My final year Presentation,2008

Three Phase AC-DC Converter Using SCR
CHAPTER-1

INTRODUCTION

1.1 PREFACE

SCRs are widely used for control motor, industrial purpose and various other control systems. However SCR controlled by gate firing pulse. For the purpose of generation gate firing pulse need a firing circuit or control scheme.

A major demand of the present day industries is to have precise and wide range of speed control for derives with good speed regulation and reproducibility

DC drives are quite old . They have many advantages and hence are used extensively in industrial and traction purposes. The starting torque a DC motor is quite high and in most suited for traction duty. A wide range of speed control above and the rated speed can be obtained, and can be achieved by less complicated control circuit.

In the 1950s electronic control came in to existence and brought about a remarkable improvement in the speed control system. In the 1960s, high –power silicon diodes and silicon-controlled rectifier were developed.

At present, thyristors are extensively used for AC to DC conversion. Conventional controllers involving magnetic amplifiers, mercury arc-rectifiers, rotating amplifiers, resistance controllers etc. have been replaced by thyristroised power controllers in almost all application.

Some of the important advantage associated with thyristroised power controllers as follows:

i. High efficiency due to the low loss in the thyristor.
ii. Long life and reduced the maintenance due to Mechanical wear.
iii. Compact size and absence of special foundation.
iv. Faster dynamic response compared to electromechanical investor.
v. Flexibility in operation due to digital controller.
vi. Lower acoustic noise compared to electromagnetic controllers e.g relays and controllers.

The conventional DC motors is used as variable speed drive in many application. But the DC drive has the following draw-backs:

i. Mechanical commutator needs regular maintenance.
ii. The commutator construction increases the cost of DC motor drive.
iii. Power/weight ratio is reduce.
iv. Brush and commutator wear occurs due to sparking and friction.
v. The mica insulation limits the voltage between the segment.
vi. Unsuitable to operate in industry and explosive environment.
The most commonly used motors for variables-speed application are the separately-excited and series-excited DC motors. The DC drive provides variable at the armature of the DC motor.

The basic method of control to achieve variable armature voltage is:

i. Phase control.
ii. Integral cycle control.
iii. Chopper control.
In all the three methods the supply voltage is impressed on the armature for a portion of the cycle and during the armature is disconnected from the supply, Since the on and off is very rapid, the motor can be respond to the average voltage only.

In phase control, the supply voltage is AC and the armature voltage is rectified AC. The phase control system is extensively used for drives but the input power factor at low speed becomes low.


For more sophisticated control the phase locked loop technique is used to achieve zero speed regulation . In more complex multi-meter drive system, micro computers are used now a day to have a greater flexibility of control .


1.2 OBJECTIVES OF WORK IN THE PROJECT

In the present work, an attempt has been made to develop a firing scheme for phase control converter. A test has been carried out to verify the performance of the system over a wide range of supply voltage and frequency .

THE PROJECT HAS THE FOLLOWING OBJECTIVES

• Develop a firing circuit
• Fabricate of firing circuit
• Experimental performance of firing circuit.

CHAPTER – 2

CONVERTER AND FIRING CONTROL SCHEME


2.1 PREFACE

To obtain controlled output voltage phase control thyristor are used instead of diodes . The output voltage of thyristor rectifier is varied by controlling the delay or firing angle of thyristors. A phase control thyristor is turned on by applying a short pulse to its gate and turned off due to natural or line commutation ; and incase of highly inductive load, it is turned off by firing another thyristor of the rectifier during the negative half cycle of input voltage .

Three phase-controlled rectifiers are simple and less expensive; and the efficiency of the rectifier is, in general, above 95%. Because these rectifier converts from ac to dc, these controlled rectifiers are also called ac-dc converters and are used to extensively in industrial applications, especially in variable speed drives, ranging from fractional horse power to megawatt power level.

The phase control converters can be classified into two types, depending on the input supply: (1) single phase converters and (2) three phase converters. Each type can be subdivided into (a) semi converter, (b) full converter (c) dual converter. A semi converter is a one-quadrant converter and it has one polarity of output voltage and current. A full converter is a two quadrant converter and the polarity of its output voltage can be either positive or negative. However, the output current of full converter has one polarity only. A dual converter can operate in four quadrants; and the both the output voltage and current can be either positive or negative. In some applications, converters are connected in series to operate at higher voltages and to improve the input power factor.

2.2 PRINCIPAL OF PHASE-CONTROLLED CONVERTER OPRATION

During the positive half cycle of input voltage, the thyristor anode is positive with respect to its cathode and thyristor is said to be forward biased. When thyristor T1 is fired at ωt = α thyristor T1 conduct and the input voltage paper across the load. When the input voltage start to be negative at ωt= π, the thyristor anode is negative with respect to cathode and thyristor T1 is said to be reverse biased, and it turned off.the time after the input voltage starts to go positive until the thytistor is fired at ωt = α is called the delay or firing angle α .



2.3 SINGLE PHASE FULL CONVERTER

The circuit arrangement of a single phase full converter is shown in with a highly inductive load so that load current is continuous and ripple free. During the positive half cycle, thyristor T1 and T2 are forward biased, and when this two thyristor are fired simultaneously at ωt = α , the load is connected to the input supply through T1 and T2. Due to the inductive load, thyristor T1 and T2 contain to conduct beyond ωt= π, even though the input voltage is already negative. During negative half cycle of input voltage, thyristor T3 and T4 are forward biased, and firing of thyristor T3 and T4 apply the supply voltage across thyristor T1 and T2 as reverse blocking voltage. T1 and T2 are turned off due to line or natural commutation and the load currant is transferred from T1 and T2 to T3 and T4.


