Separately Excited DC MotorEE 327 Signals and SystemsWest Virginia UniversitySeparately Excited DC MotorPrepared by:Cheri SettellJeannine MeyersJanet KlinkhachornSubmitted to:Dr. Ali Jalali, EE327November 22, 2002Table of ContentsAbstract 31. Introduction 42. Design Approach 63. Theory 7Figure 1, Separately Excited DC Motor Schematic 74. Calculations 9MatLab Code 105. Procedure 116. Results 13Figure 2, Initial Value Graphs 13Figure 3, Increased Rf Graphs 14Figure 4, Increased Lf Graphs 15Figure 5, Increased both Rf and Lf Graphs 16Figure 6, Decreased Rf Graphs 17Figure 7, Decreased Lf Graphs 18Figure 8, Decreased both Rf and Lf Graphs 197. Analysis 20Figure 9, Comparison of Rf Values 22Figure 10, Comparison of Lf Value 23Figure 11, Comparison of Changing Both Rf and Lf Values 238. Conclusion 249. Bibliography 262AbstractOur team was tasked with finding a way for the armature current, field current, speed,and back emf of a Separately Excited DC Motor to have no attenuation in the graph of the values vs. time. The project had to be simulated using MatLab. Our team used the differential equations pertaining to the speed, armature current, field current, and back emf of the Separately Excited DC Motor to simulate the motor in MatLab. Once the code was written, base values for the field resistance and inductance, and the base values for the armature resistor and inductor were assigned and the simulation ran. After the initial values were taken, the simulation was run again increasing and decreasing the field resistor and inductor values. From the data collected, it was concluded that optimal conditions were reached by decreasing the field inductance from 50 to 25 ohms and leaving the field resistor at 75 ohms.31. IntroductionA DC motor converts electrical power into mechanical power, because of this DC machines are used in special heavy duty applications like draglines, electric trains, and steal mills. They are used for these applications because their speed and torque can easilybe varied without suffering a reduction in the efficiency of the machine. A DC machine has two parts: stator and rotor. The stator is the outer part and it is usually stationary. The rotor rotates inside the stator. The stator houses the field windings, and the rotor houses the armature windings. A DC motor is driven by a DC current supplied to the armature windings. This is called the armature current. There is also current supplied to the field windings, this is called the field current. Since the rotor rotates inside the stator, there is an interaction between the armature current and the field current by way of the magnetic flux created by both of these windings.1 A torque Td is formed because of interaction between the flux of the armature and field windings. The speed voltage ea, also known as the back-emf, is directly proportional to motor speed. It is related by the motor speed constant kv and the field current if. The separately excited motor can be controlled by changing armature voltage, a field voltage, or an armature current. When a motor is operated using its rated armature voltage, armature current, and field current, it is said to be running at its base speed b. Armature voltage control is the method of choice if a speed below the base speed is required. If speed above the base speed is required, field current must be changed. As the power output of the motor cannot be greater than its rating, the generated torque declines in over-speed operation.2 1 http://www.mech.up.edu.au2 http://murray.newcastle.edu.au4The goal of our project was to find a way to have the values of the armature current, field current, speed, and back emf level off with no attenuation in the generated values. The following report outlines the creation of a simulated Separately Excited DC Motor using MatLab, and discusses the best way to achieve our group’s goal. From the tests conducted, it was concluded the optimal values of back emf, armature current, field current, and speed were obtained with the field resistance and inductance being 75 and 25ohms respectively. Problem Statement: The values of armature current, field current, speed, and back emf need to have no attenuation in the generated values vs. time.52. Design ApproachI. ResearchA. The team began by doing research on how a Separately Excited DC Motor worked and what it consisted of. We went to the Evansdale Library to get books on the subject. There were several books on DC Motors, but none pertaining to simulation of motors in MatLab.B. After not finding much at the library, the team got on the internet. We looked for any type of information on Separately Excited DC Motors. We found several websites with information pertaining to our project. Most of the information we found was the simulation of a Series DC Motor and the steady state MatLab code for Separately Excited DC Motors. We did not know how the steady state equations were derived from the differential equations, so we decided not to use them, but the differential equations for the characteristics of the Separately Excited DC Motor we did understand.II. Design ApproachA. After researching Separately Excited DC Motors, we decided the best way to implement our design would be to change the differential equations of the Separately Excited DC Motor into difference equations. These equations could then be put into MatLab and the Separately Excited DC Motor could be simulated.B. We decided to measure the back electromotive force, speed, armature current, andfield current. From these measurements, we wanted to find the best combination of the four measurements by changing the field resistor and inductor values.C. Once our team had decided on how to accomplish our project, we needed to learn how to change a differential equation into a difference equation. We visited Dr. Choudry. He taught us how to make a differential equation into a difference equation. Then we needed to put the equation into MatLab. We then went to Dr. Jalali to learn how to put the difference equation into MatLab. After talking to Dr.Jalali, we were able to complete our program.63. TheoryA DC motor consists of a stationary cylindrical object called the stator, and a rotating cylindrical object inside of the stator called the rotor. The stator consists of electromagnetic poles called the field windings. In Figure 1, the Separately Excited DC Motor is