CONTENTS
LIST OF TABLES ....................................................................................................................... iv
LIST OF FIGURES ...................................................................................................................... v
EXECUTIVE SUMMARY ......................................................................................................... vi
1.0 INTRODUCTION.................................................................................................................. 1
2.0 DESIGN PROBLEM STATEMENT................................................................................... 2
2.1 PROJECT GOALS.............................................................................................. 2
2.2 PROBLEM SPECIFICATIONS AND CONSTRAINTS................................. 3
3.0 DESIGN PROBLEM SOLUTION....................................................................................... 4
3.1 HARDWARE DESIGN...................................................................................... 5
3.1.1 Three-Phase Source.................................................................................. 5
3.1.2 Three-Phase Diode Bridge Rectifier ....................................................... 6
3.1.3 MOSFET and Resistor Circuit............................................................... 8
3.2 SOFTWARE DESIGN ....................................................................................... 9
3.2.1 Multisim 8 and MATLAB Simulations .................................................. 9
3.2.2 LabVIEW Measurement and Control.................................................. 10
4.0 DESIGN IMPLEMENTATION ......................................................................................... 12
4.1 THREE-PHASE SOURCE IMPLEMENTATION....................................... 12
4.2 MOSFET FIRING CIRCUIT ORIENTATION............................................ 14
4.3 MOSFET CONTROLLING ............................................................................ 16
5.0 TEST AND EVALUATION................................................................................................ 16
5.1 OP AMP CIRCUIT TESTING........................................................................ 16
5.2 MOSFET SERIES VS. PARALLEL COMBINATIONS ............................. 16
5.3 LABVIEW TESTING AND RESULTS .......................................................... 17
5.3.1 Bad Equipment ........................................................................................ 17
5.3.2 Duty Cycle Frequency ............................................................................. 17
5.3 3 Result Averaging ..................................................................................... 18
6.0 TIME AND COST CONSIDERATIONS.......................................................................... 19
7.0 SAFETY AND ETHICAL ASPECTS OF DESIGN......................................................... 21 ii

8.0 CONCLUSIONS AND RECOMMENDATIONS............................................................. 21
REFERENCES............................................................................................................................ 23
APPENDIX A – HIGH LEVEL PROJECT OVERVIEW ................................................... A-1
APPENDIX B – PROJECT GANTT CHART....................................................................... B-1
APPENDIX C – PROTOTYPE PHOTOGRAPHS ............................................................... C-3 iii

LIST OF TABLES
Table 1. Motor Tests ...................................................................................................................... 3 iv

LIST OF FIGURES
Figure 1. Torque vs. Slip................................................................................................................. 3
Figure 2. Circuit Diagram ............................................................................................................... 5
Figure 3. Three-Phase Voltages ...................................................................................................... 6
Figure 4. DBR Circuit..................................................................................................................... 7
Figure 5. DBR Output Waveform................................................................................................... 7
Figure 6. MOSFET and Resistor Model ......................................................................................... 8
Figure 7. Square Wave with Duty Cycle (D) [6] ............................................................................ 8
Figure 8. V ab
& I a
Sensing and Square Wave Output ................................................................... 10
Figure 9. Resistance Calculation................................................................................................... 11
Figure 10. Front Panel Display ..................................................................................................... 12
Figure 11. Original Op Amp Circuit Design ................................................................................ 13
Figure 12. Single-Phase to Three-Phase Converter ...................................................................... 13
Figure 13. Series Combination ..................................................................................................... 15
Figure 14. MOSFET Firing Circuit [8]......................................................................................... 15
Figure 15. Output Waveform........................................................................................................ 18
Figure 16. Loop Averaging Function............................................................................................ 18
Figure 17. Averaging Output ........................................................................................................ 19 v

