Power Circuits And Transformers - Festo

PrefaceComputer-based teaching technologies are becoming more and morewidespread in the field of education, and the Data Acquisition and Control forElectromechanical Systems (LVDAC-EMS), the Data Acquisition andManagement for Electromechanical Systems (LVDAM-EMS), and the SimulationSoftware for Electromechanical Systems (LVSIM -EMS) are witness to this newapproach.The LVDAC-EMS (or LVDAM-EMS) system is a complete set of measuringinstruments that runs on a Pentium-type personal computer under the Microsoft Windows operating environment. Computer-based instruments (voltmeters,ammeters, power meters, an oscilloscope, a phasor analyzer, and an harmonicanalyzer) provide instructors the opportunity to clearly demonstrate conceptsrelated to electric power technology that, until now, could only be presentedusing traditional textbook methods and static drawings.The LVDAC-EMS (or LVDAM-EMS) system uses a customized data acquisitionmodule to interconnect modules of the Electromechanical System with thepersonal computer. Dedicated software routes the measured values from thedata acquisition module to the computer-based instruments that provide all thestandard measurements associated with voltage, current, power, and otherelectrical parameters. However, the system does much more: it provides built-incapabilities for waveform observation and phasor analysis, data storage andgraphical representation, as well as programmable meter functions, therebyallowing unimagined possibilities for presenting courseware material.LVSIM -EMS is a software that faithfully simulates the Electromechanical System(EMS). Like the LVDAC-EMS (or LVDAM-EMS) system, LVSIM -EMS runs on aPC-type computer under the Microsoft Windows operating environment.LVSIM -EMS recreates a three-dimensional classroom laboratory on a computerscreen. Using the mouse, students can install an EMS training system in thisvirtual laboratory, make equipment setups, and perform exercises in the sameway as if actual EMS equipment were used. The EMS equipment that can beinstalled in the virtual laboratory faithfully reproduces the actual EMS equipmentincluded in the Computer-Assisted 0.2-kW Electromechanical Training System(Model 8006) in every detail. As in the actual EMS system, the operation andbehavior of the circuits simulated with LVSIM -EMS can be observed byperforming voltage, current, speed, and torque measurements, using the samecomputer-based instruments as in the LVDAC-EMS (or LVDAM-EMS) system.The existing EMS courseware has been completely revised and adapted for theLVDAC-EMS (or LVDAM-EMS) system as well as LVSIM -EMS, and the newseries is titled Electrical Power Technology Using Data Acquisition. Exerciseshave been grouped in two separate manuals: manual 1, titled Power Circuits andTransformers, and manual 2, titled AC/DC Motors and Generators.Each exercise approaches the subject matter from a practical point of view, anduses a hands-on approach to the study of electrical power technology. Studentsare guided through step-by-step exercise procedures that confirm concepts andtheory presented in the exercise discussion. A conclusion and set of reviewquestions complete each exercise, and a 10-question unit test helps evaluateknowledge gained in the courseware unit. Festo Didactic 30328-10XI

PrefaceDo you have suggestions or criticism regarding this manual?If so, send us an e-mail at did@de.festo.com.The authors and Festo Didactic look forward to your comments.XII Festo Didactic 30328-10

