Characteristics of DC Servo Motor
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Characteristics of DC Servo Motor

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Characteristics of DC Servo Motor Characteristics of DC Servo Motor Document Transcript

  • 1.0 OBJECTIVES The main objectives are: 1. Determining how a permanent magnet DC motor may be controlled by varying the magnitude and direction of its armature current. 2. Obtaining the characteristics of a DC motor by plotting torque and speed. 3. Establishing the scaling factor of the tachogenerator. EQUIPMENTS Below are the equipments and components used in the experiment. 1. Attenuator Unit (AU150B) 2. Servo amplifier (SA155BD) 3. Power supply (PS150E) 4. DC Motor (DCM150F) 5. Loading Unit (LU150L) 6. Reduction Gear Tacho Unit (GT150L) 7. Base Plate 8. Digital Multi-meter 3
  • 2.0 THEORY Servo Motor is actually a combination of four things which is a normal DC motor,a gear reduction, a position sensing device and a control circuit.The function of the servo is to receive a control signal that represents a desired output position of the servo shaft and apply power to its DC motor until its shaft turns to that position.It uses the position sensing device to determine the rotational position of the shaft.So it knows which way the motor must turn to move the shaft to the commanded position.The shaft typically does not rotate freely round and round like a DC motor,but rather can only turn 200 degress or so back and forth.The servo has 3 wire connection which is power ,ground and control.The power supply must be constantly applied so the servo has its own drive electronics that draw current from the power lead to drive the motor. To recap, there are two important differences between the control pulse of the servo motor versus the DC motor. First, on the servo motor duty cycle is on-time vs. off-time which has no meaning of the matters in the absolute duration of the positive going pulse, which corresponds to a commanded output position of the servo shaft. Second, the servo has its own power electronics, so very little power flows over the control signal. All power is draw from its power lead, which must be simply hooked up to a high-current source of 5 volts. The motor is permanent magnet type and has a single armature winding.Current flow through the armature is controlled by power amplifier as shown in Figure 5.1.As a motor accelerate the armature generated an increasing “back electromagneticforce”,Va tending to oppose the driving voltage.The armature current is roughly propotional to Voltage input,Vin toward the Va. If the speed drop because of the attached load,,Va reduces and the current and motor torque will increase.Figure 5.2 shows how the speed Varies with the load torque. 4
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  • 3.0 PROCEDURE 1) The instrumentation was set up according to the connection diagram as shows in Figure 5.3. The magnetic bases was use to organize the various apparatus.The system was provided a tachogenerator couple to the motor and the generator was necessary calibrate by finding the calibration factor,Kg,which is the volt generated per thousand rev/min of motor shaft.The switch on top of the GTX150X was used to display the tacho volt or speed as required.The potentiometer was turned until the tachogenerator voltage,Vg is 1V on the volt meter.The input voltage was recorded and output speed of DC servo motor.The result in Table 5.1 was completed by increase the one volt step until 5V.The Graph of speed against voltage of the tabulated result in Table 5.1 was plotted.The calibration factor was determine by measuring the slope of the plotted graph and the relationship between the input voltage and the output speed was discussed. 2) For the No Load characteristic experiment,the potentiometer was varied until the speed of DC servo motor shows a reading of 2000rpm.The loading unit(LU150L) was set to scale 0, the loading unit(LU150L) was inserted into a moving disc.The readings of the input voltage and the speed of corresponding tachogenerator output was record.The result in Table 5.2 was completed by increase the scale of the loading one step until to 10. 3) For Full Load characteristic experiment,the loading unit(LU150L) was set at scale 10. the potentiometer was varied until the speed of DC servo motor shows a reading of approximate 200rpm . The readings of the input voltage and the speed of corresponding tachogenerator output was record.The result in Table 5.3 was completed by decreasing the scale of the loading one step until to 0.From Table 5.2 and Table 5.3,the speed against torque was ploted and from the plotted graph was conclude 6
