OPEN LOOP CONTROL FOR A SIMPLE DC MOTOR ABSTRACT

Transcription

1 OPEN LOOP CONTROL FOR A SIMPLE DC MOTOR Pablo A. Velasquez G. Electrical and Computer Engineering Department, The Ohio State University velasquezgarrido.1@osu.edu ABSTRACT This paper consists in the construction of a simple and low cost DC motor as well as the design and implementation of an open-loop control system circuit which controls the applied voltage to the DC motor through the variation of a potentiometer. The design of the low cost DC motor is oriented to show the physic principles that occur in the functionality of it, such as the Lorentz force, which makes the coil to rotate. INTRODUCTION In the modern life, we find a lot of devices and equipment that use DC motors of all models, sizes, and power to achieve some determined work. These motors are powered with either alternate current or direct current. However, most of the devices and equipment that require low power to move their mechanism employ DC motors of small size, powered with batteries or AC to DC converters. The construction of a simple and low cost DC motor is a very interesting thing to do because it shows how a magnet is related with the flow of electrical current through a coil, which explains some electromagnetic phenomenon. A DC motor is conformed of a magnet, which is often called a permanent type magnet, and a

2 transient magnet, which is the one that is formed once an electrical current flows through a coil. Therefore, a low cost DC motor could be considered as an easy experiment to be taught and developed. CONSTRUCTION OF A DC MOTOR 1. Components: 1 meter of copper wire from 0.6 to 0.7 mm of diameter. A plastic tube of about 4 cm of diameter. 2 paper clips. 2 AA batteries. Wires. 1 AA battery holder. 1 magnet. 2. Steps for the construction of the DC motor. To build the DC motor, we have to start building the coil using the plastic tube as a base; the idea is to get a round shape for it. Then, we proceed to do about 10 to 15 rounds, leaving around 3 or 4 cm of wire on both ends of the copper wire; these will let the coil to rotate once is placed on the paper clips. Both ends of the coil should be in the same imaginary line that goes through its center (diametrically), to accomplish an acceptable balance and obtain better results.

3 Figure 1. Simple DC motor using a coil, magnet, and two AA batteries. Then, we scratch the varnish of the copper wire on both ends of the coil. This will allow the electrical current to flow through all the circuit. Finally, we place the paper clips as shown in Figure 1 on a wood or cardboard base, and then, we connect the two AA batteries to the paper clips using some wires which will close the circuit. A final prototype of the simple DC motor is depicted in Figure 1. Once the circuit is closed, the coil should start to rotate. In some cases, we have to manually rotate the coil because the electromagnetic force is not as strong as the inertia of the coil. 3. Important aspects of this experience: When electrical current flows through a coil, which is placed near a magnetic field, a forced is generated. This force is what makes the coil to rotate as long as it is greater than the coil s inertia. In addition, the magnetic force is generated because of the flow of electrical current which follows the right-hand rule.

4 For the construction of a DC motor, the elements needed are a magnet (which generates a magnetic field) and flow of electrical current through the coil. If any of these are missing, the DC motor will not work. A YouTube video of the functionality of this DC Motor is in the following link: OPEN-LOOP CONTROL SYSTEM DESIGN 1. Components: Bridge Rectifier 3N248 or similar uF capacitor kΩ resistor 2 470Ω resistor 1 100Ω resistor 1 1kΩ potentiometer A toy s DC Motor 1 Opamp zener diode 1N4734A or other with a zener voltage of 5V. 1 NPN Transistor 2N3904. Although, it is highly recommended for this experiment to use the NPN Transistor BD169, which is a power transistor. 1 PNP Transistor 2N3906. Also, it is highly recommended to use the NPN Transistor BD170. An AC source power of V. A 120V (AC) to 12 V (AC) converter can be used.

5 2. Design and calculation The circuit is practical, easy to design, and efficient in what we are looking to achieve as it is shown in Figure 2. Figure 2. Implementation of the open-loop control system on NI Multisim. Figure 3. Final version of the circuit implemented on a protoboard.

6 For its function, we had to calculate the resistors R2 and R4, as well as the capacitor C1. First of all, by looking on the zener diode datasheet, we get the following: Although in the simulation we used the zener diode 1N4734, for the calculation we consider the zener diode 1N5231B, which has a zener voltage of 5.1 V, so the difference will not be important. V! = 5.1 V; 0.5W Z! = I! = 20 ma Z!" = 1.6 I!" = 0.25mA I! = V! = 2 V Now, we wanted to have a ripple of 1 V on the capacitor, so: V!"# = ,4 = V V!"# = V!"# sen(w t! ) Then, V!"##$% = V!"# V!"# 1 V = sen w t! sen w t! = 0.92 Since, w t! = 1,16 t! = 3.07 ma I!"#$ = C.!!"##$%! (1)

7 And, t = T ma 4 I!"#$ =!!"#!! (2) We calculate R2 considering the worst case which is when there is not a load resistor in the output (R4). So: V! = V!" + Z! I! 5,1 V = V!" + 17Ω 20 ma V!" = 4.76V P!"#$ = V! I!"#$ I!"#$ = 98 ma V!"#$ = Vzo + Zz I!"#$ V!"#$ = 6.46 V Therefore, R! = V!"# V!"#$ I!"#$ R! = Ω In this case, a 1kΩ resistor was used to prevent that the current that flows through the zener is not greater than the maximum current of the zener diode. However, a 100 Ω can be used as well. Now, from equation (2): I!"#$ = V!"# R! = 0.19 A From equation (1): C =!!"#$.!!!"##$% C = 66.2 μf

8 We use a 100uF capacitor for the circuit. It all depends on the values available. In the case of R4, we calculated it assuming the worst case which is when the potentiometer has a value of 0 Ω. This means that a great amount of current will go through R4. Assuming that the current in the worst case is less than I!", so: I!"#$ = I!! + I! I!! = I!"#$ I! I!! = 0.14 A Therefore, V! = I!! R! R! = 5,1 V 0,14 A R! = Ω In this case we choose a 100 Ω resistor which would work as equally fine. SIMULATION RESULTS % Pot. V (V) I (ma) (kω) Table 1. Data acquired from the simple DC motor experiment.

9 Figure 4. Graph of V vs I on the DC motor. We can see on table 1, that every 10% of Potentiometer, the voltage on the DC motor increases or decreases 0.5 V. RESULTS The implementation of the circuit on the protoboard was successful. For every variation on the potentiometer value, the voltage on DC Motor changed, and therefore the rotation speed increased or decreased. A link of a YouTube video of the circuit implementation is in the following link: