People s Democratic Republic of Algeria Ministry of Higher Education and Scientific Research University M Hamed BOUGARA Boumerdes

Transcription

1 People s Democratic Republic of Algeria Ministry of Higher Education and Scientific Research University M Hamed BOUGARA Boumerdes Institute of Electrical and Electronic Engineering Department of Power and Control Final Year Project Report Presented in Partial Fulfilment of the Requirements for the Degree of MASTER In Electrical and Electronic Engineering Option: Control Title: Virtual Measurement System for AC Machine Using Labview Presented by: - HENNICHE Feriel - KEDDACHE Nadia Supervisor: Dr. OUADI Abderrahmane Registration Number:..../2016

2 Dedication I would like to dedicate this humble work to my dear mother and father, to my lovely brother, to all my friends without exception and for those special persons who had always been by my side during all this five years. Feriel I would like to dedicate this modest work to my dear mother and father, to my lovely brothers and sisters, to my fiancée who had been by my side, to my supervisor and to all my friends without exception. Nadia i

3 Acknowledegment First and foremost, we are thankful to God, the most gracious most merciful for helping us finish this modest work. It is our belief in him that helped us presevere at times when it seemed impossible to go on. Secondly, we are highly appreciative of the effort of our supervisor Dr «Oudi. Abd El rahman» for taking time to read through this report and his positive criticism and advices for the project We extend our thanks to our dear parents who helped us during our studies and to all our friends, specially the once who worked with us in laboratory C005 and A208. Finally we would like to express our deep gratitude to all IGEE teachers, without forgetting the library staff and security for their valuable help, all other members of INELEC community for their kidness and great help. ii

4 Abstract It is widely accepted that contemporary scientific experimentation on electrical machines requires extensive measurements and acquisition of various data. In this project the development of virtual measurements system based on labview software environment is described. This system is able to perform an essentially large number of simultaneous electrical and mechanical physical variables measurements of an AC induction machine. In the studied case only voltages, currents (of the three phase system ),speed and torque are measured and recorded in real time, while power, reactive power, apparent power,total harmonic distortion etc are calculated according to the IEEE(Institute of Electrical and Electronic Engineering) std A graphical user interface is created to allow an accurate measurements display and data recording using TDMS function (Writes data to binary measurement files (.tdm or.tdms)). iii

5 Table of contents Dedication... i Acknowledgement... ii Abstract... iii Table of contents... iv List of figures... vii List of lables... ix List of equations... x Introduction... 1 Chapter I: Induction motor theory and measurements I.1 Overview of the system... 3 I.2 Three phase load system studies... 5 I.2.1 Three phase system... 5 I.2.2 Y configuration characterization... 5 I Current and voltage... 5 I Power in three phase system... 6 I.3 induction motor... 8 I.3.1 Definition... 8 I.3.2 Operation of induction motor... 9 I.3.3 Induction motor circuit model I.4 Current measurements I.4.1 current overview I.4.2 Current measurements methods I.5 Voltage measurements I.5.1 Voltage overview I.5.2 Voltage measurements methods I.6 Torque measurements I.6.1 Force and torque measurements I.6.2 Strain gauge sensor iv

6 Table of contents Chapter II: Hardware implementation II.1 Current measurements and design II.1.1 Current sensor II.1.2 supply circuit II.1.3 practical results II.1.4 signal conditioning circuit II.2 Voltage measurements simulation and design II.2.1 Signal conditioning circuit for data acquisition II.3 Torque measurements simulation and design II.3.1 Wheatstone bridge II.3.2 Torque sensor II.3.3 supply circuit II.3.4 practical results II.3.5 Signal conditioning circuit for data acquisition II.3.6 Non linearity calculation II.4 Speed and frequency measurement simulation and design II.4.1 supply circuit II.4.2 frequency measurements II.4.3 Non linearity calculation Chapter III: Software implementation III.1 NI LABVIEW III.1.1 LABVIEW front pannel III.1.2 LABVIEW block diagram III.2 Data Acquisition(DAQ) III.3 NI-DAQ max III.4 DAQ 6009 board III.4.1 Using NI USB 6009 in LABVIEW III.5 Software implementation III.6 Experimental results v

7 Table of contents III.6.1 Discussion III.6.2 Data logging Conclusion Appendix A Appendix B bibliography vi

8 List of figures Chapter I : induction motor theory and measurements Figure 1.1: System block diagram... 4 Figure 1.2: System overview... 4 Figure 1.2.1: Three phase power system Y configuration... 5 Figure 1.2.3: Power triangle Figure 1.3.1: Equivalent Circuit Parameters of an induction Motor Figure 1.4.1: Current transformer configuration Figure 1.4.2: Rogowski Coil configuration Figure : Hall effect principle Figure 1.5.1: Voltmeter insertion Figure 1.5.2: Schematic arrangement of a capacitive divider Figure 1.5.3: Capacitive and voltage transformer device for extremely high voltage Figure 1.6.1: Strain gauge as a serpentine structure Figure 1.6.2: Alternative use of a strain gauge for measuring the force Chapter II :Hardware implementation Figure 2.1.1: Construction of a typical closed loop sensor Figure 2.1.2: Design methodology fo current sensing Figure : Supply and testing circuits Figure 2.1.4: I p current (A) measurement results Figure 2.1.5: Measurement of the output current of the LA 25-P current transduser Figure 2.1.6: Total non linearity Figure 2.1.7: Midpoint nonlinearity Figure 2.3.1: Wheatstone bridge Figure 2.3.2: Design methodology for torque sensing Figure 2.3.2: voltage output vs weight input Figure 2.4.1: Output of the speed sensor Figure 2.3.1: Voltage vs frequency Chapter III : Software description Figure 3.1.1: Front panel window Figure 3.1.2: Block diagram window vii

