ELECTRIC MACHINES (TRANSFORMERS)

Transcription

1 ELECTRIC MACHINES (TRANSFORMERS) USER MANUAL

2 CONTENTS. INTRODUCTION.... OVERVIEW..... Functionality..... Specifications SAFETY REQUIREMENTS HARDWARE AND SOFTWARE System Architecture Test Bench hardware Other required equipment Required software Test Bench Modules Description HARDWARE ASSEMBLY Installing NI PXI modules on the chassis Connections to the NI PXI data acquisition modules Grounding the chassis Connecting power SOFTWARE INSTALLATION AND SETUP Installation of NI Device Drivers Laboratory Software Installation Setting up PXI module addresses in MAX TEST BENCH SOFTWARE Running the software and getting started Lab s Interactive Block Diagram Graph window Scope window Time diagrams tab Vector diagrams window Power Modules on the Interactive Diagram Terms used in this manual: GETTING STARTED Background knowledge and skills prerequisites Preparation of the Test Bench HANDS-ON EXPERIMENTS No-load mode and determination of the transformation ratio Short circuit mode External characteristics of -phase transformer Parallel operation of -phase transformers Determining a 3-phase transformer windings polarity Asymmetric loading of 3-phase transformers TEST BENCH COMPONENTS SPECIFICATIONS TROUBLESHOOTING Problems and possible solutions Replacing a blown fuse... 6

3 . INTRODUCTION The Transformers Laboratory can be used for hands-on study of the Theory of electrical machines and transformers. The lab software has a simple and intuitive user interface. The student may choose the desired subject from the menu, working with step-by-step instructions according to individual circuits for each lab, as provided by the lab manual. Detailed help screens are included in the lab software so that the students may refresh their memory with related topics without laboratory work interruption. The experimentally obtained data can be saved in MS Excel format for offline work, reviewing, and grading. The Test Bench has been designed based on the NI PXI hardware platform with software developed in the NI LabVIEW graphical programming environment using the virtual instruments technology of National Instruments.. OVERVIEW.. Functionality The Test Bench has been developed as a tool for conducting 6 educational hands-on experiments: 4 labs on the one-phase transformer properties and labs on three-phase transformer properties. The student works with the software installed on the personal computer. Before starting the experiments the student shall register by entering his name and group. In each lab appropriate wiring diagrams are displayed on the front panel (with managed configuration items). The software allows you to control the progress of work and conduct an interactive settings of parameters. All the control functions and data acquisition are fully automated. Scope patterns, graphs, and numeric values based on the obtained experimental results are shown on the screen. The results can be saved as an MS Excel file along with the appropriate student data and date of conducting the experiment. The Test Bench provides the possibility to conduct the following hands-on experiments:. No-Load mode and determination of transformation ratio. Short-circuit mode 3. External characteristics of a -phase transformer 4. Parallel operation of -phase transformers 5. Determining a 3-phase transformer windings polarity 6. Asymmetric loading of 3-phase transformers All the hands-on labs are conducted on the Transformers Test Bench... Specifications. Power supply: three-phase mains network /38V, 5Hz. Max. power consumption:.5 kw 3. Dimensions: 5xx4mm 4. Max. weight: 3 kg The Test Bench should be used indoors, at C temperature range, with relative humidity not exceeding 8% at 5 C.

4 3. SAFETY REQUIREMENTS Please carefully read the safety instructions before starting to work with the laboratory setup. Follow the safety arrangements when connecting the hardware, during the hands-on experiments, or when disconnecting the system. The power supply of the device should not be disassembled. No third-party power sources shall be connected to the NI PXI platform and no external power or other electrical connections should be made to it. The power supplies of NI PXI and of any components of the laboratory setup should only be connected to a properly grounded wall socket. The presence of any foreign objects on the table is not allowed during the lab. The Test Bench can only be used indoors. Do not power the setup in an atmosphere containing flammable gases or in proximity of highly flammable liquids. There are no user-serviceable parts in the system. Technical maintenance shall only be provided by qualified personnel. In case of failure of the device or if a burning smell of wires or components is detected, please turn OFF the setup immediately, disconnect the power cord from the wall socket and seek qualified service from an appropriate agency. The Test Bench shall only be operated in the absence of electrical shock risk factors, such as extreme humidity, condensation, lack of grounding, etc. The body of the Test Bench should be connected to a proper grounding network through a resistive loop not exceeding 4 Ohm. Any maintenance operations can only be performed when the Test Bench is disconnected from the mains. All the students should receive obligatory safety instructions prior to starting work with the Test Bench. The Students who received the instructions shall register in a special log book, confirming with signature that they received the safety instructions and agree to the terms and conditions of using the Test Bench. Students who didn t receive the instructions are not allowed to use the Test Bench; students who violated the terms of the safety requirement shall not be allowed to perform further on the hands-on labs. The following safety requirements are obligatory:. The Test Bench should only be connected to the mains network with the approval and in the presence of the instructor. The electric connections should be made in accordance with the provided circuit diagrams and only when the Test Bench is disconnected from the mains network The following is forbidden:. Conducting any experiments or tests in the absence of the instructor. Connecting and disconnecting wires during the hands-on lab 3