About This ManualThe 29 exercises in this manual, Power Circuits and Transformers, provide afoundation for the study of electrical power technology. Completion of theseexercises allows students to continue with the second manual, AC/DC Motorsand Generators using data acquisition.This manual is divided into nine units:xUnits 1 to 4 provide a basic review of electrical concepts and theory, aswell as highlighting specific details relating to capacitors, inductors andsingle-phase circuits.xUnit 5 introduces and explores the concepts of vectors, phasors, andimpedance, and how they are used in analyzing ac circuit operation.xUnits 6 to 9 deal with three-phase circuits, single- and three-phasetransformers, as well as special transformer connections.The hands-on exercises in this manual can be performed using either theElectromechanical System (EMS system) or the Electromechanical System usingVirtual Laboratory Equipment (LVSIM -EMS). When using the EMS system, youshould turn on the computer and start Windows before each exercise. On theother hand, when using LVSIM -EMS, you should turn on the computer, startWindows , and start LVSIM -EMS before each exercise.The hands-on exercises guide students through circuit setup and operation, andexplore many of the measurement and observation capabilities of the virtualinstrumentation system. Much detailed information about circuit parameters(voltage and current levels, waveforms, phase angles, etc.) can be visualizedwith the virtual instruments, and students are encouraged to fully explore systemcapabilities.Various symbols are used in many of the circuit diagrams given in the exercises.Each symbol is a functional representation of a device used in Electrical PowerTechnology. The use of these symbols greatly simplifies the circuit diagrams byreducing the number of interconnections shown, and makes it easier tounderstand circuit operation. Appendix A lists the symbols used, the name of thedevice which each symbol represents, and a diagram showing the equipmentand connections required to obtain the device.The exercises in this manual can be carried out with ac network voltages of120 V, 220 V, and 240 V. The component values used in the different circuitsoften depend on the ac line voltage. For this reason, components in the circuitdiagrams are identified where necessary with letters and subscripts. A tableaccompanying the circuit diagram indicates the component value required foreach ac network voltage (120 V, 220 V, 240 V).Appendix B provides a table giving the usual impedance values that can beobtained with each of the 120-V, 220-V, and 240-V versions of the EMS loadmodules. Finally, Appendix C provides a chart outlining the exact equipmentrequired for each exercise. Festo Didactic 30328-10XIII

About This ManualSafety considerationsSafety symbols that may be used in this manual and on the equipment are listedin the Safety Symbols table at the beginning of the manual.Safety procedures related to the tasks that you will be asked to perform areindicated in each exercise.Make sure that you are wearing appropriate protective equipment whenperforming the tasks. You should never perform a task if you have any reason tothink that a manipulation could be dangerous for you or your teammates.XIV Festo Didactic 30328-10

To the InstructorYou will find in this Instructor Guide all the elements included in the StudentManual together with the answers to all questions, results of measurements,graphs, explanations, suggestions, and, in some cases, instructions to help youguide the students through their learning process. All the information that appliesto you is placed between markers and appears in red.Accuracy of measurementsThe numerical results of the hands-on exercises may differ from one student toanother. For this reason, the results and answers given in this manual should beconsidered as a guide. Students who correctly performed the exercises shouldexpect to demonstrate the principles involved and make observations andmeasurements similar to those given as answers. Festo Didactic 30328-10XV

Samples ExercisesExtracted fromthe Student Manualand the Instructor Guide

Exercise6-2Three-Phase Power MeasurementEXERCISE OBJECTIVEWhen you have completed this exercise, you will be able to calculate active,reactive, and apparent power in balanced, wye- or delta-connected, three-phasecircuits. You will know how to use a power meter to measure power in singlephase circuits. You will also know how to measure power in three- and four-wire,three-phase circuits.DISCUSSIONCalculating power in balanced three-phase circuitsAs seen in the previous exercise, the total active power ்ܲ supplied to abalanced three-phase load (i.e., the total active power dissipated in a circuit) canbe calculated using the following equation:்ܲ ൌ ൈ ܲ ௦ ൌ ሺ ܧ ௦ ൈ ܫ ௦ ൈ ߮ሻIn a wye-connected circuit, ܧ ௦ ൌ ܧ Τξ and the phase current ܫ ௦ isequal to the line current ܫ . The above equation then becomes:்ܲ ൌ ξ ൈ ܧ ൈ ܫ ൈ ߮The Τξ factor can be simplified to ξ , so that the final equation for the totalactive power dissipated in the wye-connected circuit is்ܲ ൌ ξ ሺ ܧ ൈ ܫ ൈ ߮ሻwhere்ܲis the total active power dissipated in the three-phase circuit,expressed in watts (W).In a delta-connected circuit, the above equation is obtained because the phasevoltage ܧ ௦ is equal to the line voltage ܧ , and ܫ ௦ ൌ ܫ Τξ . Therefore,in either a balanced wye-connected circuit or a balanced delta-connected circuit,the total active power ்ܲ dissipated in the three-phase circuit can be calculatedusing the equation above. Festo Didactic 30328-10251