  • 4.0 RESULT Table 5.1 Input voltage ,tachogenerator voltage and motor speed Tachogenerator Voltage,Vg (volts) Input Voltage,Vin (volts) Speed (r/min) 1.27 15 460 2.0 15 740 3.0 15 1080 4.0 15 1440 5.0 15 1880 Table 5.2 Brake Position,tachogenerator voltage and motor speed(Test No Load Characteristics) Brake Position (Magnetic Scale) Tachogenerator Voltage,Vg (volts) Speed (r/min) 0 5.61 2000 1 5.60 2000 2 5.53 1990 3 4.95 1780 4 4.41 1580 5 3.83 1380 6 3.46 1240 7 3.26 1170 8 2.69 960 9 2.17 780 10 1.96 700
  • 8Table 5.3 Brake position,tachogenerator voltage and motor speed(Test Full Load Characteristics) Brake Position (Magnetic Scale) Tachogenerator Voltage,Vg (volts) Speed (r/min) 10 0.564 200 9 0.614 220 8 0.684 240 7 0.762 270 6 0.822 290 5 0.884 310 4 1.011 360 3 1.162 420 2 1.277 460 1 1.290 460 0 1.290 460
  • 9 5.0 DISCUSSION In this experiment we conducted three experiment which were to know the relationship between the tachogenerator voltage and motor speed,relationship between the tachogenerator and brake position or called as Test No Load characteristic and Full Load Test. For the first experiment we conducted it by adjusted the Tachogenerator voltage with the same input voltage.We started it with 1.27 tachogenerator voltage,Vg due to the limitation of instrument which can’t adjusted lower than 1.27V.For the others we used the fixed point which are 2,3,4 and 5.The data of the experiment was tabulated in Table 5.1 and from there we realise that as Tachogenerator voltage,Vg increasing ,the speed in revolution per minutes also will increase.This can be proven as in Graph 5.1 Vg against Speed(r/min).So we can conclude that Vg is directly propotional toward Speed. As for 2nd experiment we start conducted it by set up the speed to 2000 r/min.Then we will start to take reading of Tachogenerator voltage ,Vg and recorded it in Table 5.2 by insert the loading unit toward the moving disc by one step from 0 to 10.Roughly from the Table 5.2 ,we can see that increasing in the break position will lead to decreasing on the Tachogenerator voltage,Vg and speed,r/min.This is because of the as the loading unit entered more in the disc area ,it will create greater electromagnetic force which will become the friction or the load.So we can conclude that Vg and speed are inversly toward brake position. Last experiment we conducted it same as the 2nd experiment,but with different started speed which is 200 r/min.Actually the needed speed is approximate 60~70 rpm but seem the limitation of insturement which can started with the 200rpm.We start the experiment with the loading unit scale at 10 ,then when we removed it by 1 step from 10 to 0.From the table 5.3,we can see that decreasing in the break position will lead to increasing on the Tachogenerator voltage,Vg and speed,r/min, This is because of the as the loading unit will slowly leaving the disc area ,it will lower the electromagnetic force existed which will which the friction.So we can conclude that Vg and speed are inversly toward brake position same as the 2nd experiment.So that why graph 5.2 ,we can see the same pattern of curve.
  • 10 6.0 CONCLUSION In this experiment, we learned that permanent magnet DC motor can be controlled by varying the magnitude and direction of its armature current. We also have seen the graph of torque/speed which is the characteristics of the DC motor. Also, we can now establish the scaling factor of the tachogenerator. So now we can conclude that speed of motor can be controlled easily using varies of ways.Such in this experiment ,we control the speed of motor using the load.When we can control the speed of motor ,so we can do many things with the motor,example by adjusting the speed of the motor ,we also can control the tachogenerator voltage, Vg values that we want. 10
  • 7.0 REFERENCES [1] P.C.Sen, Principe of Electric Machines and Power Electronics, 2nd Edition,John Wiley & Sons, Inc. 1997 [2] Hughes, A., Electrical Motors and Drives, Hinesmann Newnes, Nov 2005 [3] Alexander, C.K., and Sadiku, M.N.O.,Fundamentals of Electric Circuits Fourth Edition, New York: McGraw Hill. 2004 [4] H. Dadhwal .(2012 February 13). How Dc Servo Motors Can be Advantageous and Disadvantageous [Online] .Available:http://www.sooperarticles.com/technology- articles/gadgets-gizmos-articles/how-dc-servo-motors-can-advantageous- disadvantageous-826555.html 11
  • TABLE OF CONTENT 1.0 Objectives and Equipments 1 2.0 Theory 2 3.0 Procedure 4 4.0 Result 5 5.0 Discussion 9 6.0 Conclusion 10 7.0 References 11