EXECUTIVE SUMMARY
This document evaluates the design, construction, implementation, and testing of a constant speed drive for a wound rotor induction motor. A constant speed motor would best suit certain applications, such as a constant speed wind turbine or a fluid pump. A motor does not naturally maintain a constant speed as the torque varies. Our project was to design a power electronics circuit that could keep the speed constant independent of the applied torque.
The torque-speed characteristic of the motor changes as the resistance across the rotor terminal changes. Therefore, our solution is to create a circuit that can vary the resistance appropriately to maintain constant speed. We designed and developed hardware and software that work together to achieve the goal.
We did not have access to a motor; therefore, we had to design a circuit that models a motor. We achieved this by creating three-phase voltage (like a motor) from a single-phase voltage to simulate the rotor terminals. Rotor terminals of an induction motor give the user access to manipulate the motor speed. A diode bridge rectifier converts three-phase alternating current to direct current; which passes through a resistive/MOSFET circuit in a parallel combination. As the MOSFET switches more frequently, the equivalent resistance decreases. This changing resistance will, hence, keep the speed constant.
We used a software package called LabVIEW to control the MOSFET and monitor the output on the computer. LabVIEW transmitted a square wave with a certain duty cycle to the MOSFET to fire it faster or slower. As the MOSFET switches, the voltage and current at the phase terminals changes as well. A changing voltage and current means a changing resistance too. LabVIEW detects this change and outputs it to the user’s screen for evaluation.
We ran into a few stumbling blocks during implementation that hindered the project completion.
Fortunately, we were able to overcome these obstacles and successfully test and evaluate our design.
Along with the aforementioned topics, this report describes time and cost considerations, safety and ethical aspects of design, and concludes with some recommendations. vi

1.0 INTRODUCTION
This final report for our constant speed wound-rotor induction motor controller describes the journey we have taken from the beginning of the semester, the design idea we chose, the constraints that limited our progress, what we overcame, and the work we have accomplished.
For a typical induction motor, the rotational speed increases as the torque decreases, and speed decreases as torque increases. We proposed to keep the motor speed constant regardless of the torque change, and this process can be accomplished by tweaking the rotor resistance the motor senses. We did not have an actual motor to work with; therefore, we modeled the behavior and properties of an induction motor by building circuits and writing software programs to sense and adjust the change in torque to maintain constant speed. The idea of this design offers greater flexibility in manipulating larger applications, such as a wind turbine and a fluid condenser.
Throughout the semester, we have encountered numerous obstacles, such as lacking a basic understanding of motor functions, trouble acquiring circuit components, and mistakes along the way. With the help of teaching assistants and professors, we gained valuable experience and knowledge from our research, and we also learned the importance of patent-searching and notebook-keeping to avoid redeveloping prior inventions. In this document, we include the difficulties encountered, such as lack of experience with Multisim 8, LabVIEW, and MATLAB, and the inconsistent results we obtained due to machine and human error. In addition, we cover implementations and the testing procedures we went through by modeling software and putting it together with hardware, and the time and cost constraints, and safety (i.e. burning out op amps) and ethical aspect of the design. In this document, we include the purpose of designing a constant speed induction motor controller and the design solutions and ideas we came across throughout the semester. Implementations and the testing procedures were done by double checking the previous calculations and the programs written in LaBVIEW. Time and cost considerations described the problems we experienced, such as lack of knowledge on Multisim, LabVIEW, and
Matlab, and the inconsistent results we obtained due to machine and human error encountered in detail.

2.0 DESIGN PROBLEM STATEMENT
An induction motor operates at a synchronous speed when no load is attached. As the physical load is increased, more torque is required to spin the motor shaft. Unfortunately, the rotational speed of the shaft decreases as the torque increases. An analogous situation would be a car driving uphill. If one keeps a constant pressure on the accelerator as the car begins to go uphill, then the velocity will decrease. Using the cruise control feature, the car can maintain a constant speed independent of the incline that the car is traveling on. Our project is to create a “cruise control” for an induction motor. As the torque changes, our circuit will hold the motor’s speed constant.
2.1 PROJECT GOALS
Some applications make it desirable for a motor to maintain a constant speed invariant of the torque. In industry, certain fluids may slow down the speed of a motor due to the fluid’s density and other properties. It would be ideal for a motor to be able to pump any fluid at constant revolutions per minute (RPM). We achieved this goal by manipulating the torque vs. speed curve of the motor. This curve demonstrates how much the speed drops depending on a given torque.
For a wound-rotor induction motor, changing the electrical resistance across the rotor terminals will result in a different torque vs. speed characteristic. Thus, we designed a power electronics circuit that changes the resistance in response to a varying torque. Among the many motors to choose from, we used a wound-rotor induction motor because of the easy access to the rotor terminals. Figure 1 below shows the graph of torque versus slip. Slip is the difference between the synchronous speed and the actual speed the motor is spinning at. Slip is commonly calculated in percentage.
2

