CHAPTER 6
The method used to load the motor is torque mode. As many as load points can be applied until the motor is at the rated torque.
Couple the dynamometer to the motor.
The power supply of the dynamometer must be in zero position.
Now Start the motor and adjust the speed to no-load value. Start with the no-load speed at 1500 rpm at no load and gradually increase the load on the motor. Adjust the field current to maintain rated speed until the motor draws rated current.
Now turn on the readout unit and the power supply.
With constant field excitation, load the motor in increasing steps until the motor current reaches rated value.
For each step measure and record: Speed, torque, armature voltage and, field voltage and Power.
Reduce the load to zero.
Manually taken values are thoroughly analysed in MS-EXCEL plotting the graphs for torque-voltage, speed, voltage, and speed and torque.
Fig 6.1 TORQUE VS VOLTAGE MANUAL READINGS
6.2 Analysis For Torque Vs Voltage
The graphs have been plotted for a series of readings.The above graph represents the voltage with respect to torque. From the above graph it can be concluded that there is a linear relationship between torque and voltage which makes a sense that the voltage is at peak of 3.96v at 0 torque and steadily decreased as the torque increased.
Since it is linear the slope can be taken from the eqn
Y=MX+C……………………………………………………………..(1)
Fom the above graph the SLOPE can be caluclated which is given.Considering the two values
Now caluclating for the constant
Where V is the voltage
M is the slope
is the torque.
From the above eqn
Therefor torque can be derived from the eqn.
The above calucalted torque formula is used to caluclate the torque from voltage with the help of LABVIEW.
SPEED AND ARMATURE VOLTAGE CHARACTERISTICS-MANUAL OPEARTION
The below presented graphs are taken for SPEED vs VOLTAGE manually. The voltage is caluclated by varying the speed with the help of a dc drive. The speed is varied in steps of 100 and the corresponding voltage readings are taken.
FIG 6.2 ARMATURE VS VOLTAGE SPEED
6.3 ANALYSIS FOR ARMATURE SPEED-VOLTAGE
From the above voltage speed graph we can observe that at low speed the voltage is minimum and the speed of the armature is incresed the voltage increased linearly. The graph almost shows a linear relationship between speed and voltage.
At a speed of 67 the dc voltage is around 0.1 and the speed is increased to 1488 the voltage went high upto 3.4V thus moving linearly with the speed.
Since it is a linear graph slope can be caluclated by
Y=MX+C
C=0.74
APPLICATION OF LABVIEW FOR MOTOR- LOAD CHARACTERISTICS
From the above graphs and caluclations it can be concluded that it is not possible to manually check for the speed of motor each and every time it is varied. Some motors need to run at different speeds in different conditions as per the requirement. Manually taking the readings may also cause some errors due to human mistakes which may result in loss of time and accuracy in producing the goods. So labview can be used to avoid these errors which continuously keep the track on data. The visual Language is easy to understand with out much difficulty making it easy to understand for man at work. Programs can build in such a way that the conditional loops can be written which act according to the changes in the input.
Thus due to these features such as "Ease of use, , invaluable built-in functions, programming capabilities, flexible user interfaces, unambiguous compilation, comprehensive dataflow tracking features, and debugging tools, and compatibility with many hardware facilities make "LabVIEW a power tool in data acquisition, control and analysis.
WORKING PROCEDURE - PC MONITORING
A eddy current dynamometer is coupled to the dc shunt wound motor(1500 rpm). An ac tachometer is coupled to the motor which gives out an ac voltage in the range of(0-9v).
The mechanical load is applied to the motor by varying the torque from the eddy current dynamometer and by using a voltage divider voltage is taken which is of range(0-4vdc).The output voltage is pure dc which can be fed to ADC.