9 List of figures Figure3.1.3: Tools, Controls and function pallets Figure 3.2.3: The process bolck diagram Figure 3.4.1: NI USB Figure 3.4.2: DAQ max data acquisition palette Figure 3.4.3: Setting the DAQ Assistant Figure 3.4.5: Different setting to recorded the data Figure 3.5.1: Data acquisition block diagram Figure 3.6.1: instruments shows voltage, current waveforms and spectra of three phase system Figure 3.6.2: virtual instrumentation in three phase operation mode Figure 3.6.3: virtual instrument for the frequency and speed in rpm Figure 3.6.4: EXCEL representation for the recorded parameters Figure 3.6.5: Recorded voltages waveforms viii

10 List of tables Table 1.5.1: the most commonly employed voltmeters.13 ix

11 List of equations Chapter I : induction motor theory and measurements Line to line voltages in terms of line to neutral voltages Phasors expressions of the line to line voltages Line to line current in Y connection Line to line voltage in Y connection apparent power for single phase current Apparent power for three phase system Active power for single phase system Active power for three phase system Reactive power for single phase system Reactive power for three phase system Synchronous speed of rotation in rad/s Synchronous speed of rotattion in rev/min Slip equation slip speed in rad/s slip speed in rev/ min frequency of the rotor Nominal ratio of transformer Actual ration of transformer toreque equation Newton s Second Law The principle of torque comparision Force measurement using spring transducer Chapter II :Hardware implementation 2.1 instrumentation gain equation Midpoint non linear calculation percentage of non linearity Equation of the torque instrument the total frequency conversion x

12 Introduction The basic of any scientific experimentation is to perform measurement. When complex phenomena need to be studied like harmonic distortion, power dissipation, speed and torque measurements, sophisticated equipments are required. Such instruments are multichannels oscilloscopes; measurement recorders, speed measurement equipments etc are very expensive. [1] In industry, 3 phase AC motor monitoring and parameter calculation are very important for an efficient operation, a continuous observation and measurement of signals (current, voltage, power etc) is required, where sophisticated and expensive equipments are used and most of the time to give an accurate results. Data acquisition devices interfaced with LABVIEW solved all these problems by offering accurate and high speed virtual measurement system that use standard PCs to store, process and present very large amounts of measurements data. The real time electrical and mechanical variables of the electrical machine are measured using DAQ board and acquired using virtual instrument designed using LABVIEW software [2]. The designed data acquisition program allows a continuous graphical observation of all inputs and outputs, and may include protective function and generates digital outputs to shut down the system in case of exceeding the safety conditions. The main objective of this project is to design a data acquisition measurement system based on LABVIEW for measurement of 3 phase ac machine electrical and mechanical variables. Our project is subdivided into hardware and software parts. In the hardware part, we are going to design and implement the required circuitry to measure the current, voltage, speed, and torque of a squirrel cage rotor type induction motor. The hardware part is based on LA25-P current sensor, step down voltage sensor, M18 photo-cell speed sensor, SPU1 load cell force sensor, and the signal conditioning circuits for adapt the output signals of the sensors into the input range of the DAQ. In the software part, the output signals of the sensors will be acquired to LABVIEW using the NI USB The currents and voltages waveforms will be displayed in the LABVIEW front panel and the different parameters will be calculated. LABVIEW allows the data logging, 1

13 Introduction Using the TDMS function, the data will be recorded and exported to EXCEL to allow system reviewing at almost of any instant of time. 2

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15 Chapter I Induction motor theory and measurements Introduction Monitoring an ac motor is necessary for operating efficiently. There are many undesirable things that happen to electrical motors and other electrical equipment as a result of operating a power system in an over voltage manner. Operating a motor beyond its nominal range of its requirement will reduce its efficiency and cause premature failure. The economic loss from premature motor failure can be devastating. [1] So the best life and most efficient operation occur when motors are operated close to the nameplate rating. The basis of ensuring an efficient operation of the motor is the ability to perform permanent accurate measurements. This chapter presents an introduction to the various methods of measurements required. I.1 Overview of the system A system of measurement is a collection of units of measurement and rules relating them to each other. The system consists of Hall Effect typed current, voltage transformers, torque and speed sensors. Next, all this signals are collected and properly sampled by the heart of the system which is the data acquisition device. The use of signal conditioning circuit is necessary to be in the operating range of the data acquisition device where the signal is converted into digital form and is acquired by the software (LABVIEW). 3

