What are effects of source impedance on the performance of converters?1(a) shows a single phase fully controlled converter with source inductance. For simplicity it has been assumed that the converter operates in the continuous conduction mode. Further, it has been assumed that the load current ripple is negligible and the load can be replaced by a dc current source the magnitude of which equals the average load current. Fig. 15.1(b) shows the corresponding waveforms. It is assumed that the thyristors T3 and T4 were conducting at t = 0. T1 and T2 are fired at t = . If there were no source inductance T3 and T4 would have commutated as soon as T1 and T2 are turned ON. The input current polarity would have changed instantaneously. However, if a source inductance is present the commutation and change of input current polarity can not be instantaneous. Therefore, when T1 and T2 are turned ON T3 T4 does not commutate immediately. Instead, for some interval all four thyristors continue to conduct as shown in Fig. 15.1(b). This interval is called overlap interval. During this period the load current freewheels through the thyristors and the output voltage is clamped to zero. On the other hand, the input current starts changing polarity as the current through T1 and T2 increases and T3 T4 current decreases. At the end of the overlap interval the current through T3 and T4 becomes zero and they commutate, T1 and T2 starts conducting the full load current. The same process repeats during commutation from T1 T2 to T3T4 at t = + .2. Explain the principle of chopper operation. A chopper is a high speed on" or off semiconductor switch. It connects source to load and load and disconnect the load from source at a fast speed. In this manner, a chopped load voltage as shown in Fig. is obtained from a constant dc supply of magnitude Vs. For the sake of highlighting the principle of chopper operation, the circuitry used for controlling the on, off periods is not shown. During the period Ton, chopper is on and load voltage is equal to source voltage Vs. During the period Toff, chopper is off, load voltage is zero. In this manner, a chopped dc voltage is produced at the load terminals.

Chopper Circuit Voltage and Current Waveform

3. Explain the operating principle single phase bridge inverters.Single phase uncontrolled rectifiers are extensively used in a number of power electronic based converters. In most cases they are used to provide an intermediate unregulated dc voltage source which is further processed to obtain a regulated dc or ac output. They have, in general, been proved to be efficient and robust power stages. However, they suffer from a few disadvantages. The main among them is their inability to control the output dc voltage / current magnitude when the input ac voltage and load parameters remain fixed. They are also unidirectional in the sense that they allow electrical power to flow from the ac side to the dc side only. These two disadvantages are the direct consequences of using power diodes in these converters which can block voltage only in one direction. As will be shown in this module, these two disadvantages are overcome if the diodes are replaced by thyristors, the resulting converters are called fully controlled converters.Fig.10. 1(a) shows the circuit diagram of a single phase fully controlled halfwave rectifier supplying a purely resistive load. At t = 0 when the input supply voltage becomes positive the thyristor T becomes forward biased. However, unlike a diode, it does not turn ON till a gate pulse is applied at t = . During the period 0 < t , the thyristor blocks the supply voltage and the load voltage remains zero as shown in fig 10.1(b). Consequently, no load current flows during this interval. As soon as a gate pulse is applied to the thyristor at t = it turns ON. The voltage across the thyristor collapses to almost zero and the full supply voltage appears across the load. From this point onwards the load voltage follows the supply voltage. The load being purely resistive the load current io is proportional to the load voltage. At t = as the supply voltage passes through the negative going zero crossing the load voltage and hence the load current becomes zero and tries to reverse direction. In the process the thyristor undergoes reverse recovery and starts blocking the negative supply voltage. Therefore, the load voltage and the load current remains clamped at zero till the thyristor is fired again at t = 2 + . The same process repeats there after. From the discussion above and Fig 10.1 (b) one can write For < t 0i i v = v = 2 V sint (10.1) 0 i 0 v V i = = 2 sint R R (10.2)

3. What is step up chopper? Explain?Choppers may be classified on several bases.On basis of input and output voltage levels:Step-down chopperStep-up chopperComparison between step up and step down chopperIn this converter Output Voltage is greater than input voltage. This is achieved by fast switching of a semiconductor device. The increase in output voltage depends on the frequency of switching (i.e.) depends on pulse given to the device. While switch is off the output voltage almost same as input. While switch is ON mosfet is assumed as short hence current flow in a shortest path so inductor gets energized, and in continues flow we get output voltage greater than input.