Figure 1. Torque vs. Slip
Equation (1) is the equation for slip.
Slip = n
S n
S
− n
(1)
The synchronous speed for our motor is equal to 1800 RPMs and is designated as n
S
. The actual motor speed for a given torque is designated as n . Thus, a speed of 1620 RPMs would result in a slip of 0.1 (i.e. 10%). Thus, the motor’s speed slipped 10% from the synchronous speed.
2.2 PROBLEM SPECIFICATIONS AND CONSTRAINTS
For a real world situation, our project must work with an induction motor that has certain ratings and outputs. We designed our motor to operate with the 175W motor available in the lab. To determine how we should design our solution, we had to run tests on the motor. The results are in table 1 below.
No Load Test
50W
250VAr
208V
.7A
Table 1. Motor Tests
Blocked Load Test R
DC
130W
100VAr
70V
1.3A
12.6 Ω
3

The first test results came from the motor rotating at synchronous speed with no load attached.
The second test results came from the motor operating, but the shaft not spinning. This simulates full torque on the motor. From these tests we knew what component ratings we needed in order to ensure safety while working on our project.
After designing our circuit with these components, we determined that applying our circuit to an actual motor would be impossible. We could not find a lab with a three-phase outlet for the motor and a computer that could run the controlling software. Therefore, we resorted to building a circuit that could simulate the motor. There was no way to simulate both the torque and the speed. Thus, all we could do was to build a circuit that created a balanced three-phase output from a single sinusoidal wave. Unfortunately, with this solution, there is no way of knowing if the speed remains constant as the torque varies because no motor is attached. We could demonstrate, however, that changing the input torque on the computer would cause our circuit to change the resistance. This changing resistance shows that our circuit responds properly to a change in torque.
3.0 DESIGN PROBLEM SOLUTION
In order to control the speed of the motor, our circuit must be able to detect the torque and adjust the resistance accordingly. A hardware component is required in order to create a resistive circuit across the rotor terminals. Software is an integral part of the design as well. Software makes calculations based on known data and adjusts the circuit resistance appropriately. The following subsections will cover hardware first and software/coordination second. Understanding how the hardware works will make the software implementation easier.
Appendix A shows a high level diagram of our project. It shows how the LabVIEW program receives a torque input, computes a duty cycle, and outputs a square wave to the MOSFET. The
DBR then gets its power from the three-phase op amp source. LabVIEW then reads the voltage and current at the three-phase source and determines the resistance. Appendix B shows pictures of the DBR/MOSFET circuit and the three-phase source circuit.
4

3.1 HARDWARE DESIGN
Our circuit is broken down into three sub-circuits. Figure 2 below shows a diagram of our circuit.
From left to right, the circuit is divided into a three-phase power supply, three-phase diode bridge rectifier (DBR), and a MOSFET circuit.
Figure 2. Circuit Diagram
Unfortunately, we were unable to use a real three-phase power supply or motor. Thus, we had to build a circuit that took a sine wave and produced three sine waves, each 120° apart. This subcircuit simulates the three-phase motor. The six diodes in the center of the circuit are used to convert three-phase alternating current (AC) into direct current (DC). The final sub-circuit is a parallel combination of a resistor and a MOSFET switch. The MOSFET switching rate controls how much current gets to the resistor. By changing the amount of current that passes through the resistor, the equivalent resistance across the motor changes. Thus, the circuit can effectively change the resistance and control the speed.
3.1.1 Three-Phase Source
Unfortunately, the lab with computers didn’t have a three-phase source, while the lab with the three-phase source didn’t have computers. In order to test our design, we needed to create a balanced three-phase out of a single phase source signal. This circuit contains op amps that phase shift the outputs by 120° and 240°, creating balanced three-phase. Figure 3 shows the three phases plotted on one graph.
5

Figure 3. Three-Phase Voltages
This three-phase output simulates a three-phase motor. Since we cannot test our project on a real three-phase source, at least this circuit gives us the ability to simulate our circuit for test and evaluation.
3.1.2 Three-Phase Diode Bridge Rectifier
For a wound rotor induction motor, one must change the resistance across the rotor terminals to control the speed. To do this, one would have to change the resistance at all three terminals at precisely the same time. This process could be done with three resistors in parallel with three
MOSFETs across each phase. Due to cost and synchronization issues, this solution is not optimal. To simplify the solution, we built a circuit that can convert from three-phase AC to DC.
This procedure is called Diode Bride Rectification (DBR). By building this circuit, we only needed one resistor in parallel with one MOSFET. Then, the resistance sensed by the three-phase source would be equal across all three phases. Figure 4 below shows how to construct a threephase DBR.
6