2.4 PRINCIPLE OF THREE PHASE HALF WAVE CONVERTERS

Three phase converters provide higher average output voltage, and in addition the frequency of the ripples on the output is higher compared with that of single-phase converters. As a result the filtering requirements for smoothing out the load current and load voltage are simpler. For these reasons three phase converters are use extensively in higher power variable-speed drives. Three single phase half wave converters in fig can be connected to form a three-phase half wave converter as shown in fig

When thyristor T1 is fired at ωt = π/6 +α, the phase voltage υan appears across the load until thyristor T2 is fired at ωt = (5π/6 +α). When thyristor T2 is fired, thyristor T1is reversed biased, because the line to line voltage υab = (υan – υbn) is negative and thyristor T1is turned off. The phase voltage υbn appears across the load until thyristor T3 is fired at ωt = (3π/2 +α). When thyristor T3 is fired, thyristor T2 is turned off. The phase voltage υcn appears across the load until thyristor T1 is fired again at the beginning of next cycle.

2.5 THREE PHASE FULL CONVERTERS

Three phase converters are extensively used in industrial applications up to the 120-kW level, where a two quadrant operation is required. The thyristor are fired at an interval of π/3 . The frequency of output ripple voltage is 6fs and the filtering requirement is less than that of half wave converters. At ωt = π/6 +α, thyristor T6 is already conducting and thyristor T1 is turned on. During interval (π/6 +α)≤ ωt≤ ( π/2 +α), thyristor T1and T6 conduct and the line to line voltage υab =( υan – υbn) appears across the load . At ωt=π/2+α,
Thyristor T2 is fired and thyristor T6 is reversed biased immediately. T6 is turned off due to natural commutation. During interval (π/2 +α)≤ ωt≤ ( 5π/6 +α), thyristor T1and T2 conduct and the line to line voltage υac appears across the load. If the thyrtistor is numbered as shown in figure the firing sequence is T1T2, T2T3, T3T4, T4T5, T5T6 and T6T1.

If the line-to-neutral voltage are defined as

υan = Vm sin ωt
υbn = Vm sin (ωt - 2π/3)
υcn = Vm sin (ωt + 2π/3)
The corresponding line to line voltages are

υab = ( υan – υbn) = √3 Vm sin (ωt + π/6)
υbc = ( υbn – υcn) = √3 Vm sin (ωt - π/2)
υca = ( υcn – υan) = √3 Vm sin (ωt + π/2)
=
= cos α
The maximum average output voltage for delay angle α = 0, is

= cos α
The rms value of the output voltage is found from

= Vm








Fig1: Firing squence

Fig 2: Block diagram three-phase AC-DC converter


















FIRING CONTROL SCHEME

2.6 Preface

The gate control circuit are called firing or triggering circuit. These are generally low power electronic circuits, any practical power circuits configuration, e.g. chopper, converter, inverter consists of several thyristers. The firing circuit must fulfill two functions:

i. It is required to produce voltage pulses for each thyristor at the appropriate instant of time in a periodic manner and with a particular sequence depending upon the type of the power circuit. The trigger pulses with the desired sequence are generated using electronic circuits consisting of logic gates, flip-flops counters etc. The availability of these components in the integrated circuits configuration has simplified the control circuit and brought about great changes in the field of power control.
ii. The pulses produced by the control circuits are usually at low power level. They may not be able to trigger the thyristor in to conduction if fed directly. The pulses are therefore coupled to the gate cathode terminals of a thyristor through a driver circuit. The driver circuit consists of a pulse amplifier and a pulse transformer.


2.7 CLASSIFICATION OF SCR FIRING CIRCUITS :

THE SCR FIRING CIRCUITS ARE CLASSIFIED AS FOLLOWS:

i. Magnetic firing circuits.
ii. Solid –state firing circuits.

MAGNETIC FIRING CIRCUIT:

The magnetic firing circuits are either magnetic amplifiers or adjustable saturable reactors to control the firing angle in accordance with a dc signal.


SOLID –STATE FIRING CIRCUIT:

The solid –state firing circuits follow various principles to generate firing pulses proportional to a DC signal


2.8. BASIC COMMON TYPES OF SCR TRIGGERING CONTROL:

(a). Phase shift control
(b). Combined d .c and phase shift control
(c) Pulse shift control
We have chosen pulse shift control method and solid state firing circuit .

2.9 . PULSE SHIFT CONTROL

For the purpose of SCR gate pulse generation and shifting sues the principle to cosine wave crossing control which determine the firing instant of each SCR from crossing point of an associated sinusoidal wave with along reference voltage or control signal. This control signal is obtained by rectifying the secondary .6 phase voltage of the voltage sensor .

2.10 COMPARISION BETWEEN DIGITAL AND ANALOG FIRING SCHEME: We may setup a comparison chart between analog and digital firing scheme as follows:


DIGITAL FIRING CKT .(DFC) ANALOG FIRING CKT (AFC)
1. Less hardware component required in digital firing ckt.
2. It is component and small size.
3. It is simple circuitry than A.F.C
4. Low power consumption
5. It is more reliable and flexible from A.F.C
6. Firing angle can be controlled exactly from 900 to 1800 .
7. Same ckt can be used for converter as inverter.
8. Fabrication cost is lower than A.F.C

9. The digital control unit is used to adjust the firing angle. 1. More hardware components required than D.F.C
2. . It is large size.
3. Circuit is complex than D.F.C
4. High power consumption
5. It is less reliable and flexible from D.F.C
6. Firing angle can not be controlled exactly from 900 to 1800 .
7. Same ckt can not be used for converter as inverter.
8.Fabrication cost is relatively higher than A.F.C
9. The pot is used adjust the firing angle








CHAPTER-3

DESIGN OF FIRING CIRCUIT

3.1 Preface

The circuit uses the principle of cosine wave crossing control which determine the firing instant of each SCR from crossing point of an associated sinusoidal wave with analog reference voltage or control signal. In this scheme, this control signal is obtained by rectifying the secondary 6 phase voltage of the voltage sensor .