The output from ac tachometer is a ac voltage . Ac signal cannot be fed to the adc . The ac signal obtained contains noisy disturbances which when fed directly cannot be digitised. This signal can be converted to dc by using simple precision rectifier circuit. The output from the precision full wave rectifier is a dc voltage. The dc voltage obtained contains nosiy disturbances. Signal conditioning filters the ripple content present in the signal. To eleminate this ripple a RC filter is used.The output from RC filter is a pure dc without ripple which can be fed to ADC card. The output signal from the RC filter can now be fed into ADC card which is of range(0-4v). Thus the two signals ar fed to the ADC card with in the acceptable range of(+/-5v). The signals acquired by the ADC dgitise the signals so that they are understood by the PC. The signals are now analysed with the help of Graphical Programming Language " LABVIEW".A Virual Instrument is built which takes the input signals and process them to give the output voltage.
To measure the motor at a particluar load the dynamometer can set to control in two ways. The first method is by controlling the torque where speed can be measured and vice versa. In both the cases if once the desired point is given, the tester can measure speed, torque, voltage and current depending the way the dynamometer is configured.
CHAPTER-7
SOFTWARE DESIGN
A LabView program has been designed to show the motor load characteristics which include the monitoring of Speed, Torque and Power. Torque and Speed are calculated from the voltages.
Torque is calculated from the voltage coming from eddy current dynamometer
Speed is calculated from AC Tachometer.
Fig: 7.8 STEP-BY-STEP PROCEDURE FOR LABVIEW PROGRAMMING
FRONT PANEL DIGRAM FOR THE BLOCK DIAGRAM
The front panel window is the interface to the VI code and is one of the two lab view windows that comprise a virtual instrument. The front panel is of command buttons and status indicators that are used for controlling Vis. Push buttons, Knobs, Controls, Graphs acts as input devices. Controls are meant to simulate instrument input devices and supply data to the block diagram of the VI whereas Indicators simulate instrument output devices and display data the block diagram acquires or generates[25].
TORQUE ACQUISITION:
The AIN.VI acquires the voltage signal. The output obtained from the voltage divider is fed to adc card. The board number is zero and the channel number has to be specified for it t recognise and acquire the data. The output from the AI is raw information and needs to be converted to voltage units.
The output from the analog input is given to the TOENG.VI which processes the voltage signal received and converts the input signal to appropriate voltage signal.
ANALOG INPUT TO ENG.VI
This Voltage readings can be recorded with the help of a numeric indicator which is connected to the output of the TOENG.VI. The numeric indicator can be connected to the meter which is shown under.
The output from the TO ENG.vi is given to the numeric indicator through which the readings can be recorded . The voltage from the motor is of the range(0-4v) which is compatible with the ADC.
From the manual readings taken for the voltage- torque we observed that a linear relationship exists between them. Thus from the eqn we derived that
To obtain the torque applied to the motor from the voltage signal the program has been designed for the torque equation(1).
A subtract vi is used to take the difference of the voltage and constant which is then divided by the slope Value (M) to get the Torque.
The output from the subtract vi is taken and given as one input to the divide vi and the other input is the M which is the slope. The output from the divide.vi is connected to the numercal indicator and meter indicator which gives the display for the torque value .
The Torque value obtained can be analysed with the help og Graph connected to a wave form chart. The GRAPH XY has two inputs in which the torque is connected to x-axis and the y-axis is connected to the voltage.
The Graph XY can be configured to suit the scale of x-axis and y-axis. The graphs obtained are saved and analysis has been done.
The voltage and torque readings are recorded and analysedwith the help of microsoft excel .
LABVIEW ANALYSIS FOR TORQUE AND VOLTAGE
The below shown Torque vs Voltage graph has been obtained by running the Lab view program from which the accuracy of the characteristics can be studied.
Fig 7.9.2 torque vs voltage graph
CONCLUSIONS :
From the graph we can observe that under no load the voltage is maximum and as the load is increased there's a steady drop in the voltage giving a linear relationship.
Another important aspect from the above graph is that the graph clearly shows the increasing steps of the torque. The rated torque of the motor is 4.725 Nm.
As the motor is loaded above the rated torque the voltage kept decreasing and after certain time it couldn't resist the load above the rated torque and hence stalled. When the load is applied above the rated torque the motor starts drawing high currents which can damage the motor condition in long run.
The above graph gives the information of the voltage readings from no load torque to full load torque and the red line represents the voltage calculated from full load torque to no load torque.
