PC Monitoring of Dynamometer
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Published: Wed, 21 Feb 2018
The aim of the project ‘PC MONITORING OF DYNAMOMETER’ is to measure and display the Speed Torque and Power of Motor. The behaviour of motor under load and no load characteristics have been studied with the help of virtual instrumentation. The variations in the motor Speed, Torque and Power at no load torque and full load torque have been analysed. Power determines the efficiency of the motor.
Torque is varied by altering resistance in the dynamometer control circuit. The entire system will interface with a PC providing virtual instrumentation and data logging facilities. The graphical programming software has been used to study the characteristics of the motor by loading. Data logging and analysis is done by implementing the concept of LabVIEW.
The aim of the project to build and develop the LabVIEW Program to acquire and analyse the data with the help of ADC card. Speed, Torque and Power of the shunt wound dc motor are acquired and analysed.
- Implement the concept of virtual instrumentation.
- Develop LabVIEW program to acquire Torque, Speed and Power.
- Analyse the motor characteristics under load and no load using the visual programming.
- To show the difference between acquiring the data manually and applying Virtual Instrumentation.
Chapter 1 gives the introduction to the Motors, Labview and the literature review to realise the project.
Chapter 2 is the study of Virtual Instrumentation and Labview programming.
Chapter 3 explains the terms Torque, Speed & Power asscociated with the shunt motor.
Chapter 4 includes the experimental setup for the project with the specifications and the individual work of each and every component.
Chapter 5 discussion part includes the introduction to LabVIEW tools used in the programming for the project.
Chapter 6 contains the analysis for the manual operation of the motor when load is applied. Includes Torque, Speed, and Voltage characteristics.
Chapter 7 includes the labview program for Speed, Torque and Power with the analysis for motor under loaded conditions.
Chapter 8 includes the conclusions and the future scope of the project.
The extent to which electrical energy is used has influence on the development of industry and the economic prosperity of nation. Therefore rather than conserving the energy, methods need to be implemented to control the electrical energy and hence it can be possible only when the problems associated with the system are analysed and rectified with use of technology and time limitation.
In industries the shunt wound motors are used where speed control is a critical aspect. The shunt wound motors are highly flexible, versatile and even the operation costs are minimum. The speed of rollers in large rolling mills is an very important aspect to be measured. In this type of application the rpm of each motor is measured and compared to setpoints.
Under load motors draw more current which may damage the working capabilities of the motor thus causing damage. Hence there is a need for operating range for a motor to work even in extreme conditions. Characteristics such as Speed, Torque, Power, operating voltage and current are important for selection of motors. Power is the capability of the motor to do some work.
The motor characteristics at high speed are highly different from that to low speeds. The power of motors is very low at high speeds consuming very less of electric distribution. The shunt wound type of motors run basically at a constant speed, regardless of load.
In industries the shunt wound motors are used where speed control is a critical aspect. The shunt wound motors are highly flexible, versatile and even the operation costs are minimum.
Motors runs at certain speed depending on the shunt field and armature. The armature produces the back EMF to maintain a certain speed as the shaft rotates. The back EMF is produced when the rotor rotates. As the load increases it causes the armature shaft to slow down and in this case the back EMF produced will be decreased.
There are two methods for controlling the speed of a motor. The first one is by increasing or decreasing the voltage supply to the field. If less voltage is given the motor runs at low speed and as the voltage is increased the speed increases accordingly. The other method is by inserting resistance in the field circuit which makes the speed to vary upon changing the load. When load is increased apparently speed is decreased and vice versa. The change in load and speed torque and power can be acquired with the Software designed for acquisition and data analysis which is ‘LabVIEW’.
Lab view in short is for Laboratory Virtual Instrument Engineering Workbench. Labview is a very powerful and flexible instrumentation and analysis software developed by National Instruments. Labview is a programme particularly developed for engineers and scientists working in automation industry and data analysis.