3.2 VARIOUS COMPONENT OF FIRING ANALOG CIRCUIT

The complete circuit is shown in Figure below

The analog firing scheme consists of the following components:-
• Step down transformer
• Comparator
• Monostable multivibrator
• Inverter
• Optocupler
• SCR















3.2.1 STEP-DOWN TRANSFORMER






A transformer which, converts high voltage to low voltage is called step down transformer. A transformer is a stationary piece of apparatus by mean of which elective power in one circuit is transformed in to electric power of the same frequency in another circuit. It can raise or lower the voltage in a circuit but with but with a corresponding decrease or increase in current . The physical basic of transformer is neutral induction between two circuits linked by a common magnetic flux.

Three single phase transformer of 440/6-0-6 V with center taped secondary winding have been used to realize voltage sensor. The primary winding being arranged in delta are connected to the a.c side of the line commuted inverter bridge. The secondary windings of the transformer are arranged to have six phase configuration to produce six channels. Each channel produces a firing pulse to trigger a SCR.

Also another transformer used in the circuit for optocupler power supply which is single phase step-down transformer. Input- Output ratio is 220/4*4.




Basic Principle of a Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors — the transformer's coils or "windings". Except for air-core transformers, the conductors are commonly wound around a single iron-rich core, or around separate but magnetically-coupled cores. A varying current in the first or "primary" winding creates a varying magnetic field in the core (or cores) of the transformer. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the "secondary" winding. This effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will flow from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary to the number of turns in the primary as follows:

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NS less than NP.
Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of national power grids. All operate with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.
The transformer is based on two principles: firstly, that an electric current can produce a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnitude of the applied magnetic field. The changing magnetic flux extends to the secondary coil where a voltage is induced across its ends.


An ideal step-down transformer showing magnetic flux in the core.
A simplified transformer design is shown to the left. A current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron; this ensures that most of the magnetic field lines produced by the primary current are within the iron and pass through the secondary coil as well as the primary coil.
Induction law
The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:

where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Φ equals the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic field strength B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for VS and VP gives the basic equation for stepping up or stepping down the voltage

Ideal power equation


The ideal transformer as a circuit element
If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power.
Pincoming = IPVP = Poutgoing = ISVS
giving the ideal transformer equation

If the voltage is increased (stepped up) (VS > VP), then the current is decreased (stepped down) (IS < IP) by the same factor. Transformers are efficient so this formula is a reasonable approximation.
The impedance in one circuit is transformed by the square of the turn’s ratio. For example, if an impedance ZS is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of . This relationship is reciprocal, so that the impedance ZP of the primary circuit appears to the secondary to be .
Energy losses
An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%.
Experimental transformers using superconducting windings achieve efficiencies of 99.85%, while the increase in efficiency is small, when applied to large heavily-loaded transformers the annual savings in energy losses are significant.
A small transformer, such as a plug-in "wall-wart" or power adapter type used for low-power consumer electronics, may be no more than 85% efficient, with considerable loss even when not supplying any load. Though individual power loss is small, the aggregate losses from the very large number of such devices is coming under increased scrutiny.
The losses vary with load current, and may be expressed as "no-load" or "full-load" loss. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant, meaning that even an idle transformer constitutes a drain on an electrical supply, which encourages development of low-loss transformers (also see energy efficient transformer).
Transformer losses are divided into losses in the windings, termed copper loss, and those in the magnetic circuit, termed iron loss. Losses in the transformer arise from:
Winding resistance
Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.
Hysteresis losses
Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.
Eddy currents
Ferromagnetic materials are also good conductors, and a solid core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness.
Magnetostriction
Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers, and in turn causes losses due to frictional heating in susceptible cores.
Mechanical losses
In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise, and consuming a small amount of power.
Stray losses
Leakage inductance is by itself lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat.
















3.2.2 COMPARATOR


Comparator circuit has been made with the help of operational IC 741. The secondary voltage of the sensing transformer is compared with a d.c reference signal using 741 operational amplifiers. Comparator produce alternating rectangular wave form of variable pulse width . To avoid the possibility of varying the delay angle due to change in inverter output terminal voltage the d.c. reference voltage is generated by rectifying the secondary voltage of transformer and using capacitance filter. This also eliminates the possibility of collapsing the generation of firing pulse as d.c reference voltage can never be greater than the pick value of the secondary voltage. That is Vdc < Vmax . In this comparator circuit IN4001 diode and 1000µF, 25 V capacitor has been used in the rectifier circuit . This d.c output of this type of rectifier circuit is given by,
Vdc = 3√2V /π
Where, V = r.m.s per phase secondary voltage.
A 22k pot has been used to obtain variable d.c reference voltage .


Definition of 741-pin functions:

Pin 1 (Offset Null): Offset nulling, see Fig. 11. Since the op-amp is the differential type, input offset voltage must be controlled so as to minimize offset. Offset voltage is nulled by application of a voltage of opposite polarity to the offset. An offset null-adjustment potentiometer may be used to compensate for offset voltage. The null-offset potentiometer also compensates for irregularities in the operational amplifier manufacturing process which may cause an offset. Consequently, the null potentiometer is recommended for critical applications. See 'Offset Null Adjustment' for method.

Pin 2 (Inverted Input): All input signals at this pin will be inverted at output pin 6. Pins 2 and 3 are very important (obviously) to get the correct input signals or the op amp can not do its work.

Pin 3 (Non-Inverted Input): All input signals at this pin will be processed normally without inversion. The rest is the same as pin 2.

Pin 4 (-V): The V- pin (also referred to as Vss) is the negative supply voltage terminal. Supply-voltage operating range for the 741 is -4.5 volts (minimum) to -18 volts (max), and it is specified for operation between -5 and -15 Vdc. The device will operate essentially the same over this range of voltages without change in timing period. Sensitivity of time interval to supply voltage change is low, typically 0.1% per volt. (Note: Do not confuse the -V with ground).