Ex. 6-2 – Three-Phase Power Measurement DiscussionSince ሺ ܧ ௦ ൈ ܫ ௦ ൈ ߮ሻ is the expression representing the activepower ܲ ௦ dissipated in a single phase of a three-phase circuit, it follows thatthe expression ܧ ௦ ൈ ܫ ௦ represents the apparent power in a single phase.The total apparent power ்ܵ in a balanced, wye- or delta-connected, three-phasecircuit can thus be calculated using the following equation:்ܵ ൌ ሺ ܧ ௦ ൈ ܫ ௦ ሻwhere்ܵis the total apparent power in the three-phase circuit, expressed involt-amperes (VA).Following the same steps used to obtain the equation for calculating the totalactive power ்ܲ in three-phase circuits using the line voltage ܧ and the linecurrent ܫ , the equation for the total apparent power ்ܵ in a three-phase circuitcan be rewritten as follows:்ܵ ൌ ξ ሺ ܧ ൈ ܫ ሻThe power factor of a balanced three-phase circuit is the ratio of the total activepower to the total apparent power (i.e., ்ܲ Τ்ܵ ), and the relationship between ்ܲ ,்ܳ , and ்ܵ is the same as for single-phase ac circuits (i.e., ்ܵ ଶ ൌ ்ܲ ଶ ்ܳ ଶ ).Thus, the total reactive power ்ܳ in a three-phase circuit can be calculated usingthe following equation:்ܳ ൌ ට்ܵ ଶ െ ்ܲ ଶwhere்ܳis the total reactive power in the three-phase circuit, expressed inreactive volt-amperes (var).Power measurements in single-phase circuitsCommercial instruments are available to measure active, reactive, and apparentpower directly. These instruments are referred to as power meters. A selector onthe power meter usually allows the unit to measure active, reactive, or apparentpower. A power meter determines power by measuring the circuit voltage andcurrent. All power meters thus generally have at least a voltage input and acurrent input to measure the circuit voltage and current. Figure 6-9a shows thetypical connections of a power meter in a single-phase circuit and Figure 6-9bshows the equivalent connections required to measure power using the dataacquisition module.252 Festo Didactic 30328-10

Ex. 6-2 – Three-Phase Power Measurement DiscussionPower Meter CurrentinputAC powersourceAC powersourceVoltageinputܴܴ(a) Typical power meter connections(b) Equivalent connections for power measurementsusing the Data Acquisition moduleFigure 6-9. Three-phase circuit diagrams showing the connections required for powermeasurements.Measuring the total power in four-wire, three-phase circuitsMeasuring the total power in a four-wire, three-phase circuit is done by firstmeasuring the voltage and current in each phase of the circuit (i.e., the voltageacross each load element and the current flowing in each load element) andcalculating the active power and reactive power in each phase from the voltageand current measured in each phase of the circuit. The total active power (்ܲ ) inthe four-wire, three-phase circuit is simply the algebraic sum of the active powervalues obtained for the three phases of the circuit. Similarly, the total reactivepower (்ܳ ) is simply the algebraic sum of the reactive power values obtained forthe three phases of the circuit.In other words, it is like measuring the active power and reactive power in eachphase independently using three power meters and algebraically adding thethree measured power (either active or reactive) values. The total apparentpower (்ܵ ) can then be obtained by computing the vectorial sum of the totalactive power ்ܲ and the total reactive power ்ܳ . Figure 6-10 shows theconnections required to measure the total power in a four-wire, three-phasecircuit using the data acquisition module. Note that, in the circuit diagram,inputs E1 and I1, inputs E2 and I2, and inputs E3 and I3 each represent a powermeter. Festo Didactic 30328-10253