The principle Labview works is the data flow. They are broken into wires and nodes. The inputs and outputs are considered a node. It has got the built in libraries for Data Acquisition, Instrument Control and data analysis. Data acquisition is to measure electrical or physical quantities such as voltage, current, temperature, pressure, or sound. PC-based data acquisition is a combination of hardware, application software, and pc for logging the data.
Software is highly important to data acquisition systems. The reduction of hardware can be done if an effective software is designed. Data Acquisition software collects the data and displays the data. Ease in data analysis and presentation are the major reasons for using the computers in data acquisition .
LabVIEW can acquire accurate and timely measurements for monitoring industrial and control system applications. LabVIEW quickly connects to number of industrial sensors which acquires data at very high speeds. The tools provided in the LabVIEW can perform advanced signal processing, frequency analysis, digital signal processing. LabVIEW can also be applied for machine vision, motion control, and machine condition monitoring
In LabVIEW, the logic can be easily developed by using the drag drop graphical icons instead of writing lines of programs. Time taking programmes can be written in very short period using labview. This graphical language can be used in industrial applications to control instruments, build automated test systems, and acquire data virtually and many more.
The literature on fundamentals and applications of data acquisition, instrumentation, and control to engineering and technology is very extensive.
Peter T lee conducted experiments based on the torque-speed characteristics of a motor with the help of mechanical design and 3-D using the computer. The computer simulation was drawn from a mechanical model. Upon implementing the above methods a conclusion has been made on the torque- speed characteristics of dc shunt motor. The maximum speed of the motor is at no load applying zero torque. The stall torque represents a point where the motor is at zero speed. Dynamic and steady state response mathematical models were designed. The torque meter is used to calculate the torque. The torque speed curve obtained is is bit nonlinear. The mechanical design produced had few constraints which led to the nonlinearity of the curves.
As described by Comer in dc electric motor control systems series motors develop high torques at very low speeds when compared to shunt motors. For a given voltage the speed-torque characteristics show a linear decrease with the speed. Motor speed is controlled by controlling the voltage to the armature which involves construction of a circuit involving inverter amplifier, opamps, pulse width modulator which is confusing and rather time taking.
Saffet Ayasun proposed Matlab and Simpower systems for studying the steady state and transient characteristics of electrical machines. Simulink models have been designed to control the speed in three ways. Simulink has been proposed for field resistance control, armature resistance control and armature voltage control. The conclusions drawn from the simulink modelling is that the electrical machines are perfectly integrated with the software.
Pierre Guillemin proposed a technique based on fuzzy logic for controlling a dc motor. His work included on motors in food processing industries whose speed varies upon load. He worked on designing the fuzzy logic for the speed to be constant even though there’s a much change in the load. The main methodlogy is top control the voltage involving the techniques of ‘phase angle modulation’ and pulse width modulation. Use of fuzzy logic avoided the need for mathematical modelling. Real time test and acquisition has been done using the pc and eprom version of ST6 device has been used for controlling the fuzzy logic. So the main disadvantage of using ST6 is that it needs pages of text coding and has a instruction list of 40./p>
From the above proposed techniques we can conclude that it not only makes the system complicated but also it takes lot of time to design a program and create a logic for it to run. Manual way of operating may result in human errors decreasing the quality of product. Hence the need for data acquisition at high speed without much implementation of hardware components can be possible with the help of virtual instrumentation. Virtual instrumentation is the combination of modular software and I/O with software as its main tool.
Today in any type of computer aided manufacturing project work and laboratory tests, precision and reliability of instrumentation and data acquisition techniques may cause major impacts on results and outcomes. Therefore, there is a need to gain knowledge and skills to obtain any type of physical or virtual data on manufacturing, testing, measurement, and protection areas.
The data has been acquired from a dynamometer motor system and is displayed on the pc for analysis involving a communication between hardware and software. The ADC card communicates between the systems with the help of a data cable. The conditioned signals are connected to the channels of the DAQ board. The hardware system is in conjunction with the PC running LabVIEW. LabVIEW contains front panel and the block diagram which displays the data acquired and further analysis can be done. The front panel and block diagram are responsible for the understanding of programming involved.