Figure 4. DBR Circuit
The combination of the six diodes creates the DC signal. A diode only allows current to flow in the direction of the arrow. If the voltage at the top of the arrow is larger than the voltage at the bottom of the arrow, then current will attempt to flow in the opposite direction and the diode will prevent this from happening. Thus, in the configuration shown above, only the phase that has the largest instantaneous voltage will be transmitted to the output. So, when phase A drops below phase B, then only phase B is transmitted. The same characteristic is true for phase C. A graph of the output is shown in Figure 5. In this figure, the output is traced in red and a single phase is traced in light blue.
Figure 5. DBR Output Waveform
By examining the graph, one can notice that the output is not DC as it is supposed to be. The signal is a full wave rectified voltage. To make this DC, we should have placed a capacitor in parallel with the DBR in order to smooth out the voltage drops. When we were originally designing our circuit for 120VAC, we determined that a capacitor might have an adverse affect on the circuit. Even without the capacitor, the circuit will still function properly. The difference is that the resistance won’t be as stable in our final solution as in simulation. Even so, the
7

characteristic of a resistance varying circuit still holds true for our solution and proves our theory.
3.1.3 MOSFET and Resistor Circuit
The last sub-circuit of our circuit is a MOSFET in parallel with a resistor shown in figure 6. The
DBR will supply the power to this circuit and the MOSFET will change the equivalent resistance the DBR senses.
Figure 6. MOSFET and Resistor Model
This sub-circuit controls the resistance of the entire circuit. If the MOSFET is closed, then all of the current will travel through the MOSFET and the equivalent resistance of the circuit should be low. If the MOSFET is open, then all of the current will travel through the resistor causing the equivalent resistance to equal the value of the resistor. Based on this concept, if the MOSFET was closed half of the time and open the other half, then the equivalent resistance will be half of the resistor value. Therefore, changing the amount of time the MOSFET stays open or closed will control the equivalent resistance of the circuit. To achieve this, we applied a square wave pulse to the MOSFET gate terminal. Figure 7 shows an example of a square wave.
Figure 7. Square Wave with Duty Cycle (D) [6]
8

This square wave has a given duty cycle that defines the wave’s characteristic. A duty cycle determines how long the square wave’s amplitude is at Y max
and at Y min
. In the above figure, T represents the period and D represents the duty cycle. The duty cycle equals .33 and, therefore, the amplitude is at Y max
for 33% of the period. If this square wave is applied to the MOSFET gate terminal, then the MOSFET will be closed for 33% of the time and the resistance will be 1/3 of the resistor value. In conclusion, controlling the duty cycle will result in controlling the resistance and speed.
3.2 SOFTWARE DESIGN
Nowadays, very few hardware solutions exist without a software component. Every type of engineer must have some familiarity with software in real world applications. The same principle applies to our project. We used software to test design solutions in simulation, perform complex calculations, and control our circuit operation. Without software, designing our project would prove to be tedious and implementing our project would prove to be impossible. The three software packages we used were Multisim 8, MATLAB, and LabVIEW.
3.2.1 Multisim 8 and MATLAB Simulations
We developed most of the circuit models in this document using Multisim 8. Multisim 8 is a software package that can simulate real world circuits and predict circuit responses. This feature proves to be very helpful when in the design stage. If one can prove a solution in simulation, then the chances of success in the real world are very high. We built several models for our circuit in
Multisim 8. Each model increased in complexity and aided in the design process. For example, initially, we used a DC battery in place of the DBR circuit. Once we determined that a battery in parallel with a MOSFET and resistor produced the ideal result, we replaced the battery with a three-phase generator and a DBR. The circuit still operated appropriately after the swap. We then tried applying a capacitor to our circuit to further smooth out the voltage. Unfortunately, this produced some negative current that could overload the diodes at high voltages. Without proper simulation, we would have missed this valuable information.
9

We used MATLAB for a different type of simulation. MATLAB helped us to understand the torque-speed characteristics of our motor. By calculating the torque of the motor at different resistances, we were able to understand how to calculate the duty cycle. The torque equation is:
T mech
1
=
ω
S









R
1 eq n ph
V
1 eq
2


R
2 s


+



R
2 s






2
+
(
X
1 eq
+ X
2
) 2






(2)
Using the data collected from our motor tests, we were able to use this equation to plot torque versus slip. To further understand our project, we also plotted torque versus resistance. These simulations didn’t involve our actual prototype, but they proved useful in providing valuable knowledge to aid in our project’s completion.
3.2.2 LabVIEW Measurement and Control
LabVIEW proved to be the most critical software package in our project development.
LabVIEW detected voltages and currents, calculated slips and duty cycles, displayed waveforms and values, and outputted square waves. LabVIEW acted as the controller, calculator, and analyzer. Figure 8 shows how LabVIEW senses voltage and current and outputs the square wave.
Figure 8. V ab
& I a
Sensing and Square Wave Output
10