Ex. 6-2 – Three-Phase Power Measurement DiscussionL1L2L3ܴଵܴଶܴଷFigure 6-10. Three-phase power measurement using three power meters.The method of power measurement shown in Figure 6-10 works whether thethree-phase circuit is balanced or not.Measuring the total power in three-wire, three-phase circuits (twowattmeter method)A three-wire, three-phase circuit is simply a three-phase circuit with three lineconductors but no neutral conductor. Three-wire, three-phase circuits are usedcommonly because they allow three-phase power to be conveyed using threeconductors instead of four conductors. This makes three-wire, three-phasecircuits more economical than four-wire, three-phase circuits.The method for measuring the total power in four-wire, three-phase circuitsdiscussed in the previous section cannot be used to measure the total power inthree-wire, three-phase circuits. For instance, when the load is connected in awye configuration, the phase currents can be measured but the phase voltages(voltage across each load element) cannot because the neutral point generally isnot available to connect the voltage inputs of the power meters, as Figure 6-11shows.254 Festo Didactic 30328-10

Ex. 6-2 – Three-Phase Power Measurement DiscussionWye-connected loadܴଵL1ܴଶL2ܴଷL3?Figure 6-11. Diagram of a three-wire, wye-connected, three-phase circuit showing that thevoltage inputs of the power meters generally cannot be connected to the neutral point of thecircuit.Similarly, when the load is connected in a delta configuration, the phase voltagescan be measured but the phase currents (current flowing through each loadelement) cannot be measured because individual access to each load elementgenerally is not possible (i.e., it is impossible to connect the current inputs of thepower meters to measure the phase currents), as Figure 6-12 shows. Festo Didactic 30328-10255

Ex. 6-2 – Three-Phase Power Measurement DiscussionDelta-connected loadL1ܴଵL2ܴଷܴଶL3?Figure 6-12. Diagram of a three-wire, delta-connected, three-phase circuit showing that thecurrent inputs of the power meters cannot be connected to measure the phase currents.To measure the total power (either the total active power ்ܲ , the total reactivepower ்ܳ , or the total apparent power ்ܵ ) in three-wire, three-phase circuits, amethod using only two power meters can be used. This method is usuallyreferred to as the two-wattmeter method because historically, it was firstimplemented with two wattmeters instead of two power meters. Figure 6-13shows the connections of the voltage and current inputs of the two power metersrequired for the two-wattmeter method of measuring three-phase power. Notethat the voltage and current inputs of the power meters must be connected withthe polarity indicated in the figure in order to obtain correct power measurements.256 Festo Didactic 30328-10

Ex. 6-2 – Three-Phase Power Measurement ଵܴଵL2ܴଶORܴଷܴଶL3ܴଷFigure 6-13. Connections of the voltage and current inputs of the power meters to a three-wire,three-phase circuit when measuring the total power using the two-wattmeter method.The total active power (்ܲ ) in the three-wire, three-phase circuit is simply thealgebraic sum of the active power values indicated by the two power meters.Similarly, the total reactive power (்ܳ ) is simply the algebraic sum of the reactivepower values indicated by the two power meters. The total apparent power (்ܵ )can then be obtained by computing the vectorial sum of the total active power ்ܲand the total reactive power ்ܳ . This method of power measurement workswhether the three-phase circuit is balanced or not.Measuring the total power in four-wire, three-phase circuits using the twowattmeter methodThe two-wattmeter method of power measurement can also be used to measurethe total power (either active, reactive, or apparent) in four-wire, three-phasecircuits. This can be useful because the two-wattmeter method requires only twopower meters (i.e., two voltage inputs and two current

Ex. 8-2 Transformers in Parallel . 347 Connecting transformers in parallel to supply greater load power. Measuring the efficiency of parallel-connected transformers. Ex. 8-3 Distribution Transformers . 355 Introduction to basic characteristics of distribution transformers.