Virtual Instrumentation is the combination of software, Input/Output hardware for the user desired applications. Sofware is the heart of Virtual Instrumentation. Instrumentation is basically divided into two types one is natural instrumentation and the other is Virtual Instrumentation. Natural instrumentation consists of hardware components and Virtual instrumentation is of Software with limited hardware components.
The main difference between ‘natural’ instrumentation and ‘virtual’ instrumentation is that the software component of virtual instruments is more reliable and versatile when compared to the hardware components.
TRADITIONAL INSTRUMENT VS VIRTUAL INSTRUMENT
The above Fig shows the difference between natural Instrument and a Virtual Instruments. In natural instrument a embedded chip is fixed to do a particular job which is not flexible whereas in a virtual instrument software can be modified and used for different applications as per the requirement.
Labview stands for Laboratory Virtual Instrument Engineering Workbench. LabView is a visual programming language from National Instruments. The graphical language is originally named “G”. LabVIEW is used for data acquisition, industrial automation and instrument control. The programs in labview are called as virtual instruments .The graphical language can be easily identified by visual identification which makes it very easier to understand.
Labview programs are termed as virtual instruments as they generally look and work as the instruments. Labview comes with a mechanism which allows data to pass between front panel and the block diagram.
Labview is used by engineers and scientists to develop the sophisticated measurement, test, analyse control systems using graphical icons and wires. Labview can integrate with thousands of hardware devices and numerous built libraries are provided for analysis and data visualisation
The programming language used in LabVIEW is called “G” which is termed as a dataflow language. Labview Programme execution is determined by the way the block diagram is structured. Drawing wires are used to connect different function nodes. The information is transferred through the wires. The program is executed as soon as the input is available. G program is capable of parallel execution. The dataflow completely defines the execution sequence, and can be fully controlled by the programmer. The execution sequence is perfectly defined in Labview as defined in some software languages.
LabVIEW programs are called virtual instruments. Each VI has three components:
- Block diagram
- Front panel
- Connector pane.
Data Flow is the principle for the VI execution. The VI are data driven whereas the normal text coding are instruction driven as specified by the programmer. When all the input data is available then the node starts executing and the processed data is supplied to the output terminals. The graphical language makes it very easier for the programmers in which virtual instruments can be dragged and dropped as per requirement. Stand alone applications can be built with the help of advanced labview development systems.
The front panel serves as a user interface between the user and the pc. When front panel is dropped as a node onto the block diagram, the node inputs and outputs can be defined in with the help of connector pane.
DC SHUNT MOTOR
A motor is a machine which converts electrical energy to mechanical or rotating energy. DC motors are basically divided into two types. Series motors and Shunt motor.
Shunt Motor are the one in which the field is parallel to the load. In shunt motors the armature is mounted on the motor shaft. The rotating part in the dc motor is termed as the armature. The windings are located in the slots on the surface of the armature. When the current is supplied to the armature windings it creates a magnetic field that reacts with the field poles. Hence this magnetic field develops a torque which turn the rotor that is the armature. While the rotor rotates it induces a voltage which is opposite to the supplied voltage and hence emf is generated.
At start the resistance is high and as the motor picks up the speed the resistance is reduced gradually. In manual start the resistance is controlled by the man at work whereas in automatic start of motor the armature voltage or current is taken into consideration and the resistance is corrected in the begging of start .
The increase in armature voltage results in increase in speed and as the armature voltage is decreased the speed decreases. Torque is directly proportional to armature current. Increases in armature current causes the increase in torque.The rotation of the armature can be reversed by reversing the current direction.
At start of the motor the current is high. As armature starts rotating the back emf increases gradually with the speed and the current decreases. When the speed of the armature comes to constant the back emf is stable approaching the induced voltage.