16 Chapter I Induction motor theory and measurements Figure 1.1: System block diagram Figure 1.2: System overview. 4

17 Chapter I Induction motor theory and measurements I.2 Three phase load system studies I.2.1 Three phase system The voltages in the three-phase power system are produced by a synchronous generator. In a balanced system, each of the three instantaneous voltages has equal amplitudes but is separated from the other voltages by a phase angle of 120. The three voltages (or phases) are typically labeled a, b and c. The common reference point for the three phase voltages is designated as the neutral connection and is labeled as n. We may define either a positive phase sequence (abc) or a negative phase sequence (acb). The three sources V an,v bn and V cn are designated as the line-to-neutral voltages in the three-phase system. [3] Figure 1.2.1: Three phase power system Y configuration Three phase systems are used for two reasons: 1. The three vector-spaced voltages can be used to create a rotating field in a motor. Motors can thus be started without the need for additional windings. 2. A three-phase system can be connected to a load such that the amounts of copper connections required (and thus the transmission losses) are one half of what they would otherwise be. [3] I.2.2 Y configuration Characterization I Current and voltage An alternative way of defining the voltages in a balanced three-phase system is to define the voltage differences between the phases. These voltages are designated as line-to-line voltages. The line-to-line voltages can be expressed in terms of the line-to-neutral voltages by applying Kirchhoff s voltage law to the generator circuit, which yields to: [3] (1.1) (1.2) 5

18 Chapter I Induction motor theory and measurements (1.3) By inserting the line-to-neutral voltages for a positive phase sequence into the line-to-line equations yields to the following results:[3] (1.4) (1.5) (1.6) In Y connection the line current is the same as the phase current whereas the line voltage is the phase voltage scaled by. (1.7) (1.9) I Power in three phase system Total electrical power consumption depends on real power electrical energy consumption, and reactive power that can be expressed by power triangle. [4] Where: Figure1.2.3: power triangle. 6

19 Chapter I Induction motor theory and measurements 1.Apparent power Apparent power is measured in volt amperes (VA) and is the voltage on an AC system multiplied by all the current that flows in it. It is the vector sum of active and reactive power. [4] Single phase current (1.10) Where : Three phase system (1.11) Where: V: rms line voltage. I: rms line current. 2.Active power Active (real or true) power is measured in watt (w) and is the power drawn by the electrical resistance of a system doing useful work: [4] Single phase (1.12) Where: Three phase 3.Reactive power (1.13) 7

20 Chapter I Induction motor theory and measurements Reactive inductive power (Q) is measured in volt amperes reactive and is the power stored in and discharged by the inductive motors. Reactive power required by inductive loads increases the amount of apparent power measured in Kilovolt amps (KVA) in the distribution system. Increasing the reactive and apparent power causes the power factor (PF) to decrease. [4] Single phase (1.14) Three phase : (1.15) I.3 Induction motor I.3.1 Definition The three-phase induction motor, also called an asynchronous motor, is the simplest and the most commonly used type of motor in industrial applications known as the work horse of an industry. The electrical section of the three-phase induction motor consists of two parts, a fixed stator or frame and a turning rotor. There is no electrical connection between the stator and the rotor. The currents in the rotor are induced via the air gap from the stator side. Stator and rotor are made of highly magnetizable core sheet providing low eddy current and hysteresis losses. [5] The stator winding is the outer body of the motor which consists of three individual windings which overlap one another and are offset by an electrical angle of 120. When it is connected to the power supply, the incoming current will first magnetize the stator. [6] This magnetizing current generates a rotary field which turns with synchronous speed ns. The arrangement of the windings determines the number of poles that the motor has. [7] The induction motor rotors are of two types: wound and squirrel cage. The squirrel cage rotor consists of a slotted cylindrical rotor sheet package with aluminum bars which are joined at the front by rings to form a closed cage where no isolation is required between the core and 8

21 Chapter I Induction motor theory and measurements the bars. Whereas the wound rotor is used when variable speed is required the rotor and the stator have the same number of poles. In this type an external controller is used such as a variable resistor that allows changing the motor s slip rate. Compared to the squirrel cage rotors wound rotor are expensive and require maintenance of the slip rings and brunches. [6] I.3.2 Operation of induction motor The induction motor operates by virtue of currents induced from the stator field in the rotor. The operation of induction motor is similar to that of a transformer where the stator acts as a primary coil and the rotor acts as a secondary coil. Air gap acts as the dielectric medium which separates the rotor from the stator without having any physical contact. When an alternating current is provided to the rotor it produces the rotating magnetic flux around the stator. When the stator field is initially applied, the rotor field is synchronous with it, and the fields are stationary with respect to each other. Thus, a starting torque is generated. An induction motor can never reach the synchronous speed. If it did, the rotor and the rotating stator field would appear to be stationary to each other, since it would be rotating at the same speed. But in the absence of the relative motion between the stator and the rotor fields, no voltage would be induced in the rotor. Thus the speed of the induction motor is limited below the synchronous speed.[7] If the angular of the rotor, in is and that in is, then the synchronous speed of rotation is given by: (1.16) (1.17) Where p is the number of the poles and is the stator voltage frequency. The difference between the synchronous speed and the rotor speed in rated operation is called slip s and is generally expressed in percent and given by the following equation: (1.18) 9

22 Chapter I Induction motor theory and measurements Since the rotor rotates at a speed lower than the synchronous speed, the slip speed in term of is given by: (1.19) And in term of (1.20) Therefore the frequency of the rotor can be written as: (1.21) I.3.3 Induction motor circuit model The induction motor is essentially that of the transformer, where the stator is the primary and the rotor is the secondary winding. Except that the rotor values are at slip frequency. The equivalent circuit is as shown in figure 1.3.1[8] where: : Stator resistance per phase. : Rotor resistance per phase. : Stator reactance per phase. : Rotor reactance per phase. : Magnetizing (mutual) reactance. : Equivalent core-loss resistance. : Per phase induced voltage in stator winding. : Per phase induced voltage in rotor winding. : Per phase input voltage. 10