Pin 5 (Offset Null): Same pin 1,

Pin 6 (Output): Output signal's polarity will be the opposite of the input's when this signal is applied to the op-amp's inverting input. For example, a sine-wave at the inverting input will output a square-wave in the case of an inverting comparator circuit.

Pin 7 (posV): The V+ pin (also referred to as Vcc) is the positive supply voltage terminal of the 741 Op-Amp IC. Supply-voltage operating range for the 741 is +4.5 volts (minimum) to +18 volts (maximum), and it is specified for operation between +5 and +15 Vdc. The device will operate essentially the same over this range of voltages without change in timing period. Actually, the most significant operational difference is the output drive capability, which increases for both current and voltage range as the supply voltage is increased. Sensitivity of time interval to supply voltage change is low, typically 0.1% per volt.

Pin 8 (N/C): The 'N/C' stands for 'Not Connected'. There is no other explanation. There is nothing connected to this pin, it is just there to make it a standard 8-pin package.



Operation
The amplifier's differential inputs consist of V + input and a V − input, and generally the op-amp amplifies only the difference in voltage between the two. This is called the differential input voltage. Operational amplifiers are usually used with feedback loops where the output of the amplifier would influence one of its inputs. The output voltage and the input voltage it influences settles down to a stable voltage after being connected for some time, when they satisfy the internal circuit of the op amp.
In its most common use, the op-amp's output voltage is controlled by feeding a fraction of the output signal back to the inverting input. This is known as negative feedback. If that fraction is zero (i.e., there is no negative feedback) the amplifier is said to be running open loop and its output is the differential input voltage multiplied by the total gain of the amplifier, as shown by the following equation:

Where V + is the voltage at the non-inverting terminal, V − is the voltage at the inverting terminal and G is the total open-loop gain of the amplifier.
Since the magnitude of the open-loop gain is typically very large, open-loop operation results in op-amp saturation (see below in Nonlinear imperfections) unless the differential input voltage is extremely small. Finley's law states that "When the inverting and non-inverting inputs of an op-amp are not equal, its output is in saturation." Additionally, the precise magnitude of this gain is not well controlled by the manufacturing process, and so it is impractical to use an operational amplifier as a stand-alone differential amplifier. Instead, op-amps are usually used in negative-feedback configurations.
Most single, dual and quad op-amps available have a standardized pin-out which permits one type to be substituted for another without wiring changes. A specific op-amp may be chosen for its open loop gain, bandwidth, noise performance, input impedance, power consumption, or a compromise between any of these factors















Open-Loop Gain:

Let’s have a look how the 'ideal' amplifier would look like in Fig. 5-1. The search for an
Ideal amplifier is, of course, a futile exercise. The characteristics of the operational amplifier are good enough, however, to allow us to treat it as ideal. Below are some amplifier properties that make this so. (Please realize that these ratings are next to impossible to achieve).


• Gain--infinite
• Input impedance--infinite
• Output impedance--zero
• Bandwidth--infinite
• Voltage out--zero (when voltages into each other are equal)
• Current entering the amp at either
• terminal--extremely small
The "operation" referred to mathematical operations, such as addition, integration, etc. An exact equivalent of the ideal Op-Amp is called a "nullor" and it is composed of new elements -- the nullator and the norator. The input to the op-amp is the nullator (i.e. no voltage or current), while the output is the norator (i.e. any voltage or current). These two components give the device its ideal characteristics.








3.2.3 MONOSTABLE MULTIVIBRATOR:



A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors and capacitors. The most common form is the astable or oscillating type, which generates a square wave—the high level of harmonics in its output is what gives the multivibrator its common name. The multivibrator originated as a vacuum tube (valve) circuit described by William Eccles and F.W. Jordan in 1919.
There are three types of multivibrator circuit:
• astable, in which the circuit is not stable in either state—it continuously oscillates from one state to the other.
• monostable, in which one of the states is stable, but the other is not—the circuit will flip into the unstable state for a determined period, but will eventually return to the stable state. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot. A common application is in eliminating switch bounce.
• bistable, in which the circuit will remain in either state indefinitely. The circuit can be flipped from one state to the other by an external event or trigger. Such a circuit is important as the fundamental building block of a register or memory device. This circuit is also known as a flip-flop.
In its simplest form the multivibrator circuit consists of two cross-coupled transistors. Using resistor-capacitor networks within the circuit to define the time periods of the unstable states, the various types may be implemented. Multivibrators find applications in a variety of systems where square waves or timed intervals are required. Simple circuits tend to be inaccurate since many factors affect their timing, so they are rarely used where very high precision is required.
Before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. This technique was used in early electronic organs, to keep notes of different octaves accurately in tune. Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.

In this project we use DM74LS123.The DM74LS123 is a dual retriggerable multivibator capable of generating output pulses from a few nono-seconds to extremely up to 100% duty cycle. Each device has three input permitting the choice of either leading edge or trailing edge triggering. Pin A is an active low transition trigger input and pin B is an active high transition trigger input. The clear (CLR) input terminates the output pulse at a predetermined time independent of the timing component. The clear input also serves as a trigger input when it is pulsed with a low level pulse.

The length of the pulse is determined by the formula Tw = 0.33* Cext* Rext

Features:

• DC triggered from active high transition or active low transition input.
• Retriggerable to 100% duty cycle.
• Compensated for Vcc and temperature variations.
• Triggerable from CLEAR input.
• DTL, TTL component.
• Input clamp diode.