If a mechanical load is applied the speed decreases and hence the counter emf decreases. This decreases in counter emf increases the differential voltage and thus increases the input current supply to motor.
Torque is directly proportional to armature magnetic field strength. Armature magnetic field is directly proportional to armature current which in turn depends on mechanical load applied. Hence any increase in mechanical load increases the armature current and armature magnetic field strength and thus the Torque increases. Thus the speed of the motor decreases.
DC motors are differentiated based on their voltage, torque, speed and power.
Torque is the rotating force of the shaft of a motor. This rotating force is developed due to the interaction of magnetic field between field windings and the field poles. The torque of a motor can be determined by connecting it to prony brake
The torque in general terms is the force exerted by the shaft of a motor. i.e if force is applies to a lever which is free to rotate about one fixed point the lever will rotate unless restrained.
The torque is defined as t=Fr or Frsin?. The torque which is produced in clockwise is called a clockwise torque and the torque which is obtained in anticlockwise is called anticlockwise torque.
FULL LOAD TORQUE
The continous torque motor can support without overheating under the specified time rating is full load torque.
Peak torque is the maximum torque required at any point. Peak torque is delivered to motor without much overheating the motor. Peak torque is normally less than the stall torque.
PULL OUT TORQUE
This is maximum torque of the rotating shaft when operated at high speed and full voltage. Pull out torque is also considered as the breakdown torque.
Torque exerted by the motor when energized at full voltage with its shaft locked is starting torque. It is also termed as locked rotor torque.
The motors convert electrical energy into mechanical energy. The rotational energy is used to lift things, propel things, turn things, etc. When a specific voltage is supplied to a motor, it rotates the output shaft at a particular speed. The angular velocity, is measured in rad/s, rps, rpm.
Speed of a shunt motor can be controlled in three ways.
- Field Control Method
- Armature Control Method
- Armature Voltage Control method
FIELD CONTROL METHOD
In field control method the speed of the armature is controlled by adjusting the field current. upto certain speed the counter emf and the speed remains constant. Field Control method is also termed as constant speed drive.
ARMATURE RESISTANCE CONTROL
In Armature resistance control external resistance is inserted in the armature circuit for controlling the speed of motor. The insertion of resistance keeps the armature current constant in certain speed range. The main disadvantages using this techniques include powerloss in external resistance and overall low efficiency when speed is reduced to great extent.
ARMATURE VOLTAGE CONTROL
Armature voltage control is preferred over the other methods as powerloss and low efficiency is avoided. In this method the shunt field current is kept constant while varying the armature voltage. Hence the armature current and the flux generated remains constant. At certain current the torque remains constant since armature current is directly proportional to torque. Thus the speed of the motor can be controlled to certain range.
The speed regulation for a dc motor is the ratio of change in speed from no load to full loadf load speed.
The power of a motor is defined as the capability of motor to do given amount of work.Power of a motor depends on torque, speed of the shaft and the amount of time the time operates.
Electric motors are rated generally on their maximum efficiency. Thus Greater the horse power of a motor greater is the working range. Operation of a motor more than rated horse power leads to overheating of the motor thus causing a stall. This may even damage the motor in the long run. The power of motor can be determined by connecting it to a dynamometer.
Dynamometer is a device which measures force and power. Eddy current Dynamometer consists of a stator in which electromagnets and rotor disc are coupled to shaft of the engine .When a rotor is made to rotate the eddy currents are produced in the stator due to magnetic flux which is created by the filed current in electromagnets. This type of dynamometers requires some cooling arrangement as the eddy currents produced dissipates heat energy. The moment arm is used to measure the torque. In this kind of dynamometers the load is controlled by the regulation of currents in electromagnetic fields.
The selection of appropriate hardware and software was an essential part of this project. The first section of this chapter 4 gives the information of hardware components of a system, the diagrams and specifications are discussed .The second part of this chapter describes the hardware design process and software is described in the third section ,code used for the system are going to be presented.