The difference of 0.2 nm torque is due to mechanical imbalance of the swing assembly.
TORQUE VS VOLTAGE -DECREASING LOAD
The motor is started at a full load torque above the rated torque and as the torque is decreased in steps the voltage kept increasing. At full load torque the voltage is minimum and as the torque is decreased the voltage reached the peak to a max of 3.92v.
SPEED ACQUISITION
The input voltage from the ac tachometer is acquired by the AIN.VI. The output Voltage from AIN.Vi is converted to appropriate voltage with the help of the TOENG.vi. The output voltage from the TOENG.Vi is given to the numerical indicator which continuously transmits the data received and a meter to visualise the data. is connected to the output which shows the voltage readings in the front panel.
Output from gives the voltage which is further converted to speed by applying the Speed formula calculated from the Manual Operation. Voltage obtained is multiplied by 461.73 and added to 0.74 which are calculated using the slope eqn taken from the manual readings.
The graph for Speed Voltage characteristics is plotted by using XY Graph/
C=0.74
The voltage output is then caluclted by the
Thus the output voltage from the numeric indicator is multiples with the Slope M using a numeric multiply vi and then added to constant C . Armature speed is calculated from the above methodology..
The speed of the motor is taken in RPM. The speed is acquired from the ac tacho which gives the ac voltage. This voltage is converted to dc with the help of simple precision rectifier.
TACHO VOLTAGE VS SPEED
TACHOVOLTAGE VS SPEED IN LABVIEW
The above graph shows the customized view of tacho voltage vs speed graph. The green line represents the minimum voltage and the blue line represents the max speed at no load. As the motor is loaded there is a steady decrease in the speed. As the speed is decreased the voltage decreases. The graph shows that theres no huge variation in the speed as the motor is loaded to rated torque.
From the above graph it proves that there's a linear voltage speed relationship. The speed is maximum at the max voltage. Thus the speed can be controlled by minimizing the voltage.
7.4.1 TORQUE VS SPEED CHARACTERISTICS
The above graph shows the variation of motor speed starting from no load torque to full load torque. The speed declines with increasing torque. The graph can be defined by 2 load points. Torque at zero is no load point and 6 is the stall torque. From the graph it is clear that there is no much variation in the speed when loaded even above the rated torque. In shunt motors the speed variation is less when loaded and hence these motors are considered to be constant speed motors.
TORQUE VS SPEED -LABVIEW
Theoretically the torque speed of a motor has linear relationship. The above graph shows that as the torque is increasing the speed decreases. The graph shows pretty linear relationships. The ups and downs in the graph is due to the Voltage fluctuations and mechanical vibrations in the motor which is causing it to deviate from the original readings.
UNLOADING TORQUE VS SPEED
UNLOADING TORQUE vs SPEED
When the motor is started at full load torque the motor started at very low speeds and as the torque is decreased the speed increased. The graph shows a linear relationship between torque and speed. When loaded above the rated torque the power of the motor is at max drawing heavy currents but as the load is decreased the motor started gaining the speed and reached the maximum at no load torque. The torque speed characteristics are non-linear at higher current levels.
MOTOR BEHAVIOUR AT OVERLOAD CONDITIONS
Torque vs Speed over load
The above graph shows the characteristics of the motor when overloaded. From the graph we can conclude that at no load torque that is when torque is at 0 the motor is at a rated speed of nearly 1500 rpm. As the torque is steadily increased in steps the speed of motor decreased gradually till 800rpm. When the torque is set to 7Nm much higher than the rated torque the motor runs above the rated efficiency. This may lead to rise in temperature, current may exceed above the ampere rating of the motor and the windings are affected. This may lead to electrical shorts in windings and hence damage the motor in long run.
7.5 POWER
The POWER of motor is calculated from the SPEED AND TORQUE. The torque is the input from the dynamometer and the output speed from the motor is used to calculate the Power of the motor.
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The efficiency of a motor is determined by Power. The speed and torque of a motor gives the efficency of a motor. Depending on the efficiency motors are used for different requirements.