Function Table

Input Output
CLEAR A B Q Q’
L
X
X
H
H
↑ X
H
X
L
↑
L
X
X
L
↑
H
H L H
L H
L H







3.2.4 INVERTER

Pin Layout
Pin Description
Pin Descriptio









Pin Number Description
1 A Input Gate 1
2 Y Output Gate 1
3 A Input Gate 2
4 Y Output Gate 2
5 A Input Gate 3
6 Y Output Gate 3
7 Ground
8 Y Output Gate 4
9 A Input Gate 4
10 Y Output Gate 5
11 A Input Gate 5
12 Y Output Gate 6
13 A Input Gate 6
14 Positive Supply

3.2.5 OPTOUCUPLER



In thyristor converters, different potential exist at various terminal. The power circuit is subjected to a high voltage, usually greater then 100V, and the gate circuit is held at a low voltage, typically 12 to 30V.An isolation circuit is required between an individual thyristor and its gate pulse
generating circuit. The isolation can be accomplished by ither pulse transformer or optocoupler. An optocoupler could be a phototransistor or photo-silicon-controlled rectifier. We have used in this project a phototransistor optocoupler 4N25 that pin diagram show in appendix.

A common implementation involves a LED and a phototransistor, separated so that light may travel through a barrier but electrical current may not. When an electrical signal is applied to the input of the opto-isolator, its LED lights, its light sensor then activates, and a corresponding electrical signal is generated at the output. Unlike a transformer, the opto-isolator allows for DC coupling and generally provides significant protection from serious over voltage conditions in one circuit affecting the other. If high transmission ratio is required Darlington photo transistor is used, however higher transmission ratio usually results in low noise immunity and higher delay.
With a photodiode as the detector, the output current is proportional to the amount of incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a photoconductive mode. In photovoltaic mode, the diode acts like a current source in parallel with a forward-biased diode. The output current and voltage are dependent on the load impedance and light intensity. In photoconductive mode, the diode is connected to a supply voltage, and the magnitude of the current conducted is directly proportional to the intensity of light. This optocoupler type is significantly faster than those with photo transistors; however, the transmission ratio is very low. Due to the small transmission ratio it is common to integrate amplifier circuit in same package.
The optical path may be air or a dielectric waveguide. When high noise immunity is required optical conductive shield may be integrated into optical path. The transmitting and receiving elements of an optical isolator may be contained within a single compact module, for mounting, for example, on a circuit board; in this case, the module is often called an optoisolator or opto-isolator. The photosensor may be a photocell, phototransistor, or an optically triggered SCR or TRIAC. Occasionally, this device will in turn operate a power relay or contactor.
For analog isolation, special "analog" optoisolators are used. These devices have two independent, closely matched phototransistors, one of which is typically used to linearize the response using negative feedback.
Among other applications, opto-isolators can help cut down on ground loops, block voltage spikes, and provide electrical isolation.


3.2.6 SCR
An SCR is a type of rectifier, controlled by a logic gate signal. It is a four-layer, three-terminal device. A p-type layer acts as an anode and an n-type layer as a cathode; the p-type layer closer to the n-type(cathode) acts as a gate. It is unidirectional in nature.
In the normal "off" state, the device restricts current to the leakage current. When the gate to cathode voltage exceeds a certain threshold, the device turns "on" and conducts current. The device will remain in the "on" state even after gate current is removed so long as current through the device remains above the holding current. Once current falls below the holding current for an appropriate period of time, the device will switch "off".
If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge into the gate to trigger the device into the "on" state; this is referred to as "dv/dt triggering." This is usually prevented by limiting the rate of voltage rise across the device, perhaps by using a snubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly and the partially-triggered SCR may dissipate more power than is usual, possibly harming the device.
SCRs can also be triggered by increasing the forward voltage beyond their rated breakdown voltage (also called as breakover voltage), but again, this does not rapidly switch the entire device into conduction and so may be harmful so this mode of operation is also usually avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device to device.
SCRs are made with voltage ratings of up to 7500 volts, and with current ratings up to 3000 RMS amperes per device. Some of the larger ones can take over 50 kA in single-pulse operation. SCRs are used in power switching, phase control, chopper, battery chargers, and inverter circuits. Industrially they are applied to produce variable DC voltages for motors (from a few to several thousand HP) from AC line voltage. They control the bulk of the dimmers used in stage lighting, and can also be used in some electric vehicles to modulate the working voltage in a Jacobson circuit. Another common application is phase control circuits used with inductive loads. SCRs can also be found in welding power supplies where they are used to maintain a constant output current or voltage. Large silicon-controlled rectifer assemblies with many individual devices connected in series are used in high-voltage DC converter stations.
Two SCRs in "inverse parallel" are often used in place of a TRIAC for switching inductive loads on AC circuits. Because each SCR only conducts for half of the power cycle and is reverse-biased for the other half-cycle, turn-off of the SCRs is assured. By comparison, the TRIAC is capable of conducting current in both directions and assuring that it switches "off" during the brief zero-crossing of current can be difficult.
Typical electrostatic discharge (ESD) protection structures in integrated circuits produce a parasitic SCR. This SCR is undesired; if it is triggered by accident, the IC can go into latchup and potentially be destroyed.
The progression from Shockley diode to SCR is achieved with one small addition; actually nothing more than a third wire connection to the existing PNPN structure: (Figure below)