23 Chapter I Induction motor theory and measurements Figure 1.3.1: Equivalent Circuit Parameters of an Induction Motor. I.4 Current measurements I.4.1 Current overview Electric current is the flow of electric charge. The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second. Current sensing is used to perform two essential circuit functions. First, it is used to measure how much current is flowing in a circuit, which may be used for power management in a DC/DC power supply to determine essential peripheral loads to conserve power. The second function is to determine when it is too much, or a fault condition. If current exceeds safe limits, then a software or hardware interlock condition is met and provides a signal to turn off the application, as in a motor stall or short circuit condition in a battery. [9] I.4.2 Current measurement methods A signal to indicate the how much condition and the too much condition is available in a variety of different measurement methods, some of these methods are listed below: 1.Current Transformer A current transformer as shown in figure provides three key advantages: isolation from line voltage; lossless current measurement; and a large signal voltage that can provide noise immunity. This indirect current measurement method requires a changing current - such as an AC, transient current, or switched DC - to provide a changing magnetic field that is magnetically coupled into the secondary windings. The secondary measurement 11

24 Chapter I Induction motor theory and measurements voltage can be scaled according to the turn s ratio between the primary and secondary windings. This measurement method is considered lossless because the circuit current passes through the copper windings with very little resistive losses. Figure1.4.1: Current transformer configuration. 2.Rogowski Coil the Rogowski coil is an air core design as opposed to the current transformer that relies upon a high-permeability core, such as laminated steel, to magnetically couple to a secondary winding. The air core design has a lower inductance to provide a faster signal response and very linear signal voltage. Because of its design, it is often used as a temporary current measurement method on existing wiring such as a handheld meter. This could be considered a lower-cost alternative to the current transformer. [9] Figure 1.4.2: Rogowski Coil configuration. 12

25 Chapter I Induction motor theory and measurements 3.Hall Effect When a current-carrying conductor is placed into a magnetic field, a voltage will be generated perpendicular to both the current and the field. This principle is known as the Hall Effect. Figure 1.4.3: Hall Effect principle. Figure (1.4.3) [9] illustrates the basic principle of the Hall Effect. It shows a thin sheet of semiconducting material (Hall element) through which a current is passed. The output connections are perpendicular to the direction of current. When no magnetic field is present, current distribution is uniform and no potential difference is seen across the output. When a perpendicular magnetic field is present, a Lorentz force is exerted on the current. This force disturbs the current distribution, resulting in a potential difference (voltage) across the output. This voltage is the Hall voltage. Hall Effect sensors can be applied in many types of sensing devices. [9] I.5 Voltage measurements I.5.1 Voltage overview Voltage, also called electromotive force, is quantitative expression of the potential difference in charge between two points in an electrical field. Voltage can be direct or alternating. A direct voltage maintains the same polarity at all times. In an alternating voltage, the polarity reverses direction periodically. Voltage is symbolized by an uppercase italic letter V or E. the standard unit is the volt symbolized by (v). 13

26 Chapter I Induction motor theory and measurements I.5.2 Voltage measurement methods: Some of the common voltage measurement methods are listed below: 1.Meter voltage measurement: Instruments for the measurement of electric voltage are called voltmeter. Correct insertion of a voltmeter requires the connection of its terminals to the points of an electric circuit across which the voltage has to be measured as shown in the figure [10] Figure 1.5.1: Voltmeter insertion There are different types of voltmeters, the following table shows a rough classification of the most commonly employed voltmeters according to their operating principle and their application field: [10] Class Operating principle Subclass Application field -Electromagnetic -interaction between currents and magnetic fields -moving magnet -moving coil -moving iron -DC voltage -DC voltage -DC and AC voltage -Electrodynamic Interaction between current -DC and AC voltage -Electrostatic Electrostatic interactions -DC and AC voltage -Thermal Current s thermal effects -Direct action -indirect action -DC and AC voltage -DC and AC voltage -induction Magnetic induction -AC voltage -Electronic Signal processing -Analog -DC and AC voltage -Digital -DC and AC voltage Table 1.5.1: the most commonly employed voltmeters. 14

27 Chapter I Induction motor theory and measurements 2.Inductive and Capacitive voltage measurement Capacitive and inductive voltage sensors are mainly utilized in low frequency electric measurements. a.capacitive sensors: The voltage to be measured can be reduced by means of capacitive dividers as shown in figure 1.5.2: [10] Figure 1.5.2: Schematic arrangement of a capacitive divider. Capacitive dividers are affected by temperature and frequency and therefore are not important. Capacitive sensors detect voltage by different methods. 1. Electrostatic force (or torque). 2. Transparency through a liquid crystal device 3. Change in refractive index of the optic fiber b.inductive methods: (Voltage transformer): Voltage transformers have two different tasks: - Reduction in voltage values for meeting the range of normal measuring instruments or protection relays. - Insulation of the measuring circuit from power circuits. Voltage transformers are composed of two windings: Primary and secondary winding. The primary winding must be connected to power circuit, the secondary to measuring or protection circuits. Electrically, these two windings are insulated but are connected magnetically by the core. One can define: 15

