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EGRE 101 Lab 2: DC MotorAuthor: {Student name}Lab partner(s): {Lab partner name(s)}SummaryIn this lab, we built a DC motor using wire, magnets and a DC source. Once the motor was mounted on the stand and was working, we also took measurements to study the effect of its rotation on the current going through the wire. Using our measurements, we calculated the back-emf generated in the motor while it is rotating inside a magnetic field.TheoryElectric motors work on the principle of Laplace force, which is an extension of the Lorentz force. The Laplace force acting on a current – carrying wire inside a magnetic field is equal to the magnitude of the current in the wire times the cross product of the vector whose length is equal to the length of the wire inside the magnetic field and whose direction is the direction of the wire inside the magnetic field by the magnetic field vector at the corresponding point on the wire. The Laplace force equation is given in Equation 1 and visualized in REF _Ref462556309 \h \* MERGEFORMAT Figure 1.F=Il×B(1)Figure SEQ Figure \* ARABIC 1: Laplace force.One of the first people to implement this principle in the construction of a rotating wire was Faraday. We replicated his experiment, using a DC source instead of a pool of mercury, to generate the DC current.In the current lab, we used a square template to fabricate a coil made of magnetic wire. We included {insert number of turns here} turns in the coil. Other materials we used include paper clips, used as a support for the coils, a stand for the paper clips and wires that connect the paper clips to the DC power source. An image of the final motor is shown in {insert cross-reference to figure here, example: Figure 2}.{Insert a picture of your motor here}{Insert caption here, example: Figure 2: My DC motor}One important step in this process is to scrape half of the wire insulation off one of the support wires. This has the effect of cutting off the current during half of the rotation cycle, allowing the motor to perform a full rotation. If the current is not cut off during this half of the rotational cycle, the motor would oscillate instead of rotating.As the motor rotates, the field interacts with the rotating loop and creates an electromotive force that actually impedes the rotation of the motor. This means that the motor will eventually reach a terminal rotational velocity, no matter how much current is provided. In this report, we will use the equivalent circuit model shown in {insert cross-reference to figure here}.{Insert a copy of the equivalent circuit model here – use the one in the notes, but make sure you are using the correct one!}{Insert caption here, with description}Experimental procedureIn order to examine the actual circuit model shown in {insert cross-reference to the appropriate figure above} we took some measurements using a multimeter and an oscilloscope. A list of the equipment we used and their purpose can be found in {insert cross-reference to table here}.{Insert table caption, example: Table 1. Note that the table caption needs to be ABOVE the table}.InstrumentUse{Fill in table – add rows as necessary.}The first step in the procedure was to measure the resistance of the coil using a multimeter set to take a resistance measurement. The resistance of the coil was {add resistance of coil here – with unit!!}.The second step was to connect the motor to the source, inserting an external resistor in the path of the current. This connection is shown in {insert cross-reference to circuit figure here}. The series resistance allowed us to take measurements of the time-varying voltage on the oscilloscope, which does not directly measure current. Subsequently, we will use KCL, KVL and Ohm’s law to derive the current through the loop, which is also the current through the motor.{Insert figure of circuit here – you can copy/paste this from the lab instructions}{Insert caption here, with description}By using the equivalent model of the motor shown in {insert circuit figure cross-reference here}, the value of the counter-emf voltage induced in the motor due to its rotational motion in a magnetic field is calculated using Equations (2) and (3). Equation (2) is simply Ohm's law. The proof for Equation (3) is given in the Appendix.I=VRseriesRseries(2)Vemf=Vsource-IRmotor-IRseries(3)Once the circuit was constructed, the value of the DC voltage, was set to {insert value with unit here} and the motor was allowed to establish a rotational motion. Once this motion was established, the oscilloscope “cursor” and “measure” functions were used to measure the source voltage and the voltage across the resistor at various times in one cycle. These values along with the circuit analysis results of the current through the loop and the resulting back-emf voltage are recorded in {insert table reference here}.{Table caption – also, add table rows as necessary.}Measured quantitiesCalculated quantities#t [s]Vsource [V]VRseries [V]I [A]Vemf [V]12The current and back-emf values shown in {insert table cross-reference} are shown in {insert a figure cross-reference here} and demonstrate that, even though the power source is a DC source, the current going through the motor is not constant. In fact, it rises up to a maximum and is equal to zero during approximately half of the rotational cycle. This part is, of course, due to the fact that only half of the wire insulation was scraped off the connecting wire so that the current is cut off during half the cycle. The non-DC nature of the “on” cycle of the current is due to the back-emf created by the motion of the motor inside a magnetic field.{Insert two graphs here, labeled (a) and (b). In (a) plot the current as a function of time, that is, time is the horizontal axis and current is the vertical axis. In (b) plot the back-emf as a function of time. Generate these plots in MATLAB by modifying the sample code that was provided for MATLAB Hw 1.}{Insert a single caption, with descriptive text for (a) and (b)}ConclusionIn this lab, we observed the operation of the motor in some detail. The back-emf was observed through indirect measurement of the current going through the motor during its operation. This current was found to be varying even though the source we used was a DC source. This time dependence was due to the rotation of the motor in a magnetic field, which changed the number of magnetic field lines that went through the surface area of the coil at any given time. This change caused a corresponding emf to be induced at the terminals of the coil. This emf is directly related to the instantaneous frequency of rotation, which changes as the motor completes a full circle. Thus, the back-emf also changes with time. This was a way to indirectly observe the phenomenon known as back-emf.AppendixProve Equation (3). Use Question 2 in the DC Motor worksheet we completed in class as a model. The worksheet is available on the Rampages page. Modify the circuit in the worksheet to include the series resistor used in the lab. Then follow the steps to solve for Vemf in the modified circuit. NOTE: You don't plug in numbers in this proof. You work with the symbols (I, Vemf,Vsource,Rmotor,Rseries) only.ReferencesA few things about references:Why always cite references? Because most knowledge that you use in the lab is not your own invention. You are using concepts you learned from other people.Do you just list the references? No, you read them. And you refer to them in the text. If a listed item is not referenced, it most likely does not belong in the list.Where do you get proper references? You can start by using citation lists from Wikipedia, but continue your search at the VCU Library (library.vcu.edu).[1] “Working or Operating Principle of DC Motor,” Electrical Engineering and Technology. [Online]. Available: . [Accessed: 22-Sep-2019]. [2] A. Hughes, Electric motors and drives: fundamentals, types and applications, pages 1-20, 4th ed. Elsevier, 2013. Available via the VCU library at . [Accessed: 22-Sep-2019].[3] A. Filippas, Motors I: Magnetic Force, EGRE 101: Introduction to Engineering, VCU, Richmond, VA. [Accessed: 22-Sep-2019].[4] A. Filippas, Motors II: Rotational Motion, EGRE 101: Introduction to Engineering, VCU, Richmond, VA. [Accessed: 22-Sep-2019].[5] A. Filippas, Motors III: Circuit Components, EGRE 101: Introduction to Engineering, VCU, Richmond, VA. [Accessed: 22-Sep-2019].[6] A. Filippas, Motors IV: Kirchhoff’s Laws, EGRE 101: Introduction to Engineering, VCU, Richmond, VA. [Accessed: 22-Sep-2019].[7] A. Filippas, Motors V: Motor Equivalent Models, EGRE 101: Introduction to Engineering, VCU, Richmond, VA. [Accessed: 22-Sep-2019]. ................
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