The Silicon-Controlled Rectifier (SCR)
If an SCR's gate is left floating (disconnected), it behaves exactly as a Shockley diode. It may be latched by breakover voltage or by exceeding the critical rate of voltage rise between anode and cathode, just as with the Shockley diode. Dropout is accomplished by reducing current until one or both internal transistors fall into cutoff mode, also like the Shockley diode. However, because the gate terminal connects directly to the base of the lower transistor, it may be used as an alternative means to latch the SCR. By applying a small voltage between gate and cathode, the lower transistor will be forced on by the resulting base current, which will cause the upper transistor to conduct, which then supplies the lower transistor's base with current so that it no longer needs to be activated by a gate voltage. The necessary gate current to initiate latch-up, of course, will be much lower than the current through the SCR from cathode to anode, so the SCR does achieve a measure of amplification.
This method of securing SCR conduction is called triggering, and it is by far the most common way that SCRs are latched in actual practice. In fact, SCRs are usually chosen so that their breakover voltage is far beyond the greatest voltage expected to be experienced from the power source, so that it can be turned on only by an intentional voltage pulse applied to the gate.
It should be mentioned that SCRs may sometimes be turned off by directly shorting their gate and cathode terminals together, or by "reverse-triggering" the gate with a negative voltage (in reference to the cathode), so that the lower transistor is forced into cutoff. I say this is "sometimes" possible because it involves shunting all of the upper transistor's collector current past the lower transistor's base. This current may be substantial, making triggered shut-off of an SCR difficult at best. A variation of the SCR, called a Gate-Turn-Off thyristor, or GTO, makes this task easier. But even with a GTO, the gate current required to turn it off may be as much as 20% of the anode (load) current! The schematic symbol for a GTO is shown in the following illustration: (Figure below)

The Gate Turn-Off thyristor (GTO)
SCRs and GTOs share the same equivalent schematics (two transistors connected in a positive-feedback fashion), the only differences being details of construction designed to grant the NPN transistor a greater β than the PNP. This allows a smaller gate current (forward or reverse) to exert a greater degree of control over conduction from cathode to anode, with the PNP transistor's latched state being more dependent upon the NPN's than vice versa. The Gate-Turn-Off thyristor is also known by the name of Gate-Controlled Switch, or GCS.
A rudimentary test of SCR function, or at least terminal identification, may be performed with an ohmmeter. Because the internal connection between gate and cathode is a single PN junction, a meter should indicate continuity between these terminals with the red test lead on the gate and the black test lead on the cathode like this: (Figure below)

Rudimentary test of SCR
All other continuity measurements performed on an SCR will show "open" ("OL" on some digital multimeter displays). It must be understood that this test is very crude and does not constitute a comprehensive assessment of the SCR. It is possible for an SCR to give good ohmmeter indications and still be defective. Ultimately, the only way to test an SCR is to subject it to a load current.
If you are using a multimeter with a "diode check" function, the gate-to-cathode junction voltage indication you get may or may not correspond to what's expected of a silicon PN junction (approximately 0.7 volts). In some cases, you will read a much lower junction voltage: mere hundredths of a volt. This is due to an internal resistor connected between the gate and cathode incorporated within some SCRs. This resistor is added to make the SCR less susceptible to false triggering by spurious voltage spikes, from circuit "noise" or from static electric discharge. In other words, having a resistor connected across the gate-cathode junction requires that a strong triggering signal (substantial current) be applied to latch the SCR. This feature is often found in larger SCRs, not on small SCRs. Bear in mind that an SCR with an internal resistor connected between gate and cathode will indicate continuity in both directions between those two terminals: (Figure below)

Larger SCRs have gate to cathode resistor.
"Normal" SCRs, lacking this internal resistor, are sometimes referred to as sensitive gate SCRs due to their ability to be triggered by the slightest positive gate signal.
The test circuit for an SCR is both practical as a diagnostic tool for checking suspected SCRs and also an excellent aid to understanding basic SCR operation. A DC voltage source is used for powering the circuit, and two pushbutton switches are used to latch and unlatch the SCR, respectively: (Figure below)

SCR testing circuit
Actuating the normally-open "on" pushbutton switch connects the gate to the anode, allowing current from the negative terminal of the battery, through the cathode-gate PN junction, through the switch, through the load resistor, and back to the battery. This gate current should force the SCR to latch on, allowing current to go directly from cathode to anode without further triggering through the gate. When the "on" pushbutton is released, the load should remain energized.
Pushing the normally-closed "off" pushbutton switch breaks the circuit, forcing current through the SCR to halt, thus forcing it to turn off (low-current dropout).
If the SCR fails to latch, the problem may be with the load and not the SCR. A certain minimum amount of load current is required to hold the SCR latched in the "on" state. This minimum current level is called the holding current. A load with too great a resistance value may not draw enough current to keep an SCR latched when gate current ceases, thus giving the false impression of a bad (unlatchable) SCR in the test circuit. Holding current values for different SCRs should be available from the manufacturers. Typical holding current values range from 1 milliamp to 50 milliamps or more for larger units.
For the test to be fully comprehensive, more than the triggering action needs to be tested. The forward breakover voltage limit of the SCR could be tested by increasing the DC voltage supply (with no pushbuttons actuated) until the SCR latches all on its own. Beware that a breakover test may require very high voltage: many power SCRs have breakover voltage ratings of 600 volts or more! Also, if a pulse voltage generator is available, the critical rate of voltage rise for the SCR could be tested in the same way: subject it to pulsing supply voltages of different V/time rates with no pushbutton switches actuated and see when it latches.
In this simple form, the SCR test circuit could suffice as a start/stop control circuit for a DC motor, lamp, or other practical load: (Figure below)

DC motor start/stop control circuit
Another practical use for the SCR in a DC circuit is as a crowbar device for overvoltage protection. A "crowbar" circuit consists of an SCR placed in parallel with the output of a DC power supply, for placing a direct short-circuit on the output of that supply to prevent excessive voltage from reaching the load. Damage to the SCR and power supply is prevented by the judicious placement of a fuse or substantial series resistance ahead of the SCR to limit short-circuit current: (Figure below)