5 4. HARDWARE AND SOFTWARE 4.. System Architecture The overall system architecture is shown in the Fig. 4.-: Fig. 4.- The overall system architecture The system consists of the Transformers Test Bench working in conjunction with the NI PXI system. 4.. Test Bench hardware Standard hardware of the workbench:. Front Panel with the following modules:.. Power supply module.. One-phase autotransformer.3. One-phase transformer.4. Three-phase transformer.5. Adjustable load module.6. Meters. Cable for NI PXI-4 3. Connector wires set 4. Allen key wrench 5. Software CD QTY QTY 4.3. Other required equipment NI PXI-33 Integrated MXIe, 5 Periph. Slots, Port PCIe, 3m Cable NI PXI-4 Triple Output DC Power Supply with APS-4 Aux Power NI PXI-65 (6 Analog inputs, 4 Digital I/O, Analog Outputs) SHC68-68-EPM Shielded Cable, 68-D-Type to 68 VHDCI Offset, m Power Cord, 4V, A, Euro Class Personal computer (refer to System Requirements below) Minimum System Requirements CPU RAM Hard disk Other requirements Intel or AMD, GHz or faster GB or more at least 5 GB of free space a free PCI Express slot Other equipment can be possibly used upon consent of test bench manufacturer 4

6 9. HANDS-ON EXPERIMENTS 9.. No-load mode and determination of the transformation ratio Hands-on objectives In this lab we shall study the transformer in no-load mode. Based on the obtained results we shall:. Determine the transformation factor by voltage.. Determine the no-load characteristics I, P,cos j, Z = fv ( ), when I =. Theory No-load characterization is an important part of tests that each manufactured transformer has to be subjected to. The objective of these tests is to verify whether the transformer meets the standard requirements or technical specifications, according to which it was manufactured. In the process of the no-load tests it is possible to determine the transformation factor, the no-load current, magnetic losses, and the reactive power required for core magnetization. In no-load mode (idling) the transformer s primary winding is supplied from an AC voltage source, and the secondary winding is unconnected (Fig. 9.-). E E where: m Fig. 9.- Transformer circuit in no-load mode = 4.44WfФ, (9..) = 4.44W fф, (9..) m W, W the numbers of turns of the primary and secondary windings, Ф magnetic flux amplitude, m f AC frequency. In no-load mode, when the currents are insignificant: E ª V, E ª V, and the transformation factor equals to: where W E V K W E V = = =, (9..3) V, V input and output voltage effective values. The transformer s power P in no-load mode is comprised of core (iron) loss winding copper loss P = I r, сopper where I - primary winding current under no load r - primary winding active resistance P сore and the primary 8

7 Therefore, P = P + Р (9..4) core copper The no-load current I makes 5 8% of the nominal current, and copper loss P сopper is usually less than % of P. Therefore, with precision suffice for practical applications, we may consider that the power consumed in idling is mainly due to core losses, i.e. Р ª Рcore P = V I cosj (9..5) Since the magnetic flux in the transformer is virtually the same in nominal and in no-load modes, the losses in nominal mode are usually taken as equal to idling losses ( P ). The equivalent circuit of the transformer in no-load mode is shown in Fig.9.-. r and x on the diagram are the active and reactive resistances, respectively, which determine the voltage drop on the transformer s primary winding, whereas r and m x m are resistances of the magnetization circuit, which determine the active and reactive component of the EMF E. Fig.9.- Transformer s equivalent circuit in no-load mode The equivalent circuit parameters in idling can be found by the following expressions: P r = r + rm = I (9..6) V Z = I (9..7) = x + x = Z r (9..8) x m - Parameters Z, r, x vary depending on voltage V, thus the dependences between voltage V and current I are generally non-linear. Considering that the transformer s losses in idling are primarily comprised of core magnetic losses, we can take that: r = r + r m ª r m, (9..9) P r m ª. I (9..) Considering that the inductive voltage drop on the primary winding I x is insignificant compared with the value of I xm, it is reputed that: x = x+ xm ª xm, (9..) Z V m ª. (9..) I 9