We used these function blocks to gather and output data. Our program then takes the torque input from the user and calculates the equivalent resistance. Figure 9 shows the resistance calculation.
We derived this equation from the torque equation (2) using the given values for our motor.
Figure 9. Resistance Calculation
We made further similar calculations to obtain the slip and duty cycle based on a given torque.
The slip is not useful for our test situation because we are not using a real motor, but we calculated them anyway. The duty cycle is then supplied to the square wave generator in Figure
8. This generator then outputs a square wave to our MOSFET.
We displayed every value and waveform relevant to our project in the Front Panel of our
LabVIEW program. This display was essential to debugging our circuit and code. Without proper displays, we could not have determined the validity of our outcome. Figure 10 shows our
Front Panel display with a duty cycle plot, resistor plot, and voltage and current readings.
11

Figure 10. Front Panel Display
4.0 DESIGN IMPLEMENTATION
Our project had many different phases and adjustments. Our original design specifications and parameters were based on a real motor that we planned to test. Due to the higher voltages and currents, we spent many weeks working on designing and simulating circuits for this motor.
Once we found out we could not use a real motor, we realized how much time we could have saved. We made several implementation setbacks and modifications that caused our project outcome to vary from our original design. These implementations and modifications are discussed in detail in the following subsections.
4.1 THREE-PHASE SOURCE IMPLEMENTATION
From our mixed signals lab course, we developed a three-phase source solution that we thought would work. Figure 11 shows our original op amp circuit that can maintain three-phase AC without having a sine wave input source.
12

Figure 11. Original Op Amp Circuit Design
This circuit works in theory, but does not do a very good job of producing three-phase. After hours of testing, we determined the sine waves were not clean and clear enough for our project.
We turned to Dr. Bostick to help us with our design. After his guidance, we designed a circuit that took an input signal and shifted it ±120°. Our single phase source was a function generator producing a 5V and 60Hz sine wave. Figure 12 shows the op amp circuit we had to build in order to produce three phases.
Figure 12. Single-Phase to Three-Phase Converter
The op amp on the left just passes the source sine wave through to the output without any delay.
The op amp in the middle is designed to take the 5V signal and delay it by 120°, while the op amp on the right will delay the signal by 240°. This process creates a perfectly balanced three phase source. This circuit is designed to simulate the output from the motor.
13

To design this op amp circuit, we had to understand basic rules of op amps that we learned in our electronics class. To determine the output signal, we had to write an equation for the output in terms of the component values. Using basic circuit analysis, we derived an output equation for the two shifting op amp circuits. These equations for op amp B and op amp C are:
E
O , B
= −
R
4
R
3





1 +
1
1 j ω R
3
C
3





(3)
E
O , C
= −
R
2
R
1


 1 +
1 j ω C
2
R
2



(4)
The minus sign in the right hand side of the equation exists because we used the inverting terminal of the op amp. Thus, the minus sign delays the signal by 180°. Then, the term in the parenthesis controls the rest of the phase shift. One op amp circuit will add 60° to get 240° and the other op amp circuit will subtract 60° to obtain 120°. Therefore, knowing that the frequency is set at 60Hz and a phase shift of ±60°, we can determine all of the other components. Shifting the signal by 60° caused the amplitude to decrease by 50%. In order to correct this decrease, we just doubled the value of R
4
and R
2
. After building this circuit, we tested it with an oscilloscope and found it to work perfectly. Since the op amps can only handle 15V or so, the sine function input cannot be exceed this value as well. In fact, when we powered the op amps with 9V DC, the outputs would flatten out at the peaks when the input was greater than 5V AC. Fortunately, this proved to be adequate to test our circuit.
4.2 MOSFET FIRING CIRCUIT ORIENTATION
Originally we designed our MOSFET firing circuit shown in Figure 13 as a MOSFET in series with a resistor. We approached the problem this way because we wanted to avoid the short circuit situation from the parallel combination.
14