28 Chapter I Induction motor theory and measurements Nominal ratio= = (1.22) As the ratio between the magnitude of primary and secondary rated voltages: Actual ratio = k = (1.23) For extremely high voltage values, both capacitive dividers and voltage transformers are normally used as shown in the figure [10] Figure 1.5.3: Capacitive and voltage transformer device for extremely high voltage. The capacitive impedance must compensate for the effect of the transformer s internal impedance at the working frequency. [10] I.6 Torque measurements: In industry, many applications require measurements of force. The force is a vector acting in a straight line. The force can be through the center of a mass or be offset from the center of the mass to produce a torque. Two parallel forces acting to produce rotation are termed a couple. If the load or the weight is required to be measured, the sensor can be a load cell, using devices such as strain gauges. [11] Torque is a force that acts on a body to cause its rotation at a perpendicular distance the line force to the fulcrum, and is given by: [11] from (1.24) Torques can be divided into two major categories, either static or dynamic. In a discussion of static vs. dynamic torque, it is often easiest to start with an understanding of the difference between a static and dynamic force. To make it simply, a dynamic force involves acceleration, 16

29 Chapter I Induction motor theory and measurements were a static force does not. The relation between dynamic force and acceleration is described by Newton s Second Law: [11] (1.25) I.6.1 Force and torque measurements Force and weight can be measured by comparison, as in a lever-type balance which is ON/OFF system. A spring balance or load cell can be used to generate an electrical signal required in industrial applications. An analytical or level balance is a simple and accurate which operates on the principle of torque comparison. When in balance, the torque in one side of the fulcrum is equal to that on the second side, resumed in the following formula: (1.26) Where is the weight at a distance from the fulcrum and counter balancing weight at distance from the fulcrum. Many techniques are used for torque measurements. Spring transducer and piezoelectric devices are the most used. [11] a.spring transducer It measures the weight using the deflection of the spring when a force is applied. This deflection is proportional to the applied weight according to the following formula: (1.27) Where the force (in N), k is the spring constant (in N/m) and d is the spring deflection. [11] b.piezoelectric device It uses the piezoelectric effect which is the coupling between the electrical and the mechanical properties of certain material to measure force, the most known and used devices are the strain gauge sensors. [11] 17

30 Chapter I Induction motor theory and measurements I.6.2 Strain gauge sensor Strain gauges are resistive sensors which can be deposited resistors or piezoelectric resistors. The resistive conducting path in the deposited gauge is copper or nickel particles deposited onto a flexible substrate in a serpentine form (figure I.6.1). [11] If the substrate is compressed in the direction of the resistor, then the particles are forced together and the resistance decreases. If the substrate is under tension in the direction of the resistor then the particles tend to separate and the resistance increases. [11] Figure 1.6.1: Strain gauge as a serpentine structure. The piezoresistor devices are often used as strain gauge elements. In this case resistance change is due to the change in electron and hole mobility in a crystal structure when strained. Four elements are configured as Wheastone Bridge and integrated with the conditioning electronics. The resistance in a strain gauge is proportional to the degree of bending, compression or tension. Figure 1.6.2[11] shows an alternative use of a strain gauge for measuring the force applied to a cantilever beam. The force on the beam causes the beam to bend, producing a sheer stress. The resistance change in strain gauges is small and requires the use of a bridge circuit for measurement. The strain gauge elements are mounted in two arms of the bridge, and two resistors, R1 and R2, form the other two arms. The output signal from the bridge is amplified. 18

31 Chapter I Induction motor theory and measurements The strain gauge elements are in opposing arms of the bridge, so that any change in the resistance of the elements due to temperature changes will not affect the balance of the bridge, giving temperature compensation. [11] Figure 1.6.2: Alternative use of a strain gauge for measuring the force 19

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33 Chapter II Hardware implementation Introduction Measurements followed man from the very beginning of its development. Although science and methodology are developing quickly, one should always remember that a measurement principle established more than 150 years ago can still be advanced, results will only become more precise. [10] Sensors are instruments used for measuring the different physical quantities. In order to get accurate results, one should well understand their working principles. This chapter presents the different testing circuits and their results concerning all the sensors that have been used. II.1 Current measurement simulation and design Before introducing the design and simulation results. It is important to state some details about LEM-25 Hall Effect current sensor. II.1.1 Current sensor LA-25P is a current transducer, used to measure DC, AC, pulsed current with galvanic isolation between the primary circuit (high power) and the secondary circuit (electronic circuit). It is also a closed loop sensor. [8] Closed loop sensors amplify the output of the Hall Effect sensor to drive a current through a wire coil wrapped around the core. The magnetic flux created by the coil is exactly opposite of the magnetic field in the core generated by the conductor being measured (primary current). The net effect is that the total magnetic flux in the core is driven to zero, so these types of sensors are also called null balance current sensors. The secondary current in the coil is an exact image of the current being measured reduced by the number of turns in the coil. Passing the secondary current through a load resistor gives a voltage output. The closed loop sensor has several more components in addition to the core and Hall Effect sensor. The feedback electronics including an operational amplifier and the coil are the significant additions. Figure shows the construction of a typical closed loop sensor. 20

