbench. The user database is private to an individual user. It is used for components built by an individual that are not intended to be shared. The corporate database is used to store custom components that are intended to be shared across an organization. Various database management tools are supplied in order to move components between databases, merge databases, and edit them.

    Using the place component browser

    By default, the Components toolbar is enabled. If it has been turned off, select View/Toolbars/Components or right-click in the menu area and select Components from the pop-up that appears.

    Note: This section describes the placement of "real" components. Use the Virtual toolbar to place "virtual" components.

    To choose and place a component.

    1. Click on the desired group in the Component toolbar, for example, Basic. The Select a Component dialog box appears with the selected component group displayed.

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  • Alternatively, you can display the Select a Component dialog box by choosing Place/Component and selecting the desired group from the Group drop-down list.

    Note: The Select a Component browser is also referred to as the place component browser.

    Note: The default database that displays in the browser is the Master Database. If you wish to select a component from either the Corporate Database or User Database, you must select that database from the Database drop-down list before selecting a component. Once changed, the database will remain as selected for subsequent part placements.

    2. Click on the desired component family in the Family list.

    3. Click on the desired component in the Component list.

    Tip To make your scroll through the Component list faster, simply type the first few characters of the component's name.

    Note Virtual components are identified by a green icon in the Family column. You can also place virtual components by using the Virtual Toolbar.

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  • 4. To confirm that this is the component you want to place, click OK. (To cancel placing the component, click Close). The browser closes and the cursor on the circuit window changes to a ghost image of the component you wish to place. This indicates that the component is ready to be placed.

    Note If you are placing a component whose package includes multiple sections (for example, four separate gates, as in the above example), a dialog box displays, where you specify which of the sections you want to place.

    5. Move your cursor to the location where you want the component placed. The workspace automatically scrolls if you move your cursor to the edges of the workspace.

    6. Click on the circuit window where you want the component placed. The component's symbol and labels appear (unless you have specified that they are not to be displayed, as explained in Displaying Identifying Information about a Placed Component), as well as a unique RefDes made up of a letter and number. The letter represents the type of component and the number is a sequential number that indicates the order in which the components were originally placed. For example, the first digital component has the RefDes "U1", the next is "U2", the first inductor has the RefDes "L1", and so on.

    Note If the component you place is a virtual component, it is a different color from real components. This color is set in the Sheet Properties dialog box, as explained in Sheet Properties - Circuit Tab.

    Tip Some components, like resistors and capacitors will have Filter fields at the top of the Component list, to make part selection faster.

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  • Searching for Components

    Multisim comes with a powerful search engine to help you quickly locate components if you know some information about the type of component you need. Multisim searches its database for components that meet your criteria and presents them to you, enabling you to choose the component that most suits the needs of your application from the list of candidates.

    To perform a standard search of the database:

    1. Select Place/Component to display the Select a Component browser.

    2. Click Search. The Search Component dialog box appears:

    3. Optionally, click More>> to display additional search options.

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  • 4. In the desired fields, enter your search criteria (you must enter at least one item). Enter alphanumeric characters, that is, text and/or numbers. Case is not considered, and you can use the "*" wildcard to search on partial strings.

    For example, in the Footprint Type field:

    "CASE646-06" finds only the exact string "CASE646-06"

    "*06" finds any string ending with "06"

    "CASE*" finds any string starting with "CASE"

    a "?" anywhere in the string will match exactly one character. For example, "CAS?" will match "CASE", but not "CASE646-06".

    5. Click Search. When the search is complete, the Search Component Result dialog box appears.

    Tip The more specific your search criteria, the smaller the number of matching components.

    To select a component from the search results: When the search is complete, the Search Component Result dialog box appears, displaying information about the first component that matched your criteria. The Component list contains a list of all the components that matched your criteria. For example, using the search example above, the results look like this:

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  • From the Component list, select the component you are interested in. To view information about any component found by the search, simply choose it from the list and the display fields change accordingly.

    6. To place the selected component, click OK. You return to the Select a Component dialog box, where you can place the component by clicking OK.

    You can refine your search if your initial attempt yielded a large number of items.

    To refine your search: 1. Click Refine Search. The following dialog box appears.

    2. Enter desired parameters and click Search.

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