Figure 13. Series Combination
In most applications, a short will destroy a circuit. An induction motor is the exception. Since the induction motor has a built in resistance, a short across its terminals will not result in a failure. It will just complete the circuit with the equivalent resistance equal to the rotor resistance. An open circuit, on the other hand, is bad for a motor and can cause failure. An open circuit is basically a resistor with infinite resistance. This infinite resistance causes irregularities and can damage the motor. Thus, we implemented our circuit in a parallel combination in order to maintain consistency with our problem statement.
Figure 14 shows the MOSFET firing circuit, based on our background knowledge about the
MOSFET firing circuit from Dr. Grady’s Power Electronics course[8]. Our main change from his MOSFET circuit is that we are inputting the wave from LabVIEW, not a manual control.
Figure 14. MOSFET Firing Circuit [8]
15

4.3 MOSFET CONTROLLING
In previous courses, we used a microcontroller to control signals. We originally were going to use the Motorola 6812 as our controller for the duty cycle. After some thought, we determined that using a microcontroller was going to be difficult. We didn’t remember a lot about the programming language and did not know how to implement our project with a microcontroller.
Also, we needed a way to view our resulting resistance effectively. Since we determined that
LabVIEW was the best solution for viewing results, we decided to use LabVIEW to control our
MOSFET as well. This decision proved to be a very wise since the graphical user interface
(GUI) for LabVIEW is much easier to understand than an assembly level programming language.
5.0 TEST AND EVALUATION
Testing and Evaluating are important aspects to any project. We tested each circuit individually to ensure that it worked properly. We tested each function block of LabVIEW separately to locate errors in our code. Our solution worked in theory many weeks ago. But the difference between theory and reality can be quit large.
5.1 OP AMP CIRCUIT TESTING
Originally, we designed and built the self-sustaining three-phase op amp circuit. We first hooked it up to our circuit and tested the output and it did not seem right. We then tested the op amp circuit and the outputs did not make sense either. The waves weren’t clear or clean and did not shift very well. This testing caused us to reevaluate our op amp design until we determined a better solution. We then designed and built the new op amp circuit. The output was perfect and clearly represented balanced three-phase AC. The only issue is that we could not change the frequency from 60Hz because we chose our capacitor and resistor values with 60Hz frequency.
Changing the frequency would cause the amplitudes to differ from each other.
5.2 MOSFET SERIES VS. PARALLEL COMBINATIONS
After testing, we discovered that our circuit worked with a parallel MOSFET/resistor combination. As the duty cycle decreased from 1 to 0, the resistance decreased from 25 Ω to
16

about 10 Ω . Now, we wanted to test and see if our circuit would work for a series combination, since that was our original design. After testing it, we found out that it also worked. At a high duty cycle, the resistance nearly equaled 25 Ω , while at a low duty cycle, the resistance was around 500 Ω . This high resistance indicates an open circuit. Thus, our circuit worked for both series and parallel combinations. We kept our final design in parallel because a real world application demands a parallel combination as opposed to a series combination.
5.3 LABVIEW TESTING AND RESULTS
LabVIEW gave us the most problems throughout the end of the semester. We knew our circuit worked, but we had to get LabVIEW to show that it worked, read inputs and outputs, and display data properly. A solution is not valid without substantial proof. Finally, we were able to overcome our LabVIEW obstacles and complete the project successfully.
5.3.1 Bad Equipment
Something new seemed wrong with the equipment every day we hooked it up to our circuit.
Some days, the inputs didn’t look right, while other days, the waveforms looked choppy. Every time we hooked up the circuit, something else seemed to fail. More times than not, the problem was LabVIEW equipment. The main issue was the terminals. Since these are public terminals that any student can use, some of them don’t work. If a student tries to force too much current back into the console, then that port may malfunction. We ran into this issue during open house and almost could not demonstrate our project. The solution was to move from the first ports to the last ones. The theory was that the last terminals were used less frequently.
5.3.2 Duty Cycle Frequency
We figured that LabVIEW would output a perfect square wave without a problem. Not considering the square wave to be a possible issue, we gave it a frequency of 10Hz. While testing our circuit, we determined that something was failing. We connected the square wave to an oscilloscope to show the waveform. We did not see the square wave that we expected. We tried changing the frequency to 100Hz. This value seemed to be the minimum frequency for the square wave to function properly. After making this change, the square wave properly controlled the MOSFET.
17