Step up Chopper or Boost ConverterStep up chopper or boost converter is used to increase the input voltage level of its output side. Its circuit diagram and waveforms are shown below in figure

Operation of Step up ChopperWhen CH is ON it short circuits the load. Hence output voltage during TONis zero. During this period inductor gets charged. So, VS= VL

Where I is the peak to peak inductor current.When CH is OFF inductor L discharges through the load. So, we will get summation of both source voltage VSand inductor Voltage VLas output voltage, i.e.

TYPE OF AC VOLTAGE CONTROLLERS The ac voltage controllers are classified into two types based on the type of nput ac supply applied to the circuit. Single Phase AC Controllers. Three Phase AC Controllers. Single phase ac controllers operate with single phase ac supply voltage of 230V RMS at 50Hz in our country. Three phase ac controllers operate with 3 phase ac supply of 400V RMS at 50Hz supply frequency. Each type of controller may be sub divided into Uni-directional or half wave ac controller. Bi-directional or full wave ac controller. In brief different types of ac voltage controllers are Single phase half wave ac voltage controller (uni-directional controller). Single phase full wave ac voltage controller (bi-directional controller). Three phase half wave ac voltage controller (uni-directional controller). Three phase full wave ac voltage controller (bi-directional controller). APPLICATIONS OF AC VOLTAGE CONTROLLERS Lighting / Illumination control in ac power circuits. Induction heating. Industrial heating & Domestic heating. Transformer tap changing (on load transformer tap changing). Speed control of induction motors (single phase and poly phase ac induction motor control). AC magnet controls.Explain the principle of operation of cycloconverter.A cycloconverter is a device that converts AC, power at one frequency into AC power of an adjustable but lower frequency without any direct current, or DC, stage in between. It can likewise be acknowledged as a static recurrence charger and holds silicon-regulated rectifiers. Cyclo-converters are used in very large variable frequency drives with ratings from few megawatts up to many tens of megawatts.Principles of Cycloconverters:The operation principles of cycloconverters can be classified into the following three types based on the type of input AC supply applied to the circuit.Single-Phase to Single-Phase Cycloconverter:

Understanding of operation principles of cycloconverters should begin with single-phase to single-phase cycloconverter. This converter is having back to back connection of two full wave rectifiers. Suppose for getting one fourth of input voltage at the output, for the first two cycles of Vs the positive converter operates supplying current to the load and it rectifies the input voltage. In the next two cycles the negative converter operates supplying current in the reverse direction. When one of the converters operates the other one is disabled, so that there is no current circulating between rectifiers. In the below figure Vs represents input supply voltage and Vo is the required output voltage which is one fourth of supply voltage.Image for One fourth of input voltage at the output using 1-phase to 1-phase Cycloconverter