8 To obtain the idling characteristics we shall vary the voltage supplied to the primary winding, measure the supplied voltage, current and power factor, and calculate the power and resistance values. Based on the obtained results the dependences I, P,cos j, Z = fv ( ) are to be plotted. The no-load characteristics are shown in Fig Hands-on procedure Fig.9.-3 Transformer no-load characteristics During the lab the secondary winding is disconnected, and the primary winding is supplied with AC voltage of nominal frequency from a regulated voltage source (Fig. 9.-4). Measurements should be done at several values of the supply voltage in the.3v r -.V r range. The transformation factor is to be determined at nominal voltage or lower. Required equipment. -phase transformer. Power Switch module 3. Autotransformer 4. Voltmeters (V, V ) 5. Ammeter (A ) Preparations Fig The no-load lab circuit. Select No-load mode and determination of the transformation ratio from the labs menu.. Assemble the circuit in Fig Note: To measure the value of cosj correctly pay attention to the polarity of ammeters and voltmeters: whenever an ammeter and a voltmeter have a common connection point, make sure that they are connected to that point by the plug of same color. 3

9 Fig.9.-5 Schematic diagram of lab connections Request the instructor to check the circuit connections before continuing your work on the lab. 3. Make sure that the switch K is disconnected and autotransformer knob is in the Min position. 4. Set the automatic switch (, Fig. 4.5-) to the ON position and activate the turnkey (5, Fig. 4.5-) to enable the supply of power to all circuits on the Test Bench. 5. Check the tick marks near A, V, V and Phase Meter to enable the corresponding meters. In the drop-down menus for j and j on the Phase Meter module select A and V, respectively. Step-by-step instructions. Activate the switch K on the lab Front Panel.. Gradually turn the autotransformer knob clockwise to increase the voltage up to the value of.v r (V =4V) and record the indications of the meters by clicking Record on the Front Panel after each measurement. 3. Gradually reduce the voltage up to.3v r (V =66V), recording the indications of the meters (4-5 points). For visual representation of the obtained graphs in the process of the experiment use the Graph window 4. When finished, click Stop. 5. Recorded indications of the meters are automatically saved in an Excel file, which can be opened by clicking Excel on the Front Panel. Report The report shall include:. A table with meter readings at each step, along with calculated values (Table 9-).. Characteristic graphs of I, P,cos j, Z as a function of voltage fv ( ), obtained through the experiment. Table 9-3 Experimental values Calculated values I V V cosφ P K Z x r A V V - W - Ohm Ohm Ohm 3

10 4 5 Expressions V K = V P = VIcosj V Z = I P r = r + rm = I x = x + x = Z - r m Test questions. How are the EMF and the voltage of the transformer secondary winding correlated?. How are the EMFs of the primary and secondary windings of the transformer correlated? 3. What comprises power consumed by the transformer in idling? 4. How are the core losses in the nominal mode and under no-load correlated? 3

11 . TEST BENCH COMPONENTS SPECIFICATIONS Specifications for power modules are given in Table -: Table - Power modules Power module Power and 3-phase network connection/disconnection P (kw) f (Hz) V (V) V out (V) /38 Adjustable AC voltage Variable Load Specifications of the transformers are given in Table -. Table - Transformers Power module P (kw) f (Hz) V r (V) V r (V) I r (A) I r (A) One-phase transformers Three-phase transformer The three phase transformer module contains wafer switches S and S. Depending on the position of these switches, different interconnections are provided between transformer windings and terminals on the Front Panel. Connections of windings in position of the switches are given in Table -3 and Table -4. Table -3 Primary winding switch (S) in position Terminal # Marking Terminal # Marking Terminal # Marking A 3 B 5 C X 4 Y 6 Z Table -4 Secondary winding switch (S) in position Terminal # Marking Terminal # Marking Terminal # Marking 7 a 9 b c 8 x y z Before starting the lab 9.5 Determining a 3-phase transformer windings polarity the instructor changes the positions of switches S and S. During the experiment the student must determine the connection (phasing, the beginning and end) of each winding to the terminals. In all other labs the switches S and S should remain in position. 6