5.3 3 Result Averaging
After completing our project, we got an output that varied a lot. The output did vary about a certain value consistently, but the average value was unclear to the naked eye. Our output would seemingly vary from 9 Ω to 18 Ω randomly. Figure 15 shows what this output looked like. We believed that this random nature was due to not having true DC voltage across the resistor.
Figure 15. Output Waveform
In order to solve this, we had to develop a LabVIEW function that would average all of the points and plot the average. This process took several weeks to complete. We tried nearly every function, used every reference, and followed every example. Finally, on the last day, we found a solution that works.
First, a loop runs 26 times collecting 26 points from the output. Then, those points are placed into an array, added together, and then divided by 26. This procedure produced a very nice average. Figure 16 shows the LabVIEW function that implements this design.
Figure 16. Loop Averaging Function
18

Unfortunately, 26 may not have been enough data points. One hundred data points would be desirable. Using 100 data points took a long time to compute the data and was not practical in this application. Twenty six points, however, still demonstrates the idea of averaging. Figure 17 shows what the output looks like with the averaging in affect. Notice how the value on the right varies between 11 Ω and 14 Ω instead of 9 Ω and 18 Ω . Also, notice the drop from around 23 Ω to 12 Ω . This drop shows how the resistance changes at different duty cycles. Thus, our project works according to specifications.
Figure 17. Averaging Output
6.0 TIME AND COST CONSIDERATIONS
According to the Project Gantt Chart in Appendix C, our team started falling behind around mid-
October. Several setbacks and delays occurred throughout the semester. First, we were late in choosing a project for the semester because we were in a different TAs group. We had chosen power to be our focus area and a project was “assigned” to us; but we wanted to come up with an original invention that no one has done in EE464K before. Solar and wind energy are two of the most concentrated power-related area one can choose from, but many great project ideas were already taken. The next best thing would be hydro power. We planned to construct a small water pumped storage reservoir that could allow the water to turn a turbine and create power. The TAs and professors favored the objective, but they thought a reservoir was not feasible enough to complete in one semester and demonstrating it on a small scale was very problematic. At that point, a week had already gone by. To prevent further delay, we settled with the original assigned project.
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Second, since we had no prior knowledge or taken any courses on electric machinery. We had to do a lot of research and reading to get the basic concept of how a motor works and how to control the motor speed. Third, we spent a significant amount of time modeling induction motor’s torque-speed characteristic curve using MATLAB, mathematical software that neither of us were very familiar with. Fourth, Mohit, the power TA, not our actual TA, recommended constructing a schematic of wound-rotor induction motor in Multisim to help us have a better understanding of how induction motor operates. Once again, we spent an ample amount of time learning, drawing, and testing our circuit. We could not continue with the rest of the project, because the motor concept was crucial in aiding us further down the road. Although, MATLAB and Multisim were not the focal point of our project, their graphs and schematics were a big help in understanding and explaining our project to someone who is or was not familiar with the motor concept.
Fifth, we had already calculated the component values needed to model our wound-rotor induction motor controller. We knew exactly what and how the circuits were to be built; the only thing left to do was to acquire electronic parts from Dr. Grady, because he is the only person who controls the flow of parts to students. We emailed, talked, and looked for Dr. Grady several times for three weeks straight, but he was always unavailable, due to his busy schedule. We finally caught him one day in his office, convinced him of our project idea, and got the parts we needed.
Sixth, assembling software with hardware caused a lot more trouble than anticipated. The software program, LabVIEW, and the circuits we built, functioned perfectly when not connected to each other. But once we hooked up the circuits, the measurements made across a point with one measuring device conflicted with the value measured by another device. At that time, we were done with all the software design and hardware implementations except testing; therefore, when something goes wrong, it was impossible to tell what the source of the problem was or where the bug originated. On Open House day, we had trouble setting up the circuit. After much tweaking, we replaced several wires for bad connections, changed stations because LabVIEW interface or the function generator did not work properly, and LabVIEW interface had been misused by students from previous years, causing the ports to give wrong readings.
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At the beginning of the semester, we estimated the total project cost to be around $200, about