Three-Phase to Single-Phase Cycloconverters:Like as above converters, three-phase to single-phase cycloconverter applies rectified voltage to the load. Positive Cycloconverters will supply positive current only while negative converters will supply negative current only. The cycloconverters can operate in four quadrants as (+v, +i), (+v, -i) rectification modes and (-v, +i), (-v, -i) inverting modes. The polarity of the current determines if the positive or negative converter should me supplying power to the load. When there is a change in current polarity, the converter previously supplying current is disabled and the other one is enabled. During the current polarity reversal, the average voltage supplied by both the converters should be equal.Three-Phase to Three-Phase Cycloconverter:Two basic configurations are available for three-phase cycloconverters such as delta and wye. If the outputs of above converter are connected in wye or delta and if the output voltages are 120 phase shifted the resulting converter is three-phase to three-phase converter. The three-phase converters are mainly used in machine drive systems running three-phase synchronous and induction machines.Explain switched mode power supply.Aswitched-mode power supply(switching-mode power supply,switch-mode power supply,SMPS, orswitcher) is an electronicpower supplythat incorporates a switching regulator toconvert electrical powerefficiently. Like other power supplies, an SMPS transfers power from a source, likemains power, to a load, such as apersonal computer, while convertingvoltageandcurrentcharacteristics. Unlike alinear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions, which minimizes wasted energy. Ideally, a switched-mode power supply dissipates no power.Voltage regulationis achieved by varying the ratio of on-to-off time. In contrast, a linear power supply regulates the output voltage by continually dissipating power in the passtransistor. This higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight.Alinear regulatorprovides the desired outputvoltageby dissipating excess power inohmic losses(e.g., in a resistor or in the collectoremitter region of a pass transistor in its active mode). A linear regulator regulates either output voltage or current by dissipating the excess electric power in the form ofheat, and hence its maximum power efficiency is voltage-out/voltage-in since the volt difference is wasted.In contrast, a switched-mode power supply regulates either output voltage or current by switching ideal storage elements, likeinductorsandcapacitors, into and out of different electrical configurations. Ideal switching elements (e.g., transistors operated outside of their active mode) have no resistance when "closed" and carry no current when "open", and so the converters can theoretically operate with 100% efficiency (i.e., all input power is delivered to the load; no power is wasted as dissipated heat).

The basic schematic of a boost converter.For example, if a DC source, an inductor, a switch, and the correspondingelectrical groundare placed in series and the switch is driven by asquare wave, the peak-to-peak voltage of the waveform measured across the switch can exceed the input voltage from the DC source. This is because the inductor responds to changes in current by inducing its own voltage to counter the change in current, and this voltage adds to the source voltage while the switch is open. If a diode-and-capacitor combination is placed in parallel to the switch, the peak voltage can be stored in the capacitor, and the capacitor can be used as a DC source with an output voltage greater than the DC voltage driving the circuit. Thisboost converteracts like astep-up transformerfor DC signals. Abuckboost converterworks in a similar manner, but yields an output voltage which is opposite in polarity to the input voltage. Other buck circuits exist to boost the average output current with a reduction of voltage.Explain fly back converter.Theflyback converteris used in bothAC/DCandDC/DCconversion withgalvanic isolationbetween the input and any outputs. The flyback converter is abuck-boost converterwith the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. When driving for example aplasma lampor avoltage multiplierthe rectifyingdiodeof the boost converter is left out and the device is called aflyback transformer.

The schematic of a flyback converter can be seen in Fig. 1. It is equivalent to that of abuck-boost converter,[1]with the inductor split to form a transformer. Therefore the operating principle of both converters is very close: When the switch is closed (top of Fig. 2), the primary of the transformer is directly connected to the input voltage source. The primary current and magnetic flux in the transformer increases, storing energy in the transformer. The voltage induced in the secondary winding is negative, so the diode is reverse-biased (i.e., blocked). The output capacitor supplies energy to the output load. When the switch is opened (bottom of Fig. 2), the primary current and magnetic flux drops. The secondary voltage is positive, forward-biasing the diode, allowing current to flow from the transformer. The energy from the transformer core recharges the capacitor and supplies the load.The operation of storing energy in the transformer before transferring to the output of the converter allows the topology to easily generate multiple outputs with little additional circuitry, although the output voltages have to be able to match each other through the turns ratio. Also there is a need for a controlling rail which has to be loaded before load is applied to the uncontrolled rails, this is to allow thePWMto open up and supply enough energy to the transformer.

Applications1. Low-power switch-mode power supplies (cell phone charger, standby power supply in PCs)2. Low-cost multiple-output power supplies (e.g., main PC supplies