34 Chapter II Hardware implementation Figure 2.1.1: Construction of a typical closed loop sensor The primary current being measured (Ip) creates a magnetic flux in the core. The core is made up of thin pieces of steel stacked together to give high frequency response. The Hall Effect sensor in the core gap measures the amount of flux in the core. The voltage output of the Hall effect sensor is proportional to the current Ip. The output of the Hall sensor is amplified in the compensation electronics. The current output of the compensation electronics (Is) creates a second magnetic field in the coil. The magnitude of this secondary field is the product of current Is times the number of turns in the coil (Ns). The magnetic flux from the secondary coil cancels out the flux from the primary to zero. The system thus operates at zero magnetic flux. [12] After explaining the principle of the Hall effect current sensor LEM25, we can introduce the design methodology. Figure 2.1.2: Design methodology for current sensing. II.1.2 Supply circuit From the LA-25P datasheet we know that the typical range of voltage supply is ±12 ±15 V. For the supply circuit we used the supply voltage generator it generates up to 30V with 3 Amperes limit for both supply and testing circuit which will limit our testing to a maximum of 3A. 21

35 Chapter II Hardware implementation In order to implement the testing circuit four resistors were used 10Ω and three 5Ω with 25W each, to make sure they can support up to 3A current. In order to measure the output current I S a 10Ω resistor was used.the circuit is shown in figure Figure 2.1.3: Supply and testing circuits II.1.3 practical results After implementing the circuit we measured the I p current (the input current to the sensor) the results are shown in the following graph : Figure 2.1.4: Ip current (A) measurement results. After that, we measured the voltage across the measurement resistor R m calculated the output current by dividing by the value of the resistor, the Figure2.1.4 shows the graphical representation of the measurements. 22

36 Chapter II Hardware implementation Figure 2.1.5: measurement of the output current of the LA-25P current transducer II.1.4 Signal conditioning circuit As the data acquisition board has a range of -10V to 10v input; the range of mV of the voltage across the resistor R m (10Ω) needs to be mapped to the range 0 3V. An instrumentation amplifier with a gain of 100 was designed. For this purpose we used the INA 101HP which is a high accuracy instrumentation amplifier designed for low level signal amplification and general purpose data acquisition. [13] The gain of the INA101HP is set by connecting a single external resistor R G (2.1) In this case II.1.5 Non-linearity calculation [13] From the measurement we can calculate the nonlinearity which is the measurement of the difference in output current offset of two lines of equal slope, one going through the minimum points and one going through the maximum points of the output curve as shown in figure 2.1.5: 23

37 Chapter II Hardware implementation Total Non-Linearity Figure 2.1.6: Total non-linearity Under most cases that involves this condition, the total non-linearity can be closely approximated by calculating the maximum deviation at the X midpoint, which typically has the largest error between the actual values taken and the calculated baseline. The midpoint (X m ) for a zero based dataset is easily found as: X m X max X 2 min X min (2.2) So: The actual Y value at this X value (Y s ) minus the Y value, at this X value (Y m ), of the line connecting end points represents the total amount of non-linearity. Total Non-Linearity (X s,y s) (X m,y m) 2 point straight line from (x,y) min to (x,y)max X m Figure 2.1.7: Midpoint nonlinearity. 24

38 Chapter II Hardware implementation ymax ymin ymin ys 2 % Linearity 100% ymax ymin ymin 2 (2.3) Using this formula the total non-linearity of LA-25P current transducer is found to be II.2 Voltage measurement simulation and design In order to determine the voltage between the phases and the neutral of the machine, a meter voltage measurement was used. The two terminals of the volt meter were connected to the points across which the voltage has to be measured. After that a step down transformer was used in order to step down the voltage to 12 v (rms). The turn ration of the transformer is given by Turn ratio= = II.2.1 Signal conditioning circuit for data acquisition As the data acquisition board has a range of -1 0v to 10 v input voltage. The simple circuit is voltage divider. The resistors used are, which produces a gain of. Hence the input voltage is : 6 v(rms). II.3 Torque measurement design Before talking about the operation of the Torque sensor, brief introduction to Wheatstone bridge and its functionality is needed. II.3.1 Wheatstone bridge In this part, the Wheatstone bridge circuit will only be considered with respect to its application in strain gauge technique. The presentation of the Wheatstone bridge is shown in Figure 2.3.1: 25

39 Chapter II Hardware implementation Figure 2.3.1: Wheatstone bridge The four arms of the bridge are formed by the resistors R1 to R4. If at nodes A and D so called (excitation diagonal) are connected to a known voltage (e in ) then a voltage (e out ) appears between nodes B and C so called (measurement diagonal). The value of the output voltage depends on the ratio of the resistors (R1:R2) and (R3:R4). For the balanced case the ratio is 1 and e out = 0. [15] II.3.2 Torque sensor [15] The load cell SUP1 is a strain gauge based measuring instruments whose output voltage is proportional to applied torque. The output voltage produced by a resistance change in strain gages that are bounded to the torque sensor structure. The magnitude of the resistance change is proportional to the deformation of the torque sensor and therefore the applied torque. The four arms Wheatstone bridge configuration shown in Figure depicts the strain gage geometry used in the torque sensor structures. This configuration allows for temperature compensation and cancellation of signals caused by forces not directly applied about the axis of the applied torque. So an excitation voltage is required and is applied between points A and D of the Wheatstone bridge. When torque is applied to the transducer structure the Wheatstone bridge becomes unbalanced, causing an output voltage between points B and C. After explaining the principle of the torque sensor SUP1, we can propose the following design measurement circuit block. 26