DC SHUNT MOTOR-WORKING
The DC motor has two basic parts:
The choosen shunt Dc motor is the motor which converts electrical power to mechanical power. The shunt motor is different from that of series motor. In the shunt type dc motor field winding is connected in parallel with the armature. The field winding is parallel to armature and this is referred as shunt winding and the motor is called a shunt motor.
The rated rpm of the motor is 1500. The speed of the motor can be increased or decreased with the help of a drive. The current can be increased or decreased to set the speed. To reduce the speed of the motor mechanical load can be applied. Torque can be applied to reduce the speed of the motor. Under loaded conditions the speed of the motor reduces.
Motor can be applied until it reaches the rated torque of the motor. If the load goes beyond the rated torque it starts drawing high curent and after some time it comes to stall.
CHARACTERISTICS OF SHUNT MOTOR
Shunt motors run at constant speed even when there is huge variation in the load. The Speed of the shunt-wound motors may be regulated in two ways. The first way is by putting resistance in series with the armature, through which the speed is decreased and the second method is by inserting resistance in the field by which the speed will vary as the motor is loaded.
The characteristics of a shunt-wound motor is that it has a very good speed regulation, and is aconstant speed motor, though the speed slightly decreases as load is increased. Shunt-wound motors are used mostly in industrial and automotive applications where accurate measurement of Torque, Speed are highly necessary.
EDDY CURRENT DYNAMOMETER
A dynamometer is a device used to load the motor which also indicates the torque. The dynamometer used in this project works on hysteresis brake principle. The dynamometer system mainly consists of three parts:
- Torque indicator
- Power Supply
- Eddy current dynamometers are the “actuated braking systems.” This braking system develops load torque with the interaction from the dc magnetic field produced by the windings on the stator and induced eddy currents in the rotor. When the magnetic field is added around the rotor it causes eddy currents. These eddy currents help in slowing down the speed.
The torque developed in the dynamometer is transmitted to the stator which is free to rotate over an arc and this is used to deflect a spring balance and so the torque can be directly measured on a scale. Eddy current dynamometer comes with a rotary potentiometer which works on the principle
Rotary potentiometers come with a spiral resistive strip, and a wiper which moves axially as it rotates. As the wiper moves across the resistive strip resistance changes.
The voltage divider is used to know the output voltage when input voltage is given across the resistance. The ouput voltage depends upon the input voltage given and the resistor chosen. The resistor choosen is 10k? and an input voltage of 5v is given to the voltage divider.
The input voltage is given across the termicals 1 and 2 and the output voltage is taken across the terminals 2 and 3.
Connect the voltmeter across the resistance . Now switch the output voltage off and manipulate the input voltage to 5v. Now switch on the output toggle button on . The input voltage is set to 5v.
Tachometers are used to measure the angular speed of a rotating shaft. The speed is measured in revolutions per minute (rpm). A tachometer works on the principle that it ” the speed of rotating shaft is determined by the variation in output frequency signal or voltage. As the speed increases the voltage level and the frequency increases.
CONTACT TYPE AC TACHOMETER
The above fig shows the tachometer coupled to the motor. Ac tachometer used in the design is rated at 0.5v/100rpm. It shows that increase in speed increases. For every 100rpm variation the voltage increases by 0.5 rpm. The voltage signal from the ac tachometer is given to the simple presicion rectifier which further converts the signal to dc voltage.
SIMPLE PRECISION FULL WAVE RECTIFIER
A simple precision full wave rectifier converts the ac voltage to dc voltage. The simple precision full wave rectifier is built with operation amplifier which works on the principle of differential voltage.
An operational amplifier is a DC-coupled high-gain electronic voltage amplifier which has differential inputs and a single output. Negative feed back controls the gain in the opamps. The output goes positive when the non inverting input(+) goes more positive than the inverting input (-) and vice versa
When the input signal goes positive again, the op amp’s output voltage will take time to go back to zero, then to forward bias the diode and produce an output. The time taken is determined by the opamp’s slew rate. Slew rate is the maximum attainable range of the output voltage. Slew rate limits the highest frequency of the sin wave.