$100 per person. $50 to $100 was for sensing devices that detect the change in torque, and automatically adjusts the rotor resistance to maintain constant motor speed. The remaining $100 or so was allocated for construction and unforeseen costs. Now that the semester has ended, we are proud to say that we did not need to spend a dime on the project. All the circuit parts were acquired from the Power Lab, the Engineering Laboratory on ENS 2 nd
floor, and Dr. Grady.
Despite the major issues mentioned earlier in this section, we managed to suffice, and presented
Kapil a functional project.
7.0 SAFETY AND ETHICAL ASPECTS OF DESIGN
Our senior design has a very safe circuit design. None of the circuits would blow up or explode because the power going into the circuit is quite small. At the ratings of the motor, our circuit should still work properly. But since we have not tested this, we cannot be for certain. We do have to be careful not to overcharge the op-amps because burning the op-amps would not be good for our circuit. One more precaution we should pay attention is that when everything is set up and the software and hardware are communicating, we need to remember to keep the wires apart and do not let them accidentally short-circuited other parts of the circuit. If we had finished our circuits a couple of weeks in advance, we might have been able to utilize the actual motor.
Our design remains ethical in nature. We could not exhaust all avenues searching for similar inventions, but our research told us that most constant speed drives us induction motors that don’t have access to the rotor terminals. Therefore, our project should be fairly original and avoid law infringement.
8.0 CONCLUSIONS AND RECOMMENDATIONS
We recommend students who are interested in this project to get ahead start, know all the resources, and do not procrastinate. For our three-phase op-amp shifter, we combined three or more resistors to get one desired resistive value. Near the end of the semester, we realized that
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the check-out laboratories on ENS 2 nd
laboratory offers a greater selection of resistors. If we had known we could check out resistors, our circuit would look nicer and less time would be spent on making the circuit. Time is also a very important factor. Previous 464 students have warned us not to fall too far behind and to keep up with the schedule. We did fine at the beginning of the semester, but as the semester progressed, we fell a little behind.
In the real world, a constant speed control for an induction motor would be ideal. With the ability to maintain a constant rotational speed, the motor is more flexible and can be used for a wide range of applications. During this semester, we learned a lot about induction machines, power electronics circuits, op amp circuits, testing procedures, LabVIEW coding, and debugging problems.
Our goal was to determine if our circuit would correctly adjust the resistance to maintain a constant speed. Originally, we thought we could use an induction motor and sense if the circuit is working properly or not. As the project developed, we determined that testing on a real motor would be very unlikely. Therefore, we could not sense a torque or speed and our outcome changed to determining if resistance changed when the duty cycle changed. We found that building an op amp circuit to create three-phase voltage was tricky. We were locked in on a certain frequency and had a maximum voltage signal we could produce due to the op amp specifications. The DBR worked perfectly according to plan. Placing an oscilloscope across the output of the DBR gave us the desired signal. We were also happy with the results we viewed on the LabVIEW Front Panel because the resistance changed as the duty cycle changed. The only issue was that the signal varied dramatically. We determined this dramatic oscillation was due to the DBR voltage not being real DC. Creating an averaging function proved to be a difficult task for us because we were unfamiliar with LabVIEW and didn’t know where to begin. Fortunately, we were able create a function that averages the waveform with decent accuracy. Thus, we were able to complete our solution and meet project requirements and specifications.
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REFERENCES
[1] http://www.tecowestinghouse.com/PDF/woundrotor.pdf#search=%22woundrotor%22 (current 20 September 2006).
[2] A. E. Fitzgerald, C. Kingsley, Jr., and S. D. Umans, Electric Machinery , McGraw
Hill, New York, N.Y., 2003, p. 306.
[3] “Electric Motor” http://en.wikipedia.org/wiki/Induction_motor#Singlephase_AC_induction_motors (current 25 September 2006).
[4] “Wound Rotor” http://www.tecowestinghouse.com/PDF/woundrotor.pdf#search=%22woundrotor%22 (current 26 September 2006).
[5] “Control tutorials for MATLAB” http://www.engin.umich.edu/group/ctm/extras/plot.html
(current 06 December
2006).
[6] “Pulse Width Modulation” http://www.answers.com/topic/pulse-width-modulation (current 06 December
2006)
[7] “EE362L, Power Electronics, Capacitor Filtered Diode Bridge Rectifier” http://www.ece.utexas.edu/~grady/EE362L_Diode_Bridge_Rectifier.pdf (current
06 December 2006)
[8] “EE362L, Power Electronics, MOSFET Firing Circuit” http://www.ece.utexas.edu/~grady/EE362L_MOSFET_Firing_Circuit.pdf
(current 06 December 2006)
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APPENDIX A – HIGH LEVEL PROJECT OVERVIEW
A-1

APPENDIX A – LEVEL PROJECT OVERVIEW
A-2

APPENDIX B – PROJECT GANTT CHART
B-1

APPENDIX A – CHART
B-2

APPENDIX C – PROTOTYPE PHOTOGRAPHS
C-3

APPENDIX C – PROTOTYPE PHOTOGRAPHS
C-4