40 Chapter II Hardware implementation Figure 2.3.2: Design methodology for torque sensing. II.3.3 Supply circuit From the off center load cell SUP 1 datasheet we know that the typical range of voltage supply is 0.1v 15 V. For the supply circuit we used the supply voltage generator it generates up to 30V. We supplied the torque sensor with 5v from its input terminals ( red(+) and black(-)). The sensor was suspended about 10 mm above the table and different masses were applied to it. Using a voltmeter the output voltage was recorded from its output terminal (green (+) and white (-)). II.3.4 Practical results: The output voltage results are shown in the following graph Figure 2.3.3: Vout vs Weight From the curve, the output has a linear relation of the form: (2.4) Where: and So: K=

41 Chapter II Hardware implementation II.3.5 Signal conditioning circuit for data acquisition As the data acquisition board has a range of -10v to 10 v input voltage; the range of mV of the torque output voltage needs to be mapped to the range 0 10V. An instrumentation amplifier with a gain of 1000 was designed. For that we used the same instrumentation amplifier as the one of the current with: II.3.6 Non linearity calculation Using the equation of x midpoint calculation we get: The by using the formula of the midpoint calculation the nonlinearity of the Off center load cell SUP 1 is found to be: 0.02 %. II.4 Speed and frequency measurement simulation and design II.4.1 Supply circuit From the Fotocell M18 datasheet we know that the typical range of voltage supply is 12v 30v. Using the supply voltage generator we supplied the circuit with15v from its input terminals. Following the wiring diagram of PNP connection a resistance of 100 Ω is connected between the output terminal and the ground. After that, the output of the sensor was displayed on the scope, the result was similar to the one shown in Figure 2.4.1: Figure 2.4.1: output of the speed sensor When even a light is detected from the sensor, a pulse is generated. 28

42 Chapter II Hardware implementation II.4.2 Frequency measurement In order to read the frequency from the measured speed, a frequency to voltage convertor circuit is used. For this purpose the LM2907 is used. With this configuration the LM2904 will saturate at an output voltage of v. The size of is dependent only on the amount of ripple voltage allowable and the required time. The total conversion equation is: (2.5) K is the gain constant, typically 1.0 [17] The output results of the voltage and the frequency are shown in the following graph: II.4.3 nonlinearity calculation Figure 2.4.3: Voltage vs frequency. Using the equation of x midpoint calculation we get: The by using the formula of the midpoint calculation the nonlinearity of the LM2907 is found to be: 0.4 %. Conclusion In this chapter we did test all the sensors that will be used later in measurement system analysis and their appropriate signal conditioning circuit for data acquisition which will help us to get accurate results from the three phase machine. 29

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44 Chapter III Software description Introduction The outputs data from the design and implementation of the hardware system described in chapter II, need to be recorded according to their software requirement. This chapter discusses the function of the software used in this project with respect to the measurement system. III.1 NI LABVIEW NI LABVIEW (Laboratory Virtual Instrumentation Engineering Workbench) is a graphical program (G-programming) development environment from National Instruments ( Programs written in LABVIEW are called VIs (virtual instruments). One advantage of programming in LABVIEW is that we don t have the overhead to write huge codes. Just the function of the different blocks needs to be known. Also, another advantage is that it works on Dataflow Programming principle, i.e. the output is only obtained when all the inputs get their input data. LABVIEW consists of two windows called the Front Panel and the Block Diagram. III.1.1 LABVIEW front panel: The LABVIEW front panel is the window where, different controls (such as switch, knobs, numeric inputs, etc.) and indicators (such as LEDs, graphs, numeric outputs, etc.) can be viewed; it can also be called as LABVIEW HMI (Human Machine Interface). [17] Figure Front panel window 30

45 Chapter III Software description III-1.2 LABVIEW block diagram: LABVIEW block diagram serves as the brain of the program. It contains the graphical source code that defines the functionality of the VI. Front panel objects appear as terminals on the block diagram. The block diagram basically consists of different functions such as mathematical functions, Boolean functions, programming loops, etc. [17] Figure 3.1.2: Block diagram window LABVIEW program also contains the following three types of pallet which give you the options you need to create and edit the front panel and block diagram: Tools Palette: The Tools palette is available on the front panel and the block diagram. A tool is a special operating mode of the mouse cursor. When you select a tool the cursor icon changes to the tool icon. Use the tools to operate and modify front panel and block diagram objects. Controls Palette: The Controls palette is available only on the front panel. The Controls palette contains the controls and indicators you use to create the front panel. Functions Palette: The Functions palette is available only on the block diagram. The Functions palette contains the VIs and functions you use to build the block diagram. The figure below shows the three types of pallets: 31

46 Chapter III Software description Figure : Tools, Controls and function pallets. III.2 Data Acquisition (DAQ) [18] Data acquisition is the process of measuring an electrical or physical phenomenon such as voltage, current, temperature, pressure, or sound with a computer. The system consists of sensors, DAQ measurement hardware, and a computer with programmable software. So, Data Acquisition is the process of: Acquiring signals from real-world phenomena. Digitizing the signals. Analyzing, presenting and saving the data. Lab View software enabled us to acquire data using DAQ board the process block diagram is shown in figure Figure 3.2.1: the process block diagram 32