STEP BY STEP PROCEDURE TO ACQUIRE POWER
Torque is acquired from voltage. Speed is acquired from the ac tacho. The product of torque and speed gives the Power. The torque is in NM and needs to be converted to lb-ft to get the standard units for power. Hence the torque is multiplied with 0.74 to get the output in lb-ft.
Speed obtained is in rpm and need not be further changed.
The product of speed and torque is taken by Multiply.VI. The output is divided by 5252 to get the result in HP.
To convert the hp to KW units the obtained results are multiplied by 0.75.
TORQUE VS POWER
LABVIEW TORQUE VS POWER
TORQUE VS POWER CHARACTERISTICS
From the above graph we can observe that the power is minimum at no load and as the torque is increase the power is increased showing a linear graph. The power is 0 at o torque and reached a maximum of 0.9Kw at 6.6 nm. The small deviations in the graph are due to the voltage fluctuations caused by the noisy disturbances and vibrations of the motor. Hence from the above graph it proves heavy loading of the motor causes power to be consumed more
When the decrease in torque is small it cannot offset the RPM and the power still increases. When the decrease in torque is large enough it there is a steady increase in RPM and hence the power start to drop.
POWER VS SPEED
The graphs are plotted for POWER SPEED to analyse the characterisitics of the motor.
SPEED VS POWER USING LABVIEW
SPEED VS POWER FULL LOAD TORQUE TO NO LOAD
From the above graphs we can see that as the speed increases power decreases. At a speed of 1500 RPM the power is at nearly 0KW as no load is applied on the motor. As soon we start loading the motor the POWER kept changing according to the change in speed. At low speed the power is high. This shows that under loaded conditions motor is using its maximum efficiency to turn the motor. If motor is loaded above rated power it starts drawing heavy currents and gets demagnetised before stalling. This Problem can be avoided by designing over load protection
ACCURACY
TORQUE:
The torque meter is 0.9% inaccurate. This is due to the mechanical imbalance of the meter.
TACHOMETER
The speed obtained is nearly 0.1% inaccurate.
Thus from the above accuracy we can see that the system is not that inaccurate. It has very minute errors which are almost negligible.
CONCLUSION:
The design, implementation and investigation of "pc monitoring of dynamometer" measuring device have been studied in detail.
Factors like speed , torque and power have been briefly discussed.
After the whole analysis , it can be clearly observed that the speed of shunt wound motors run nearly at constant speed even though loaded.
Motor characteristics when loaded above full load torque is studied.
Data acquisition terminology is used to log the results which are much more accurate than the manually logged data.
The characteristics of the motor at no load and at high load are thoroughly studied.
Concept of Virtual Instrumentation has been implemented to study the voltage, speed, torque, power characteristics.
With the help of Labview the real time moments of the motor are captured and analysed.
The results obtained practically are not that accurate when compared to the theoretical analysis due to mechanical vibrations and noisy disturbances of motor.
The advantages of Data acquisition through Labview is shown visually with the help of graphs plotted between manually logged results and results obtained in real time using Labview.
The need and importance of virtual instrumentation in industry is shown.
HURDLES FACED DURING PROJECT
ADC card has been used instead of NI DAQ card which is termed as the standard Data Acqusition Card. With out the DAQ card most of the VI s doesn't work. Only basic VI's work with the ADC card which has been a major constraint in my project. DAQ cards have the feature of signal-conditioning devices with built-in features.
Sampling rate is a major feature which determines how often analog to digital conversion takes place. Due to problem with instrument drivers loaded this feature has an error which was not used to know the sampling rate of the signal.
Labview has built in filtering VI's which reduces the ripple and noisy disturbances associated with the signal. This feature can be only used with NI DAQ card which has been another major constraint.
Some of the VI's built using ADC card is not seen with the systems loaded with NI DAQ cards.
RECOMMENDATIONS AND FUTURE WORK
Developing a programme which directly controls the motor. Conditional statements can be written to control the entire motor process.
Digital filtering concept can be implemented instead of the electronic circuits which are confusing.
Motor Vibrations can be with the help of hardware designed by National Instruments.
NI Labview FPGA model can be implemented and compact RIO hardware can be used to minimise the vibrations of motors for accurate results.