Crowbar circuit used in DC power supply
Some device or circuit sensing the output voltage will be connected to the gate of the SCR, so that when an overvoltage condition occurs, voltage will be applied between the gate and cathode, triggering the SCR and forcing the fuse to blow. The effect will be approximately the same as dropping a solid steel crowbar directly across the output terminals of the power supply, hence the name of the circuit.
Most applications of the SCR are for AC power control, despite the fact that SCRs are inherently DC (unidirectional) devices. If bidirectional circuit current is required, multiple SCRs may be used, with one or more facing each direction to handle current through both half-cycles of the AC wave. The primary reason SCRs are used at all for AC power control applications is the unique response of a thyristor to an alternating current. As we saw, the thyratron tube (the electron tube version of the SCR) and the DIAC, a hysteretic device triggered on during a portion of an AC half-cycle will latch and remain on throughout the remainder of the half-cycle until the AC current decreases to zero, as it must to begin the next half-cycle. Just prior to the zero-crossover point of the current waveform, the thyristor will turn off due to insufficient current (this behavior is also known as natural commutation) and must be fired again during the next cycle. The result is a circuit current equivalent to a "chopped up" sine wave. For review, here is the graph of a DIAC's response to an AC voltage whose peak exceeds the breakover voltage of the DIAC: (Figure below)

DIAC bidirectional response
With the DIAC, that breakover voltage limit was a fixed quantity. With the SCR, we have control over exactly when the device becomes latched by triggering the gate at any point in time along the waveform. By connecting a suitable control circuit to the gate of an SCR, we can "chop" the sine wave at any point to allow for time-proportioned power control to a load.
Take the circuit in Figure below as an example. Here, an SCR is positioned in a circuit to control power to a load from an AC source.

SCR control of AC power
Being a unidirectional (one-way) device, at most we can only deliver half-wave power to the load, in the half-cycle of AC where the supply voltage polarity is positive on the top and negative on the bottom. However, for demonstrating the basic concept of time-proportional control, this simple circuit is better than one controlling full-wave power (which would require two SCRs).
With no triggering to the gate, and the AC source voltage well below the SCR's breakover voltage rating, the SCR will never turn on. Connecting the SCR gate to the anode through a standard rectifying diode (to prevent reverse current through the gate in the event of the SCR containing a built-in gate-cathode resistor), will allow the SCR to be triggered almost immediately at the beginning of every positive half-cycle: (Figure below)

Gate connected directly to anode through a diode; nearly complete half-wave current through load.
We can delay the triggering of the SCR, however, by inserting some resistance into the gate circuit, thus increasing the amount of voltage drop required before enough gate current triggers the SCR. In other words, if we make it harder for electrons to flow through the gate by adding a resistance, the AC voltage will have to reach a higher point in its cycle before there will be enough gate current to turn the SCR on. The result is in Figure below.

Resistance inserted in gate circuit; less than half-wave current through load.
With the half-sine wave chopped up to a greater degree by delayed triggering of the SCR, the load receives less average power (power is delivered for less time throughout a cycle). By making the series gate resistor variable, we can make adjustments to the time-proportioned power: (Figure below)

Increasing the resistance raises the threshold level, causing less power to be delivered to the load. Decreasing the resistance lowers the threshold level, causing more power to be delivered to the load.
Unfortunately, this control scheme has a significant limitation. In using the AC source waveform for our SCR triggering signal, we limit control to the first half of the waveform's half-cycle. In other words, it is not possible for us to wait until after the wave's peak to trigger the SCR. This means we can turn down the power only to the point where the SCR turns on at the very peak of the wave: (Figure below)

Circuit at minimum power setting
Raising the trigger threshold any more will cause the circuit to not trigger at all, since not even the peak of the AC power voltage will be enough to trigger the SCR. The result will be no power to the load.
An ingenious solution to this control dilemma is found in the addition of a phase-shifting capacitor to the circuit: (Figure below)

Addition of a phase-shifting capacitor to the circuit
The smaller waveform shown on the graph is voltage across the capacitor. For the sake of illustrating the phase shift, I'm assuming a condition of maximum control resistance where the SCR is not triggering at all with no load current, save for what little current goes through the control resistor and capacitor. This capacitor voltage will be phase-shifted anywhere from 0o to 90o lagging behind the power source AC waveform. When this phase-shifted voltage reaches a high enough level, the SCR will trigger.
With enough voltage across the capacitor to periodically trigger the SCR, the resulting load current waveform will look something like Figure below)

Phase-shifted signal triggers SCR into conduction.
Because the capacitor waveform is still rising after the main AC power waveform has reached its peak, it becomes possible to trigger the SCR at a threshold level beyond that peak, thus chopping the load current wave further than it was possible with the simpler circuit. In reality, the capacitor voltage waveform is a bit more complex that what is shown here, its sinusoidal shape distorted every time the SCR latches on. However, what I'm trying to illustrate here is the delayed triggering action gained with the phase-shifting RC network; thus, a simplified, undistorted waveform serves the purpose well.
SCRs may also be triggered, or "fired," by more complex circuits. While the circuit previously shown is sufficient for a simple application like a lamp control, large industrial motor controls often rely on more sophisticated triggering methods. Sometimes, pulse transformers are used to couple a triggering circuit to the gate and cathode of an SCR to provide electrical isolation between the triggering and power circuits: (Figure below)

Transformer coupling of trigger signal provides isolation.
When multiple SCRs are used to control power, their cathodes are often not electrically common, making it difficult to connect a single triggering circuit to all SCRs equally. An example of this is the controlled bridge rectifier shown in Figure below.