47 Chapter III Software description The parts are: 1- Physical input/output signals: A physical input/output signal is typically a voltage or current signal. 2- DAQ device/hardware: DAQ hardware acts as the interface between the computer and the outside world. It primarily functions as a device that digitizes incoming analog signals so that the computer can interpret them. 3- Driver software and software application: Driver software is the layer of software for easily communicating with the hardware. It forms the middle layer between the application software and the hardware. Driver software also prevents a programmer from having to do register-level programming or complicated commands in order to access the hardware functions. Application software adds analysis and presentation capabilities to the driver software. The software application normally does such tasks as: real-time monitoring, data analysis, data logging, control algorithms and human machine interface (HMI).[19] III.3 NI-DAQ max [18] Driver software is the layer of software for easily communicating with the hardware. It prevents a programmer from having from having to do register level programming or complicated commands in order to access the hardware functions. The driver software from national instrument are : - NI DAQ max - NI DAQ max Base. The DAQ Assistant, included with NI DAQ mx, is graphical interactive guide for configuring, testing and acquiring measurement data. 33

48 Chapter III Software description III.4 DAQ 6009 board [18] NI USB 6009 is a simple and low cost multifunction I/O device from national instruments. The device is shown in figure and has the following specifications: - 8 analog inputs (14-bit input resolusion, 48 ks/s) - 2 analog outputs (12- bit input resolusion, 150 ks/s) - 12 digital I/O - USB connection, no extra power, supply needed - Compatible with labview, labwindows/cvi, and measurment studio for visual studio.net - NI_DAQ max driver software. - Figure 3.4.1: NI USB III.4.1 Using NI USB 6009 in LABVIEW The DAQ max functions, are used in order to use the NI USB6009 in lab view as shown in the figure Figure 3.4.2: DAQmasx DATA acquisition palette. 34

49 Chapter III Software description By dragging the DAQ assistant icon, a block diagram appears in order to select either «Acquire signals» or «generate signals». In the next window the type of the input analog and the appropriate channel from where the data is acquired are selected all these steps are shown in figure Figure 3.4.3: Setting the DAQ Assistant In order to acquire the needed data correctly, the DAQ Assistant also allow the user to select the acquisition mode, the signal input range, rating frequency and rename the name of the channels, as in figure Figure 3.4.5: Different setting to record the data 35

50 Chapter III Software description In this part we are going to record three analog inputs which are voltage, current and speed as explained before, once these parameters are recorded, they will be used to calculate the different output powers of the three phase system. The program implemented in labview follow the standards shown in Appendix A. III.5 Software implementation Figure shows the structure block diagram of the data acquisition system. Figure3.5.1: Data Acquisition block diagram The DAQ 6009 interface is used to record four different signals namely voltage, current, speed and torque. In order to obtain the rms voltage and current, the Harmonic distortion analyser is used to determine the total harmonic distortion and the fundamental frequency, then from the FFT spectrum (Mag-phase).vi the magnitude and the phase at the fundamental frequency (V 1,I 1 ) are obtained to find the harmonic voltage and the current. 36

51 Chapter III Software description The revolution per minute (rpm) speed is derived from the conversion of the speed frequency to voltage. In the other hand, the static torque is given by the applied force of the generator. The overall circuit for the implementation is shown in Appendix B. III.6 Experimental results The virtual instrumentation developed through the front panel, graphical user interface provides analysis function according to the standard set in Appendix A. Magnitude of the supply voltage and current, frequency, voltage, current harmonics and other analysis conform the IEEE standard : real and reactive power, frequency and power factor etc. The system was tested and calibrated in laboratory provides continuous monitoring of a three phase system the input currents and voltages wave forms with their harmonics graphs are shown in figure Figure 3.6.1: virtual instrument shows voltage,current waveforms and spectra of three phase system 37

52 Chapter III Software description III.6.1 Discission a. Voltage input source The three voltage phases are shifted by 120 which results in balanced three phase systems, with an amplitude around 238 V, the same value measured by using the voltmeter. Since the input is not a pure sine wave, the Fast Fourier Transform of the signals results in more than one harmonics, the fundamental harmonic is at frequency which is the operating frequency of the induction motor with an amplitude of 238 V b. Current input source The three current phases are shifted by 120, with an amplitude around 1.2 A, as the voltages the Fast Fourier Transform for the currents results in more than one harmonic where the fundamental harmonic is at frequency (fundamental frequency) with an amplitude of 0.93 A(rms). Figure shows the virtual instrument Graphical User Interface (GUI), which consists of Visual indicators. i.e. gauges and graphs. The indicators are used to display the measured values of interest. The GUI also provides the possibility of saving data. Figure 3.6.2: virtual instrumentation in three phase operation mode 38

53 Chapter III Software description From the user interface panel, the three phase operation mode of the system shows the characteristics operation mode of the motor. The system is operating with real power of W. The power factor is current THD %, voltage THD 11.16% the other parameters can be read from figure The user interface for the speed rpm is depicted in rpm Figure III.6.3 Data logging Figure 3.6.3: virtual instrument for the speed in rpm The application is capable for logging measured values in real time and calculated parameters. All measured parameters are stored in test file for later use. In this case the parameters were stored in EXCEL file with their corresponding day and time of measurement as indicated in figure Figure 3.6.4: EXCEL representation for the recorded parameters 39

54 Chapter III Software description Using these parameters recorded Figure 3.6.4, the voltage waveforms have been plotted, and the graphs are shown in figure Figure 3.6.5: Recorded voltages waveforms This method of recording data helps in the diagnosis of the system malfunction either in the online or offline measurements. It allows the user to determine the exact time and date of the malfunction of the system.the different calculated parameters serves in detecting the problems quickly. 40