The opamp 741 choosen in the project has a slew rate of 0.5v per micro seconds
SIMPLE PRECISION FULL WAVE RECTIFIER
The negative voltage at the input through the diode and resistor. The positive half cycles appear at the output of the second diode. When positive voltage is given the feed back is given by the diodes and hence negative cycles . The positive cycles and negative cycles are summed differentially to get the output voltage.
The opamp 741 choosen in the project has a slew rate of 0.5v per micro seconds
For measurement applications the signal has to be ripple free. Hence low pass filter can be used to achieve the signal free from ripples.
RC LOW PASS FILTER
Resistor Capacitance generally termed as RC circuits are used in filtering a signal waveform, thus changing the relative amounts of high frequency and low information in their output signals compared to their input signals. RC filter is a common application for smoothing a signal.
The RC circuit has a capacitor and a resistor in which are connected in series. The charged capacitor would discharge its energy into the resistor placed in series with it. This voltage across the capacitor is found through Kirchhoff’s current law which says that the “current coming from capacitor is equal the current flowing out through the resistor”. The linear differential eqn. can be given by
ANALOG TO DIGITAL CARD
Analog to Digital conversion is interfacinf the analog I/O with the Digital I/O. ADC cnversion takes place in three stages. They are Sampling, Quantisation and encoding. The ADC card which is used in the Project is CIO-DASO8/JR-A0.
The analog input cannot be directly sent to the pc. The ADC digitises the analog input signal continusly. The proper reconstruction of signal is possible only when the sampling rate is twice the highest frequency component. If the sampling rate is under that the problem of Alaising occurs.
To eleminate the problems of aliasing the signal must be sampled at a rate higher than Nyquist frequency rate. The fig shows the sampling rates and the construction of the signal.
If a signal is sampled at too low rate perfect reconstruction of signal is nt possible which results in the loss of data.
Quantisation represents the signal in discrete and certainj voltage levels. Quantisation actually is approximating the signals to the lowest possible range. The fig below shows the sampling of a singal which is then quatises indicated by a red line. The quantization is necessary as during the sampling the small range of signals may not be digitised. This results in loss of information. By approximating the values the reconstruction of signal is perfect.
Coding is the process of converting the sampled signals to n bits which are represented by 0 and 1’s. These bits represent certain voltage levels which is given by the resolution.
When acquiring data to a computer, an analog to digital Conveter takes an analog signal and digitises the signal. The sigal is digitised to sertain binary numbers. These binary numbers represents respective voltage levels. Resolution refers to the number of binary levels ADC can represent a signal. The resolution of a n bit ADC can be by taking the value .
The ADC card which is used in the Project is CIO-DASO8/JR-A0.
DATA ACQUISITION AND CONTROL ARCHITECTURE
Data acquisition and cntrol architectlog input is given to the pc via ADC which converts the analog input to digital input. The data can be transmitted bidirectionally with the help of data acquisition device. Labview Software is loaded in the pc which acquires the data with the help of built in libraraies.
Instacal is software which manages the data acquisition hardware. It is used in calibration of the boards attached. It scans all the internal registers and the electronic equipment and if any fault found error messages are shown.
The ADC is calibrated with the help of instacal to check whether the data logger is accurate or not.
Labview environment is opened by when new VI is selected from the start up screen. The file menu contains commands for file manipulations. Edit menu is used to modify the block diagram and front panel objects. By default the undo or redo settings for a VI are 8. It can be manipulated as per the requirement.
Operate menu acts in running or stopping a VI or to change the settings of VI. The tools menu acts as interface in communicating with the data acquisition boards to build the applications and in enabling the web server.
Labview programming is mainly divided into two panels. The first one is the Front Panel and all the controls and indicators are in l
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