Controlled bridge rectifier
In any bridge rectifier circuit, the rectifying diodes (in this example, the rectifying SCRs) must conduct in opposite pairs. SCR1 and SCR3 must be fired simultaneously, and SCR2 and SCR4 must be fired together as a pair. As you will notice, though, these pairs of SCRs do not share the same cathode connections, meaning that it would not work to simply parallel their respective gate connections and connect a single voltage source to trigger both: (Figure below)

This strategy will not work for triggering SCR2 and SCR4 as a pair.
Although the triggering voltage source shown will trigger SCR4, it will not trigger SCR2 properly because the two thyristors do not share a common cathode connection to reference that triggering voltage. Pulse transformers connecting the two thyristor gates to a common triggering voltage source will work, however: (Figure below)

Transformer coupling of the gates allows triggering of SCR2 and SCR4 .
Bear in mind that this circuit only shows the gate connections for two out of the four SCRs. Pulse transformers and triggering sources for SCR1 and SCR3, as well as the details of the pulse sources themselves, have been omitted for the sake of simplicity.
Controlled bridge rectifiers are not limited to single-phase designs. In most industrial control systems, AC power is available in three-phase form for maximum efficiency, and solid-state control circuits are built to take advantage of that. A three-phase controlled rectifier circuit built with SCRs, without pulse transformers or triggering circuitry shown, would look like Figure below.

Three-phase bridge SCR control of load
• A Silicon-Controlled Rectifier, or SCR, is essentially a Shockley diode with an extra terminal added. This extra terminal is called the gate, and it is used to trigger the device into conduction (latch it) by the application of a small voltage.
• To trigger, or fire, an SCR, voltage must be applied between the gate and cathode, positive to the gate and negative to the cathode. When testing an SCR, a momentary connection between the gate and anode is sufficient in polarity, intensity, and duration to trigger it.
• SCRs may be fired by intentional triggering of the gate terminal, excessive voltage (breakdown) between anode and cathode, or excessive rate of voltage rise between anode and cathode. SCRs may be turned off by anode current falling below the holding current value (low-current dropout), or by "reverse-firing" the gate (applying a negative voltage to the gate). Reverse-firing is only sometimes effective, and always involves high gate current.
• A variant of the SCR, called a Gate-Turn-Off thyristor (GTO), is specifically designed to be turned off by means of reverse triggering. Even then, reverse triggering requires fairly high current: typically 20% of the anode current.
• SCR terminals may be identified by a continuity meter: the only two terminals showing any continuity between them at all should be the gate and cathode. Gate and cathode terminals connect to a PN junction inside the SCR, so a continuity meter should obtain a diode-like reading between these two terminals with the red (+) lead on the gate and the black (-) lead on the cathode. Beware, though, that some large SCRs have an internal resistor connected between gate and cathode, which will affect any continuity readings taken by a meter.
• SCRs are true rectifiers: they only allow current through them in one direction. This means they cannot be used alone for full-wave AC power control.
• If the diodes in a rectifier circuit are replaced by SCRs, you have the makings of a controlled rectifier circuit, whereby DC power to a load may be time-proportioned by triggering the SCRs at different points along the AC power waveform.











CHAPTER-4

EXPERIMENTAL RESULT


• The output signal of Diode In 4007 is pulsating d.c.
• The input signal of IC 741 in pin 2 is pure d.c.
• The output of IC 741 is rectangular.
• The output of IC 74LS123 is Monoshot.
• Firing sequence is T1T2, T2T3, T3T4, T4T5, T5T6, and T6T1
• The input of IC 7404 is output of oscillator and T1, T2, T3, T4, T5, and T6
• The input of optocupler ( IC 4N25 ) is output of IC 7404
• The input of thyristor gate is output of opto-coupler.





Fig1: Practical Circuit implementation

:

Fig 2: Wave shape of output voltage of step-down transformer




Fig 3: Output Wave shape of Comparator



Fig 4: Wave shape of comparator across line voltage







Fig 5: output wave shape of Monostable Multivibrator







Fig 6: output wave shape of Inverter








Fig 7: output wave shape of Optocoupler




Fig 5: output wave shape of opto-coupler with line voltage









CHAPTER-5

CONCLUSION AND SUGGESTION FOR FURTHER WORK


5.1 CONCLUSION

The work presented in this thesis covers the design and fabrication of analog firing control scheme. The power circuits and an open loop firing angle control scheme using less hardware components have been designed and fabricated. The recorded waveforms at the different points of firing circuit are observed identical to the theoretical ones.


5.2 SUGGESTIONS FOR FURTHER WORK

The basic objective of this project work was develop and fabrication of firing a scheme for the three phase converter. The main objective of this thesis has been successfully realized. This work can be extended as listed below for further work.

 The entire work is was an open loop from however, for fast response and better stability.
 The present work is designed and fabricated for the three phase converter. It may be designed for other types of converter.
 Firing circuit using phase locked loop system can be designed and fabricated for better reliability.
 The present scheme may be tested under the presence of spike in the supply voltage.
 It may be used microprocessor based load.













REFERENCE



1. Harunur Rashid Muhamud { ( C ) 1988 By prentice –Hall, Inc.}, “POWER ELECTRONIC” Printed in USA
2. A project on “Design and fabrication of a control scheme for Inverter”, by Dr. Tapan Kumer Chakraborty
3. Electronic Circuits (Fifth edition) by Sedra/Smith
4. A.P. Malvino, Electronic Principles (2nd Ed. 1979. ISBN 0-07-039867-4) p. 476.
5. Mohammad Lutifar Rahman Dr.(First Edition June 1989), “Digital Electronics” ( Part -1 and 2)
6. Central Electricity Generating Board (1982). Modern Power Station Practice. Pergamon. ISBN 0-08-016436-6.
7. Flanagan, William (1993). Handbook of Transformer Design and Applications. McGraw-Hill. ISBN 0-0702-1291-0.
8. Pansini, Anthony (1999). Electrical Transformers and Power Equipment. CRC Press. pp. p23. ISBN 0-8817-3311-3.
9. Winders, John (2002). Power Transformer Principles and Applications. CRC. ISBN 0-8247-0766-4.
10. SEN P.C. ( C ) 1987, Third Reprint 1990, “Power Electronic” , Tata McGRAW-Hall Publishing Company Limited



THE END